ese sate rat SAS ase vets stays Ss cS ehesrotetey= Es ie nana ; rai Ee Was peers int stitutes Vota tetarahy Stennis nesters Ke : Sets : ase § pris seins ast Ne fesecet fataes bt pee sf Be Sed ae Bere FR b fit itreree ts ts gates Tiesstiatstetsteaetacass retin op petites 2h s: Spears & Tite nanan eee ys fa ray : 3 x or tk 5 sean Patni se eke a enptet See Pt . he i se aR ore ASS Sasa bye she peieaessas eatanatee f woge . sete oe oe “SS, ft ; \ THE UNIVERSITY OF ILLINOIS LIBRARY ): BE C. cq | Gout BV Ss Set Purchased, 1918. 5S SSI : ae mS A Of wen et Caan . Jr ai QO 2h S R lee? A RH A Oe 7 Beith the collection of Julius Doerner, Chicago | _ CENTRAL CIRCULATION BOOKSTACKS The person charging this material is re- _ sponsible for its renewal or its return to | the library from which it was borrowed on or before the Latest Date stamped below. You may be charged a minimum fee of $75.00 for each lost book. Theft, mutilation, and underlining of books are reasons for disciplinary action and may result in dismissal from the University. TO RENEW CALL TELEPHONE CENTER, 333-8400 UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN a When renewing by phone, write new due date below previous due date. L162 PeeCTRIC LIGHTING A PRACTICAL EXPOSITION OF THE ART FOR THE USE OF ENGINEERS, STUDENTS, AND OTHERS INTERESTED IN THE INSTALLATION OR OPERATION OF ELECTRICAL SECANTS VOLUME II. DISTRIBUTING SYSTEM AND LAMPS BY Pia Nl be CROC KOR JE i Pay: PROFESSOR OF ELECTRICAL ENGINEERING IN COLUMBIA UNIVERSITY, NEW YORK. PAST—PRESIDENT OF THE AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS FR He Di Ga] Oe. NEW YORK , D. VAN NOSTRAND COMPANY LONDON Pec te NE SPON «LINER: 57 HAYMARKET, S. W. 1904 . Copy RIGHT, 1961, By D. Van NostTrRAnpD ComMPany. eM kal Si nw Od wh In presenting this second volume on the subject ‘of Electric Lighting, attention is called to the fact that it relates to the conductors for transmitting and distributing the current, commonly called the distribution system, to the lamps, the supply of which is the final object of the entire system, and to the various auxiliary devices, such.as switches, cut-outs, meters, etc., employed in con- nection with the same. In short, the present volume covers all parts of electric lighting systems outside of the generating plants, the first volume being devoted to the latter. The properties of conductors and various systems of electrical distribution, including direct current, as well as single and polyphase currents, occupy the first half of the book. Overhead and underground conductors are next discussed, and then arc lamps are treated: in considerable detail, since they are important features in electric lighting, and have not been very fully treated in other publications. Interior wiring, incandescent and other forms of lamps, and finally electric meters are given considerable attention. An appendix, containing the National Electrical Code, and another the Report of the Com- mittee on Standardization are included, these being the rules which must, or at least should be, followed in constructing or operating any electrical system. In treating each branch of the subject, the principles have first been given with considerable fullness, being followed by practical examples of the prominent methods and forms of apparatus employed in actual practice. In the space available it has been impossible to go deeply into any subject, but the attempt has been made to cover the important elements and their relation, so that they may be understood and used success- POLL Veer | Both volumes are intended as text books for engineering schools and as hand books for practicing engineers, and for that 800242 1V PREFACE. reason abstruse and detailed matter has been omitted as far as possible. The National Electrical Code, containing the requirements ac- cording to which all electric lighting and other installations should be made, is so important that itis printed in full in Appendix I. The corrections made in December, 1900, were anticipated, and have been incorporated. The Report of the Committee on Stand- ardization of the American Institute of Electrical Engineers being also of fundamental importance, is given in full in Appendix II. The author is glad to take this opportunity to thank many friends for information and assistance. Messrs. J. W. Lieb, C. W. Rice, and P. Torchio of New York, and Mr. W. S. Barstow of Brooklyn, kindly gave the benefit of their wide experience in con- nection with electrical distribution. Messrs. Clark and McMullen of New York rendered valuable assistance in connection with overhead and underground conductors. To Mr. Joseph Bijur the author is specially indebted for a great deal contained in the chapters on the electric arc and arc lamps. - Mr. John W. Howell, of the General Electric Lamp Works, very kindly read over the proof of the chapter on incandescent lamps, in which subject no one has had greater and more successful experience. Mr. C. S. Aylmer-Smail assisted the author in collecting material, in proof-reading, and in other ways. Finally, thanks are due to the General Electric, Westinghouse, and other companies, which have freely given information and illustrations. (ORIN EE INR LDS: CHAPTERS 1. PAGE ELECTRICAL DISTRIBUTION, PHYSICAL PROPERTIES OF CONDUCTORS , . j CHAPTER II. : SHRIPNSEOVSEEMSEON sie RGrRIGA TWD TS DRIP UWL TONG se cea 6 allen oe i CHAPTER Til. RARAR EMD OVSLEMS OFLU bCERICALIOISDRIBUTIONG sii. ela con cae) 2 28 CHAPTER IV. THREE— AND FIVE-WIRE SYSTEMS OF DISTRIBUTION . . . : 2. » © . 70 CHAPTER Va DIRECT CURRENT TRANSFORMER SYSTEMS OF ELECTRICAL DISTRIBUTION . 93 CHAPTER VI. NET WOR KSVORNILLECT RICAL? CONDUCTORS. 4) Bsc ts + Shad cova. ten el, oh enk s LOS CHAPTER VII. PRINCES OLE AL TERNATINGECMRRENTS tk.) otek so. a ep a8 a) a) ee LOS CHAPTER VIIL- RRINCEEME STORE AE TL DRNA TING EOLYPHASE, (GURRENTSH 6. <6 ene cose) 4 CHAPTER IX. Porshe Eh Smee ey en ee Pn lan SPM) Gabe el oh “olf on ae AAD CHAPTER X. ATrTorwvarTine: CURRENT OYSTEMS OF. DISTRIBUTION 4 9/7. 6 6: * ‘s «6 18 CHAPTER: CALCULATION OF ATLERNATFING CURRENT CIRCUITS 2)... 6 eh oe » of 224 CHAPTER XII. UE CAGMELECERICAL CONDUCTORS .. . 2 «on ee ee dee) se (ees oOL CHAPTER XIII. UNDERCROUNITeHTROTRICAT, CONDUCTORS 9. 2. eu 0s 20 we 60 ae cw ss 208 V1 CONTENTS. CHAPTER XIV. ie PAGE Tuk /EVyECTRIC ARC This is not only true of the whole cir- cuit, but also applies to any portion or branch of the circuit ; and ordinarily it is far simpler and more likely to avoid errors if each part of the circuit is considered separately. In the case of a very complicated electrical: system, it would be practically out of the question to treat the circuit as a whole; but it is always possible to divide the system of conductors into separate lengths, in each of which we can determine the current, the resistance, and therefore the fall of potential which takes place. In most practical work the current in amperes is given, since it is usually known how many lamps or how much power are to be supplied. It then becomes necessary to calculate the value of the resistance in order to have the proper value for the drop, the latter being assumed or fixed by the conditions in each case. The common idea that a short con- ductor of very large diameter has no appreciable resistence is quite fallacious. For example, a bar of copper one foot long and one inch in diameter has about one hundred-thousandth of an ohm re- sistance. While this may be a negligible amount in most cases, it is always perfectly definite, and is often quite appreciable. Such a rod would carry one thousand amperes with a drop of one hun- 19 ELECTRIC LIGHTING. dredth of a volt between its ends, or ten volts per thousand feet, which is by no means insignificant. Bars of this size or larger are often used in practice carrying correspondingly heavy currents ; hence it is not safe to ignore resistance, even in the case of very large conductors. Loss of Energy. — The second objectionable effect which re- sistance produces in electrical distribution is the loss of energy’ which it occasions. This loss is absolute, and must always occur whenever a current flows through a resistance. The exact value of this loss is given by the expressions : — fe? Watts lost, = 1A Bie in which / is the current in amperes, 7 is the resistance in ohms, and £& is the drop or lost.pressure in volts, being applicable either to the whole circuit or to any part of it. From one of these ex- pressions the loss of energy can always be ascertained, provided any two of the three quantities are known. These equations give the loss which occurs continuously so long as the current flows ; that is, the vate of dissipation of energy or the power wasted. For a given time ¢ in seconds — i? Lt Loss of energy (in joulés or watt-seconds) = 7 *A7—= 2 77 == To find the loss of energy in heat units, any of the above values may be multiplied by .24 for calories (gram-degree cent.), or by .00095 to obtain British thermal units (pound-degree Fahr.). This loss of energy, while quite considerable in almost every electrical system, usually amounting to from 5 to 25 per cent, is | rarely the controlling consideration in electric lighting. The dof, which has already been considered, and the heating limzt, which will be discussed later, are usually of more consequence than the mere waste of a small fraction of the total energy, the success or failure of an electric lighting plant being dependent upon keeping them. within certain limits. | Economy in Design of Conductors. In many cases, particu- larly for long-distance transmission in contradistinction to local distribution, the relation between the first cost of the conductors and the energy lost in them may be a matter of prime importance. This subject was first attacked in 1881 by Lord “elvin, then Sir PLeCLTIGAL LIS 7KIBU LION. idl William Thomson, who read before the British Association a paper on “The Economy of Metal Conductors of Electricity,” in which he attempted to give a general solution of the problem. The con- clusion reached by him, and now known as “ Kelvin’s Law,” may be stated in the following language: The most economical size of conductor ts that for which the annual interest on capital outlay equals the annual cost of energy wasted. In other words, the total annual expenditure for interest on the investment and energy lost on the line 1s a minimum when these two ttems are equal to each other. The importance of this law has usually been greatly overesti- mated, but gradually its hmitations have been brought out. In 1886 Professors Ayrton and Perry showed, in papers before the Society of Telegraph Engineers and Electricians, that Kelvin’s Law applies only in certain cases ; and they gave various modifica- tions and extensions of it. Professor George Forbes has also con- tributed to this subject in his Cantor Lectures of 1885,* in which he showed that the portion of the investment which is not propor- tional to the cross-section of the conductor should be kept separate, so that the amended law becomes: Zhe most economical area of conductor ts that for which the annual cost of energy wasted ts equal to the annual interest on that portion of the capital outlay which ts proportional to the area or weight of metal used. Professor William A. Anthony, in an article on “ Economy in Conductors, and the Limitations in the Applicability of Kelvin’s Law,” + demonstrates that in some cases Kelvin’s Law gives absurd results, and' may, for example, require that a// of the energy should be wasted tn order to secure the highest economy. This is due to the fact that the minimum expense of operation is considered, and the energy deliv- ered at the end of the line, which is still more important, is entirely ignored. In fact, a great many laws of this kind can be deduced according to what factors are considered. Kilgour ¢ and Abbott § give 15 possible combinations of the six variable factors involved in the problem, but state that only 11 of these are likely to be of any practical importance. The six factors and the 11 cases are as follows: -* London Liectrician, vols. xv. and xvi. + Electrical Engineer (N. Y.), Oct. 31, 1894. t Electrical Distribution, London, 1893, p. 115. § Electrical Transmission of Energy, N. Y., 1895, p. 457. Te, ELECTRIC. LICHIING: - V = the pressure at'the receiving end of the conductor ; v = the pressure at the delivering end of the conductor; W = the power given to the receiving end of the conductor; zw == the power obtained at the delivering end of the conductor ; / = the current in amperes, and SS = the cross-section of the conductor. CASE NO-e GIVEN. REQUIRED. CASE NO. GIVEN. REQUIRED. iJ eee a Ly hls 20, Se . nine Veit 2 Naan & v7, W,w, S. 8 1 SES. V, Daal, aes 3 V, w. WAT AE , as 9 W, w. V, Ula .4 VS v, 1, W, w. 10 WES. Viv, I, w. 5 OF: View Oh aos 11 w, S. Vo 0 ERG 6 v, W. Ped, Wye: This is a far more complete treatment of the question than that originally given, Kelvin’s Law being only one (No. 5) of these 11 different cases. But any of these solutions of the prob- lem is of somewhat doubtful practical value; and it is probably true that Kelvin’s Law, or any modification or extension of it that has yet been brought out, has done more harm than good in elec- trical engineering. It gives a false confidence in the results of calculations which may be totally at variance with real commercial economy. The reason for this difficulty lies chiefly in the fact that the actual costs of some of the items cannot be expressed, even approximately, as mathematical functions. Furthermore, various incidental factors and particular conditions arise, such as the avail- able sizes of machines, which render a general solution of this problem of questionable value in the actual cases which are found in practical work. It is a common mistake to forget that the interest and depre- ciation on the investment is a fixed and irretrievable expense, while the energy lost on the conductor depends upon the power transmitted. When the plant is lightly loaded, or is shut down entirely, owing to hard times, strikes, etc., the fixed charges run on as usual, but the energy loss is greatly reduced, or stopped altogether. Hence it is not wise to lay the full amount of copper corresponding to the maximum or even ordinary demands, as there is no control over the investment after it is once made, whereas the energy loss adjusts itself to the working conditions, Probably the safest, as well as the quickest, method to arrive at a correct result would be to obtain a general solution of the PREGURICAL DISTRIBEGTION 13 problem by means of some form of Kelvin’s Law; then this re- sult should be carefully checked by assuming a larger and also a smaller wire, and estimating the economy that would be secured if they were substituted for the size of wire obtained by the calcu- lation. The difficulties of determining the various items of ex- pense are greatly reduced by assuming a certain size of wire, and the several factors that are almost impossible to cover by a gen- eral formula, immediately become definite. Scientific and rational methods should always be preferred to empirical ones; but every experienced engineer will admit that when complicated questions of cost arise it is unwise to rely entirely upon general formule, which are almost necessarily abstract and incomplete. The at- tempt to force science beyond its legitimate limits has done great injury to many industrial enterprises as well as to science itself. Specific examples of this problem will be considered later in the case of constant-current arc-lighting circuits and feeders for con- stant potential systems. Current-Carrying Capacity of Conductors. — The third objec- tionable effect of resistance in electrical distribution is the heating which it causes. The production of heat in an electrical conduc- tor has already been stated in terms of the various quantities in- volved. This heat is an absolutely definite and unavoidable result cmethestowsottie. current. [ts etiect, is to. raise the tempera- ture of the conductor, and this rise continues until the rate at which heat is lost equals the rate at which it is generated; then the temperature becomes constant. It is obvious, therefore, that any electrical conductor is only capable of carrying a certain cur- rent with a given elevation of temperature, and in practical work the allowable temperature is limited by considerations of injury to insulation, danger of fire, etc. No exact general rule for current capacity can be given, as much depends upon the conditions in each case. But, since a wide margin must be allowed between the danger point and the permissible current capacity, it is possible to establish rules which are somewhat arbitrary, but sufficiently safe in almost any case. This is practicably the basis upon which tables are made giving the current that it is allowable for any size of wire to carry. These tables are partly based upon general experience, and partly the results of experiment and calculation. The first rule of this kind originated with Lord Kelvin, and 14 BEBOCTRICOLIGIUT ING: was adopted by the Board of Trade (London). It stated that the current density in copper conductors should not exceed 1,000 amperes per square inch of cross section. Professor George Forbes discussed this problem in a paper read before the Institution of Electrical Engineers (London) in March, 1884, and showed that the Board of Trade rule was hardly safe for very large conductors, and gave an unnecessarily large margin for small wires. This fact is very evident when it is con- sidered that the current at a given density and also the heating increase in proportion to the square of the diameter of a wire, while the heat-dissipating surface only increases as the diameter. Dr. A. E. Kennelly has given the results of his investigations in two papers before the Association of Edison Illuminating Com- panies, Aug. 18, 1889, and Aug. 11, 1898.* He determined by calculation and experiment the heating of conductors submerged in water, buried in the earth, inclosed in wooden molding, and suspended in air. It is found that there is not such a great difference between the heating effects under these various conditions. An insulated cable in water is the simplest case; since the rise in temperature of the conductor depends merely upon the thermal resistance of the insulation, the outer surface (or sheathing) of the latter being kept at a constant temperature by the water. An underground conductor only differs from the foregoing in the fact that its sheathing may rise in temperature because heat is not taken from it rapidly enough by the surrounding soil or conduit. In other words, the thermal resistance of the conduit and soil is added to that of the insulating covering. For underground conductors in iron pipe conduits laid in cement, the temperature elevation due to this cause would be small, probably not more than 10 or 20 per cent greater than that of the same cables submerged in water. The heating of conductors in wooden or even earthenware con- duits would be considerably greater, and in the case of the former might be considered to be the same as for those placed in wooden panels or molding, the rules for which will be given later. Insu- lated wires suspended in air are more highly heated than similar submarine or most underground conductors, for the reason that the thermal losses by radiation and convection through the air are * Electrical World, Nov. 23 and 30, 1889 and Sept. 2 and 9, 1893. ELECTRICAL DISTRIBUTION. 15 less than those through water and solid bodies, except those which are very poor conductors of heat, such as wood. For similar rea- sons the temperature rise of a bare wire in air is usually greater than that of the same wire covered with insulating material. The effect of the latter is to increase the surface from which heat is radiated and carried away by convection. In most cases a con- siderable increase in the temperature of a bare wire is not objec- tionable .except so far as it represents loss of energy. The real limitation to the heating of electrical conductors is the point at which their insulation is likely to be injured. The following are standard tables, giving the maximum current- carrying capacity of different sizes of insulated copper conductors : TABLES, Of.) CURRENT =—~CARRVINGSCAPACITY. TABLE 1. TABLE 2. TABLE 3. TABLE 2. TABLE 3. A. W. Ge AMPERES. AMPERES. AMPERES. CIRCULAR MILLS. AMPERES. AMPERES, 18 3 3 5 200,000 200 300 16 5 6 8 300,000 270 400 14 10 12 16 400,000 330 500 12 15 17 23 500,000 390 590 10 20 24 32 600,000 450 680 8 25 a8 46 700,000 500 760 6 35 46 65 800,000 550 840 5 45 54 77 900,000 600 920 4 50 65 92 1,000,000 650 1,000 3 60 16 110 1,100,000 690 1,080 2 70 90 131 1,200,000 730 14150 1 85 107 156 1,300,000 70m 1,220 0 100 127 185 1,400,000 810 1,290 00 120" = 9 150 220 1,500,000 850 1,360 000 145 177 262 1,600,000 890 1,430 0000 175 210 312 1,700,000 930 1,490 1,800,000 970 1,550 1,900,000 1,010 1,610 2,000,000 1,050 1,670 Table No. 1 is based upon Kennelly’s experiments, and is in- tended to allow a rise in temperature of 75° F. for twice the cur- rent specified, thus giving an ample factor of safety. The normal current would only raise the temperature 18%° F., since the heating effect is proportional to the square of the current. The National Electrical Code permits a current density 20. to 25 per cent greater than the foregoing, the figures being given in Table 2. 16 ELECTRICWIGATING. This would give a temperature elevation of 27° to 30° F., and still allows a considerable increase (about 60 per cent) in current above the rated value without injurious effects. This applies to rubber-covered wires, which should never be heated above 150° F., and should have a normal working temperature considerably below this limit, in order to have a margin for safety. Table 3 permits a still greater current density, and is used for wires with “ weather proof” insulation, which is not so susceptible as rubber to injury by heat. BIBLIOGRAPHY OF ELECTRICAL TRANSMISSION AND DISTRIBUTION, INCLUDING OVERHEAD AND UNDERGROUND CONDUCTORS AND INTERIOR WIRING. ABBOTT, A. V., Electric Transmission of Energy, N.Y., 1895. Bapt, F. B., Zucandescent Wiring Handbook, Chicago, 1894. BELL, Louis, -lectric Power Transmission, N.Y., 1897. Davis, C. M., Standard Tables for Electric Wiremen, N.Y., 1896. HERING, CARL, Universal Wiring Computer, N.Y., 1894. Kapp, G., electric Transmission of Energy, London, 1894. KILGOUR, SWAN, AND BiaGs, Electrical Distribution, [ts Theory and Practice, London, 1898. No.1, A., How to Wire Buzldings, N.Y., 1893. RAPHAEL, F. C., Localization of Faults in Electric Light Mains, N.Y. and London, 1897. Ross, R., Llectric Wiring, N.Y. and London, 1896. RUSSELL, S. A., Hlectric Light Cables and the Distribution of Electricity, London, 1892. WATSON, A. E., Handbook of Wiring Tables, N.Y., 1892. WEILLER ET VIVAREZ, Lignes et Transmissions Electriques, Paris, 1892. SERIES SVSTEMS OF ELECTRICAL DISTRIBUTION. Ay Cua rrside: 1s Agee idl SERIES SYSTEMS OF ELECTRICAL DISTRIBUTION. THE various systems of electrical transmission and distribution are Classified in the following table. They are especially selected with reference to their use in electric lighting; but they include those employed for power transmission and other electrical pur- poses, the same principles and methods being generally applicable. SYSTEMS OF ELECTRICAL DISTRIBUTION. DERIES SYSTEMS, Constant Current. Voltage usually varied. Direct Current. Series arc lighting. Usually operated at about 10 amperes and 50 volts per lamp. Series incandescent lighting. About 10 amperes and 10 to 380 volts per lamp (about 3 candle- power per volt). Series incandescent lighting (“ Municipal systems ”’). Three to 3.5 amperes and 20 to 50 volts per lamp (1 volt per candle- power). Series-parallel incandescent lighting. Similar to No. 2, but single lamps replaced by groups in parallel. Direct current-converter systems for incandescent or arc lighting. Motor-dynamos in series, lamps supplied by secondary circuits, Alternating Current, 6, 7, 8, 9, and 10. Alternating current systems corresponding to Nos, 1, 2,3, 4, and 5. PARALLEL SYSTEMS. Constant Potential. Current varies with number of lamps. Direct Current, II. 12. 13. 14. Two-wire incandescent and arc lighting (about 110 or 220 volts). Three-wire incandescent and arc lighting (about 220 or 440 volts). Five-wire incandescent and arc lighting (about 440 volts). Two-wire with motor converters in parallel (primary 1,000 to 5,000 volts). 18 BPLECTRIC LIGHTING. Single Phase Alternating Current. 15. Low tension incandescent and arc lighting without transformers. This corresponds to No. 11. Other alternating current systems similar to Nos. 12, 18, and 14 have not been introduced. 16. High-tension incandescent and arc lighting with transformers. Primary circuit 1,000 to 5,000 volts, two- and three-wire secondary circuits at about 50, 100, or 200 volts. 17. Very high tension systems with step-up and step-down transformers. Long distance transmission circuit 5,000 to 25,000 volts. Polyphase Alternating Current. 18. Two-phase system. 19. Three-phase system. 20. Monocyclic system. For the sake of completeness, the above table includes almost every possible system of electrical distribution, but many of them are unimportant or entirely obsolete at the present time. The systems which are now more or less generally used are Nos. 1, 11, 12,13, 14, 46317, 8, 1O;-and/s202 ) Uhevlastethrcesare tprimamin intended to operate motors, but are also employed in many cases for electric lighting. SERIES SYSTEMS OF DISTRIBUTION. The simplest arrangement of lamps or other devices to be sup- plied with electrical energy is a series system in which the cur- rent from the + terminal of the dynamo, J, passes first through Fig. 1. Series Arc Circuit. one lamp, Z, and then through another, and so on, finally returning to the — terminal of the dynamo, as shown in Fig. 1. In such cases the current is usually constant, hence the expression constant current is practically synonymous with sevzes in electrical distribu- SERIES SYSTEMS OF ELECTRICAL DISTRIBUTION. 1) tion. The term igh tension also applies, since the voltage usually employed is high, being equal to the sum of the pressures con- sumed in all of the lamps on the circuit. For example, sixty lamps are commonly placed upon a single arc-lighting circuit ; and since each lamp (open arc) requires about fifty volts, it follows that the total pressure approximates 3,000 volts. The problem of designing or studying series circuits is not difficult, the path of the current being usually simple, and the current constant throughout the circuit. This last statement is only true, however, if the leak- age of current is insignificant, which is generally the case in elec- tric light and power distribution. Distribution of Potential on Series Systems. — The potential on a series system falls throughout the circuit in direct proportion to the resistance. That is, E=/R, the difference of potential £ in volts between any two points being equal to the product of the current / in amperes and the resistance # in ohms included between them. This simple fact completely covers any possi- ble problem that can arise in connection with a series system, pro- vided a direct current is used, and is easily applied in almost any L ib u ~ MA a ¢ i] e R M Gia ! : : P Q Fig. 2. Distribution of Potential on Series System. case. In Fig. 2 an arc-lighting system is represented, D being the dynamo and Z, Z, Z, the lamps, connected in series. The total difference of potential generated by the dynamo is assumed to be 1,000 volts, measured between the two brushes marked + and —. This potential falls as the current traverses the circuit, fifty volts being consumed by each of the twenty lamps. This is made up of forty-five volts actually used in the lamp itself, and a drop of five volts on the conductor between two lamps. That is, the drop on the line wire is usually about 10 per cent of the total AJIZF. The relative potential of the various points on the circuit is easily 20 ELE CERI CHIIGHIING. found. For exampie, between the + brush and the middle point M of the circuit, there is a difference of potential of 500 volts, and the same amount between the middle point and the — brush. Similarly any two points on the circuit will have a difference of potential equal to fifty volts, multiplied by the number of lamps included between them. Personal Danger from Series Circuits. — If a man standing on the ground touches a very highly insulated circuit, only a very slight current will pass through his body; but if the insulation is low or any defect exists at any particular point, then a considerable current may flow through his body. In Fig. 2 the line is supposed to have a ground connection at the point /, due to a defect in the insulation. This will cause the potential of the circuit to be zero at that point ; consequently a man may stand on the ground, and touch the line at that point with perfect impunity. If he touches the wire at point Q he will receive a barely perceptible shock, due to 100 volts, since there are two lamps between that point and the ground connection; but if the circuit be touched at the point X&, the difference of potential between it and the ground connection being 18 x 50 = 900 volts, will produce a dangerous, or perhaps fatal, current. When the defect in the insulation does not amount. to what is called ‘dead ground,” but has a resistance, for example, of 1,000 ohms, then a man touching the wire at the point & will receive a shock due to 900 volts as before; but the resistance of the ground connection, which is 1,000 ohms, will be in series with his body. Consequently the current will be less; and, assuming the resistance of his body to be 1,000 ohms, the current will be one-half as great as in the first case. If the ground connection has a resistance of 8,000 ohms, the current through the body would 900 i} be = 8,000 + 1,000 10 We may sum up these various cases as follows : — 1. A very highly insulated direct-current electrical circuit may be touched at any one point without danger by a man standing on of an ampere, which is not dangerous. or in connection with the ground. 2. If a ground connection exists on a series electrical circuit, the danger of touching the circuit increases directly with the resist- ance between the ground connection and the point of contact. 3. The resistance of the ground connection is in series with Dirt OS wists OF ELECTRICAL DISTRIL ULION, 21 the body of any one connected with the ground and touching the wire at some other point. 4. It is never safe, however, to assume the insulation to be perfect, or that a ground connection exists at some particular point, or that it has a high resistance. The circuit should always be treated as if the most dangerous possible conditions existed. Regulation of Series Systems.— The condition required on series circuits is usually the maintenance of a constant current. This is accomplished by designing the dynamo so that it will automatically generate a nearly constant current. The various | J Me) dynamos used in series arc-lighting, such as the Brush, Thomson- Houston, and Wood machines, are well-known examples of this type of generator. They are provided with regulating devices, which either shift the brushes or vary the strength of the field, OL (DC ming Ordersto keep rthe current, at«a, constant value. In addition to these special regulators, such machines are so designed that they have considerable self-induction, resistance, and armature- reaction, all of which tend to prevent the current from rising to a high value, even when the machine is short-circuited.* eries Arc-Lighting System. — The general arrangement of the apparatus and circuit is represented in Fig. 1. The dynamo and lamps may be selected from the various well-known and thoroughly successful forms of constant-current arc-lighting apparatus. The determination of the proper size of wire is not very difficult. Gen- eral custom and considerations of strength require that no wire smaller than No. 8, A. W. G., should be used. Similarly it would not usually be necessary to employ a conductor larger than No. 4, because the potential being high, and the current small, the loss of \energy is not great, even in a wire several miles in length. To # NG ; G A 5 ‘\ take a specific case, let us assume a circuit five miles long, supply- Xe ing 80 arc lamps, the potential being 4,000 volts and the current 10 amperes. If No. 6 wire is used, the resistance would be 2.1 ohms per mile, or 10.5 ohms for the whole line. This involves a drop of 105 volts and a loss of energy of 1,050 watts, which is only 2.63 per cent ; consequently it is evident that the use of a little larger or a little smaller wire would not seriously affect the economical working of such a line. The substitution of No. 3 for No. 6 wire would save one-half the loss of energy, the cross- * For a description of such machines see Vol. I., p. 330. Fag ELECTRICAIAGHLING section and weight being twice as great, and the cost of the insu- lated conductor would be nearly doubled. With No. 6 wire the total weight of copper would be 2,098 pounds, and the cost of the wire (insulated) would be about $500. It is doubtful if it would be wise to invest an additional $500 in order to use No. 3 wire and save one-half the energy or 025 watts. The New Brush Arc-Lighting System is an interesting case of series distribution. In the original type of Brush dynamo the armature is provided with two or more separate open-coil windings, connected to a corresponding number of commutators. ‘The cir- cuit leads through these windings in series, so that the construc- tion may be regarded as equivalent to several armatures in series. P 7+ 3000 C 2000 — 3000 Fig.4. Figs. 3 and 4. Original Arrangement of Brush Are Lighting System. This arrangement is represented diagrammatically in Fig. 3, in which A, #, and C are three commutators connected in series with each other and with the line that supplies a number of lamps, L, £, etc., in the usual manner. Assuming each armature wind- ing to generate 2,000 volts, the total £.47/ of the machine will be 6,000 volts. The distribution of potential in this case is shown in Fig. 4, the + brush of C being + 8,000 volts, and the — brush of A being — 8,000 volts, with respect to the potential of the eatth, which as’ represented ‘by thegzero line "COs welnesiallvor potential through the circuit is indicated by the inclined lines, P O and O WN, the total amount being 6,000 volts, and the middle point being zero. This assumes an ideal case with a uniform distribu- tion of conductor resistance and insulation resistance, but would SERIES SVSTEMS OF ELECTRICAL DISTRIBUTION. aes be approximately true for a practical case in which the system was in good condition. If the insulation of some portion of the circuit became poor, it would _- tend to make the poten- tial at that point ap- P.+6000 G ; 2000 proach zero, producing a corresponding change B in the rest of the cir- 2000 Cll sHOmsexampic.. a ground connection at ei the negative terminal VV 0 would bring that point N Fig. 5. Distribution of Potential with Grounded +to zero, and the positive : Terminal. terminal P would then become + 6,000 volts, as represented in Fig. 5. The new Brush system, illustrated in Fig. 6, differs from the old in the fact that the lamps, Z, Z, are inserted in the circuit between the commutators 4, A, and C, in which case the line con- sists of three loops. With this arrangement, the .J/F. generated by each of the three armature windings is consumed by the lamps Fig. 6, Fig.7, Figs. 6 and 7. New Arrangement of Brush Arc Lighting System. between it and the next armature winding, so that the potential does not rise above + 1,000 volts, or fall below — 1,000 volts, in the ideal case represented in Fig. 7. Even if the circuit becomes grounded at any point, the potential will nowhere exceed 2,000 volts, and the maximum difference of potential existing between any portions of the circuit will not be greater than this amount. A voltmeter connected across from the — brush of B to the + brush of C would only indicate 2,000 volts, in spite of the fact that the £.17.F. generated between those points is 4,000 volts, the 24 ELECTRICALIGHIING: remaining 2,000 volts being used in the lamps between B& and C. This reduction or subdivision of the total £.47.F; is the advantage of this system, and avoids the dangers involved in the use of the ordinary types of machine for supplying a large number (50 to 200) of arc lamps in series. On the other hand, it is necessary to arrange the line in several loops instead of having one long cir- cuit. In Fig. 6, for example, there would be six wires running out from the station, while Fig. 8 would only require two. Never- theless, the former plan may be preferable. to the operation of three separate dynamos, which would be less efficient, occupy more space, and demand more attention than a single large machine. If desired, the number of Jamps on any loop may be increased or decreased, since the current is kept constant by’a regulator on the dynamo; and it is quite immaterial where the resistance is introduced in a series circuit. In fact, any or all of the lamps may be cut out, or they may be put upon two loops and none on the third, or the full load may be placed on a single loop, in which case the arrangement reduces to the ordinary one shown in Fig. 3. When the number of lamps on any loop is augmented or diminished, the potential difference between its terminals varies in direct pro- portion, so that two-thirds of the lamps on one loop would require a P.D. of 4,000 volts between the brushes to which it is connected. This gives great flexibility to the system, and provided the lamps are not very unequally divided, the pressure is not excessive on any one loop. It should be noted that in either the old or the new system, the full /.JZ.F. of 6,000 volts would be found to exist if the circuit be opened at any point. Indeed, the P.D. would tend to rise momentarily considerably above the normal voltage.* Series Incandescent Lamps on Arc Circuits. — Several forms of incandescent lamps have been designed and manufactured for use on the regular 10-ampere arc circuits. These consist of lamps similar in general principle and construction to those used for con- stant potential, parallel distribution, but containing a shorter fila- ment of larger cross-section that is sufficiently heavy to carry the full current of 10 amperes. The most important consideration is that of maintaining the continuity of the circuit when the filament of any lamp happens to break, which might occur at any time. This may be accomplished * Volalwep eget. nie lesestolLiis OF ELECTRICAL DISTRIBUTION. 25 by some form of cut-out, which short-circuits the lamp when the filament is broken. One type of this device is called a “film cut- out,” and consists of a thin sheet, /; of paper or other material interposed between the points P and P connected to the conduc- tors 4 and & which enter and leave the lamp, as represented in Fig. 8. This film obliges the current to pass through the lamp so long as the fil- ament is intact; but when the latter breaks, the difference of potential rises from its ordinary value of 10 or 20 volts to the full A.AZF. of the circuit, which is usually several thousand volts. This high pressure is sufficient to puncture the film, allowing the current to pass di- rectly across between the points P and Pa ee ae P, thus short-circuiting the lamp and re- descent Lamp. establishing the continuity of the circuit. In some cases a small automatic switch is employed, which is caused to close and short-circuit the lamp by means of a magnet connected across as a shunt between the leads of the lamp. The coils of this magnet are of high resistance, and carry little current until the filament is broken, when the full current is thrown through them, causing the switch to close. ‘¢ Municipal’? Series Incandescent Lighting Systems. — These are similar to the preceding; but instead of operating with a’stan- dard arc-lighting current of 10 amperes, they are usually designed for about 8 or 38.5 amperes. This gives a filament of sufficient length and cross-section to be durable, and yet does not require excessively large leading-in wires. The lamps are made of various sizes, requiring about one volt per candle-power. The practice with this system is to feed the circuit with a constant potential, usually from 590 to 1,000 volts, several of such circuits being or- dinarily operated in parallel by the same dynamo, YD, as represented in Fig. 9. This arrangement is therefore a parallel-series system. When the filament of a lamp breaks, and it is automatically cut out of the circuit, the current increases in strength, since the total resistance is reduced, the potential remaining constant. This in- crease of current is indicated by an ampere meter, or current indi- cator 4 placed in each circuit, and is corrected and brought back Fig. 8. 26 ELECTRIC LIGHTING. to its normal value by switching in an extra or “relief” lamp Z,, at the station. This is usually done by an attendant who is kept on duty to watch the various circu:ts. The system is rather a crude one, and is rarely used except for street-lighting in place of arc lights where the more powerful light of the latter is not required. Either the direct or alternating current is applicable to this method of distribution, and both have been used. ‘The current capacity of the dynamo must be sufficient to supply the various circuits in parallel. In the case shown (Fig. 9), the current required would be 15 amperes, since there are 5 rows of lamps, each taking 3 am- peres. With 10 lamps of 50-candle-power and 50 volts in series, the dynamo should operate at a constant potential of 500 volts. A shunt or compound wound direct current machine, or a separately excited or composite alternator, would be suitable for the purpose. ee eM eo ke te le [ 24O-O-O-O0-0-0-0-0-0-O7F] 4 & E}-0-0-0-0-0-0-0-0-0-0 9.75. , [| -}-O-O-O-O-O-O-O-O-O-O i DO eo fA Se / ke Fig. 9. Parallel-Series System of Distribution. In the case of the compound or composite machines they should ~. | ; {= simply give an absolutely constant potential, since the number of | lamps, and therefore the drop, on each circuit is constant. Series-Parallel Incandescent Lighting Systems may be arranged in the manner indicated in Fig. 10. Several lamps are arranged in parallel to form a group, and a number of such sets are con- nected in series, as shown. It is not necessary for the groups to be identical, provided they are all adapted to take the same current in amperes, which should be kept constant, and provided the lamps of each set agree in voltage. For example, on the ordinary 10-ampere arc circuit, one group might consist of 5 lamps, each requiring 50 volts and 2 amperes; the next might be composed of 10 lamps, each taking 100 volts and 1 ampere, and so on. Such groups have been used directly on the ordinary series arc- lighting circuits (constant current), like the series incandescent 4 Aan i £ SLRIES SYSTEMS OF ELECTRICAL DISTRIBUTION. 27 lamps described on page 24. The former arrangement is even less practical than the latter, and is also inferior to the “muni- cipal”’ system, since a lamp which breaks or burns out cannot be either short-circuited or compensated for by adding a lamp in the station. To provide for this contingency, which is likely to be of frequent occurrence, a local device ds required for each group, which will either connect fv new, Jan Payee ia sone of the Fig. 10. Series-Parallel System of Distribution. lamps fails, or short-circuit the entire group. This is such a com- plicated and unreliable arrangement that the system is not a very practical one. Alternating Current Series Systems. — Each of the direct cur- rent series systems that have been described has, or at least might have, a counterpart alternating current system. The general arrangement and method of operation would remain substantially the same; but as the phenomena of alternating currents differ in some respects from those of direct currents, the discussian of such systems will be given in the chapters on Alternating Current Distribution. 28 ELECTRICAINGH TING. en CE Agmaue Ra Tt PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. In contradistinction to the series connection of lamps or other devices to be supplied with electrical energy, the other common method of distribution is the favallel or multiple arc arrangement represented in Fig. 11. Assuming that four lamps, each taking one ampere, are to be fed, the current generated by the dynamo D should be 4 amperes, which divides at the point where the first lamp is connected, and 1 ampere flows through it. The remaining 3 amperes pass on to the next lamp, and so on. The current sup- plied by the source should be equal to the sum of the amperes Fig. 11. Principle of Parallel Distribution. required by all of the lamps or other devices that are connected at any given time. The voltage should be as nearly constant as pos- sible ; hence the system is designated as constant potential, but this is only approximately true. In the case illustrated, the dynamo generates 112 volts, which is slightly reduced by the resistance of the wires until it falls to 110 volts at the last lamp. Parallel systems are far more important in electrical distribu- tion than series systems; since practically all incandescent lamps, a large proportion of arc lamps, and nearly all electric motors, are supplied by them. Constant potential circuits are usually more complicated than the simple series systems, there being only a LANA fis DIGLLMS (OF ELECTRICAL DISTRIBUTION. 29 single path for the current in the latter case, while with parallel connections there are a number of branching paths. Furthermore, the maintenance of a uniform voltage over a large district is exceedingly difficult. The “drop” or loss of voltage, due to the resistance of conductors, which has already been discussed on page 9, is particularly objectionable in incandescent lighting, since the slightest decrease of potential produces a very considerable diminution of light. For example, the candle-power of an ordinary lamp is reduced from 16 to 15, which is more than 6 per cent, wken the pressure falls from 110 to 109 volts, or less than 1 per cent. Such a very small variation in pressure would hardly be appreciable in any other practical work, such as steam or gas distribution. The drop in pressure produces three different effects in the lamps or other devices supplied by parallel circuits : — (1) All of the lamps receive a lower voltage than that gene- rated by the source of electrical energy. (2) Some lamps may be supplied with a lower pressure than others. | (3) The potential at some lamps may vary when others are thrown on-or off the same circuit. The least harmful of these effects is the first, which merely requires the generator to be run at a little higher voltage, and does not necessarily involve any difference between the candle-power of the lamps, since the drop may be made substantially the same for all of them by some of the methods described later, On the other hand, varzations in the candle-power of lamps, due to either of the last two effects, are extremely objectionable and difficult to overcome. In order to study these problems let us take a specific case, and assume that 100 incandescent lamps are to be supplied with electric current. They are supposed to be divided into five groups of 20 lamps each; each lamp requires a current of 110 volts and. one-half ampere, and gives 16 candle-power ; there- fore one group takes 10 amperes, the total current being 50 am- peres. The members of each group of lamps are connected in parallel in the usual manner, but will be indicated by a single line in the following diagrams in order to avoid confusion. These groups are assumed to be 200 feet apart in a straight line, making a total distance of 800 feet between the extreme groups, as shown 3 ELECTRICILIGHIING. in Fig. 13. The five groups of lamps represented by the light vertical lines are connected together by two conductors, which are shown as heavy horizontal lines. [hese conductors correspond to the so-called szazzs in electrical distribution systems, to which are connected the /eads or small branch wires actually supplying the Fig. 12. Arrangement of Feeders and Mains. lamps. The mains receive their current through feeders, AA and LL, which connect them with the generating plant J, as represented in Fig. 12. .As a general rule no lamps are connected directly to the feeders. The celebrated “Feeder and Main” patent of Edi- son * covered this arrangement of electrical conductors. In the first case, represented in Fig. 18, the mains are supposed to be fed at one end, the feeding-points being represented by short vertical lines marked + and — respectively. The mains are as- sumed to consist of No. 0000 wire,.A. W. G., which would weigh 1,025 pounds for 1,600 feet required. Each section of the mains consists of 200 feet of No. 0000 wire, and has a resistance of about 800 ft..No.0000 Se ey Current =|10 AMP, Drop = "4 VOLTS ; : P.D.= 111 VOLTS 110.2 109.6 109.2 (09. Fig. 13. Feeding at One End of Mains; 1025 Ibs. Copper; 2 Volts Max. Difference Between Lamps; 111 Volts at Feeding-Points ; 1.2 Volts Average Drop. .01 ohm. The current in the first section of the + main is 40 amperes, since it supplies 4 groups of lamps taking 10 amperes each, hence the drop is 40 x .01 = .4 volt. Similarly the drops in the other three sections are found to be .8 .2 and .1 volts respec- tively. The drop in the — main has exactly the same values, but is in the opposite direction, the fall of potential being always in the * U.S. Patent, No. 264,642, Sept. 19, 1882. PARADE Eien hie OF RLECTRICAL DISTRIBUTION. of direction in which the current flows. The distribution of potential is shown in an exaggerated manner in Fig. 14. It will be seen that a potential of 111 volts, supplied at the feeding-points, gives 109 volts at the other end, therefore no lamp receives a pressure more than one volt greater or less than the normal value of 110 volts. The horizontal axis OO would represent the line of zero poten- tial when the system is uniformly insulated, in which case the potentials of the mains at the feeding-points would be + 55.5 volts and — 59.5 volts respectively. A defect in the insulation at any point would tend to cause the potential of that point to approach zero, as already explained in connection with Figs. 4 and 5; and if the — feeding-point were grounded, the + feeding-point would + Drop= AVolt 109, Drop= Fig.\4, Fig. 14. Potential Diagram Corresponding to Fig. 73. become + 111 volts, all the potentials having positive values. ok the potential dzfference would remain the same in all cases. | Tapering Conductors. — The use of tapering or “conical ear ductors in place of the ordinary cylindrical ones is hardly practica- ble, on account of the difficulty of making a wire or rod of that form. It is possible, however, to use a jointed conductor com- posed of sections of different sizes of wire. The object of such an arrangement is to proportion the cross-section of the conductor to the current which it has to carry in cases where the current varies from point to point, this being the usual condition in parallel distribution. If Fig. 13 be modified in such a way that the size of each section of the main is proportional to the current passing through it, Fig. 15 is obtained. In this case the drop in each section will be .25 volts, being the same for all. Hence the po- tential falls uniformly from the + feeding-point to the end of the b2 ELECTRICVLIGH TING. main, and would be represented by a straight line, instead of the broken one in Fig. 14. It is sometimes stated that the use of tapering mains secures economy in copper, but such is not the case in ordinary parallel distribution. The weight of copper required in Fig. 15 is 1,013 lbs., which is practically the same as the 1,025 Ibs. called for in Fig. 18. The fallacy arises from the fact that the conductor is assumed to be a true cone, the elements of which are straight lines. As a matter of fact, the elements would curve outward since the i cone should be one-half the cross-section, or Vee ofethe diameter at a point midway between the base and the apex, in- stead of one-half the diameter. fxreemesteae 200) Phen = 200" == Bfea toes 200! ee Bigaene B00’ =F 317 2. No.000 9 SF PS. Nol ie oNo.000.55 8 No.l) oa Resis. = .00625 Ohm 0083 0125 025 Current= Current= |]OAmp. 10 Drop= .25 Volt PD.= volts 110.5 HO 109.5 109 Fig. 15, Tapering Mains; 1,013 Ibs. of Copper; 2 Volts Max. Difference Between Lamps; 1171 Volts at Feeding-Points; 1 Volt Average Drop. Tapering conductors give a uniform drop, as already stated ; and the average drop is slightly less than with cylindrical wires, being 1.2 ‘volt.in Fig. 48)"and divoltsing ie lo). SE his asenot a) matter of great consequence, however, as it is customary to con- sider the szaxzmum drop in electrical distribution, and that is the same for the two cases when all the lamps are connected. If only the first groups of lamps were lighted, the tapering conductors would give considerably less drop than cylindrical ones. Never- theless, it is doubtful in practice if the advantages are worth the extra trouble of laying and connecting several different sizes of wire. Where the distances are considerable, and where joints or cut-outs would be introduced in any event, it may be desirable to vary the size of a main in proportion to the current it is to carry at different points. In this discussion it is of course assumed that the conductor must always have sufficient current capacity, whether it be tapering or cylindrical. PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. 88 In the next case, Fig. 16, the mains are supposed to be fed at their centers, as shown. In this arrangement No. 2. wire, weighing 321.5 lbs., gives almost exactly the same variations of potential as in the two preceding cases, the maximum pressure being 111 Resis. —= .032 OHM Current = 10 AMP. Current =|10 Amp. Drop = 232 VOLTS . P.D.= 109.08 voLTs 109.72 : 109,08 Fig. 16. Feeding at Middle of Mains; 321.5 Ibs. Copper; 1.92 Volts Max. Difference Between Lamps; 111 Volts at Feeding-Points; 1.3 Volts Average Drop. volts and the minimum 109.08 volts. This shows that a great saving of copper is effected by simply feeding the mains in the middle rather than at the ends. Theoretically, it would only re- quire one-quarter as much copper in the former case. ‘This is easily seen, when it is considered that the mains in Fig. 16, on each side of the feeding-point, are one-half as long, and carry about one-half as much current, as those in Fig. 13, consequently the conductor need only have one-quarter of the cross-section to give the same drop. The weight is found to be slightly more than one-quarter in the example, because the average current is in the proportion of 15 to 25 instead of 1 to 2. The next case, Fig. 17, represents the mains fed at opposite points. This was formerly called the lWerdermann system, after SS se SRS pre ara AN mn mr Resis. =— ff 205 OHMS os 05 OS Current = 40 Amp, 30. 20. 10- Drop = 2 VOLTS I 10/AMP, A Current = 10:AMP, Drop — .5 VOLTS P.D, = 111 VoLTs 109.5 109. 109.5 lik Fig. 17. Feeding at Opposite Ends of Mains; 202.2 Ibs. Copper; 2 Volts Max. Difference Between Lamps; 116 Volts Between Feeding-Points ; 6 Volts Average Drop. its inventor, but is now known as the anti-parallel or return loop method of distribution. In this case the same length (1,600 feet) of No. 4 wire, weighing only 202.2 lbs., gives an equally good distribution of potential. It is sometimes supposed that this ar- 34 ELECTRIC LIGHTING. rangement must give a perfectly uniform pressure at the lamps, since the sum of the distances of each lamp from the feeding- points measured on the two mains is a constant. As a matter of fact, however, the middle lamps will receive a lower voltage than those at the ends, as shown in the diagram. This is due to the fact that the former are supplied through the portions of the main conductors which carry heavy currents, and in which the drop is greatest. For example, the drop on the mains in the case of the central group of lamps is 2+15+1.5 + 2 =7T volts, but for the end group of lamps it is only 24+15+4+1-+ 0.5 = 5 volts. It is possible, however, to secure a perfectly uniform pressure at all points between the mains, if their cross-section is made propor- tional to the current in each section by the use of the so-called conical conductors already described. In this way the drop in each section will be the same, and each group of lamps will receive exactly the same pressure, being equal to the difference of poten- tial between the feeding-points minus the drop in four sections. In the next example the mains are fed at distributed points as represented in Fig. 18. In this case No. 7 wire, weighing only ee ee ee Resis. — +1 OHM zi Al Jl 1 Current — 10 AMP. 0. 20. 10- Drop — 1 VOLT T | A Current 10 Amp. 10- Drop = 1 VOLT 2 3 1 P.D,— 109 VOLTS 11 TI! wid * “109 Elec. World Fig. 18. Feeding at Distributed Points on Mains; 1017 Ibs. of Copper; 2 Volts Max. Difference Between Lamps; 116 Volts Between Feeding-Points; 6 Volts Average Drop. 101 lbs., gives no greater variation in voltage (i.e. one volt from the normal) than No. 0000 wire, weighing 1,025 lbs., in Fig. 18. These examples show the great difference that is made by chan- ging the points at which the feeders are connected to the mains. It should be carefully noted, however, that in both the last two cases (Figs. 17 and 18) the feeders must supply 116 volts to the mains instead of only 111 volts, as in the preceding examples PARALLEL SSL LIIS OF ELECTRICAL DISTRIBUTION. 30D (Figs, 18, 15,and 16). In Fig. 17, for instance, the difference of potential between the feeding-points + and — must be 116 volts, in order that the end group of lamps A shall receive 111 volts, since there is a drop of 2+15+4+1-+40.5 = 5 volts. in the upper main. Similar reasoning applies to the group A in Fig. 18, the drop being 3 + 2 = 5 volts. This necessity for sup- plying a considerably higher voltage at the feeding-points of the mains is disadvantageous in two respects. First, it involves a loss of power in watts equal to the extra pressure multiplied by the total current ; and second, it may allow great variations in poten- + 200 FT. NO.8. .13 OHM. 20AMP. 2.6 VOLTS DROP ~ 111 VOLTS. 200 FT. NO.8. 13 OHM. 5 AMP. .65 VOLTS DROP 109.05 VOLTS 409.05 VOLTS 200 FT. NO.8. 13 OHM, 15 AMP. 1.95 VOLTS DROP 200 FT. NO.8. 13 OHM. 10 AMP. 1.3 VOLTS DROP 100 FT. NO.8. .065 OHM. 25AMP. 1.63 VOLTS 109.7 VOLTS&. 109.7 VOLTS. ; Fig. 19. Closed Ring; 2,000 ft. No. 8; 100 Ibs. Copper; 115.23 Volts Between Feeding- Points; 1.95 Volts Max. Difference Between Lamps; 5.58 Volts Average Drop. tial to occur when a large number of lamps are thrown on or off the circuit. For example, if all the lamps except one were put out, the remaining one would receive practically the full pressure of 116 volts. This may be overcome by reducing the voltage of the feeders when lamps are disconnected, either by automatic or hand regulation, employing some of the methods described later ; but it is evidently simpler to maintain the same pressure at the feeding- points. On the other hand, the drop in the feeders themselves must be overcome by raising the voltage at the generating plant when the current carried by them increases. In such cases it may not involve very much additional trouble to regulate for the drop in the mains as well as for that in the feeders. ead 36. ELECTRIC TIGHGING. A further extension of the principles shown in Figs. 17 and 18 is indicated in Fig. 19, in which five groups of lamps are connected across the mains, which form complete circles, being fed at diamet- rically opposite points. In this case, 2,000 feet of No. 8 wire, weighing 100 lbs., is used, instead of 1,600 feet, as in the previ- ous examples. A similar arrangement is shown in Fig. 20; but the lamps are assumed to be divided into four groups, of 25 lamps each. All the lamps receive exactly the same voltage, 1,600 feet of No. 10 wire, weighing only 50 lbs., being required. This exact equality in voltage is due to this being a special case, in which the lamps happen to be symmetrically placed with respect to the feed- ing-points. In Fig. 17, for example, the second and fourth groups 200 FT. NO.10..2 OHM. 4110 VOLTS. 12.5 AMP. 2.5 VOLTS DROP 110 VOLTS&s 100 FT. NO. 10. .1 OHM. 25 AMP. 2.5 VOLTS. DROP 200 FT. NO.10..2 OHM. 12.5 AMP. 2.5 VOLTS DROP 100 FT. NO.10. 1 OHM. 25 AMP. 2.5 VOLTS DROP 110 VOLTS. 110 VOLTS. Fig. 20. Closed Square; 7,600 ft. No. 10; 50.3 lbs. Copper; No Difference Between Lamps; 117.5 Volts Between Feeding-Points; 7.5 Volts Average Drop. of lamps have exactly the same voltage, since they are equally dis- tant from the feeders. The pressure at the feeding-points is 117.5 _ volts in Fig, 20, being higher than in any of the other cases. “Individual Conductors. — The most certain way to obtain a constant voltage in parallel distribution is to provide each lamp “or group of lamps with its own particular conductors. One arrange- . ment of this kind is illustrated in Fig. 21, five groups of lamps, : rf each taking 10 amperes and placed 200 feet apart, being assumed, ~ as in the previous examples. The feeding-points, marked + and SW —, are supposed to be located at some distance from the lamps, \ as shown. The pair of conductors that supply each group are so X proportioned in size and length that the drop has an equal value ) for all of the groups. This condition will be secured if the cross- TARA emt LIMO OF LLDECTRICAL DISTRIBUTION. 37 sections of the various conductors are respectively proportional to their lengths. For example, a conductor twice as long as another should have double the cross-section, so that the resis- tance of the two will be equal. If the currents are not the same for the different conductors, the cross-sections should be further modified in proportion to the currents. In other words, for all of pee the pairs of conductors, the fraction “ should have the same value, a z being the current in amperes, / the total length of both conduc- tors, and a the cross-section. It is not apparent what advantages this plan of using individual wires has over the arrangements already described, the weight of copper being even greater than that in Fig. 18, for example. The Length = Size = Resis =]. Fig. 21, Individual Conductors; Unequal Lengths; 186 Ibs. Copper; No Difference of Voltage Between Lamps; 175 Volts Between Feeding-Points; 5 Volts Drop. answer is to be found in the fact that the groups of lamps in Fig. 21 are not only equal in potential when all are burning, but they are also independent of one another, the turning on or off of one or more groups not affecting the others, provided that the voltage at the feeding-points + and — be kept constant. In the preceding cases, the throwing off of some lamps would vary the pressure of all the others. In fact, it was pointed out that disconnecting every lamp but one would raise its potential practi- cally the whole amount of the drop, which was five or six volts in some instances. It was also stated that the remedy for this vari- ation consists in regulating the pressure at the feeding-points. Thus it appears that it is necessary to maintain a constant voltage at the feeding-points, with some arrangements of conductors, and a variable voltage with others. These questions will be considered later under feeder regulation. 3 38 RLEGERIC WLC TING Fig, 22 represents another example of individual conductors, but in this case each group of lamps is supplied through the same total length of conductor ; 1.e., 800 feet of No. 8 wire, having 0.5 ohm resistance. Consequently the drop is five volts for all, since each group takes 10 amperes. The advantage of this plan over that shown in Fig. 17, which it somewhat resembles, is the free- dom from interference already explained. It should be noted, however, that in either Fig. 21 or 22 the turning off of a portion of the lamps in one particular cluster would affect the remaining ones in that group. In order to secure complete independence of operation for every lamp in a system, it would be necessary to provide each one with its own individual wires. This is practi- cally out of the question in almost all cases; but it can be approx- imated more or less closely, the tendency in the best practice being to subdivide the circuits and reduce the number of lamps on each, as far as economy and simplicity will reasonably allow. oe ee eee BA aie SETA Current =|1O amps. 10 P.D. at lamps=| 110 volts 110 Drop=5 volts for each group Fig. 22. Individual Conductors; Equal Total Lengths; 200 lbs. Copper; No Difference of Voltage Between Lamps; 115 Volts Between Feeding-Points; 5 Volts Drop. Calculations of Drop, Weight, etc., of Mains. — The examples already given (Figs. 18 to 22) show the results obtained by differ- ent arrangements of mains and feeding-points in parallel distribu- tion. These cases having been treated concretely with definite sizes of wire, voltages, currents, etc., bring out the facts clearly, and are intelligible to those who may not possess special mathe- matical knowledge. It will be well, however, to discuss these im- portant problems in a more general way before dismissing them. For this purpose the following symbols may be adopted : — L is the length of each main in any desired units ; 7, the length of each section of main (i.e. between adjacent lamps) ; J, and —/,, the currents in the two mains at the feeding-points ; z and —z’, the currents in the two mains at the point x; V, and v,, the potentials on the two mains at the feeding-points ; uy, the potential difference between the two feeding-points, or between one feeding-point and the opposite point on the other main ; PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. 39 V’ —v' =u, the potential difference between the two mains at any point distant x units from + feeding-point ; D, the fall of potential or “drop” at any given lamp with respect to the difference of potential between the two feeding-points ; C, the current consumed by each lamp ; LV, the number of lamps ; #, the resistance of each main per unit of length. Considering first the ordinary parallel circuit represented in Fig. 18, the drop on both mains from the first to the second lamp (or group of lamps) is 2 R7C (CV — 1), and the total drop from the feeding-points to the last lamp D 2 PAO = yeaa a el ae Pn In this equation there are V— 1 terms, having an average N value of So hence we have — = RIC (N7— NV), (1) and the resistance per unit of length which will give this maximum drop D, is — D gS ZC (WV? — NV) (2) In the case illustrated in Fig. 16 the current divides, one-half flowing in each direction, so that it is only necessary to substitute if a for JV in the above formule, or Des A (wv - 2.N) (3) 4 D 2 > ee (4) 1C(N?— 2) and its — In the case of anti-parallel distribution (Fig. 17) the drop to any lamp, say the zth from the + feeding-point, is R7C [CV — 1) + (N—2)+...-a]onthe + main, and R/C[(V— 1) + WWV—- 2) +... (V—-+z-+1)] on the — main. Hence the total drop on both mains is the sum of these values which is — CEG, sy od D= (N242Nx—83N+42x—22%, (5) From this equation it is evident that the drop depends upon ~. Differentiating (5), we find that D is a maximum when Naat ©) ad 3 40 ELECTRIC LIGHTING that is, in the middle of the circuit. In fact, this is evident without calculation, for the reasons given on page 34. Substituting this value in (5), we find that Dae a (3.N2—4N +1). (6) The drop is a minimum when x = WV or 1, that is, at either end of the circuit. Hence substituting in (0), we have — Gy io LEEK aa ae Ouait (2VG <% V). (7) This last equation might have been obtained directly from (1) ; for evidently in the anti-parallel system (Fig. 17), the drop at the first or last lamp is one-half the drop at the last lamp in the ordi- nary parallel circuit (Fig. 13), provided the mains are of the same size, By subtracting (7) from (6) we obtain the greatest difference in pressure between any two lamps in the circuit — Pia Lage TOs ane aoe Cy —2 NV+ 1); (8) (Dee eae R —. Max min i 9 ae hE Gl NEA Ty | ) The relative economy of the three systems can now be found. The weights of copper required are inversely proportional to the resistances ; hence calling A,, A,, and A, respectively the cross- sections (in circular mils, for example) of the mains, which will produce the same maximum difference in pressure between any two lamps, we have from (2), (4), and (9) — pip AD ee be teat ia cede Ria ee =4(N*—N):(M—2N):(NV?-2N+1). Hence the simple parallel system (Fig. 13) requires more than four times as much copper as when the mains are fed in the middle (Fig. 16), but there is very little difference between the latter and the anti-parallel method (Fig. 17). This comparison is made on the basis of a certain maximum dfference in pressure between any two lamps on the circuit. If we consider the same /ofal drop, the advantage of the plan illustrated in Fig. 16 is much greater. The PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. Al relative weights of copper required by the three systems then be- come from (2), (4), and (6) — A,:A,:4, =4(N?— N):(N?—2N):(3N?—-4N+1). As an example to illustrate the use of the above formula, let us assume that twenty 16-candle-power lamps, each taking one-half an ampere, are placed ten feet apart on mains each 200 feet long, the maximum allowable difference between any two lamps being one volt. Hence /= 10, V= 20,C = .5, and D=1. The resis- tance per foot by the ordinary parallel arrangement would be from (2) — | 1 Peeper Oe tT, 00053 ob 1 10 x .5 (400 — 20) Bay which corresponds to a No. 7 wire (A. W. G.). By the second method (Fig. 16) it would be — 4 2 eh eee ee PN HE 50999. olin: ; 10 x .5 (400 — 40) corresponding to No. 18 wire. And by the anti-parallel system — 4 ee 10 x .6 (400 — 40 4 1) == UAV VAL Nis, which also corresponds to No. 18 wire. By the first two systems, however, the total drop would be but one volt, while with the anti-parallel plan, the adzfference between the lamps having the highest and lowest pressure would be one volt. This is proved by finding the drop to the first lamp,’ from 10 x .00221 « .5 2 Chi wig | L/int (400 — 20) = 2.09 volts, and the drop to the middle of the circuit, from (6) — LOD KOO: = : 71 X 8 (4200 — 80 + 1) = 3.09 volts. Hence the greatest difference in pressure is one volt, but the total drop to the middle lamp is 3.09 volts for the anti-parallel sys- tem. The same size of wire (No. 13) gives a total drop of only one volt, if arranged according to Fig. 16 ; and for the simple par- allel method (Fig. 13), the maximum drop with No. 138 wire is found by (1) to be — D=10 X .00221 X .5 (400 — 20) = 4.18 volts. 49 ELECTRIC WIGHTING: In short, for the same maximum difference in voltage in the three systems, the relative weights of copper are, roughly, 4:1 :1, and the total drop, 1:1 : 3.09 volts; while for the same weights of copper, the maximum differences in pressure are 4.18 :1:1 volts, and the total drop, 4.18: 1 : 3.09 volts. The problems of calculating and comparing the results obtained by different arrangements of conductors in parallel distribution be- ing of great importance in electrical engineering, it will be well to give other general methods of solving them. These are largely derived from Abbott’s work on Electric Transmission of Energy, with certain modifications and corrections. Let it be supposed that the mains supply an indefinite number of lamps or other devices uniformly distributed along their entire length. This is equivalent to assuming that the current supplied by the generating plant flows between the two mains in a uniform sheet throughout their entire length. CasE I. Cylindrical Conductors. Parallel System. — Fig. 23 represents two parallel cylindrical conductors connected to the source of supply at 4 and C. From each element, dr, of the + main AS, along its entire length, an elementary amount of current will pass to the other main, CD. Hence the current decreases uniformly from its maximum value /,, at the point A, to 0 at the point 4. At any point x, the current in the mains will be the total current /,, minus all the current which has flowed across from one conductor to the other, between the point A and the point x under consideration. This latter quantity will be 4, / Z, since the flow of current per unit of length is /,/ Z. By Ohm’s law the variation in potential in any conductor is fiz Ri, Uhe resistance “of theyelement(727for both ainaincees 2 Rdx ; hence the drop in pressure for the element dr is given by the expression : — d (ty — W) = 2 Rdx(Z,— 2%) 2 R11 — 2) ax, (10) Integrating between + = 0 and x = L — ty — Ww =2R1f (1-2 )dx = 22X72): (11) This equation, which gives the drop on both mains between the feeding-points 4 and C and any point x, represents a branch PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. 438 of a parabola, to which the conductor may be considered as an asymptote. When +=0, u — uv = 0, showing no drop at AC which is evident; when += LZ, u — uw’ = RIL, being obviously the maximum value of the drop, its average value being 3 RL, L. Asvan example, letrit be assumed that — i.) Jat peres: V,— v, = 40. I, = 60 feet. 02 12x nae ee (120 — x), he .02 ohm per toot, - ‘ 60 ( 5) If x be successively taken as 10, 20, 30, 40, 50, and 60, the corresponding values for the drop are 4.4, 8.0, 10.8, 12.8, 14.0, and 14.4 volts, from which the curve ZF in Fig. 23 is plotted. The curves in Figs, 24, 25, and 26 are also plotted from the same m Th G / a Keane K once Agile Lower AY? Dee Ata igs me a B Vo Vo & -I, Y -lo oe ale Pee se) ’ 0 ; v! D Kee + Leet Kitnmmnennats | sctmnanre | Fig. 23. Fig. 24. Figs. 23 and 24, Parallel Distribution. Cases !. and II, data. An inspection of this curve indicates an unequal drop along the conductors, evidently due to varying current density in‘ the mains. To avoid this variation it is possible to employ tapering, or so-called “conical,” conductors, already referred to on page 31. CasE II. Tapering Conductors. Parallel System.—In Fig. 24, AB and CD are two parallel tapering conductors, supplied with current at A and C, and having a cross-section which is proportional to the current at any point, so that the current density will be constant. The same notation as in Case I. will be used, except that R, is the resistance of each main per unit of length at A or C, the resistance per unit of length at any other point, x, being represented by 7 which is evidently a variable. The drop or vari- ation in potential for any element now becomes — Cm = Oda (7, 3 12). (12) 44 HLEOCTRICILLGH Live, But 7 =p/S at any point, p being the specific resistance and S$ the cross-section of the conductor at that point; hence — 2pJ,(1 —— 2) as “ ; d (uy —u’) = ao ly, — 12) = G (13) But the current density which, by hypothesis, is constant, is — See eee S Hence by integrating (13), we have — Uo Ué = 2K, 1,x: (14) This is the equation of a straight line, indicating a uniform drop from AC to BD; uw — uw being a maximum when x = Z, and having a value 2 R,/,Z, which is twice as great as in Case I. This demonstrates that, with a tapering conductor having the same resistance per unit of length at the supply point as a cylin- drical one, there is twice’ the drop. if 1s also-a viact thatacne weight of copper is one-half as much for the former ; since it is not a true cone, the diameter at the middle section being .707 in- stead of half that at the base, as already explained on page 32. Such a conductor might also be considered as a wedge, two of the sides of which are parallel. Consequently, with the same weight of copper, there is no reduction in the maxzzmum drop when so-called conical conductors are employed, as has been claimed. There is, however, a saving in the average drop, which is readily seen by comparing the curves EF and GH in Figs. 28 and 24, or by substituting 4 Z for x in 11) and (14), which give BALL as the drop at the middle point ( g p p of the/mains im Case’ [and sel in Case II. The latter value assumes that the area of the base of the tapering mains is made twice as large as for the cylindrical ones, in order that the weight of copper shall be the same for both. Hence the drop at the middle point is % as great in Case II. as in Case I., the maximum drop being the same, and the average drop being ? as much. The loss of energy corresponds to the average drop, hence it is also $ as great for the tapering conductors. Usually, however, the PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. 45 maximum drop is the controlling consideration in designing elec- trical conductors, particularly for electric lighting. Case III. Cylindrical Conductors. Anti-Parallel System. — In this case the mains are fed from opposite ends as already de- scribed in connection with Fig. 17. It is evident that this arrange- ment differs from the two preceding in the fact that no lamp receives the full voltage delivered to the mains, because % is at one end of one main, and vz, is at the opposite end of the other. i K M N URNS eco he---- ee \se = L. fs al. es yet san Figs. 25 and 26. Anti-Parallel Distribution. Cases Ill. and IV. A study of Fig. 25 shows that the variation in pressure between the ends of any element dr for both mains is equal to the aiffer- ence between the drop in one main and that in the other, whereas in the two previous cases it was the sam of the two drops, hence — a(t, — uw’) = Rdx(t—2'), (15) tz and —z' being the currents in the respective mains at the point Ld x, and having the following values : — eae fem La and, ae See Ly ye vs substituting these in (15), we have — Des . a (Up th ome Le L, (1 — =) dX (16) Integrating — Uy — Wy = fea Ay [ This equation is also that of a parabola, but its axis is perpen- dicular to the mains at their middle point. When zx =0orr= lL, ut — u' = 0, showing that at each end the lamps receive the same voltage. To locate the maximum difference in pressure between =*) pp Sg aeey a alg L the lamps —- 46 ELECTRIC LIGHTING. sgt) man (ta an 2eherao aX (18) that is, the greatest drop is at the center of the mains, and has the value R/,L /4 obtained by substituting (18) in (17). But it should be carefully noted that this represents the dzfference be- tween the voltage of the middle lamp and that of either end lamp. For the latter the pressure is less than the difference of potential between the feeding-points (= V, — uv) by the quantity RAL / 2, which is the total drop in either one of the mains. Hence the middle lamp receives a voltage which is less than that supplied to the feeding-points by an amount — Kel a L aah Lo en ee 0 Ores on" 19 4 a Zz 4 OH This value is only three-quarters as large as the maximum drop in Case I., which was found to be A/,LZ, and the greatest aiffer- ence petween the voltage of lamps is only one-quarter as much, or RIpL / 4, the weight of copper being the same. Case IV. Tapering Conductors. Anti-Parallel System. — The plan of feeding from the opposite ends of the mains may be applied to tapering conductors with even greater advantage than in the case of cylindrical conductors. By applying the equations in Cases II. and III. to this arrangement, shown in Fig. 26, the following expression is obtained : — a (Uy — Wy) = (r2 — 2’) ax. vy and 7’, as well as z and —z’, being respectively the resistances and currents in the two mains at the point x Hence by a train of reasoning similar to that in the previous cases, 7 = p/S, and r' = p/ S'; but pz/ S and pz’ / S' are constants for each main, by hypothesis, and are equal to each other, hence — qo #) 9, (20) Dax “Uy — w' =a constant, and LVS ge mae] eT (21) In other words, there is no difference in the voltage supplied to the various lamps, the pressure at any lamp being the difference PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. Af in potential between the feeding-points less the quantity R,/,Z, which latter is therefore the maximum drop for all of the lamps. This is the same value as in Case I., but the amount of copper is only one-half as great; hence the maximum drop is one-half as much for the same weight of copper, and all lamps receive the same voltage. Drop in Voltage with Irregular Distribution of Lamps. — In the various cases heretofore considered (Figs. 13 to 26 inclusive), the lamps were assumed to be uniformly distributed on the mains. _This represents not only ideal conditions, but also applies fairly well to actual practice at full load; that is, when the maximum number of lamps are lighted. In fact, the circuits should be care- fully designed to approximate this condition as closely as practica- ble in most cases. When only a fraction of the lamps are turned on, it is evident that they may be very irregularly distributed. This would give rise to an almost infinite number of special prob- lems corresponding to the possible arrangements that might be made ; but there are certain general facts that apply to such cases. If in Fig. 13 it be assumed that only the last or right-hand group of lamps is connected, the drop would be equal to the cur- rent multiplied by the total resistance of both mains, or 10 x .08 = .8 volt. Hence the potential difference supplied to this group of lamps would be the pressure at the feeding-points minus the drop, that is, 111 — .8 =110.2 volts. If now the middle group of lamps be turned on also, the potential difference which they re- ceive would be 111 — 20 x .04 = 110.2 volts, and the pressure at the last group becomes 111 — (20 x .04 + 10 x .04) = 109.8 volts. Thus the various groups may be lighted successively, and it will be found that — 1. The addition of each group reduces the pressure for all of those already connected. 2. The maximum drop occurs when all of the lamps are con- nected. | 3. The greatest difference between the voltage of any two lamps will usually exist when all are turned on. The first statement might be contradicted on the ground that the pressure at the first group of lamps connected directly to the feeding-points would remain the same whether the others were 48 FLECLTRACCLIGLHZ ING. lighted or not. ‘Theoretically this is true; but practically there would be some drop on the mains even for this group, unless it’ were connected exactly at the feeding-points ; and there would always be a drop on the feeders when any lamps were turned on, unless it is overcome by some of the special methods of feeder regulation which will be described later. The same statements apply to Fig. 16, in which the portions of the mains on each side of the feeding-points may be considered as corresponding to the whole mains in Fig. 18. Even though all the lamps on one side were connected, and only one on the other side, the total drop and the difference between the voltage of lamps would be no greater than for the full-load conditions represented in the diagram. Similar reasoning is applicable to the arrangements shown in Figs. 17 and 18; in fact, any two groups of lamps would have the same pressure in the case of the former, and any number less than all would give no greater total or difference in drop than the full load of lamps. If the first three groups were lighted, and only a single lamp out of the last group was turned on, the latter might receive a potential about three volts higher than that of the others. This is greater than the maximum difference when the cir- cuit is fully loaded, which is only two volts. Hence it appears that when one end of a pair of anti-parallel mains is heavily loaded, and there are very few lamps in circuit at the other end, the difference between the voltage of lamps at the two ends is greater than when the full load is turned on. Consequently this is an exception to statement 3 above. But even in this case, the maximum drop and the average drop are less with a fractional load. In Fig. 27 the curves AB and CD represent the potentials on two cylindrical mains, which are fed according to the anti-parallel method at 4 and D respectively, being fully and uniformly loaded. The drop between A and £ is greater than between C and //, because the average value of the current is greater for the former, as will be seen by comparing Fig, l{- “Hence the pressure sup- plied to the middle lamps #7 is less than that at the end lamps AC, as already explained in connection with Figs. 17 and 25. If now ail the lamps on the right-hand half of the mains be discon- - nected, there will be no drop between & and /, and the fall of poten- tial from HY to D will be constant, and will be represented by the PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. 49 straight line HD. It is evident from an inspection of this diagram that (D — EH > BD — LY, or in other words, there is a greater aifference between the voltage at #D and EH when half the lamps are thrown off. Hence anti-parallel mains, when very- unequally loaded, may show an exception to statement 3 (page 47) as already explained. It is also apparent, however, that the maximum as well as average drop are smaller with any load less than the full amount. The substitution of tapering mains for cylindrical ones makes the pressure still more uniform for fractional loads, since the drop is more nearly equal for the different portions of the conductors. When lamps are irregularly connected to a closed loop or ring arrangement of mains, the problem becomes somewhat more difh- Ant , Fig. 27. Effect of Unequal Distribution of Lamps on Anti-Parallel System. cult, since there are two paths for the current.* Such acase is rep- resented in Fig. 28, in which a pair of ring mains are supplied at the feeding-points, with a voltage V, the total value of the current being /, The resistance of a semicircular portion of each main is R. Two equal lamps are assumed to be connected as shown, the current in each being $7 Let x be the current that flows down- ward from the + feeding-point, then the current in the upper half of mains is J — x, and their combined resistance is 2 2, hence, — ON (Tc) ot RI— =>“? = Re, (22) ani Ome, (23) also Mi —Ve= R(w—P)or i + Af = Re (24) * This matter will be discussed further under the head of ‘* Networks ’’ of conduc: tors. 50 ELECTRIC LIGHLING. From (23) and (24), Vy —-W=ni-ht+ ee. (25) From (22) and (23), ANE Y -W= RI (26) From (20), rere GR ACMAN Kg tag (27) Substituting this value of V, in (26), V Bobet ee § V —-V,=R/J—-—+7),-—-—+—- 1 : 5 eal 9 +- 1 (28) Simplifying, we have, ys a (29) From (27) and (29), v= V— S42. (30) From (23), wb Ail R (31) To take a specific case, let us assume V7 = 110 volts, 7= 8 amperes, and K=1ohm. Then we find from (29), (380), and rs (I~ x) =3 Fe BR(x- ty ys Fig.29. Figs. 28 and 29. Irregular Distribution of Lamps on Ring Circuit. (31) that Y, =105 volts, Y,= 104 volts, x =5 amperes, the current that flows upward from the + feeding-point is 8 —5 = 3 amperes, the current in each lamp being 4 amperes. Fig. 29 represents the distribution of potential in this case, including the MARA ae Bl Le LAI CAL DISTRIBUTION. ol drop in each portion of the mains. The addition of more lamps to this circuit, whether symmetrically or unsymmetrically placed, would increase the total and average drop, also the maximum dif- ference in voltage between lamps. The general conclusion is, that the three statements made on page 47 apply not only to the simple arrangement shown in Fig. 13, but also to almost any parallel system of mains, with the exception of peculiar conditions on an anti-parallel circuit, as ex- plained in connection with Fig. 27. Hence it is ordinarily suffi- cient in practice to calculate the distribution of potential on a parallel system for full load, since the total or average drop, and the greatest difference in voltage between lamps, will almost always be smaller for any number or arrangement of lamps less than the maximum load. MHeretofore the principal point that has been con- sidered in discussing parallel systems has been the difference between the pressures supplied to the various lamps which are burning at the same tzme. But it has already been explained on page 85 that in cases where the total drop is considerable, — 10 per cent, for example, — the voltage at the lamps will rise nearly that same percentage when the full load is thrown off, leaving only a few lamps. It will now be well to study the means employed to prevent variations in the voltage of a given lamp when others are thrown on or off the same circuit. —Regulation of Voltage Supplied to Parallel Systems. — The pe -points in the various diagrams (Figs. 13 to 29) might in some cases be supplied with current directly from the generator, or the feeders may be so short that their resistance is insignificant. Under those circumstances it would only be necessary for the dy- namo or other source to generate a constant pressure in order to supply the mains represented in Figs, 13, 15, and 16. If this were kept at 111 volts, the last group of lamps would receive’ 109 volts at full load, and no lamp could receive more than 111 volts, even if all but one were turned out, so that the extreme variation would be but one volt from the normal pressure of 110 volts. The large amount of copper used in these cases saves the trouble of regulation, and often might be worth the extra first cost. This is practically the way that the majority of isolated plants are operated, the size of the wires being made sufficient to limit the drop to a small amount so that the dynamos may be run at a 52 ELECTRIC LIGHTING. fixed voltage. With wiring designed for a total drop of 4 per cent, the greatest variation from the average pressure would only be about 2 volts, and usually the maximum drop is between 2 and 4 per cent for isolated plants where the distances are moderate. In most cases, even the simplest, feeders are employed to connect the mains with the generators, so that the pressure lost in them must be included in determining the total drop. When the dis- tances are greater, or it is attempted to save copper by using smaller conductors or by adopting such arrangements of mains as those represented in Figs. 17 to 22, the drop becomes too large to warrant the maintaining of a constant potential at the dynamo. Nevertheless, many small central stations and isolated generating plants are operated at an approximately fixed voltage, in spite of the fact that the drop may be 5 per cent or more. The usual practice in such cases is to run the dynamos about 2 per cent above the normal voltage of the lamps, the consequence being that at full load the latter receive about 3 per cent less pressure than that for which they are intended, assuming the drop to be 5 percent. This custom arises from the fear of shortening the life of incandescent lamps by feeding them with too high a voltage. It appears to be a generally accepted idea that the rated pres- sure of a lamp is a /z#22¢ above which it should never be allowed to rise. Asa matter of fact, the voltage marked on the lamp should be considered as an average value, to be approximated as closely as possible at all times. It is a rare thing to see incandescent lamps burning even one or two per cent above their rated pressure, while they are very often operated considerably below this point. The author has observed in thousands of cases in America and Europe that incandescent lamps are usually run perceptibly below their proper voltage, and at least half of them are so low that they are positively dim. This is partly due to the usual falling off in the candle-power of lamps which occurs after they have burned for some time, amounting to a considerable loss after a run of 500 hours. This matter will be treated fully under the head of incan- descent lamps. In isolated plants, and in many central stations where lamp renewals are paid for by the user, this diminution in candle-power is great because of the tendency to unduly prolong the life of the lamps. But in stations or plants where it is desired to render good service, the lamps are renewed more frequently. PARALLEL SVSTEMS OF ELECTRICAL: DISTRIBUTION. 583 The reason for generating a constant potential is the simplicity and convenience secured by so doing. The attendant merely has to keep the index of the volt meter at a certain point, by means of the ordinary rheostat in the shunt field circuit, being either in- structed to do so, or naturally falling into that habit. It would greatly improve the service, however, if the pressure were kept at a given value for any number of amperes up to half load, and raised a certain percentage when the current exceeds that amount. For example, below half load the dynamo could be regulated to generate 2 volts higher potential than the normal voltage of lamps, which would again be increased 2 volts when the current is greater than half load, the total drop being 5 volts. In this way the pressure at the lamps would not be more than one or two volts high or low at any time, and the extra trouble or intelligence required would certainly be insignificant. Indeed, it would seem to be perfectly practicable to carry this plan further, and subdivide the load into three or even four parts instead of two. The instructions could be just as definite and almost as easily carried out as for one fixed potential. If this regulation were effected by hand, using the ordinary rheostat in the field circuit, it would be a rough approxi- mation to the rise in voltage with increasing load which occurs automatically in an “over-cox:pound” dynamo Regulation by Means of Compound or Over-Compound Dynamos. — It would seem that an excellent way to operate systems in which a constant potential is required at the lamps or other re- ceivers, is to employ generators which are over-compound wound to give a rise in voltage from no load to full load the same in amount as the total drop, thus automatically securing the desired Tesulte een objection to this plan is the tendency for the £47 F. of the dynamo to rise, and a very excessive current to flow in case of a short-circuit. The 4.47.F. of a plain shunt machine, on the other hand, tends to fall with a short-circuit. But when properly protected by fuses or circuit-breakers this difficulty is not likely to be serious. Another difficulty that may arise in such a case is the fact that when there are two or more over-compound generators the pressure may be too high when only one is in use. For exam- ple, when one machine out of two is running with one-half of the total load, it will raise the voltage just as much as if both were working at full load, whereas it should only increase the pressure 54 ELECTRICALIGHTING. one-half of the maximum percentage of drop. This trouble may be avoided by always leaving in circuit the series coils of all the dynamos, or preferably by substituting an equivalent resistance for them when they are disconnected.* The manner of connecting two or more compound dynamos to operate in parallel is represented in Fig. 30. 4 is the armature, B the series, and C the shunt field coils, X the field rheostat, D, /, are switches connecting the main terminals of the dynamo with the ‘bus bars G and / respectively, and & is a switch to connect R ) Q ( F OO 00-0 070 Os 000009000C Fig. 30. Compound Dynamos in Parallel. the equalizer f // with the brush from which the series coil B leads. It is a common practice to mechanically join D, £, and F bya cross-bar so that they move together and form a three-pole switch. In such cases, when a dynamo is about to be connected to the cir- cuit, the switches D, /, and F are left open, and the field magnet is excited by the shunt coil C, being regulated by the rheostat R until the pressure generated is a little greater (about one per cent) than the difference of potential between the ‘bus bars G and f. This fact may be ascertained by comparing two volt meters * This matter is explained in vol. i., p. 349. t Vol. i., p. 348. PARALLEL aI oLEMS OF ELACTRICAL DISTRIBUTION. 50 respectively connected to the dynamo and to the ’bus bars, or by connecting a single volt meter first to one and then to the other, which avoids the error due to a difference between two instru- ments. A still better plan is to connect the dynamo to the bus bars through a high resistance and a galvanometer which deflects one way or the other according to whether the dynamo voltage is higher or lower than that of the circuit. For this purpose it is very convenient to use a volt meter having a scale on both sides of the zero point. After the pressure of the dynamo has been prop- erly regulated, the three switches, D, £, and /, are closed. When this is done simultaneously with a three-pole switch, a considerable current will flow through the series coil 2, which tends to still further increase the voltage of this dynamo, at the same time tak- ing current away from the series coils of the other machines, and thereby reducing their potential. The shifting of load thus pro- duced may be so sudden and so great as to be objectionable. To avoid this difficulty the two switches & and / are sometimes com- bined to form a double-pole switch, the other one, D, being operated independently. With this arrangement the double-pole switch EF is closed first, allowing the current to flow through the series coil 4, and the regulation of voltage is made under these condi- tions. The switch JD is then closed, and the -.4/7F. of that ma- chine will not change materially. The current which it generates will also be small, provided its voltage was adjusted to be only slightly greater than that of the ’bus bars. When the three switches, D, /, /, are simultaneously closed, it is found in practice that armature reaction, etc., tend to lower the potential of the generator about as much as the current in the series coil tends to raise it, hence the effects counteract each other. But it is merely an accident if such is the case, and it can only be determined by trial. It often happens that the two actions do not balance each other, the rise of 4.4/./. being greater than the fall. In these cases, which are common in electric railway stations, the attendants learn by experience that the pressure of a dynamo should be regulated a certain number of volts below that of the *bus bars before it is connected to them, in order that it shall act properly when the three-pole switch is closed. This is certainly a crude method of working, and increases the chance of having a back current flow through the series coil, which would tend to 56 ET ECTRICLICHLING demagnetize the field if the E.7F. of the dynamo is considerably less than the pressure at the ‘bus bars, particularly when the equalizer is somewhat long or is too small in cross section. It would seem to be generally desirable to separate the switch PD in order to have independent control of the equalizer. Another advantage secured by this arrangement is the field excitation that is positively produced when the switch £ F is closed, avoid- ing the delay and uncertainty which are always involved when self- excitation alone is depended upon. In fact, self-exciting dynamos often fail to generate, or become reversed in polarity.* The use of separate switches also enables the series coil to be left in cir- cuit when a dynamo is not working, for the reasons explained on page 58. The three switches D, &, and / might. all be made independent ; but there would then be a chance for D and # to be closed, and the equalizer switch & left open, which is likely to cause serious trouble, due to an excessive or reversed current in the series coil £; or the switches D and & might happen to be closed with / open, in which event the series coil would not be in circuit, and the dynamo could not generate sufficient voltage when the load increased. Compound or over-compound generators are generally used in isolated plants and smaller central stations, and are almost uni- versally employed in electric railway power-houses; but in large electric-lighting stations plain shunt dynamos are often employed in order to give greater flexibility of regulation. In such systems the lamps and other devices. are supplied through a number of feeders, which are fed with different pressures at the station according to their length and the load upon them. The methods employed will be described later under the head of “ Feeder Regulation.” It should also be noted in this connection that the business of large stations warrants the constant employment of one or more men to regulate the voltage, while in small plants the regulation should be automatic as far as possible, in order to reduce the required attendance toa mimimum.. It is not unusual for such plants to be left to take care of themselves for consider- able periods of time. In most cases automatic regulation has to be supplemented more or less by hand adjustment of the field rheostat to make up for change in speed due to variations in * Vol. i, p. 362. PARALLEL SYSTEMS OF ELECTRICAL DISTRIBUTION. O51 steam pressure or water pressure in the case of hydraulic power. The heating of the field coils and resistances in shunt and com- pound dynamos, as well as hysteresis in the magnets, also cause variations in the voltage which usually have to be overcome by hand regulation. { Automatic Constant-Potential Regulators. — To entirely avoid the necessity for hand regulation, or to reduce it to a minimum, Filganate Belknap-Chapman Automatic Voltage Regulator. automatic devices may be used to maintain constant potential. One of these regulators, illustrated in Fig. 81,* consists essentially of a rheostat in the shunt field circuit, whose moving arm, J, is operated by a solenoid, Q. A relay solenoid, A, is connected across the main conductors through the binding-posts f 7’, and has contact points, z, that govern the admission of current to the working solenoid Q. The latter is differentially wound with four coils, two of which have a small continuous current flowing through them, and are in opposition to each other, the current being sup- * Electrical World, N.Y., March 20, 1897, p. 395. 58 FLECTRICOIIGHTING plied through the switch G, and binding-posts £28. The other two coils, when the circuit through them is closed by the relay, act to neutralize one of the continuously excited coils. This method of operation avoids the injurious sparking that would occur if the circuit of the main solenoid were actually broken, and thus per- mits a close adjustment of the relay contact points, and secures a more sensitive regulation. The cores of the solenoid are composed of small, soft iron wires to give quick action and reduce hysteresis effects. Two lamps, / F", constitute a non-inductive resistance for the relay circuit. An ordinary hand rheostat, £, is included in the shunt field circuit in order to adjust the resistance, and also give independent control. The voltage for which the device is set may be altered by shifting the small weight / on the relay lever. In this device the consumption of energy is not large, being only 60 or 70 watts for a large regulator. The construction as de- scribed would tend to maintain a constant potential at the points on the circuit to which the binding-posts 7 f' are connected. It is evident that these may be in the station or at any desired posi- tion on the system of conductors, thus producing the effect of over- compound winding. This would necessitate the running of special pressure wires to some distance, which may be avoided by provid- ing the relay solenoid, A, with a series coil in addition to the shunt coil, or, in short, by compounding the regulator instead of the field magnets themselves. The series coil would have to be connected in the main circuit, or shunted around a resistance placed in it. In order to maintain a constant current for charging storage batteries and for other purposes, the relay 4 is wound with a series coil only. These devices are also made for alternating current regulation. . .375 (chree-wire, system, neutral one-half size. 0). .. . 313 Four-wire system, all four Wires of same'size™ 2 2%... 6222 Five-wire system, all five wires of same size. . . . 2. . «156 Five-wire system, three inside wires one-half size . . . . .109 Seven-wire system, all seven wiresof same size . . . . .097 88 ELECTRIC LIGHTING. It is evident that similar systems having a greater number of wires might be designed, but they would be extremely complicated, and of very doubtful advantage. In fact, the desirability of a five- wire system is questionable, since the use of 220-volt lamps ena- bles three-wire circuits to be operated at 440 volts. vertical mains included between the points A Y X and Z are uni- formly loaded, and not considering the effect of any load outside of this region, it follows that one-quarter of the current will flow out from the feeding-point / on each of the four mains leading there- from. If ten lamps, each taking one ampere, are connected to each section of the mains, the initial current in the main / a will be 30 amperes, since three sections (/a,@ A, and a Y) must be supplied by it. When the current reaches the point a, it will have been reduced to 20 amperes, since 10 amperes are consumed in the sec- ae rts | | : Fig. 75. Two-Wire Network for Parallel Distribution. tion 7a. Hence the average current between / and a is 26 am- peres ; and if the resistance for each section be taken as .04 ohm, the drop will be 1 volt. The initial current from a to 4 is 10 am- peres, and its final value is zero; hence it averages 5 amperes, and the drop is .2 volts for that portion. The same is true of the sec- tion a Y, provided that the load beyond Y be ignored, as already stated. Having thus determined the drop in voltage on the posi- tive mains, it is evident that precisely the same drop will also occur on the negative conductors. A lamp at_/ will receive the full pres- sure supplied by the feeder, which may be assumed to be 112 volts ; a lamp at a will have 112 — (1 + 1) = 110 volts; and a lamp at 4 will be sféd* with 112-— ‘(1 4-1 4592 282) = 109.6) volts™ sSimilar statements apply to the lamps at the other points, W Z, etc. NEDTWORRSRGL LLEOTRICAL, CONDUCTORS: 105 When the lamps are not equally distributed, the problem is much more difficult to solve. In the apparently simple case of a single lamp connected at W, a large portion of its current will flow directly from / to W, but a considerable fraction of it will take the path / a Y W, and also the path /zZ W. Since there is a flow of current from /to A, the latter must be of lower potential than the former; hence a small amount of current will take the course /7 A Y, and if the network were extended beyond 4, some current would follow still more indirect routes. Current would also pass through the remainder of the network shown in Fig. 75, as well as through the portion represented in Fig. 76; in fact, a single lamp connected at any point of a network would cause cur- rent to flow in every section except a few that might happen to have no potential difference between their ends. With a number of lamps irregularly located, the conditions be- come even more complex. It might be supposed that a solution of the problem could be obtained by compar: ing the resistances of the va- Fig. 76. Portion of Network Represented in Fig. 75. rious paths. For example, the course /a Y W has three times the resistance of / W, hence the current in the former should be one-third as much as in the latter. But this is not true, because Jj A aqis in parallel with / a. If we attempt to allow for this by calculating the joint resistance, the case is further complicated by the fact that the section /7 carries current that flows vza X as well as through A, and so on. A correct method would consist in applying Kirchhoff’s Laws, which are as follows : 1. The algebraic sum of the currents in all the conductors that meet at any point ts Zero. 2. The algebraic sum of all the products of the currents and resis- tances in conductors forming a closed loop equals the algebraic sum of all the E.M. Fs in the loop. In the networks under consideration there is usually no E.M.F. working within each loop, as, for example, the loop /a Y W; therefore we may simplify the second law as follows: 106 ELECTRIC LIGHTING. - The algebraic sum of all the products of the currents and resistances in conductors forming a closed loop ts zero. The application of these principles to simple cases has already been given on pages 35 and 49; but it would be difficult to apply them to the extensive and complicated networks used in practice, particularly when the lamps are unequally distributed. Neverthe- less, methods for making such calculations have been given by Herzog and Stark,* Herzog,+ Coltri,t Muellendorff,§ and others. Electrical Model of Network. — In the pioneer work of Edison in 1882, the designing of the underground network of conductors was aided by constructing models in which the conductors were represented in miniature by copper wires. If the model is correct in scale, it is possible to determine from it the distribution of cur- rent and drop with various amounts and positions of load. This plan can be followed in any case, but would ordinarily be consid- ered too much trouble, although it might often effect a considerable saving in, or better arrangement of, the copper. | Mechanical Model of Network. — Another method of solving this problem was devised by H. Helberger || of Munich, and con- sists in employing a mechanical model in which the conductors are represented in length and position by horizontal strings stretched with a certain force corresponding to the cross-section of the con- ductor, the load being applied in the form of weights that are hung > upon the strings, and are proportional to the current consumed at the various points. The amount that the strings are depressed indicates the drop in voltage, being usually limited to a certain value in a given case. The points at which the strings are sup- ported correspond to the feeding-points, being raised or lowered with respect to each other a distance proportional to the difference in the electrical pressures with which they are fed. Actual Design of Network. — These methods for determining the size of the mains in a network are not much used in America, although they are applied quite generally in Germany. Experience * Llektrotechnische Zeitschrift, 1890, p. 221, and Electrical World, vol. xv., p. 300. + Llektrotechnische Zeitschrift, 1893, p. 10. t Zdzd., 1893, p. 425. § Zbid., 1894, pp. 67 and 236. || German Patent No. 68918, Class 21, April 5, 1892. See also Western Elec- trician, April 27, 1897. : NETWORKS OF ELECTRICAL CONDUCTORS. 107 in this country has shown that it is sufficient to employ a few stan- dard sizes of mains. In New York City, for example, each of the three-wire mains has a cross-section of 350,000 circular mils in the central and heavily loaded portions of the network, and a cross- section of 200,000 circular mils in the outlying or less heavily loaded districts, and in some cases conductors of 150,000 circular mils are large enough. It is not found necessary to specially determine the size of each individual main or section thereof ; the same size being used throughout a large district, and having been selected with reference to the general rather than local conditions. The justification for this apparently crude practice is, first, the simplicity of laying and maintaining a network composed of only two or three standard sizes of mains, larger or smaller sizes being either too clumsy or too weak mechanically ; second, it is prac- tically impossible to predetermine the current that a main will carry, the demand upon it being often much greater or less than was expected; third, an excess of copper in one portion of a network tends to help other portions that are more heavily loaded, and conversely a small section of main acts as a weak link in the chain. 7 It is important to appreciate this interdependence of the parts of a network of conductors, as it constitutes its chief advantage. In the mechanical model already referred to, it is evident that all of the strings would aid each other in supporting a weight hung at any point upon them. The electrical analogue acts in a similar manner. As already stated, it is customary to construct networks with certain sizes of mains which have been found by experience to be suitable for towns having a certain density of population, character of service, etc. In cases where this very empirical method cannot be followed, or when it is desired to check it by calculation, a care- ful plan of the given district should be made, and lines representing the proposed mains are then drawn. A main (2- or 3-wire as the case may be) is run through each street, or two mains may be laid, one on each side, in: order to reduce the trouble of making house connections. In the case of unimportant streets in which there are no customers, the mains may be omitted or put in later. Where the mains intersect they are connected, all the + conduc- tors (of which there are usually four or eight) being brought to- 108 ELECTRIC LIGHTING -* gether, the same for the — conductors, and also for the + conduc- tors in a three-wire system. The connection of four + wires and four — wires in a two-wire system is represented at /and JW in Fig. 75. Having thus laid out the entire network, certain feeding- points are then chosen. ‘There is no absolute rule for determining their position, but they should be located to give as nearly uniform voltage as possible throughout the system. They should be ar- ranged so that the feeders can be run conveniently to them and connected to the network, and should be nearer together where the load is great, and wzce versa. In case it is subsequently found that they are too far apart, others may be added without disturbing the feeders and mains already laid. In this way increase of load upon the system may be provided for at any time. It is also possible to reénforce the mains by laying others parallel to them, but prac- tically the same result is obtained when additional feeders are put in. If, for example, the average distance between feeding-points is reduced to one-half, the average current on a given main will also be one-half as great as before, and since it flows only one-half as far, the drop would be one-quarter as much. Extending a network of mains in any direction will also tend to help the conductors already laid, because it provides more paths for the current, as explained on page 104. The Edison system of underground conductors originally adopted for network distribution and still very generally employed for the purpose will be described in the chapter on Underground Conductors. Other methods of constructing such networks will be given under the same heading. PRINCIPLES OF ALTERNATING CURRENTS. 109 CHAPTE.REAViT: PRINCIPLES OF ALTERNATING CURRENTS. Introduction. -— The various principles and facts concerning direct current distribution set forth in the preceding chapters, ap- ply also to alternating current systems. But in addition to the simple phenomena due to resistance, which occur in the former case, there are certain additional factors that must be considered in connection with alternating current transmission. The flow of a direct current, which is steady, is entirely determined by the ohmic resistance of the various parts of the circuit ; and if all these resistances are known the distribution of potential and current can be determined exactly. The flow of an alternating current de- pends not only upon the resistance, but also upon any ¢tuductance (self or mutual inductance) or capaczty that may be contained in or connected with the circuit. These two factors have absolutely no effect upon a direct current after a steady flow has been estab- lished, which usually requires only a small fraction of a second. But in an alternating circuit either or both of them may be far ‘more important than resistance, and in some cases may entirely control the action of ‘the current, the effect of resistance being insignificant. Since alternating current problems involve a consideration of three factors, they are usually more complicated and difficult to solve than those relating to direct currents. Nevertheless, by an extension of the principles and methods already explained, it will be found that alternating current systems can be designed correctly and without great difficulty. Practically the only reason for employing alternating currents in electric lighting is to enable the cost of the conductors to be re- duced by using high voltages and transformers. It has already been shown that the cross-section of a wire needed to convey a 110 ELLE CLE GLLIGET UNG. given amount of electrical energy in watts, with a given percentage of “drop” or loss of potential in volts, is inversely proportional to the square of the £.47./. employed: hence it requires a wire of only one-quarter the cross-section and weight if the initial voltage is doubled. The great advantage thus obtained by the use of high tension can be realized either by a saving in the weight of wire required or by transmitting the current to a greater distance with the same weight of copper. In alternating current electric lighting the primary £.J7.F. is usually at least 1,000 and often 2,000 to 10,000 volts. Even at a pressure of 1,000 volts an advantage of 100 to 1 is gained over a system operating at 100 volts. This enormous difference enables a given number of lamps to be sup- plied at a far greater distance, and at the same time the conductors weigh very much less. ‘These facts make it unnecessary to design a system of alternating current conductors with the great care that is required for direct current distribution, since the use of. slightly larger conductors will make up for any small differences in the arrangement of the feeders and mains. The result is that ordinary alternating current systems of conductors are less complicated than those used for direct currents. The very elaborate network of mains, for example, often employed for the latter is seldom required for the former except in the low voltage secondary distribution. On the other hand, the actual uniformity of voltage secured in direct current circuits is usually superior to that obtained on alter- nating current systems. This is partly due to the fact just stated, that less care is required in designing the circuits, consequently there is a tendency to exercise too little care. The difference also arises from the effects of inductance and capacity, which produce variations in potential as great as, or greater than, those due to re- sistance alone. The exact influence of these factors under various conditions will be considered later. The reason that the alternating current can be used at the high pressure of 1,000 volts or more, while the direct current is limited to about 110, 220, or 440 volts for constant potential lighting is due to the greater facility with which the alternating current can be transformed from a higher to a lower pressure, and vce versa. This is accomplished by simple transformers, consisting merely of two or more coils of wire wound upon an iron core. Since there are no moving parts, the attention demanded and the likelihood of TAENOIEL BoVOPreALILRINALTING CURRENTS, BLE the apparatus getting out of order are small. This enables the al- ternating current to be generated at or transformed to a high pres- sure suitable for transmission over long distances with small con- ductors, the potential being locally transformed to that required by the lamps, usually about 100 volts. In order to convert a direct current from one potential to another it 1s necessary to employ a motor-dynamo, which is practically a combination of a motor and a dynamo costing considerably more than an alternating current transformer, having a lower efficiency, and being more troublesome to take care of. In almost every other respect the direct current is preferable for electric lighting; and where the distances are not great, as, for example, in isolated plants and central stations in thickly populated cities, the direct current has been the system most generally and successfully employed. Two-phase and three-phase alternating current systems are often employed to supply incandescent and arc lights; but they are only advantageous for operating motors or rotary converters, and so far as lamps are concerned, they are more complicated, and possess no compensating superiority over the single-phase system. The latter, on the other hand, is not desirable when there are a number of motors of anything more than small size, such as fan motors. Hence polyphase systems are used in cases where both lamps and motors are to be supplied with alternating currents. The principles of alternating currents will now be given; but it is not intended to treat the subject exhaustively, as there are several excellent works entirely devoted to it. It is sufficient herein to consider briefly the chief facts, to serve as a basis for study and calculations concerning alternating current lines, trans- formers, etc. Principles of Alternating Currents. — Each armature coil of a dynamo tends to generate an £.4/./, which rises to a certain max- imum value, then falls to zero, then reverses in direction, and again returns to zero. This cycle of changes, which can be represented by a curve (Fig. 77), constitutes a complete period ; and since it is repeated indefinitely at each revolution of the armature in a bipolar field, the currents produced by such an 4£.1/.¥. are called pertodic currents. The number of complete periods in one second is called the frequency of the pressure, or current. In Fig. 77 the period is completed in .01 second, hence the frequency is 100. Since the Ly ELECTRICOTAGHIING current changes its direction at each half-period, it follows that the number of adternations or reversals is twice the frequency. Various forms of pressure or current waves may be generated, depending upon the arrangement of the armature winding, pole- Figs. 78 and 79. Flat-topped and Peaked Fig 77. Representation of Alternating E.M.F. Wave Forms. pieces, etc. It is possible, by having the pole-faces either consider- ably wider or considerably narrower than the armature coils to pro- duce a flat-topped wave (Fig. 78); or by making the coils exactly the same width as the pole-pieces, a peaked wave (Fig. 79) may be obtained. If the lines of force are excessive at the edges of the poles, extra waves,-or upper harmonics J] B W, are superimposed upon the main or fundamental wave AC (Fig. 80). The extra D A E ies 4 H Fig. 80. Alternating E.M.F. with Third Harmonic. wave, /B W, shown is the third harmonic, being always an odd number; but in some cases the fifth, seventh, or almost any odd harmonic may be present. These harmonics are alternating pres- sure or current waves of three, five, seven, etc., times the frequency of the fundamental wave AC, which are generated simultaneously PRINCIPLES OF ALTERNATING CURRENTS. 113 with the latter, and modify its form. In the case represented, DVEG Fis the wave of £.1/.F. that is actually produced, being the combination of the fundamental AC and the third harmonic, JBW. For example, the voltage at D is the sum of STZ and /7. At V it is the algebraic sum of A and &, and so on. The ideal Fig. 81. Sine Curve. form of wave generated by a coil of wire revolving about an axis in a uniform field is the szve-curve, in which the £.47./. at any point, P, is proportional to the sine of the angle 6, through which the coil’ has moved (Fig. 81). fiethesmaxinune value otuthe: 4:47. /\-at 17 is A. thenithe instantaneous value ¢ at any point is Cp ieee ea Os (32) When the current wave is also a sine-curve, a similar expression gives the instantaneous value as follows: ham pee ATU (33) The sine-wave being the ideal form, practically all calculations are based upon it; other forms tend to be converted into the sine curve; and it seems to be the best for general use, so it should be accepted for the same reason that standard weights and measures, screw-threads, etc., are adopted. This is true, even though some other form might have advantages for certain purposes. The question has been much discussed ; * but the tendency has been for _ manufacturers generally to adopt the sine-form, the actual waves of pressure and current in most commercial apparatus being close approximations to the true sine-curve. If all alternating current apparatus is designed for the sine-wave * Electrical World, Aug. 4, 1894, p. 107, and many other issues to Dec. 1, 1894. 114 ELLE CLRICALIG HY ING: it is possible to operate dynamos, motors, measuring instruments, etc., on any circuit, thus avoiding the endless confusion and difficul- ties that would arise if a different form of wave were adopted by each manufacturer and for each particular purpose. Effective Values of Alternating Pressures and Currents. — Since the value of an alternating current is continually varying, it is usually more convenient to consider its mean value; but this zs not the ordinary arithmetical average. If an alternating current is flowing through a conductor, its heating effect at any instant will be proportional to the square of the current strength ; and the aver- age heating effect for the whole time during which it flows will be the average of these squares, —that is, the mean square. It fol- lows, therefore, that a direct current, to produce the same heating effect, would have a value equal to the square root of this quantity, that is, Vmean square. The same is true of alternating current 2 E.M.F., since the heating effect is Rs 80 that, with a constant re- sistance, the heating is proportional to the square of the voltage. The square root of the mean square of the voltage or current is called its effectzve value, and is the quantity which is indicated by alternating current volt- or ampere-meters. For a sine-wave the ee effective pressure or current is —= = .707, or about 71 per cent of V2 the maximum value, and conversely, the maximum is V2 = 1.41 times the effective value, or 41 per cent greater. These relations may be summed up as follows: Se ers a a EE efec = ‘Mean Square of = 50 dine WOT Tee ome: (34) St ste oie) AL kA - / efec = */ mean square of 7 biter = 107 Lmaz. (35) In practical cases it is usually sufficient to determine the effec- tive volts and amperes of alternating currents, the instantaneous values being rarely considered except for the purpose of deducing formulas, studying phenomena, and other investigations. The term virtual is sometimes applied to the Vmean2 values instead of the word effective ; but the latter word is now generally adopted. Inductance is one of the three fundamental quantities which affect the flow of an alternating or other varying current, the other two being resistance and capacity. It is due to inductive action PRINCIPLES OF ALTERNATING CURRENTS. rls ee of the circuit on itself, or of one portion of the circuit on another portion of the same circuit, in which cases it is called se/f-7nduction ; or it may be due to the action of one circuit upon another inde- pendent circuit, in which case it is called mutual induction. The former is the one generally considered in transmission, and will be treated first. The unit of inductance, or the “coefficient of self- or mutual in- duction,” is called the “exry, which is the inductance of a circuit when the £.4/./. induced in it is one volt, while the inducing current varies at the rate of one ampere per second. For example, if a counter £.A/7.F. of one volt is set. up in a coil when the cur- rent is increased at the rate of one ampere per second, then the self-inductance of that coil is one henry. The physical cause of the phenomenon of self-induction is the fact that a current flowing in a conductor tends to produce mag- netic lines of force around itself. If the conductor is a helix of wire, the lines produced by each turn pass through that turn and through most of the others, so that the total flux through the helix is large. When the current varies, the lines of force also vary in number, and necessarily cut the turns of wire, thereby setting up an E.M.F. in the latter. With increasing current this Z.JZF. is counter, and opposes the flow ; with decreasing current it aids it ; but when the current is steady no £.J/.F. is induced, since the lines of force do not vary or cut the conductor. In the case of mutual induction, it 1s evident that a second coil & in the neighborhood will be cut by the lines of force produced by the first, tending to set up an £.M/.F. in £, which will cause a current to flow in it, or will oppose or aid a current already flowing, according to the relative directions of the lines of force and the current. Inductance was defined by the Chicago Electrical Congress of 1893 in terms of the £.47.F. generated, but it is also proportional to the number of turns of wire and to the average flux through each when unit current is flowing. This is similar to the first definition, since the production of a certain number of lines of force by one ampere in one second tends to generate a certain £.1/.F- A third definition of inductance may be based upon the electro- magnetic energy stored in a coil when a unit current is flowing, which energy is proportional to the square of the flux density, other things being equal. 116 ELECTRICOLIGHTIING. These three definitions may be summed up as follows: Three Definitions of Inductance. — Calling Z the inductance in henrys, ¢ and 7 the instantaneous values of the £.1/.F. in volts and the current in amperes respectively, z the instantaneous value of the average flux through each turn of wire, Z the number of turns, LIers ai Tp W the energy in joules and oF the time rate of variation of the current, we have: in terms of £.47.F. Ny See (36) Sg (37) in terms of energy Ne W=43Li?, \ (38) Reactance due to Self-Induction. — It has been shown that the effect of inductance in an alternating current circuit is to oppose the flow of current on account of the counter /.J/7.F. which it sets up. This opposition may be considered as an apparent resistance, and is called reactance to distinguish it from true ohmic resistance. The value of the reactance due to inductance is given by the fol- lowing expression, in which fis the frequency in periods per second, and Z is the inductance measured in henrys Reactance = 27fZ. (39) The result obtained gives the equivalent or apparent resistance in ohms. Example.— A coil of wire having a self-inductance of 25 millihenrys = .025 henry is supplied with an alternating current at a frequency of 100 periods per second. Its reactance, assuming its ohmic resistance to be negligible, would be 2nfL =2 X 3.1416 X 100 X .025 = 15.7 ohms, Such a coil would have the same apparent resistance as a non-inductive circuit of 15.7 true ohms, and if connected to an alternating current source giving 1000 volts at 100 frequency, the effective current flowing through the coil [O00 eee would be i 63.7 amperes. Impedance due to Resistance and Inductance. — Actual circuits always have resistance as well as inductance, and in most cases the former cannot be neglected. The combined effect of resistance PRINCIPLES OF ALTERNATING CURRENTS. 117 and inductance is called zwpedance to distinguish it from the other two, and has the following value in ohms (apparent resistance). Impedance = V RK? + (2 2fL)*. | (40) Example.—A coil of wire has a resistance of 20 ohms and an inductance of .o25 henry. For an alternating current having a frequency of 100 the im- pedance of the coil is J 2+ (2 rfl)? = Sf 202+ 15.72 = 25.4 ohms. Supplied with 1000 volts the coil would receive a current el ee 39.3 25.4 amperes. The relations expressed analytically in (40) are evidently repre- sented graphically by the right- angle trianglean Hig. $2, The re- sistance A in ohms is laid off on ‘a convenient scale to form the aS 2Tr¢L base, the reactance 2 z/fZ is laid off also in ohms to form the perpen- dicular, and the impedance in ohms is found by-measuring the hypoth- Fig. 82. Graphical Representation. enuse of the triangle, since it is equal to the square root of the sum of the squares of the other two. Lag of Current due to Inductance. — Besides opposition or re- actance to an alternating current, inductance also causes the latter to lag behind the 4.47.7. which produces it. The curve £F in Fig. 83 represents the waves of an alternating /£.J7.F. impressed upon a circuit containing ohmic resistance without inductance or capacity. In such a case the resulting current will reach its maxi- mum as well as zero values at the same instants as the A.JZF, and may be represented by the curve CD. If now a self-induction coil be introduced into the circuit in series with the resistance, the current waves will lag with respect to those of E.47.F.; that is, the maximum current will flow a little later than the instant of maximum £./7.F., as indicated by the dotted curve GH. The amount of this lag is measured as an angle called the angle of lag, assuming one complete period to correspond to 360°. In Fig. 83 the current wave is shown as having its zero value one-eighth of a period, or 45° behind the zero £.J/.F, and the same for the maxi. mum and other corresponding points, hence the angle of lag is 45°. 118 ELECTRIC LIGHTING. The tangent of the angle of lag with a given resistance R and inductance Z in the circuit is t 2nfL ee reactance rf . (40a) resistance vs Referring to Fig. 82, it is evident that the tangent of the angle ¢ is equal to 2xfL +R; therefore ¢ represents the angle of lag, which may be easily determined graphically in this way. It is apparent, from Fig. 82, that the angle of lag ¢ is small if the resistance is Fig. 83. Lag of Alternating Current. large compared with the inductance Z, unless the frequency is high. It is a fact also, that however large the inductance or fre- quency, and however small the resistance, the angle of lag can never be greater than a right angle, or 90°. This is evident in (40a), since ¢ = 90° when its tangent is infinity. Example.— A circuit has a resistance of 2 ohms and an inductance of .0016 henry. What is the angle of lag for an alternating current having a frequency of 100? 2nfL = 2 X 3.1416 X 100 X .0016=2 ohms. The resistance # is also 2 ohms, therefore tan ¢ = ?=1 and ¢ is 45°. This is the condition shown in Fig. 88, the current wave GH (dotted) being 45° be- hind the 2.4.7. wave EF. The curve CD represents the current that would flow if a wire of 2 ohms resistance without inductance were supplied with 100 volts alternating “.47./., the current at any instant having one half the nu- merical value of the £.4/./. its effective value being 100 + 2=50 amperes with no lag. The addition of .0016 henry inductance produces a reactance of 2 ohms, which combined with the resistance of 1 ohms, makes an impedance of J 22 + 22 = 2.82 ohms, which is much less than their arithmetical sum. The current is 100 + 2.82 = 35.5 amperes, so that the effect of inductance is to diminish the current, and cause it to lag as shown by comparing curves CD and G// in Fig. 83. PRIVG LEW OLr ALTERVA TING CURKELNIS. 119 Determination of the Power of an Alternating Current. — Ina circuit containing ohmic resistance only, the current wave C does not lag with respect to the 4.17.7. wave £, and the power is represented by the curve PQ in Fig. 84. At any instant the power in watts is the product of the 4.477. and current at that instant, but for convenience these values (curve PQ) are plotted on a smaller scale than & and C. The power is positive at all times, since the product of the positive values of & and C’as well as the negative values of D and & are always posi- tive, and its effective value is the V mean? of these products, which is simply the product of the effective -.47.F., and current, as read on a volt and an ampere-meter, that is Fig. 84. Power of Alternating Current with no Lag. Power =) fee /: Gaels) With inductance in the circuit, the current lags behind the £.M.F.and the power may be represented by the curve PRQS in Fig. 85. The negative values R and S of the power are due to the fact that the current C is positive when the -.J7-F. is nega- tive, or vice versa ; hence the actual power is reduced, being the algebraic sum of these a Q quantities. When the re- actance is very great com- pared with the resistance, the current lags 90°; and the negative power at 7 and U, in Fig. 86, is equal S to the positive power at P and Oso that ‘the actual power is zero. _ All that Fig. 85. Power of Alternating Current with 45° Lag. A c occurs is a charging and discharging of electro-magnetic energy in the coil, the amount re- turned being nearly equal to that stored. It should be noted that the frequency of the power curves in Figs. 85 and 86 is twice that 120 ELECTRIC. LIGHTING. of the £.1/-F. or current. The effective power, when the E.A/.F. and current differ in pase — that is, one lags behind the other — is Power =Z/cos ¢. (42) In this expression cos @ is the cosine of the angle of lag, and is. called the power factor, since it is the ratio of the veal power to the ap- parent power ET. To measure alternat- ing current power, it is necessary to know the Fig. 86. Power of Alternating Current with 90° Lag. angle of lag if separate volt- and ampere-meters are used, or to employ a watt-meter, which gives the true reading directly. Example. — Taking the same case as before in which the £.47.F. is 100 volts, the current is 35.5 amperes and the angle of lag is 45°, the real power would be 100 X 35.5 X cos 45° = 100 X 35.5 X .71 = 2520 watts, while the ap- 25) OYE parent power is 100 X 35.5 = 3550 watts. Capacity is the third quantity which effects the flow of an alter- nating or other variable current. This physical quantity is familiar in the case of the electrostatic capacity of a Leyden jar or a con- denser, and is measured in terms of the favad as a unit, being the capacity of a condenser which will contain one coulomb of charge at a potential of one volt. Since this unit is much too large for ordinary use, the wzcrofarad, or millionth of a farad, is generally employed. Reactance due to Capacity. — When the two terminals D and G of a condenser are connected respectively to the two wires 5D of an alternating current source A, as = indicated in Fig. 87, the condenser | will be charged and discharged continu- OK ally, so that current will flow in the Fig. 87. Condenser in Alternating wires B and D in spite of the fact that ikea sf th the two sides D and G of the condenser are insulated from each other, which prevents the actual passage of current through it. Thus we see that a condenser is equivalent to a closed circuit PRINCIPLES OF ALTERNATING CURRENTS, Peal having a certain resistance, or, in other words, it has an apparent resistance in ohms which is called its reactance, corresponding to that due to inductance. Evidently the flow of current increases directly with the capacity and with the frequency, therefore the reactance is inversely proportional to these quantities. Calling K the capacity in farads, the reactance in ohms is: i reactance = Daf (43) CUITOT tee at, (44) Example.— What is the reactance of a 50-microfarad condenser to an alter- nating current of 100 frequency? The reactance from (48) is: 1 1 1 7 oafk 2X 3.1416 X 100 x .000050 ~ 081416 ~ 21-8 ohms. If the Z.A47.F. of the supply is 100 volts, a current of oS = 3.14 amperes would flow in the connecting wires. Lead of Current due to Capacity. — A condenser is supplied with an alternating £.47.F. represented by the curve EF GA in Fig. 88. It is evident that current will flow into the condenser in one direction while the £.1/.F. varies from its greatest negative value £& to its highest positive value /; and its direction is the same as that of the positive 4.47. /., therefore, a positive wave of current C is produced during that time. The condenser is fully charged when the A..F. reaches its maximum value F/ so the flow into the condenser ceases and the current is zero. The 4.//.F. then falls as shown by the line /GH,, and the condenser discharges, producing the: negative current wave Y and so on. By comparing the two curves, it appears that the maximum current into the condenser occurs at a point C, which is 90° ahead of the maximum £.4/7.F. at F, and the same for other corresponding points. Hence, the char- ging current of a condenser has a /ead with respect to the im- pressed #.17.F. The tangent of the angle of lead or negative lag is given by an expression analogous to (40a). Fig. 88. Lead of Alternating Current. Reactance = 2 aK. (45) Resistance | R ; fal, == — 12 ELECT RIGALICHIING This angle may be determined graphically by a triangle, as in- dicated in Fig. 89, similar to Fig. 82; but in this case the reactance pode 1 -=> Impedance due to Resistance, Inductance, and Capacity. Gouhes a certain ohmic resistance is in series with a condenser, the - impedance or combined apparent resistance in ohms is given by an \ expression corresponding to (40). is laid off downward, since it produces a lead instead of a lag. Impedance = \/e 24 (46) if 2 saa) The same result is obtained graphically in Fig. 89. When in- ductance and capacity are both present in a circuit, the reactance of one tends to balance that of the other, so that the combined reactance is the algebraic sum of the two, that is: u afk (47) When all three quantities — resistance, inductance, and capacity — are present in a circuit, the combined impedance is Reactance = 2 7fZ — Impedance = \/ 23+ (2afL — 7K} (48) 1 DPE IRS saci EE pe ae (49) Tangent of angle of lag or lead = R The graphical solution is shown in Fig. 90; the inductance re- actance being laid off upward, and the capacity reactance down- ve DEN + Fig. 89. Impedance and Lead Fig. 90. Impedance and Lag due to Shown Graphically. Three Quantities. ward, the difference being the perpendicular of the triangle ct which the hypothenuse is the impedance and the angle ¢ is the lag or lead, as the case may be. For convenience, the constant 27/ is designated as /. PRINCIPLES OF ALTERNATING CURRENTS. 123 Example.—A circuit consists of a coil having 20 ohms resistance and .025 henry inductance in series with a condenser of 50 microfarad. What is the combined impedance and angle of lag when supplied with alternating current at a frequency of 100? The impedance from (48) is VY 202+ (15.7 — 81.8)? = 25.6 ohms, which is less than the capacity reactance (31.8 ohms) alone. The tangent of the angle of lead (the capacity reactance being greater than the in- ductance reactance) is 15.7— 381.8 16.1 2 hoe ieee ON) bi ——_—— .805, Power Factor with Resistance, Inductance, and Capacity.—It has already been stated (42) that Power = E/ cos ¢, when the cur- rent lags on account of inductance. It applies also to a leading current in a capacity circuit, and to a circuit containing resistance, inductance, and capacity. All that is necessary is to determine the angle of lag or lead from (49), or Fig. 90, and the cosine of that angle is the power factor. It is evident that the angle of lag, 4, is small when the induction and capacity reactances are nearly equal, so that they practically neutralize each other, in which case cos 4, the power factor, is almost 100 per cent 1 ; Resonance. — If 27/Z = Inf K in (48), then 1 and the impedance of the circuit reduces to R simply, just as if neither inductance nor capacity was present. In other words, the two reactances exactly neutralize each other. The electrostatic energy in the condenser discharges when the electromagnetic energy in the inductance is being stored up, and vce versa. This condition is called electrical vesonance; and the circuit is said to be tuned for the particular frequency indicated in (50), since with a given £.M/.F., the current will be much stronger for that fre- quency than for any other, the impedance being a minimum and equal to the resistance only. The induction- or capacity-reactance R L K may each be very high, in which WS/\/V/0 UII case the difference of potential i | S ie U Fig. 91. Reactances in Series. across either pair of terminals will be much greater than the impressed Z.J/.F., since the drop in voltage for each part of the circuit is equal to its reactance multiplied by the current. This can best be made clear by an example. 124 BLECTRIGQIIGR TING Example.— In Fig. 91 a non-inductive resistance 2 of 2 ohms is connected in series with a coil Z, having .051 henry inductance (with insignificant resis- tance), and with a condenser, A’, of 50 microfarads. What are the conditions when 100 volts alternating £.47./. at 100 frequency is applied to the terminals Pand U? The impedance from (48) is /2?+ (32. — 31.8)? —/4+ .04 = 2.01 ohms. This is only .01 ohm, or one-half of one per cent more than the resistance alone. With 100 volts the current is 100 + 2.01 = 49.8 amperes. Since the re- actance of the coil Z is 32 ohms, the potential difference across its terminals and 7 is 32 X 49.8 = 1593.6 volts, and between the terminals Z and U of the condenser X it is 31.8 X 49.8 = 1583.6 volts. Each of these is nearly sixteen times as great as the impressed voltage at P and UV, which is only 100. This multiplication of pressure when resonance happens to occur is sometimes the cause of breakdown in the insulation of conductors and apparatus. Resonance may be set up by the frequency of the fundamental wave or by that of any upper harmonics (Fig. 80). With the fundamental frequency and with the third and fifth harmonics present as is often the case, there would be three values of the frequency / in (50), any one of which might satisfy the equa- tion and give resonance, if the product Z A happened to have a corresponding value. Circuits Containing Reactances in Series. — Equation (48), or the graphical methods shown in Figs. 82, 89, and 90, may be used to find the impedance of any combination of resistances, induc- tances, or capacities in series. In such a case, the sum of all the resistances should be substituted for R, the sum of all the induc- tances for Z, and the sum of all the capacities for K in (48). These total values may be used also in the diagram represented in Fig. 90. The #./.F. is then divided by the total impedance thus found in order to obtain the current. The potential difference be- tween any two points of the circuit is the product of this current and the impedance between those points. The angle of lag is found by (49) using total values for Rk, Z, and A. If one or two of these quantities are not present in any case, they disappear from the equations or graphical representations without affecting those quantities remaining. | Reactance E.M.F.— In Fig. 92 the horizontal line OA is laid off in proportion to the resistance of a certain coil, and the vertical line OB is made proportional to its reactance = pl, in which p = 2xf. When the, rectangle is completed, the diagonal OD represents the impedance = V R2 + (pL)?, being the same as the hypothenuse in Fig. 82. Assuming a current of one ampere in the coil, the voltage across its terminal is 7 V R?+ (p/)?, which is numerically equal to the impedance ; hence the line OD in Fig. 92 PRINCIPLES OF ALTERNATING CURRENTS. 125 may be taken to represent that voltage. Similarly the line OA gives the drop due to the ohmic resistance, and the line OL shows the voltage required to overcome the reactance. That is, the im- pressed £.17.F. OD is resolved into two components at right angles to each other, one of which, 7/2, overcomes the resistance, and the other, g//, overcomes the reactance. The reactance is, in fact, an opposing /.4/.F., having an effective value, OF, equal but directly opposite to pZ/, and lagging 90° behind the current, the phase of the latter being represented by the line OZ. These relations, which are very important, may be stated as follows: With a given current / the effect of resistance is equiva- lent to a counter £.M/./. equal to /R, and represented by the line OG. With the same current, the reactance is actually an opposing £.MF. having an effective value equal to pZ/, and indicated by he oe eee OO Fig. 92. Components of E.M.F. OF. The combined effect of the two is equivalent to a counter E.M.F, equal to OH, which is their resultant. Hence OD, the impressed £.47./., must exactly balance O/7; that is, it is equal and opposite as shown. The phase of the impressed /.A/.F. is represented by the line OD, with respect to which the current phase OA has an angle of lag ¢, and OF, the reactance EL. MF, has a further lag of 90° behind the current. Precisely similar diagrams and reasoning apply to the capacity reactance and angle of lead in Fig. 89 and to the combined -induc- tance and capacity reactances in Fig. 90. Composition and Resolution of E.M.F. and Current. —JIn the manner explained above, two or more alternating /.AZ.F’.’s may be combined to form a resultant, or one £.47.F. may be resolved 126 ELECTRIC LIGHTING. into two or more components; and the same is true of alternating currents. For example, two alternators are running in series, the value and phase of the /.J7.F. of one being represented by O17 in Fig. 98, and the value and relative phase of the other’s F.J/.F. being represented by OM. The combined effect is the same as that of one 4.477. having the value and phase O/, which is the resultant of the two. If OM and OW represent the phase and values of two alternating currents, the phase and amount of the resultant current are given by OP. This method applies to any 0 9) ea PRINCIPLES OF ALTERNATING CURRENTS. 129 Self-Inductance of Lines and Circuits. — It is possible to calcu- late the inductance of coils of wire, but difficult to do so, and in most cases it is determined by comparison with standard induc- tances. The inductance of aérial lines is easily calculated, since they are generally parallel, and the medium has a fixed permeabil- ity practically equal to one. Wires laid upon a wall of wood, or other non-conducting, non-magnetic substance, or otherwise placed out of proximity to such a substance, have practically the same in- ductance as aérial lines. The following formulas may be used to determine the self- inductance of two parallel aérial wires forming part of the same circuit, and composed of copper or other non-magnetic matertal. A Peper centimeter = (5 + 2 log, =| VE (51) In this expression / is the self-inductance in henrys per centi- meter of each wire, A is the interaxial distance between the two wires, and v7 is the radius of each. The dimensions 4 and 7 may be expressed in terms of any unit, provided it is the same for both. Since the Napierian logarithms in the above equation are 2.302585 times the common logarithms, and as it is more convenient to use the diameter d of the wire instead of its radius, we have without appreciable error : 2A Pape cconumMetein= (5 + 4.6 log 7 ae : (52) i) per foot = (45, 24 + 140.3 log a 10—-* (53) se pen mile: = (80.5 + 740. log ) 1) arta) For each of two parallel z7oz wires we have the following expres- sions, in which the only change is the first constant : | 2A L per centimeter = (75 aloe 7 il eae (00) Jf per centimeter = 2A 75 + 46 log = jo (56) ea 2A L per foot =(2 286 + 140.3 log =) 10- ~9 (57) = ie L per mile = (12070 + 740 log jo (58) a0) ELECTRIC LIGHTING. In order to save the trouble of calculating the inductance with various sizes of and distances between wires, the following table is given: INDUCTANCE, IN MILLIHENRYS PER MILE, FOR EACH OF TWO PARALLEL COPPER WIRES. A.W.G. Interaxial No. 0000 000 Distance. Diam. Inches. 0.460 0.4096 inches. 0.907 1.130 1.260 1.555 1.484 1.576 1.648 L019, i 12056 1.242 | 1.280 1.372 | 1.410 1.465 | 1.502 1.596 | 1.6385 1.688 | 1.726 157605 (A 1eto7, I~ oa eon) fp) Se LOTR RES — IDE wb 5 Oo1c GW © 0 Hon C9 rR CO CH Or Orb L10t é 1818 | 1.856 1.799 : scl ih 1.949 LEST .946 1.982 | 2.023 1.930 ; 2.042 |. 2.079 1.971 05: 2.092 | 2.128 2.023 2. 2.1384 | 2.172 > e+ Or CO WH BG SO Go or me Od © So DNNNRE HEE e ee eee on) DH HOD MHD. interaxial Distance. Inches. 1.355 ieee! 1.709 1.801 1.931 2.025 2.097 2.155 2248 2.319 2.376 ae DAE AE | 2.395 .433 2.470 DWN NWNDNWH HEHE OP PR WDE HOD OAG OO SCOHTMMOWADWARES AIRMAN RHDOaAMD LE-xamples.— To show the use of the above formulas and table, let it be required to determine the inductance of an overhead line, 13 miles long, con- sisting of two No. 0000 copper wires, 48 inches apart. Since it is a metallic circuit with two conductors, the total inductance is due to 2 X 1;=3 miles of wire. From (54) the inductance per mile is ( 2 =80.5 + 740 log eS aD ANie 2) PRINGIPEBS ION ALTERNATING CORKENTS. eee Substituting, in this expression, the value of the distance between the wires A = 48 inches, and the diameter of each wire, a= .46 inch, we have 6 —6 L per mile = (80.5 + 740.3 log oy 10. = .001799 henry. This is equal to 1.799 millihenry, and is the same value as that given in the first column of the table, and shows how those figures were obtained. The total inductance of the circuit is 8 X 1.799 = 5.397 millihenrys. Impedance of Circuits. — Having obtained the inductance of a given line or circuit, by calculation or from the table, the reactance from (38) is 2r f Z, and the impedance from (89) is VR? + (224fL). The resistance R may be found in the table on page 8. Tables are often given showing the impedance of jines; but in order to cover the various frequencies, sizes of wire, and distance apart, they become too bulky to include in the ipresent volume. Mutual Inductance of Circuits. — ‘The inductive effect of one circuit upon another separate circuit is called mutual inductance, ‘The most familiar example in electrical engineering is to be found in the action between the primary and secondary coils of a trans- former, and will be considered later under that head. If two con- ductors run parallel to each other, as, for example, two overhead jines upon the same poles, an alternating current in one tends to induce an alternating 4.A7,F. in the other, the direction of which is opposite to that of the inducing current. Consequently two parallel alternating currents which are exactly in phase tend to oppose each other; but if they differ by 180° in phase, that is, flow in opposite directions at the same time, they tend to aid each other. The currents in two or more parallel wires leading from the same terminal of an alternating current source would have about the same phase, assuming their angles of lag to be nearly equal, hence they tend to oppose each other. This opposition has the effect of increasing the drops in voltage similar to that due to self-induction ; in fact, the action of these wires upon one another is practically the same as that of one element of a wire upon the other elements, but in the latter case it is called self-induction. In practice, two alternating currents from independent gene- rators would not be likely to remain exactly in phase, except for a few seconds at a time, so that their mutual induction upon each 132 FLECTRIC LIGHTING. other would produce opposing effects at one time, aiding effects at another, and so on as the phase changed. Supposing the frequency of one current to be 100 and of the other 1004 periods per second, the difference would be one period in two seconds, so that the voltage on each circuit would be raised once and lowered once every two seconds, causing very objectionable flickering in incan- descent lamps. This is avoided by increasing the difference in frequency between the two currents. For example, if one were raised 5 per cent and the other lowered 5 per cent, the difference would be 10 periods per second, and the fluctuations in voltage\ occurring at that rate would be hardly noticeable. It is better, however, to have a still higher rate of 15 or 20 per second in order | not to be perceptible or injurious to the eye. It is also possible to eliminate this effect by arranging or transposing the wires as described later. Means of Reducing Self-Inductance. — In equations (51) to (58) it is evident that self-inductance is decreased by diminish- ing A the interaxial distance between two wires forming a metallic circuit. This somewhat paradoxical fact is understood when we consider that self-induction is proportional to the number of mag- netic lines linked with a circuit, as defined on page 116. Conse- quently, the greater the distance between the two wires which constitute it, the more lines will be enclosed. Hence the wires should be as close together as possible in order to reduce self-in- ductance, the limit being the distance necessary for proper insula- tion, and in the case of overhead wires they must be sufficiently far apart not to swing too near each other. If two insulated wires are laid side by side, or twisted together, their self-inductance becomes insignificant ; and if concentric con- ductors are used, it disappears entirely, since the tendency to pro- duce magnetic lines by one is neutralized by the other, the currents being equal and opposite. One wire, carrying an alternating cur- rent and running through an iron pipe, will have large self-induc- tance, on account of the great number of lines which are set up around it; but if both wires of a metallic circuit are put in the pipe, the self-inductance is very small. Another way to reduce the drop due to self-induction is to szd- divide the conductor, using several smaller wires having the same total sectional area. PRINCIPLES OF ALTERNATING ‘CURRENTS 133 Example, — An overhead line 1 mile long consists of two No. 0000 wires forming a metallic circuit, the distance between the wires being 24 inches. One mile of No. 0000 has .258 ohms resistance at 20° C., so the resistance of the circuit is 2 X .258=.516 ohms. The self-inductance per mile from the table on page 130, is 1.576 millihenrys, or .003152 henrys for the circuit. Ata frequency of 100 the impedance is /.5162 + (628 x .003152)? = 2.05 ohms. With acurrent of 40 amperes the drop due to resistance is 40 x .516 = 20.64 volts, and the total drop is 40 x 2.05 = 82 volts. Using eight No. 6 wires in parallel the joint resistance would be .52 ohms, being almost exactly the same as before, or .52 X 8 = 4.16 ohms for each wire. Assuming the distance apart to be the same, or 24 inches for each pair, and neglecting the mutual induc- tance between the pairs, the self-inductance from the table would be 1.912 millihenrys per mile, or .003824 for the circuit, and the impedance \/.4162 + (.628 X .003824)2 = 5 ohms. The current in each wire is 40+8=5 amperes, so the resistance drop is 5 X 4.16 = 20.8 volts, and the total drop is 5 X 5=25o0hms. In this case the resistance drop is practically the same as before, and the impedance drop is only 25 volts, or about 20 per cent greater than the simple resistance drop, while in the previous case it was 82 volts, or four times the resistance drop. The above example proves the great reduction in inductance drop effected by subdividing the conductor. This is sometimes said to be due to the use of sma/ler wires, but this is not true; in fact, the inductance itself is increased: by reducing the size of wire, as shown in the foregoing example, and in equation (53). In reality, the impedance drop in the second case would probably be greater than that calculated, on account of the mutual inductance between the corresponding wires of each pair; but this need not be very great if they are not put close together, and may be practically neutralized by arranging or transposing the wires, as explained under the next heading. ~jAnother method of reducing the effect of inductance is to balance it by the effect of capacity. It was shown in connection with Fig. 90 and equations (46) and (49), that certain values of capacity in a circuit may completely or partially neutralize the reactance due to inductance. In the Stanley electric power system condensers are used upon the circuit in connection with the motors to balance their induc- tance, so that the watless current is much reduced. In other words, the power-factor is raised, and the drop on the line is dimin- ished. Theamount of capacity, A, in farads, required to neutralize a certain inductance, 7, in henrys, at a frequency, f, is obtained from (49) and has the following value: 134 ELECTRIC LIGHTING. iE ~ Lf} ee Synchronous alternating current motors may also be used to balance inductance, since they have the effect of capacity in caus- ing the current to lead when their field magnets are over excited. By regulating the field excitation the power-factor can be raised to practically 100 per cent. The same effect is produced by rotary converters, and will be considered more fully later in connection with the polyphase transmission and direct-current distribution system. | Means of Reducing Mutual Inductance. — The simplest plan is to increase the distance between the conductors, as already stated ; but this is limited by practical considerations, such as requirements for carrying the wires on the same pole. In such cases the effect of mutual induction would be great if the wires were arranged as A@ @c E G ] oe) J K L M N Q e | P O B® @D H F Figs. 104, 105, and 106. Arrangement of Conductors to Neutralize Induction. represented in Fig. 104, in which A and & indicate the wires of one circuit, and C and J those of another parallel circuit. The wire A being very near the corresponding wire C of the other cir- cuit, would tend to set up an opposing £.//.F. in it, and C would react upon A in a similar manner. The mutual induction of B and JY would also have the same effect. If, however, the wires are placed equidistant, as shown in Fig. 105, any one conductor, £, will be acted upon equally by both wires, G and A, of the other circuit, since they are at the same distance from it. Consequently mutual induction between the two circuits 1s neutralized. This can also be accomplished by transposing the wires with respect to each other, at certain intervals, as shown in Fig. 106, in which the por- tion /V of one wire, counteracts the effect of the part Q of the other wire of the same circuit. Consequently the inductive action upon the other circuit, /AK and LJ, is nil. PRINCIPLES OF ALTERNATING CURRENTS. 135 It should be noted, however, that the self-induction of either circuit is not materially altered by these arrangements, being de- pendent upon the average distance between the two wires of each circuit, and not upon the presence of the other circuit. ‘¢ Skin Effect’? is the name given to the phenomenon accord- ing to which alternating currents tend to have a greater density near the surface than they have along the axis of a conductor. If we imagine a wire to be made up of elementary filaments parallel to its length, it is evident that the central or axial filament will be surrounded by a greater number of magnetic lines than an element at the surface, since each filament tends to set up lines around itself. This fact produces no effect upon a steady current after it has been established, there being no variation in the number or position of the lines. Hence a steady current has a perfectly uni- form distribution throughout the entire cross-section of a conductor having a uniform specific resistance. In the case of an alternating current, the additional lines of force that inclose the filaments near the axis are reversed twice during each period, the effect being to generate a greater back f:.M.F. of self-induction than for the outer filaments of the wire. Consequently the current density is less near the axis than it is near the surface. With high frequency and large conductors this action may be so great that there is actually a dack flow of current at ornear the axis. But with ordinary sizes of wire and frequen- cies, the effect is small. This “skin effect” is generally treated as an increased appar- ent resistance of a conductor, being called its vzrtual resistance ; and since it involves a larger drop in voltage and a greater loss of energy, it is practically the same as true resistance. In Fig. 107, which shows graphically the values of virtual resistance, RK, is the apparent or virtual resistance for a given alternating current, R,is the true ohmic resistance of a copper conductor at 20° C. (68° F.), A is the area of cross-section of the latter in circular mils, and fis the frequency. A conductor one inch in diameter has a cross-section of one million circular mils, so that at a frequency of 100, the product of A and f, is 100,000,000. Referring to Fig. 107, we find that oes that is, the. virtual fesistance is 21) percent greater than the true resistance, consequently this is too large a 136 ELECTRIC LIGHTING. conductor to use at that frequency. On the other hand, No. 0 wire has a sectional area of 105,500 circular mils, and with the same frequency of 100, the product A f = 10,550,000, which would give a virtual resistance less than one-half of one per cent greater than the true value, and need not be considered practically. Frequencies higher than 135 are rarely used in practice, and with a conductor one-half inch in diameter f A = 135 x 250,000 = 33,700,000, and R, + R, = 1.03 approximately. The conclusion is that with conductors smaller than one-half inch diameter, the increased resistance due to “skin effect ” is less than 3 per cent for commercial frequencies. If a larger cross-section than this is required it should be subdivided among several wires in parallel, or a) roel OF Al & F_IN MILLIONS i 0 10 20 30 40 50 60 70 SO 90 100 Fig. 107. Curve Showing Corrections for ‘Skin Effect.’’ the conductor may be made hollow or in the form of a flat bar, the “skin effect ’’ being greatly reduced thereby. It has already been shown, on page 132, that the self-inductance drop is reduced by subdividing conductors, but the present phenomenon is a different one, and should be considered separately. With tron conductors the virtual resistance is much greater than with copper or other non-magnetic metal; but iron is not often used to carry alternating currents, and the exact value of the per- meability is not easily determined,* so that formulas will not be given. Capacity of Overhead, Underground, and Submarine Conductors. —It is possible to predetermine the electrostatic capacity of elec- * Merritt, Physzcal Review, November, 1899. PRINCIPLES OF ALTERNATING CURRENTS. 1s We trical conductors, and almost all cases will come under one of the following heads : Case 1. Jusulated conductor with metallic protection ; for ex- ample, an iron-armored submarine cable, or a lead-covered under- ground conductor, having the metallic sheathing connected with the earth, which is the usual condition. Case 2. Szngle aérial conductor with earth return. Case 3. Metallic circuit consisting of two parallel aérial con- ductors. In the following expressions, A is the capacity in farads, * is the dielectric constant, Y is the internal diameter of the metallic covering, @ is the diameter of the conductor, % is the height above the ground of an aérial wire, and 4 is the interaxial distance be- tween two parallel wires. In cases 2 and 3, the medium being air, k = 1, and does not appear in the equations. This assumes that the conductors are bare; but if they are covered with insulation of ordinary thickness it would only slightly increase the capacity, & being greater than 1 for insulating materials. The proximity of other conductors may increase the capacity considerably, but their effect is difficult to calculate. Case 1. Insulated con- ; 2 Oy ee Oe Ue peracentimeter. = ductor with metallic ae D (59) covering. ed, Tooke 10? K foot 7-2 per foo shy D (60) Bid E =o K per mile = 38.83 & 10 ids D (61) ae: 9 sve nie 9 2 nat Caseuno Single aérial Popeeentineren 241.5 x 10 | conductor with earth me 4h (62) return. | a , br ety K per foot = 1,361, x 10 4h (63) | ayed ae aay. 9 = K per mile = 38.83 x 10 4h (64) joy = og 7 138 ELECTRICULIGATING: - Case 8. Two parallel aérial ‘conductors Avper centimeter 71120 Saxetoe forming metallic cir- of each wire ~ 08 2A (65) cuit. a Apper foot ee 30s lel of eachiwire 0 VR 2AM eGo) loge d ASpermiles | 1) 42a of each wire 2A (67) log =] Examples, — What is the capacity of one mile of No. 0 (A. W. G.) lead- covered cable, with rubber insulation .15 inch thick? Substituting in (61) for d, the diameter of No. 0 wire = .825 inch, and for D the external diameter of the insulation = .3825 + (2 X .15) = .625 inch, and for & the dielectric constant of pure rubber = 2.5, we have: 38.83 & 10—9 625 log 355 K per mile= = 342. x 10—-° farad = .342 microfarads. What is the capacity of one mile of single overhead bare No. 0 wire, 10 feet above the ground, with earth return? Substituting in (64) the values of A and d, both in inches, we have: ‘6 , x earls | 5 AC per mile eae = 12.2 x 10— farad = .0122 microfarad. 4 xX 120 10g — 395 What is the capacity of two parallel overhead bare No. 0 wires, 12 inches apart, and each one mile long? Substituting in (67), we have: 2x 19.42 x 10—° DD < 1G log —355 ie = 20.8 x 10—9 farad = .0208 microfarad. Means of Reducing Capacity. —It is evident from equation (59) that the capacity of a given length of insulated conductor with. metallic covering is decreased by diminishing &, the dielectric con- stant of the insulation, by increasing LD, the internal diameter of the metallic covering, or by reducing d, the diameter of the con- ductor. Since the capacity varies in direct proportion to &, the insulating material should have the minimum dielectric constant. Unfortunately the best insulators usually have high values for &, notably india rubber, gutta-percha, paraffin, and mica. ‘The dielec- tric constant of paper is comparatively low, and largely for that reason it is used for insulating the wires in a telephone cable. Paper is also used for the insulation of electric light and power PRINCIPLES OF ALTERNATING CURRENTS. 139 cables, and would have special advantages when it is desired to make the capacity as low as possible. ‘This question will be con- sidered further under the head of insulated and underground con- ductors. _ The reduction of capacity by diminishing d@, the diameter of the conductor, is limited in practice by the necessity for using a cer- tain size in order to give sufficient current capacity, and not have too much resistance. It is also a fact that it is not practicable to materially reduce electrostatic capacity by augmenting LY, or in other words, by increasing the thickness of the insulation. Fig. 108 represents a lead-covered cable, in which dis the diameter of Figs. 108 and 109. Reducing Capacity by Increasing Thickness of Insulation. the copper conductor, and JZ is the internal diameter of the lead covering. If the former is 4 inch, and the latter 3 inch, the thick- ness of insulation is } inch. In Fig. 109 the thickness of insula- tion is twice as great, or } inch; so that D’ becomes 3 inch, d’ being + inch the same as @. The capacity in the two cases will be in the ratio : 1 1 Lona Le a 7 ~ log 2° log 8 OO: bo jo That is, the capacity is reduced only 36 per cent by doubling the thickness of insulation. The volume of insulation in the two cases would be in the proportion (D? — a’) :(D" — ad") = 3: 8, which is an increase of 267 per cent, or almost three times as much. Since the amount of insulating material affects directly the cost and size of the cable, it would seldom pay to nearly treble this material in order to diminish the capacity to the extent of only 36 per cent. Hence in almost all cases the thickness of insulation is determined by its insulating qualities, and strength to withstand breakdown by electrical and mechanical pressures. 140 RLECTRICOLIGHTING. To reduce the capacity of overhead wirés, the distance between them and from the ground should be increased. But even in this case the reduction is small compared with the increase in distance. Assume a horizontal wire } inch in diameter, and one mile long, to be strung 380 feet above the earth, and another wire of the same size and length to be strung 60 feet above the earth. From (64) the capacities in the two cases will be respectively : SO Coe Ome log 7200 Pilfeterse yc MUS log 14400 The difference between the two values is only about 8 per cent, although one wire is twice as high as the other. The capac.ty with respect to each other of two parallel overhead wires 8 feet apart, each being } inch in diameter and one mile long, is found from (67) to be ED sent) am log 860 Increasing the distance between the wires to 6 feet, the capacity = 10" < 1.0" tarade—0.010 1 microtaradaana = 19193 < 10 = *ifaradi =" OU 9sasnicrolarad = 7'6 X 10—*tarade= 0076" microtarac: becomes TOM eels Aer = 6. 8ax «10 @'tarad =U 0G6S8emicroiancd: log 720 . In this case the capacity is reduced 104 per cent by doubling the distance between the wires. From these examples it is evident that this way to diminish capacity is hardly economical where the cost of construction is greatly affected by the height and distance apart of ‘wires, as 1s ‘the case in@pole. jinesiy) Phe smethodwor balancing the reactance of capacity and inductance, already set forth on page 133, can be applied to reducing the effect of capacity in electrical circuits. PRINCIPLES OF ALTERNATING FOLYPHASE CURRENTS. 141 Series (banked Lets PRINCIPLES OF ALTERNATING POLYPHASE CURRENTS. THE advantages of two- and three-phase, or other polyphase sys- tems, apply solely to the operation of motors. In fact, such cur- rents are positively disadvantageous for supplying arc or incandes- cent lamps. Consequently this subject comes under the head of electric power rather than electric lighting. | However, electric lamps are often used upon the same circuits with polyphase motors, and in many cases energy is transmitted over long distances by polyphase currents, to be converted into direct currents for local distribution to lamps; so it is necessary in the present volume to consider the principles of polyphase systems, and the methods of operating lamps upon them. A two-phase current may be regarded as, and in most cases actually consists of, two dzstzmct single-phase currents, flowing in { 1 ' ! 1 E B H Fig. 110. Two-Phase Current. separate circuits. There is often no electrical connection between them, their only relation being that of ¢zme. That is, they differ in phase. This condition is shown in Fig. 110, in which the curve ABCD represents one alternating current, and E/GAH// represents 142 BLBCTRICOTILAIING - another, the difference in phase being 90°, the maximum value G of the second occurring 90° behind the maximum point A of the first, and so on for other corresponding points. If there is no lag of either current, the same curves can be taken to represent the two E.M.Fs, and with the same lag for both currents they would still be 90° apart in phase. If the lags were not equal, then the phase relation would be altered correspondingly. The two /.4/.F:s or B E Fig. 111. Two-Phase, Four-Wire Circuit. currents might have different maximum values, or different wave forms, but in practice they are usually made as nearly alike as pos- sible. It is evident also that the difference in phase might be made anything between 0° and 360°; but it is almost always designed to be 90°, or one-quarter of a period, and for that reason is often called a guarter-phase current. ‘“[wo-phase currents may be gene- rated by two separate alternators, but in order to preserve the phase relation it would be necessary to have their shafts coupled or posi- B E Fig, 172. Two-Phase, Three-Wire Circuit. tively connected together. In practice, a two-phase current is usu- ally generated by two separate windings upon one armature, the machine having the same general form as a single-phase alternator. The two circuits may be kept entirely separate, as in Fig. 111, lamps Z being connected to each, in which case four wires are required. In order to save one wire it is possible to use a common PRINCIPLES OF ALTERNATING POLYPHASE CURRENTS. 143 return conductor for both circuits, as in Fig. 112, the dotted por- tion of one wire, J, being eliminated by connecting across to C at Mand N. For long lines this is economical, but the interconnec- tion of the circuits increases the chance of trouble from grounds or short circuits. It is alsoa fact that the current in the conductor C will be the resultant of the two currents, dif- fering by. JO" siniephase, ~ brom:-thee principle’~ P shown in Fig. 93, the value of this resultant ievtoundsin Piewils to be Ok = V2 OP = 1.41 x OP the two-phase currents being repre- sented by the components OP and OQ at right Oo angles to each other. Consequently the result- 1%. 773 Resullant of ant current in C is 1.41 times that flowing in either B or & in Fig. 112 and the cross-section of the wire C should be 41 per cent greater. A three-phase current consists of three alternating currents, differing in phase, as indicated in Fig. 114. One current is repre- sented by the curve /AZ, another by the curve J/WVO, and the third by the curve PQR, the maxima points /, W/, and Q (or other corresponding points) being 120° apart in the ideal case, and ap- § M Q O Fig. 114, Three-Phase Currents. proximately so in practice. These three currents might be carried in three entirely separate circuits requiring six wires, being analo- gous to the two-phase, four-wire system in Fig. 111; or one com- mon return conductor may be used, thereby saving two wires, and reducing the total number to four, as shown in Fig. 115. The armature windings and their phase relation are represented dia- 144 ELECTRIC LIGHTING grammatically by the coils J7A, A7B, and MC, the three main con- ductors by AZ, BG, and C/, the common conductor being indicated by the dotted lme 47. The lamps Z, Z, Z, are connected across between the common point /V and the three main conductors. If the three circuits are balanced (i. e., have equal currents) the common conductor M/W will carry no current, and may be dis- pensed with. This is a most interesting and important feature of Fig. 115. Three-Phase Circuits with Y Gonuecuen. the three-phase system. The simplest way to understand it is to consider that each wire acts as the return conductor for the other two. In other words, the algebraic sum of the three currents meeting at the common point /V is equal to zero; consequently Kirchhoff’s law is fulfilled. This fact is shown in Fig. 114, the algebraic sum of the ordinates of the three curves being equal to zero at any point. For example, at SR the ordinate of curve J7/VO is zero, and the ordinates of the other two are equal in value, but opposite in sign. At 7A the sum of the two positive ordinates of the curves MN and PQ are equal to the negative ordinate of the other curve /KZ, be- cause. 1 7) = "sins 0° == teand kh sem 90° =\1, and so on for other points. Fig. 116. Kesultant Current, re pees Tirbe PAu The same principle is proved in Fig. 116, in which a balanced three-phase current is represented by three equal vectors at 120° with respect to each other. Two of these currents, OZ and OU, are equivalent to their resultant OR, which is equal and opposite to the third cur- rent OS; consequently the resultant of all three currents is zero. In the operation of motors the three currents are usually equal, all three wires being connected to each machine, so that the fourth PRINCIPLES OF ALTERNATING POLVPHASE CURRENTS. 145 wire WN, in Fig. 115, is superfluous ; but for electric lighting this extra conductor is required, unless the lamps on the three circuits are balanced. If the currents in the three branches are not equal, Fig. 117. Three-Phase Circu.ts with A Connection. then the wire J/W carries the difference between them, so that its function corresponds closely with that of the neutral conductor in the ordinary three-wire system described on page 70. Another method of connecting three-phase circuits is shown in Fig. 117, and is called the A (delta) connection, the arrangement in Fig. 115 being designated as the Y connection. In either of these cases any lamp Z is fed simply by the £.M/.F. due to a single arma- ture winding, J7A in Fig. 115, or OP einmebion slits elf ehowever, v2 lamp is connected across the outer terminals of the Y circuits, it re- ceives a voltage which is the resul- fate Ore two 17778" that’ are in series, but differ by 120° in phase. minseissshown mo bio. sl18, 0A, DB, and DC representing respec- tively the £.17.F.s of a three-phase armature winding with Y connection. Assuming the £.J/.F. of each phase DC to be 100 volts, then the £.17./. between A and C will be V3 = 1.78 times as great, or 173 volts. Production of Rotary Field by Two-Phase Current. — An iron ring, wound with insulated wire, as represented in Fig. 119, is sup- Fig. 118. Relative Voltage of A and Y ; Connections. plied with two-phase currents at the four equidistant points A, BA, C, and D, the two conductors of one phase being connected at A and #, and those of the other phase at Cand D. Considering only one current, and assuming it to enter at A, the direction of wind- ing. is such that it will produce a south pole at A, and a north pole 146 ELECTRIC LICHTING. at B, so that a compass needle placed inside the ring would tend to point vertically upward as indicated by the dotted arrow. ‘This | condition is represented at 1, in Figs 20, the tcurment eee having its maximum positive value, and the other current, PEO, being ezeromeaten Davee stant. A moment later, the first current has decreased somewhat, and the other has increased, so that they are equal eine iis case, each will tend to produce. a south pole where it enters the ring, at A and VL respectively, so that a resultant polarity is produced midway between, as shown at 2 by the arrow inclined at 45°. The next instant, at 90°, the: current 2 27 has fallen to?zerofand. the currents 14s reached its maximum, so that the arrow takes a horizontal position, as represented at 8. Again at 135°, the current Z J/ has reversed, tending to make a south pole at the bottom of the ring, and the needle will incline downward at an angle of 45°, as shown at 4. Fig. 119. Ring Supplied with Two-Phase Current. oO 1 2 3 4 5 6 7 8 9 Fig. 120. Magnetic Resultants due to Two-Phase Current. By following the successive conditions, the needle will be found to take the various positions represented at 5, 6, 7, 8, and finally at 9, it returns to its original vertical direction, the current having completed one period. Thus the needle tends to be carried around continuously by the shifting resultant field, so long as the ring is supplied with two-phase currents. PRINCIPLES OF ALTERNATING POLVPHASE CURRENTS. 147 Principle of Polyphase Motors. — It is this capability of produ- cing continuous rotation that gives the polyphase currents their interest and value, since it enables motors to be operated very suc- cessfully. The ring with the magnetic needle, in Fig. 119, illus- trates the principle of the syzchronous polyphase motor, since the armature revolves in exact synchronism with the phases of the currents. If the needle is replaced by a cylinder of laminated iron wound with conductors, like an ordinary armature, except that they are short-circuited, it is found that it will revolve also; but in this case the speed is a little less than that of a synchronous armature, the difference being called the s/f, usually amounting to from 1 to © per cent. This slip represents a relative motion of the rotating field, with respect to the armature conductors; consequently the latter are cut by the lines of force, thereby inducing currents in them. It is the action of the field upon these induced currents which causes the armature to revolve, this type being called the induction motor. It is aremarkable fact that no current is sup- plied to the moving part, so that it need have no electrical connec- tions made to it except for purposes of starting and regulation. In some cases the construction is modified so that the part in which the currents are induced revolves, and the other part is stationary. For this reason, and because no energy is supplied directly to the so-called arma- ture, it is considered more correct to distinguish the two elements of an induction motor as vofor and sfavor, or primary and secondary. The Action of Three-Phase Currents Fig. 121. Ring Supplied with in producing a rotary field is quite three Rips es Curent similar to that described for two- phase currents. The ring in Fig. 121 is wound as before, but ToeecuLrentmismicdiiledt three equidistant points, /7, Y,.and 2, instead of at four points. Taking the instant when the current flowing in at H is a maximum, two currents flow out at Y and Z, each having one-half the value of the current entering at 7 This tends to produce a south pole at /, and two north poles at Y and Z respectively. The resultant due to the latter is a south pole at 148 ELECTRICALIGHITING. 7, midway between Y and Z, consequently a magnetic needle would take the position shown by the dotted arrow. (This condi- tion is represented at 0° in Fig. 114.) A moment later (at 60° in Fig. 114) currents enter at both / and 7, and a maximum current flows out at Y, hence the needle would point toward VY. At the end of another one-sixth of a period (at 120° in Fig. 114), the maximum current will enter at Z, and the needle would turn to that point, and so on until it had made a complete revolution in one period of the alternating current. Actual Forms of Polyphase Motors. — The synchronous type of polyphase motor is similar in principle and construction to the cor- responding generator, in fact, two identical machines may be used, one as generator and the other as motor, the same being true of single-phase alternating, as well as direct-current machines. In all these cases the field magnets must be supplied with direct current either from a separate exciter or from the machine itself, which, if it is an alternator, must be provided with a commutating device for that purpose. The advantage of the polyphase over the single-phase synchro- nous motor is. the fact that the former is self-starting, owing to the fact that it exerts some rotary effort even when standing still. In this case it acts as an induction motor, the armature being supplied with polyphase current, but the field circuit is left open until syn- chronous speed is reached. On the other hand, the single-phase motor has no starting torque, and has to be provided with some special device in order to bring it up to synchronism. The practical forms of induction motor are self-starting with considerable torque, but they are generally arranged with some means for introducing resistance into the secondary circuit, in order to give them full torque when starting, and to prevent a great rush of current at-that time. ay ALR af LL ph AY’ anf Cf g A ; ee. ; o 1e) ie) 8TATIC BALANCE GROUNDING DEVICE GROUND Fig. 139. Automatic Grounding Device. is similar to the Cardew device, the difference being that thin paper or other insulating material is used in place of the air-gap. This is punctured by excessive voltage, and the secondary circuit is TRANSFORMERS. jens srounded, as is evident from Fig. 138. A similar automatic device is indicated in Fig. 139, the secondary being grounded when an electrostatic balance is caused to act by abnormal potential. It is not necessary that these or other devices should be connected to both wires of the secondary circuit, as in Figs. 138 and 139; one ground connection being sufficient, but two are less likely to fail. Another automatic means of ground- ing consists of an electromagnet in 1OOOV} series with a vacuum tube between the secondary circuit and the ground. When the potential rises above a cer- tain value, it produces current enough =| MWY through the magnet to cause it to operate a mechanism that grounds the secondary circuit. All of these automatic grounding devices are open to the objections that er ig. 140. Transformer Protection. they do not act instantly, so that the | insulation might break down before they operated ; besides which they might fail because they depend upon contact points and mechanism. | A grounded metallic shield between primary and secondary coils is another form of protective device due to Professor Thomson. It is simply a covering of sheet metal, placed between the primary and secondary windings in such a manner that it is impossible for current to leap from one to the other without passing through the shield, which naturally leads it away to the ground (Fig. 140), thus protecting the low-ten- sion circuit. GROUNDED SHIELD 1000V GROUND Fig. 141. Grounded Secondary The permanent grounding of the Circuit, : Sy Gee 3 : secondary circuit, indicated in Fig. 141, is perhaps the most positive means of protection, but is open to some objections. It was formerly forbidden by insurance rules in this country, but is now permitted. The ground connec- tion should be a very good one, similar to that required for light- ning arresters (Vol. I., p. 437), and the wire leading to it should 178 ELECTRICODIGCHIING. have a current capacity fully equal to that of those portions of the primary and secondary circuits through which the current might pass to the ground. The objections to this arrangement are: 1. Insurance authorities have opposed any grounding of strong current electric circuits used inside of buildings, because a single fault would then cause a short circuit or leak, whereas with com- pletely insulated circuits ¢zwo s¢multaneous faults are required. 2. A permanent ground connection zzvzfes trouble, since with it a break-down in insulation between primary and See ee is more likely to occur than without it. 8. Certain conditions may arise under which trouble will be aggravated by the ground connection. The first of the above objections is minimized by requiring the ground connection to be made at the neutral ° 6 point of the secondary circuit. In case the latter is a three-wire system, this is the middle or neutral wire as represent- on edun bio alt 2 Wath alt wo-wireacecs = ondary circuit the middle point of the secondary coil may be grounded, a con- 1000 Vv Ere tee nection being brought out for the pur- Fig. 142. Secondary Circuit with ; : Ns a A) pose. By grounding the neutral point instead of one of the outer conductors, the voltage is divided in halves, so that the tendency to break down insulation is reduced in still greater proportion. The fact that the insulation between primary and secondary is: more likely to break down if the latter is grounded, is self-evident ; but it may be answered that the consequences are provided for, and the total danger is reduced. There are, however, certain possible conditions which might cause serious trouble on a grounded sec- ondary circuit. For example, an accidental connection between the primary and secondary circuits may allow the primary current to flow through the whole or part of the secondary coil. This will tend to produce an abnormally high voltage in the latter that may rise to several times the ordinary value, so that lamps, sockets, insulation, etc., will be burnt out. In order for this to take place the primary circuit must happen to have a ground connection on TRANSFORMERS. 179 the side opposite to that on which the accidenta. contact with the secondary exists. It is necessary also that this last-named fault should be on the primary wires before they reach the transformer, otherwise the primary fuses would blow, and cut off the current entirely. In short, this combination of circumstances is ‘not likely to occur, and if it did the danger would be great whether the sec- ondary were grounded or not. Test wires may be run from the central station and connected to the secondary circuits of the various transformers. This per- mits the insulation resistance between each of the primary wires and the secondary circuits to be determined at the central station. In fact, a ground detector may be used which would instantly indi- cate a fault. A single test wire might be used; but it is better to divide the transformers into groups, each of which has its own wire, so that any trouble may be located more readily. These wires may be quite small. Pertodical tests of the insulation of each transformer should be made at least once a year. A small step-up transformer is carried to the places where the tests are to be made. Its secondary vol- tage should be at least twice the primary voltage of the system, and its current capacity at least four times the charging current that flows during any test. Its primary is connected to the high-voltage lines, and one terminal of its secondary is connected to the primary and the other to the secondary of the house transformer, which must be disconnected previously from both primary and secondary circuits. This pressure is applied for one minute, thus subjecting the insulation between the primary and secondary circuits to twice the working voltage, which is likely to develop any fault. A fuse is put in the circuit to protect it in case the insulation is punctured. Insulation tests of the primary circuit by means of ground detectors and special measurements are very important on systems using transformers, since any defect in the insulation of the latter will almost certainly lower the general insulation of the primary circuit, giving warning of some trouble. By keeping careful watch on the insulation, and promptly following up any indications, serious consequences may be avoided. This method differs. from the pre- ceding one in the fact that no test wires are required. Transformer Fuse Blocks or Cut-outs. To protect transformers from excessive currents, fuses are inserted in the primary circuit. 180 ELECTRIC LIGHTING. The boxes or blocks which contain these fuses may be attached to or combined with the transformer as illustrated in Fig. 128, or they may be entirely separate from it as shown in Fig. 143. In either case the fuse itself is usually inclosed in or carried by a tube or plug of porcelain which is easily inserted and withdrawn through a hole in the box in order to facilitate the inspection or renewal of a fuse. Fuse-blocks are made either double- or single-pole as repre- sented in the two illustrations cited. The presence of a fuse in the primary circuit protects the secondary circuit also, since an abnormal current in the latter causes a corresponding increase in the primary current which will blow the fuse and open the circuit. Fig. 143. Transformer Cut-outs. In most cases the secondary circuit is further protected by fuses inserted in the local or house wiring. Testing Transformers. For determining efficzency various methods have been employed. That used by Professor Ryan * con- sisted in tracing out by means of instantaneous contacts the curves of primary and secondary /.4/.F. and of primary current, the sec- ondary current being measured by an ammeter. Having obtained these curves, the power in each circuit was calculated, and the ratio gave the efficiency. This method also has the advantage that the exact form and phase relations of the several waves are brought * Trans. Amer. Inst. Elec. Eng., Dec., 1889, TRANSFORMERS. 181 out. Mr. W. Mordey * proposed to find the efficiency of a trans- former by running it at the given load until a constant temperature is reached as determined by a thermometer or by a resistance test. Direct currents are then passed through the coils of such strength that their heating effect maintains the same constant temperature. It follows that the direct current power (= /2R=£J), which is easily measured by volt- and ampere-meters or by a watt-meter, must be equal to the total losses with the alternating current. Calorimetric methods have been used by Dr. L. Duncan, + the total losses being determined by placing the transformer in a water, oil, or*ice calorimeter. Both of these last methods, depending upon heat measurements, are laborious and liable to error. . Volt- and ampere-meters may be employed to measure the pres- sures and currents in the primary and secondary circuits. If the load is non-inductive and more than one-tenth of full value the pro- duct of secondary volts and amperes, divided by the product of pri- mary volts and amperes, is the efficiency. With very light load or with inductive load the current lags behind the £.4/-F., and the voltamperes must be multiplied by the power factor (cos ¢) to get the true watts. By means of one of the various three-instrument methods, the true power can be determined; but the simplest plan is to measure the true watts in the primary and in the secondary circuits with wattmeters. Stray power methods are convenient and accurate, the losses being determined individually. The iron losses, which we have seen are constant (page 154), are determined by a wattmeter in the primary circuit when the secondary is open. The copper losses may be calculated for any load by (69) if the primary and second- ary currents as well as resistances are known or can be measured, which is usually an easy matter. Since the efficiency is always found for a definite load, the secondary current is fixed by that fact. The primary current /’ is ieee ee (76) in which /” is the secondary current, & the ratio of transformation, and /, the exciting current which flows with open secondary. If * Jour. Inst. Elec. Eng., London, vol. XVIII. p. 608. + Electrical World, vol. IX. p. 188. 182 ELECTRIC LICHLIING. /, is assumed to be 3 per cent of /” + & the error in the efficiency will be very slight. Having determined the iron and copper losses the efficiency is equal to the secondary watts divided by the second- ary watts plus the losses as given by (72). Potential Transformers are used to furnish current for volt- meters or wattmeters. They are small transformers (Fig. 144), usually mounted on the switchboard, their function being to con- vert high voltages to lower values that are more convenient and safer to measure. With a definite ratio of trans- formation, a volt- or watt- meter supplied from the low-voltage secondary circuit, can be calibrated to indicate the original or primary voltage. If the currents consumed _pro- iB duce a certain percentage ecient ee ae | of drop in the secondary voltage a corresponding error 1s introduced, un- less the instrument is specially calibrated to allow for this. A simpler plan is to use a trans- former having sufficient capacity so that the drop is insignificant. One should not connect additional instruments or pilot lamps to a potential transformer until it has been ascertained _ that they do not cause an objectionable fall in secondary voltage. ~— Auto-Transformers. In these devices the primary and second- ary currents both flow in a single winding. The circuits of one form of auto-transformer are represented in Fig. 145, A & being a coil of insulated wire wound upon an iron core as in an ordinary transformer. When the coil ABZ is supplied with alternating cur- rent from the primary circuit on the left, differences of potential are established between the various parts of the coil. If connec- tions are made to it at the points Cand JP, which divide it into three equal parts, the potential difference between D and £ will Fig. 144. Potential Transformer. TRANSFORMERS. 183 be one-third of the total voltage applied at A and BZ, and between C £ it will be two-thirds of that value. Assuming, for example, that 300 volts are supplied at A and 4, then 100 volts may be tapped off from D and — and 200 volts between Cand £. These might be used in almost exactly the same way as the common types of transformer with separate primary and secondary circuits, since a certain number of watts at one voltage may be converted into a nearly equal number of watts at another voltage. There is an objection, however, to auto-transformers, arising from the fact that the secondary is connected directly to the pri- mary circuit, as at B and £ in Fig. 145. Although the actual vol- tage between the secondary wires may not be high, nevertheless conditions may arise that will make the secondary circuits very dangerous. For example, an acci- dental ground anywhere on the primary conductor A will subject to the full primary voltage a per- € son who is connected to the earth and happens to touch the secondary wire &. It is practically the same as if the primary current breaks through to 3 = the secondary circuit in an ordinary transformer, Fig. 145. and we have seen in Figs. 188 to 142 what pre- — Auto-Transformer. cautions are taken to make this danger as small as possible. On this account auto-transformers are not suitable for general use on high-tension systems. They are employed chiefly for series circuits in electric lighting, as described under that head in the next chapter. They are used also in place of dead resistance for starting alternating current motors. It is evident that they may be applied as compensators to subdivide the voltage in three- and five-wire systems instead of the machines described on page 79. The action of an auto-transformer is similar to that of the ordi- nary transformer. In either case the primary current sets up an alternating magnetic flux which induces an F.//.F. in each turn of winding. In an auto-transformer there is only one winding; but if any two points,as D and £& in Fig. 145, are connected to a suitable circuit, a current will flow through it. This tends to produce a demagnetizing effect similar to that due to the secondary current of the common transformer; hence the primary current increases, in order to maintain the same magnetization, and automatically adjusts itself to supply the energy drawn in the secondary circuit. 184 ELECTRIC LIGHTING. Reactive and choke coils, which are somewhat similar in con- struction and action, will be described as means of regulation in the next chapter. Standard Types of Transformers. The following table gives data concerning standard commercial transformers of from .6 to 50 k.w. capacity. It will be noted that the 125 cycle type has less core loss, and higher efficiency, but poorer regulation, than the 60 cycle type; the differences, however, are not very great. GENERAL ELECTRIC TYPE H OIL TRANSFORMERS. ADAPTED FOR USE ON 50 to 140 Cycle Circuits. Data Based on 1040 or 2080 Volts Primary and 60 Cycles (Column A), or 125 Cycles (Column B). EFFICIENCY. Watts Capacity. Core Loss, Watts. Full Load Copper Loss, Watts. Regulation, Per Cent Quarters Load. Quarter Load. One- Three- One- Half- Load 600 1,000 1,500 2.000 2.500 Ca — - => CROSS CO Oo OC CO ec CO taest ~~ amd we eS i en) OOO OO Se Ng aici S Ob eo cc TO em OTTO bh OO ee Pore Co OO CO SOS ae ates) He © Os © OO ve) co © ERM Sp me or © Or Cc SERS ae Onoon eave) OOH BRa ve) ¢ coe 3,000 4.000 5,000 7,500 10,000 co bo OO 6 6 SINS wmowonn aire mvelvelive) DRAAAS OADPPeH OO 6 tO 20 SINT SIS OD wWwreHoon He bO bc oH OO 0 0 Se ess) SOc O69 © 6 Hs bo G9 00 bo st +] OOO Oe 15,000 20,000 25,000 30,000) § 40,000 50,000 TA -1 =—j = . SSS S13 3 C3 Sa Ov Oxy Ox N44 Or O1 09 bS CO IO ont to Ht CO 2 CO CO CO CO Ps ES NG NS ES SScaoo OO OH OH OO CO 8 OH CO CO CO NOWOOR HU OD OH OO CO SS ie eae ae OOO CO ~1r COO OO C29 See shoe es ee C CO CO AI OS CO COCO Oo OO PHD HOT mt OCH Or OOo mo 6 OTH CO Te Temperature rise not exceeding 45° C. for A and 40° C, for B in 8 hoars full load. Temperature determined by increase of resistance method. For comparison with data based on 1000 or 2000 volts primary, deduct 7% from the above core loss and add 0.1 to the per cent regulation. The above transformers are suitable for operation on circuits having voltage within 10% above or below the rating. Polyphase Transformers. Exactly the same types of trans- formers as those used‘ for single-phase currents may be employed with two- and three-phase currents, each phase or branch having its own transformer or set of transformers. It is possible, -also, to construct special polyphase transformers in which the magnetic circuits are combined in a manner analogous to that in which the TRANSFORMERS. 185 electric circuits are interconnected, as explained with reference to Fig. 115. In this way a certain saving in the material of the iron core is effected; but they are more complicated in construction than ordinary transformers, and are seldom used in this country. A description of them may be found in Jackson’s Alternating Currents, page 683. The arrangement and operation of transformers in connection with polyphase systems will be described in the next chapter. For further information regarding the theory, construction, and operation of transformers reference may be made to the following works : The Alternate Current Transformer, by J. A. Fleming, new edition, 2 vols. N. Y. and London, 1896. Alternating Current Phenomena, by C. P. Steinmetz, N. Y., LSTAG) Alternating Currents, by D.C. and J. P. Jackson, N. Y. and London, 1896. The Principles of the Transformer, by F. Bedell, N. Y. and London, 1896. CEUAE Tike x ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION. THE facility with which alternating currents may be trans- formed from one voltage to another gives possibilities of variation in systems of distribution that are greater than with direct cur- rents. Adding to this the transformation from two- to three-phase, and from alternating to direct currents, or vice versa, by rectifiers and rotary converters, and the opportunity for elaboration becomes almost unlimited. There has been a tendency to yield to this temptation, and go too far in the complication of circuits and appa- ratus. Certain systems have become more or less standardized and generally accepted, but alternating current practice is still far less definite than direct current work. The more important meth- ods will be classified and described in the present chapter. Alternating Current Series Systems. — Series circuits corre- sponding to the direct current arrangements shown in Chapter II. may be operated by alternating currents. The principal systems that have been used are — 1. Simple series circuit with constant current alternator. 2. Series circuits supplied by constant current transformers. 3. Parallel-series circuits. Several forms of constant current alternators have been intro- duced, analogous to the well-known Brush and Thomson-Houston series arc dynamos, the principal example being the Stanley ma- chine made by the Westinghouse Company. No regulating device is required to keep the current constant; but armature reaction and self-induction are purposely exaggerated in the design, so that the current does not increase very much, even when the machine is short-circuited. The same is true to a certain extent of a constant direct current dynamo, but self-induction has a much greater effect with alternating currents. On the other hand, the voltage of a 186 ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION. 18T constant current alternator rises very high if the circuit is opened, since it is entirely relieved of armature reaction and inductance drop. This is likely to break down insulation unless it is pre- vented by providing a film cut-out similar to that shown on page 25, or some other device connected to the terminals of the ma- chines, so that it will short-circuit the latter if the voltage becomes too great. Such machines have no advantage over constant direct current dynamos, except that the main current is generated with- out a commutator; but they require some source of direct current for field excitation. Furthermore, there are many examples of the direct current type that are very successful, hence they are gen- erally adopted for arc-lighting on a simple series circuit, the direct current lamp being preferred when other considerations are equal. Constant current transformers have been illustrated in Figs. Fig. 146. Constant Current Transformer System. 135, 136, and 137, and their operation described. They are not used on a true series system, since their primaries P are supplied in parallel at constant potential, as represented in Fig. 146, the secondary circuits S only being arranged in series fashion, and car- rying constant currents which feed the lamps L.- The advantage of this method is the fact that a large number of lights can be operated from the same source of current. For example, each of the circuits: in 2,/,G, Fig. 146,.may have as many lamps as an entire dynamo in the direct current series system; so that one large alternator of 1000 k.w. capacity’ can supply about 2000 lights ; whereas it would require 16 to 20 direct current machines, since the number of lamps that can be fed by a single dynamo is usually limited to 100 or 125. In simplicity and in economy of operation the single large alternator would have considerable ad- Valitave-s ees transformers“ 7) Porcelain -; Insulators Lifting Table -Lifting Cam 1] 4 Ze aa idddigp bitdig EZ 14 F SS a —— —} Fig. 172. Oil Circuit Breakers. 214 ELECTRICMUIGHTING. on US 2 FIELD &.P.D.T. SWITCHES 1000 FIELD S.P2D.7% 400 AMP. SWITCH 1 oO AMP. SwiTCHES [2% SWITCHES n 409 AMP. SWITCH 1 1000 ave SWITCHES 7 Z " r Oz: 5 STARTING BOX SHUNT , 1000000 C.M. ASBESTOS CABLE vad: Ll} 100 K.W. ABC a ROTARY 1000000 C.M. ASBESTOS CABLE 1000000 C.M. 4000000 C.M. ASBESTOS CABLE ASBESTOS CABLE 1000 AMP.SWITCHES 1000 AMP.SWITCHES benny 34S) SYNCHRONIZING LAMPS LAMPS mmog C) O 000000 C.M 1000000 C.M. Sees bee a) A.C. AMMETER ASBESTOS CABLE A.C. AMMETER Se SERIES TRANSFORMER SERIES TRANSFORMER le O OIL COOLED REGULATOR OIL COOLEO REGULATOR 209 P50 1000000 C.M. | "LEAD COVERED CABLE 1000000 C.M. Bitiens LEAD COVERED CABLE = 750000 C.M. te LEAD COVERED CABLE Sarie ae { Al g80 K.W' OIL COOLED TRANSFORMERS «— — -4500—_> uw pm) (3) =< (} > co e o oO ui oO + fe} z | HIGH TENSION FUGES: wi =) o <= (3) > TO GROUND 2 . i=) : von esr LIGHTNING ARRESTERS Wi (a) <= fia TENSION JOINTS |] o! NO.6 LEAD COVERED CABLE Fig. 173. Connections at Sub-station. version from direct to alternating current, the efficiency of these systems is fairly high. The following figures have actually been obtained in practice.* The step-down transformers had an efficiency * Street Railway Journal, October, 1899, p. 710. a ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION. 215 of 98.2 per cent at full load of 350 k.w. each, and the rotary con- verters of 990 k.w. each gave 96 per cent efficiency at full load, making a combined efficiency of 94.2 per cent. In comparing this with a system using alternating current throughout, it should be re- membered that the latter would require two sets of transformers to bring down the pressure from 6600 volts to that used in the lamps. In other words, a set of static transformers would be substituted for the rotary converters, so that the combined efficiency would be U3s20x 98/2" 96.4 instead of 94.2 percent. On the other hand, it would not be possible to use direct current arc lamps or motors, storage batteries, or any electrolytic apparatus in connection with a purely alternating current system. Furthermore, losses and the difficulties of regulation, etc., caused by inductance, capacity, and low-power factor would occur with alternating current in the dis- tributing conductors and house-wiring. In most practical cases these would more than offset the additional loss of 2.2 per cent in the rotary converters. A form of oil circuit-breaker used in the very high-tension (6600 volts) circuits of this system is illustrated in Fig. 172. It consists of a magnet operating the valve of a pneumatic cylinder the piston of which raises or lowers a metal cross-head carrying three wooden rods that extend down into three cells, each contain- ing the switching apparatus for one phase of the circuit. A _ sec- tion through one of these cells is shown in Fig. 172, the others being the same, and separated from each other by 4-inch brick par- titions to act as barriers and prevent arcing between the switches. The actual circuit-breaking parts are connected to the movable rods and are submerged in oil. This type of circuit-breaker is rated at 10,900 volts and 800 amperes per phase. In connection with the above-described system of three-phase transmission and direct current distribution, it is common practice to employ double current generators, rectifiers, and frequency changers, which will now be explained under their respective headings. Double Current Generators. — Since a rotary converter is pro- vided with a direct current commutator and with alternating cur- rent collecting rings connected to its armature winding (page 97), it may be employed as a generator if driven by an engine or other source of power, and polyphase or direct currents or 216 ELECTRICALIGUTING. - both may be obtained from it. In some plants these machines are used as converters at one time and as generators at other times. They may be run as polyphase generators to supply energy at a distance through step-up transformers, and can also be utilized to charge storage batteries with direct current, these two functions being performed at different hours of the day or at the same time, if desired. When so used they are termed double cur- rent generators. The use of these machines in the stations of the Chicago Edison Company is described in The Electrical World and Lngineer, May 19, 1900, which also contains complete illustrations and description of the three-phase transmission and direct current distribution system employed ona very large scale by that com- pany. Rectifiers. — This name is given to.those forms of apparatus in which single or polyphase alternating currents are changed into direct currents by means of commutators; these machines being without field magnets or armatures. This distinguishes them from rotary converters which are complete dynamo machines. Recti- fiers are much simpler, cheaper, and more efficient than converters; _ nevertheless, they have not been very generally introduced, chiefly on account of practical difficulties in keeping them in adjustment and avoiding sparking. In principle a rectifier is a reversing commutator Cin Fig. 174, similar to that of an open-coil arc dynamo (Vol. I. p. 382). In Fig. 174. Rectifier of Alternating Currents. this case a two-part commutator is represented, one segment being continuously connected to one wire J7 and the other segment to the other wire VV of the supply circuit. These connections are not shown, but are made through a pair of brushes and rings connected respectively to the two commutator segments. The wires (7 N lead from a single-phase generator G through a con- stant current transformer 7, and it is evident that the connections ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION. 217 of the brushes A B will be reversed at every half revolution of the commutator. If a circuit feeding arc lamps Z in series be con- nected to brushes A #, it is evident that the current will flow through the lamps always in the same direction, provided the connections are reversed exactly when the alternating current reverses. In Fig. 175 the alternating current 4, B,C, D,F, etc., is converted into the pulsating direct current 4, B,C, £,F, etc., if the Leversals occur ate theapoints: ©, 2: 7; etc. It should be noted also that the 2 E S 2 current is zero at the instant of re- ee Ns versal, hence sparking is avoided. In A 1K order that the action shall take place correctly the commutator C must re- ae = volve synchronously and in proper Fig. 175. Rectified Single-phase phase with the alternating current. ha For a two-part commutator the number of revolutions per second must equal the frequency, and the brushes 44 must be set very carefully so that they pass from one commutator segment to an- other at the instant when the current is zero. The constant current transformer 7 may be one of the forms already shown in Figs. 185, 186, and 137; and it will feed the lamps practically the same as if they were connected to it directly, except that the current through them will be unidirectional instead of alternating, thus allowing ordinary types of direct current arc lamps to be used. The fact that the current is pulsatory is not objectionable for arc lighting, since the standard Brush and Thom- son-Houston arc dynamos produce currents of this character. Incandescent lamps may also be operated equally well with this current. A low frequency of 25 is too low for very satisfactory running of either kind of lamp, but at 40 or more periods per second both work well. The pulsating current is also applicable to the charging of storage batteries and to other electrochemical purposes ; but for the operation of the ordinary direct current motors it would be likely to cause sparking unless the pulsations were “smoothed out” by inductance, storage batteries, or other suitable means. Rectifiers have been more generally employed in England than in America, the Ferranti type being used in a number of stations. This consists essentially of a synchronous alternating current 918 ELECT RICAIIOCHTING motor driving the rectifying commutator. A constant current transformer with movable coils somewhat similar to that shown in Fig. 135 is employed in connection with this rectifier, which is usually applied to arc lighting. An interesting example of rectifier is that installed by Mr. W. S. Barstow in Brooklyn. The 6600 volt three-phase current from the main generating station is supplied to the primary of a con stant current transformer of the type illustrated in Fig. 135, the secondary circuit at 6600 volts is led through the rectifier, which consists simply of a three-part commutator driven by a synchron- ous motor, the three-phase conductors being connected respectively: to the three segments of the commutator. Two brushes set dia- metrically opposite each other are applied to the commutator, and are connected to a series circuit of arc lamps. The standard form of Thomson-Houston commutator is employed with the usual blower attachment to suppress sparking. Since the Thomson- Houston armature has practically a three-phase winding of the Y form, the current supplied to its commutator segments is practi- cally the same as in a three-phase rectifier. In the latter case the commutator is placed at a distance from the armature winding, and is driven by a synchronous motor. Owing to changes in the phase of a motor when variations in load, etc., occur on the circuit, the position of the brushes may not agree exactly with the points of zero current, so that sparking will occur. Since the maximum potential exists between adjacent commutator segments separated only by a small air-gap, there is a strong tendency to flashing or “ring-fire’’ around the commutator, thus short-circuiting a pres- sure of several thousand volts. This constitutes the chief difficulty in the operation of rectifiers. The direct current obtained by rectifying a two- or three-phase current does not pulsate so much as the rectified single-phase cur- rent in Fig. 175, for the reason that in the former-case two or three waves are superimposed. If, for example, we reverse all the waves below the zero line in Figs. 110 and 114, the resulting direct current would be represented by a curve obtained by sum- | ming up the ordinates at every point, the fluctuations being much / less than in Fig. 175. beer eee Changers. — As their name implies, these machines are used for changing the frequency of an alternating current. ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION. 219 Ordinarily the object is to increase a low frequency of say 25 periods per second, which is hardly high enough for arc or incan- descent lighting, to 60 periods for example, which is more satisfac- tory for the purpose and is also suited to the usual types of motors and transformers. The type of frequency changer made by the General Electric Company is essentially a polyphase transformer with a movable secondary. The latter consists of a secondary or armature suitably wound for any desired voltage and phase, which is mechanically revolved and acted upon by the rotating field of a polyphase primary. If the secondary is revolved in a direction opposite to that of the rotary field, obviously the frequency of the current in the secondary will be higher than the frequency of the current supplied to the primary, and vice versa. When the secondary rotates against the rotary field, it acts as a generator and requires power to drive it, and when it turns with the field the machine acts as an induction motor. Thus we see that there is a combined generator and transformer action when the frequency is raised. The secondary may be rotated by any suitable mechanical means, the synchronous polyphase motor being ordinarily used for this purpose. By over-exciting the field of the latter, the lead- ing current thus produced (p. 134) may be made to balance the lag caused by the primary of the frequency changer, thereby raising the power factor on the supply circuit. The output of a frequency changer when the frequency is increased is equal to the sum of the mechanical power applied to it and of the electrical imput in the primary, less the losses. The frequency of the secondary current is equal to the number of poles of the primary, multiplied by the sum of the revolutions per second of the shaft and of the field (when they run in opposite directions). Ifthe primary has a three-phase winding and the secondary is provided with a two- phase winding, the current is changed from three- to two-phase at the same time that the frequency is raised. It is evident also that the opposite change may be effected by transposing the windings. Tests of a 200 k.w. General Electric frequency changer of this type gave the following results : Primary wound for 6000 volts, 3 phase and 25 cycles. Secondary “ 5S DAVOS im 2 tae 6 ‘ae G24 aNce 220 ELECTRICOLIGH TING. The above machine was directly connected to a 4-pole, 3-phase, 6000-volt, 25-cycle (hence 750 7f.m) synchronous motor with a stationary armature. ‘The efficiency was 73} per cent at 80 k.w. output, 814 at 120 k.w., 874 per cent at 160 k.w., and 91 per cent at 200 k.w. or full load. The power factor was practically 100 per cent at all these loads, showing the balancing of the lag- ging and leading currents as already pointed out. The wave forms of £.4/./. and current approximated closely to the simple sine curve. ‘The efficiencies stated above signify the true watt output divided by the true watt imput, the latter including the true watts consumed by the motor. Transforming from Two- to Three-Phase. — It has just been explained how the frequency changes may be employed to trans- form currents from two- to three-phase, or wvece versa. This requires, however, two rather expensive machines demanding at- tention, so that when no change in frequency is desired, it is more economical to transform from two- to three-phase, or the converse, by means of simple static transformers. A method of this kind, devised by Mr. C. F. Scott,* ‘is llustrated in Figs. 176-178. It Figs. 176 and 177. Transforming from Two- to Three-phase. involves the use of two transformers, one wound for a ratio of transformation of say 1000: 100, and the other for a ratio of 1000: 86.7. In, Fig. 176, H/ ‘represents the primary:and AB the secondary of the first transformer, A/V and C/ being respect- ively the primary and secondary of the second transformer. The two primaries are fed from the two-phase circuit D/ and /G, and one terminal of the secondary C/ is connected to the middle: point of the secondary AZ. The three-phrase circuit is connected to the points AB and C. In the diagram of potentials (Fig. 177) it is evident that AC and CB will each be equal to AS, so that ABC * Electric World, March 17 and 24, 1894. ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION. 221 will be an equilateral triangle when C7: AB:: V3:2::86.7: 100,. hence the three secondary £.J/./’s. represented by AA’, LC and CA, are equal in value and differ by 120° in phase. This is the proper condition for supplying a three-phase circuit connected to the points A, B and C. In practice, especially in small trans- formers, it is a sufficiently close approximation if the A.JZ./. of the secondary C/ is 90 instead of 86.7 per cent of AB. This method is often employed when two-phase energy is pro- duced by the generators (see Fig. 178) and it is desired to trans- mit some or all of it to a considerable distance. By transforma- tion from two- to three-phase, a saving of twenty-five per cent in copper is secured in the Lranemittines conguctors:A A.C. At the receiving station the energy may be distributed in three-phase form or may be transformed back again into two-phase current as indicated in Fig. 178. The large generators at Niagara are of the Fig. 178. Phase Transformation. two-phase type, and some of the energy produced by them is trans- formed in the above-described manner, so that it may be trans- mitted in three-phase form. — Transforming from Single to Polyphase.— It is often very Wenge: able to accomplish this result when it is required to operate motors from single-phase circuits, but the subject belongs to electric power more than to electric lighting. A method of this sort, invented by Mr. C. S. Bradley,* consists in causing, by means of a condenser, a lead of current in one branch of a circuit, and in combining this with lagging currents in another branch so as to produce a three- phase current in the secondary circuit. Size and Location of Transformers.— Most systems of alternat- ing current distribution employ transformers, and it is of great importance to exercise special care in deciding upon their sizes and locations. The constant core loss results in the course of a year * Phasing Transformers 7rans. Amer. Inst. Elec. Eng., September, 1895. IAA ELECTRIC LIGHTING: -in a waste of a large amount of energy, and every effort should be made to reduce this to a minimum. When transformers were first introduced, it was customary to use a large number, of them in small sizes; but it was soon found that the core loss consumed too great a fraction of the total output of the station, often amount- ing to 25 and sometimes to 50 per cent. This was partly owing to the fact that transformers at that time were not as well designed and constructed as at present, but it was due also to the custom of using too many small sizes. This is made evident by inspecting the table on page 184, which shows that a 600 watt, 125-cycle transformer has a core loss of 20 watts, while one of 50,000 watt ne 8) = Le: ce 5 = Second Ave. Fig. 179. Number and Arrangement of Transformer. capacity, or 83.3 times as great, has a core loss of 354 watts, which is only 17.7 times as much. In the first case the loss’ is 3.3 per cent, and in the second it is but .7 per cent, or about one-fifth as large. If the comparison be made between the 1 k.w. and the 10 k.w. transformers, it is found that the former has a core loss of 2.5 per cent (25 watts) and the latter of 1.08 per cent ; these being the limits of sizes ordinarily used in electric lighting. The actual case of a district in a small town is represented in Fig. 179. Originally there were installed 25 small transformers (indicated by dots and numbers) having a combined capacity of 340 lamps of 16 c.p. and a core loss of 1664 watts. These were after- ALTERNATING CURRENT SYSTEMS OF DISTRIBUTION. 223 wards replaced by two larger transformers (indicated by black rectangles), which had only 238 watts core loss, or about one- seventh as much, the saving being 1426 watts. If the first plant operated for 24 hours per day the core loss would have aggregated 40 k.w. hours for each day in the year, and probably this was greater than the useful energy consumed in the lamps. This may be a rather extreme case ; but there were many others equally bad, and at one time the average practice was little better.” Besides the advantage of lower percentage of losses in large transformers, a gain is made in the fact that the total required capacity is less. If, for example, a small transformer is installed for each house, it is necessary that its size should be sufficient to supply the maxi- mum number of lights that will ever be used in that house at one time, ~ Ordinarily, in fact for fully 99 percent of thésyear, the number of lamps burning will be much less than this maximum, hence the transformer and its core loss are far out of proportion to the average useful current. On the other hand, a larger trans- former, supplying ten houses for example, need not have ten times the capacity, because it is practically impossible that all of the houses will burn the maximum number of lights at the same time. In short, one 7.5 k.w. transformer will safely take the place of ten transformers of 1 k.w. each ; and the former would have a core loss of only 85 watts compared with 10 x 25 = 250 watts for the latter. 7 A further saving may be effected by having sub-stations in which the transformers are concentrated or “ banked,” and con- nected in parallel. During the hours when the load is light, only one transformer need be operated, the primary circuits of all the others being open; but when the load increases transformers are added as required, thus the core loss is kept in reasonable propor- tion to the useful energy. 224 ELECTRIC LICLLING: - CELA PET: CALCULATION OF ALTERNATING CURRENT CIRCUITS. THE properties of' electrical conductors were given in Chapter I., which included a general discussion of economy in their design. The principles there laid down apply to alternating as well as to- direct current conductors, but additional factors enter in connec- tion with the former. In the long-distance transmission of power, these questions are of prime consequence; but in electric lighting the distances are ordinarily shorter, so that the problem is not so difficult or important. Hence it will not be necessary to consider the matter in detail or at great length in this work. Choice of Frequency. — One of the first points to be decided in designing an alternating current system is the best frequency to employ. Those generally used in the United States are 25, 40, 60, 125, and 133 cycles per second. It would be well if the Standardization Report of the American Institute of Electrical Engineers were followed and three standard frequencies of 30, 60, and 120 became generally adopted. These would cover almost all cases that arise, and being simple multiples of each other would facilitate the design and construction of apparatus in regard to number of poles, windings, etc. In other countries many different frequencies are employed, 100 cycles being a common value in Ingland and on the Continent. Sixty cycles or less is considered to be “low frequency,” and above that is called “high frequency,” but anything between 60 and 120 is rarely used in America. A frequency of 135 cycles was originally adopted when the alternat- ing current was introduced for electric lighting, and is still used in many plants, especially those installed by the Westinghouse Com- pany. A standard of 125 cycles is adopted for electric lighting apparatus by the General Electric Company. These high frequen- cies possess the advantage that the size and cost of transformers are less when they are selected. At the present time a 10 k.w. trans- CALCULATION OF ALTERNATING CURRENT CIRCUITS. 225 former costs about 25 per cent more for 60 cycles and about 60 per cent more for 25 cycles than for the high frequency of 125 cycles. | In the early history of alternating current lighting, the gene- rators were belt-driven and ran at about 1000 r.p.m. Consequently 133 cycles could be obtained with a 16-pole machine. At present large direct-connected alternators running at about 100 r.p.m. are generally installed, and would require 160 poles to give the same frequency. This would make a complicated and expensive con- struction, so that 60 cycles, requiring 72 poles, would be much more practical. Another objection to high frequency is the fact that inductance or capacity effects are greater. The drop in voltage due to the former and the charging current due to the latter are both directly proportional to the frequency, and the tangent of the angle of lag is also proportional to it. For example, the voltage drop on a No. 0 A.W.G. wire due to its resistance and reactance (p. 116) is only one-half as much at 265 as it is at 125 cycles. Tables showing this difference will be given later in the present chapter. Since the drop is greater it follows that the regulation is poorer on high frequency circuits. The greater wattless cur- rents cause greater heating in generators, lines, transformers, etc. Still another disadvantage of high frequency is the fact that it renders more difficult the parallel operation of generators and rotary converters, as well as the running of motors. The disadvantages of low frequency, besides the higher cost of transformers already noted, are the difficulties involved in operat- ing arc and incandescent lamps. It is not yet practicable to run the former below 40 cycles and the latter below 25 cycles per second, and even at those values the results are not very satisfactory. Low-voltage or large candle-power incandescent lamps flicker less than the standard 110 volt 16 c.p. lamps, but the practice is deter- mined by the latter. The 220 volt lamp would be still more sen- ‘sitive to the waves of current, on account of its thinner filament. In conclusion, it may be said that for lighting alone at moder- ate distances a frequency of 125 or 133 may be adopted ; but even in such cases it would probably be wiser to choose 60 cycles, in order to permit the operation of motors and the extension of the system to greater distances. For supplying power as well as light 60 cycles are very satisfactory where the circuits are not too long. 226 ELECTRIC LIGHTING, To transmit energy to great distances, a low frequency, such as 25 or 80 cycles, is suitable. The same is true of long underground or submarine cables where the capacity effects would be great. A low frequency of 20 is generally selected also for the simple trans- mission of energy between generating and distributing stations, where the energy is converted into direct current before it is used, so that the frequency makes no difference so far as the lights are concerned. This question, which is almost always a serious one, is often complicated by the fact that a certain frequency has already been adopted in the original plant, so that there is a great tempta- tion to adhere to it in making additions. In many cases it may be necessary to do so; but frequently it would be wiser and cheaper in the end if the old-fashioned apparatus were sold, even at a great sacrifice, and a new plant designed and installed in accordance with the best practice. Relative Weights of Copper for Various Systems. — This question is exceedingly important, but belongs more to long-dis- tance power transmission than to electric lighting. Nevertheless, the problem often enters directly or indirectly in electric light engineering, and it will be well to consider briefly the principles involved, and the methods of calculation that are employed. A comparison between the weights of copper required for the different direct-current systems was given on page 87. In at- tempting to apply similar reasoning to the alternating current, the difficulty arises that the voltage ordinarily measured is not the max- zmum value; and since the insulation is subjected to the strain of the latter, the relative figures obtained depend upon which basis of comparison is adopted. For long-distance transmission the highest voltage that is practicable under the circumstances would ordinarily be chosen, but for local distribution the effective pressure would determine the question. The most important systems of transmission and distribution are represented in Fig. 180, and the relative weights of copper required are given in terms of the common two-wire circuit taken as 100. For equal effectzve values of £./.F. the direct and alter- nating currents demand the same weight of copper; but if equal maximum values are considered, the latter requires twice as much copper, distances, power in watts, percentage of drop, etc., being equivalent. This is easily understood when we remember that the CALCULATION OF ALTERNATING CURRENT CIRCUITS, 227 maximum value of an alternating Z.I.F. is V2 = 1.41 times its effective rating, consequently an alternating 4.1/7.7. of 100 volts would have the same maximum as a direct £.M.F. of 141 volts. System. Connections. Weight of Copper. Single Phase 75 Wire : 100. Single Fhase 375 3 Wire 15 0.0 Two Phase 3 Wire Lente Two Phase 3 Wire (1457 Three Phase ; Wire TEE _ Three Phase 4 Wire O55 Monocyclic { 28 150. Fig. 180. Relative Weights of Copper. For the same number of watts the amperes of the direct current are as great, so that the drop in volts is in the same propor- 00 141 tion with equal resistances. But the percentage of drop is only LOORen LOOM = —as large for the current, or one-half as much copper aL Oakey ee 228 | ELECTRIC LIGHTING. would be required for the same percentage of drop. This is sim- ply a particular case under the general law that wezght of copper zs inversely proportional to the squares of the voltages, other things being equal. The single-phase, three-wire (Fig. 150) requires 374 per cent as much copper as the two-wire system, and by making the neutral one-half the cross-section of either of the outside conductors the copper is reduced to 314 per cent. These percentages are in the same proportion as for direct-current systems, the figures for which were explained on pages 72 and 87. The above ratios assume that the lamp voltages are equal in all cases ; but it is evident that this will give twice the zofa/ voltage for the three-wire circuit, since it practically involves the placing of two lamps in series. For the same ¢ofa/ voltage, the three-wire would require 50 per cent more copper than the two-wire system, since the former would have three conductors instead of two, and everything else would be the same. Two-phase four-wire require the same amount of copper as the ordinary single-phase two-wire circuits, the former being equivalent to two single-phase systems. With the two-phase three-wire sys- tem (p. 142) the case is not so simple, but may be determined as follows: Assume the voltage V between either outside wire and the common return wire to be the same as ina single-phase cir- cuit. The total power transmitted is V/ for the latter, where / is the current, and to transmit the same power by the two-phase system the power must be 2 lz, in which z is the current in either outside wire, and is equal to 7+ 2. The current in the common conductor is z v2. consequently to have the same current density, which is the condition of maximum efficiency, its cross-section must be V2 times that of either of the others, and its resistance is *-+ V2 in which r is the ohmic resistance of one of the outer wires. The loss of power for each of the latter is 277, and for the middle wire it is 2 727+ V2 = 7%, V2, hence the total loss in the three wires is 2227 + 72rV2 = 7% (2+ V2) =/7 (24+ V2)+ 4, since 7= 7+ 2. The loss in the equivalent single-phase circuit is 27?R, in which R is the resistance of one of the conductors, and this must be the same as the loss for the two-phase system ; hence [27 (2 a2 ) 2 = =. V2) re oe a epee Eee id lial 4 or 7 a4 ai; CALCULATION OF ALTERNATING CURRENT CIRCUITS. 229 Therefore each outer wire must be (2+ V2) +8 times, and the middle wire V2 (2+ V/2) + 8 times as large as each single-phase conductor. It follows that the wezght of copper in the two-phase, three-wire conductors 1s re) " 2 ae (Ces UC ue) 2 = 1.457 com- pared with 2 for the singlephase, two-wire system, or tn other words 1s 12.9 per cent as great at the same minimum voltage. If this comparison is made on the basis of maximum voltage, it is necessary to make the potential difference between the outer con- ductors equal to that in the single-phase, two-wire circuit, hence the voltage between either outside wire and the common return is V + “V2, and the current in each branch z, = / + v2; so that the power in both branches is 2(V+ V2)(7+ V2) = VJ, which is the same as that in the single-phase system. The current in the common return is 7, V2 = /, and its resistance should be 4 + V2 if 7, is the resistance of each outer wire. The total loss is . 29 b. ee This must be equal to 2/2, the loss in the single-phase, hence 2 2 4k 17, (2+ V2) — May ae. or V5) — ere ee ares a © 2 PE A) . ee aN Ose That is, each outside wire requires ae times, and the middle wire times as much copper as each of the (2+ V2) V3 4 single-phase conductors. Hence the total system demands Di Dee Dye (2 bea DAD ta gle-phase, or 1.457 times as much copper; that is, the two-phase three-wire system requires 45.1 per cent more copper than the two- = 2.914 compared with 2 for the sin- phase at the same maximum voltage. Considering a three-phase, three-wire system (p. 145), having a voltage VY between the lines measured as A potential, the current in each line, or Y current, is 7, and the current from line to line, or A current, is 2, V3. Hence the total power in all three branches is 3/7,-+ V3 = 17, V3, and if this is to equal V/, the power of the single-phase circuit, then V7 = Vz, V3, or 7, =7+ “V3. 230 ELECTRIOULIGAHLING If ~ is the resistance of each three-phase conductor, then the loss per wire is /77, + 38, and the total loss is /*7,, while in the single- phase system it is 272. Hence, to get the same loss, /47, = 2/2R, or ry = 2R; that is, each three-phase wire has twice the resistance and half the copper of each single-phase conductor, or in other words, the three-phase, three-wire system requires TS per cent as much copper as the single-phase at the same maximum voltage. In the three-phase, three-wire system, with the lamps connected be- tween the neutral point and the three outer wires, — that is, in Y fashion (p. 144),— the voltage between the outer wires, or A poten- tial, will be / V3, if Vis the lamp or Y voltage. In other words, the potential between lines is raised from V to VV8; and since it has been shown previously that the weight of copper is inversely as the square of the voltage, the weight of copper for the three wires will be one-third as great as in the preceding case, and3 + 3 = 4as much as for the two-phase. The addition of a fourth or neutral conductor (Fig. 166) will increase this to 4 x 4 = 3, so that the three-phase, four-wire requires 33.5 per cent as much copper as the single-phase, two-wtire system at the same minimum voltage. The monocyclic system, when supplying lamps, is practically the same as a single-phase circuit (p. 208); and most of the energy for motors is carried by the two main conductors, except in starting, when the auxiliary wire furnishes a certain amount of current. If the extra wire is omitted on certain circuits, then the copper re- quired is the same as for the single-phase, two-wire system. If the third wire is one-half as large as each of the other two, then the monocyclic calls for 125 per cent ; and if the three conductors are all of the same size, then the copper demanded is 150 per cent of that used by the single-phase circuit. Hence the monocyclic system requires as much copper as an equal voltage single-phase, two-wire system, plus the copper in the auxiliary conductor. This table is issued by the General Electric Company, and gives in convenient form the constants to be used in calculating over- head electric transmission lines, the various quantities having the . following significance : — x = Ohmic resistance. LZ = Inductance in milhenrys per 1000 feet of conductor. C= Capacity in microfarads per 1000 feet of conductor. 7, = Charging current at 100 cycles and 10,000 volts to neutral, that is, ina 20,000 volt single-phase, and a 17,300 volt three-phase line. CALCULATIONS OFMALTERNATING CURRENT CIRCUITS.” 231 to= 2 X mw X frequency X CX E X 10 —*; where Z is the Z.AZF. between a line and neutral. : x = reactance = 2 X m X frequency X Z X 10—%, The £.4Z.F. consumed by resistance 7, of the line, is = /y, and in phase with the current /. The £.4Z/. consumed by the reactance x, of the line, is = 7x, and in quadrature with the current 7. The £.47./. consumed in the line is neither /r nor /x, but depends upon the phase relation of current in the receiving circuit. The loss of energy in the line is= 727, hence does not depend upon the reac- tance, but only upon the resistance. Two wires in parallel have the same resistance and about half the reactance (if strung on separate insulators and intermixed) of a single wire of double cross-section. Thus replacing one No. 0000 wire by two No. 0 wires, the resistance, weight of cop- per, etc., will remain the same, but the reactance will be reduced practically to half, so where lower reactance is desired, the use of several conductors, strung on indepen- dent insulators and intermixed, is advisable. The values given for Z, C, 2, and x are calculated for sine waves of current and EMF. LINE CONSTANTS FOR ELECTRIC TRANSMISSION. Per 1000 Feet of Eacu WIRE. REACTANCE AT 25, 60, 125 CycuEs, (toe? is MILHENRYS. CAPACITY IN MICROFARADS, RENT. IN OHMs. S1zE oF WIRE A.W.G., WEIGHT DIAMETER RESISTANCE AT INDUCTANCE IN CHARGING CuR- C.M. 211600 | . : .00388 167805 | . : .00578 133079 | . P .00368 105592 | . : .00358 5694 | . ‘ .00351 66373 | . : .00342 52633 | . : .00334 41742 |. ‘ .00326 38102 | . : .00320 26250 | . : .00315 20816 | . ‘ .00306 16509 | . : .00300 13094 | . : .00294 10382 | . as) .00288 OD ON OO RW NH rea The above figures apply to parallel wires that are 18 inches apart, but are not much modified by moderate changes in interaxial distance. For example, the inductance Z is decreased about 10 232 ELECTRIC LIGHTING. per cent when the wires are 12 inches apart; and is increased about 10 per cent when they are put 30 inches apart. A similar change would be produced in the reactance 2rfL. The capacity C and charging current z, are increased about 10 per cent, if the inter- axial distance is reduced to 12 inches; and is decreased about 10 per cent when it is raised to 80 inches. or wires placed close together in cables, the inductance and reactance would be greatly reduced, and the capacity and charging current greatly augmented, so that their values in the table would not be even approximately true. In such cases, or whenever there may be any doubt, the general formule for inductance and capacity, given on pages 129 and 187 respectively, should be used. The table on page 150 shows the inductance of wires from No. .0000 to No. 12, and for interaxial distances from 3 to 96 inches. The equations on page 138 give results for capacity that are twice as great as those found in the above table, the reason being that each conductor is considered separately with respect to zevo potential, which would ordinarily exist at a point midway between two wires forming a circuit ; whereas the equations (p. 158) give the capacity with respect to the other were, which is one-half as much. Thecalculated charging current would be the same in both cases, because the total voltage would be used in one instance, and the potential with respect to zero is taken in the table. The latter plan is generally better. Other convenient tables issued by the General Electric Company are given below. They apply particu- larly to overhead circuits with wires 18 inches apart, and are suffi- ciently accurate for most practical purposes, when the distance between wires is approximately that amount. If the conductors are less than 18 inches apart, the loss in voltage is lower than that given by the formule, and if they are close together, as in cables or interior wiring, the loss will be only that due to resistance. The following general formulz may be used to determine the size of copper conductors, current per conductor, volts loss in lines, and weight of copper per circuit for various systems of electrical distribution. ! DD SGV LG Area of conductor, Circular Mils = Piers (77) Wax: Current in ‘main conductors = —_-- (78) ie CALCULATION OF ALTERNATING CURRENT CIRCUITS: -233 PERL, Volts loss in lines = 100 ; GEG) TD ye NES I em ere | P x E? x 1,000,000 Lbs. copper = (80) W = Total watts delivered. D = Distance of transmission (one way) in feet. P = Loss in line in per-cent of power delivered, that is of W. £ = Voltage between main conductors at receiving or consumer’s end of circuit. Homcontinuousrcurrent 7 = 2100 710 17 = 1 and A = 6.04. CONSTANTS FOR ELECTRICAL CIRCUITS. VALUES OF K. VALUES OF T. SYSTEM. Per Cent Power Factor. [ Per CeEnT Power FAcror. VALUES OF A. 100 95 80 Single-phase . . 2160 | 2400 | 3000 Two-phase (4-wire) 1080 | 1200 | 1500 Three-phase (3-wire) 1080 | 1200 | 1500 Soe ES oo > SOR VALUES OF M. 25 CYCLES. 60 CYCLEs. 125) Cy CLES, Per Centr Power PER CENT POWER Per Cent PowsER FACTOR. FAcTOR. FACTOR. i 2 = fh ° zi A. W. GAUGE. No. oF WIRE. A. W. GAUGE. ee) —) ~ i) ea) —) 90 | 85 90 | “13 es) Ore CO S > S SS bo co “ICO rR AbD CO ON Coo wo NHK pt et ho bo =o wo Oo eH He CO an Bo am —_} ef Ino Oc bo on S sic oO (ate gpa fain, pear ery See eee no eo et a ee et et ee me bo bo bo na SO Od CO Od OID ON GMO WG He CO be bo co SB be Co co LY: oS So -oo (ome cs ES SS Syog Hr bow Bo bb wb Oa oe “IO © bv = OO oA Wo OW Oe aa co cet ce ec ed we cll wc 25 CSO ODO tO HE SS 65 Sh Ea WS Cl CO COO HH bw BO CS 565 BOK OD BSH G25 OS CO HH pw BRO ie eee nee SOD Ou tw Re of O- > oO etl tn cell eet cel oe cl cee eo CO SSO COO HH Nw Ba Do SS SS OWS HHO oR GO AS Some con oC Ome hoo ec ye tls mes ie ie RS i TS) Lc ae eee ee ee cl oe cc eld a a et bo bo bo 0 SOD OH NW BOA NO ob Ao Fe wa aA Sore o> SOW) So Sooo Ro 16:5 3Se Sie Os oe co SCO WI On = oODoD ON Oo HCO SS (==) (==) Qs) SS Si=) oo. h6hcOrF H= OD © bo W Od jot jt _—_ > Oo 234 ELECTRIC UIGHTING. The value of A for any particular power factor is obtained by dividing 2160, the value for continuous current, by the square of that power factor for single-phase, and by twice the square of that power factor for three-wire, three-phase, or four-wire, two-phase. The value of 17 depends on the size of wire, frequency, and power factor. It is equal to 1 for continuous current, and for alternating current with 100 per cent power factor and sizes of wire given in the preceding table of wiring constants. The value of Z depends upon the system and power factor. It is equal to 1 for direct current and for single-phase current of 100 per cent power factor. The value of A and the weights of wire in the table are based upon .00000502 lb. as the weight of one mil foot of copper. It should be observed that P stands for the per cent loss of the delivered power, and not the per cent loss of the power at the gen- erator; and that £& is the potential at the end of the line, and not at the generator. ¢ When the power factor cannot be more accurately determined, it may be assumed to be as follows for any alternating current sys- tem operating under average conditions: Lighting with no motors, 95 per cent; lighting and motors together, 85 per cent ; motors alone, 80 per cent. The size of wire in (77) is for each of the main or outside conductors of a given system, for example, the three wires of a three-phase, the four wires of a two-phase, or the main two wires of a monocyclic system. The neutral wire in the three- wire system for direct (p. 72) or alternating currents (p. 192) may in the case of feeders be made one-half or one-third size, or omitted entirely (p. 76), depending upon how well the system may be bal- anced. For local or secondary circuits it should generally be the same size as each of the main wires. These statements also apply to the middle wire of the four-wire, three-phase arrangement (Fig. 166). The size of the auxiliary conductor in the monocyclic sys- tem should be in the same proportion to either main wire as the motor load in amperes is to the total load in amperes. A simple method of calculating the drop and other data of alternating current circuits is represented in Fig. 181, being derived from the little book on Alternating Current Wiring and Distribution, by W. L. R. Emmet. Assume a case where 500 incandescent lamps, of 57.5 watts each, are to be fed from the sec- CALCULATION OF ALTERNATING CURRENT CIRCUITS, 235 ondaries of transformers of different sizes, being half loaded on the average. The primaries are supplied by two No. 2 wires from the generator two miles distant, the wires being 18 inches apart, and the frequency of 125 periods per second. The voltage of the lamps is 100, and the ratio of transformation is 10. In Fig. 181 the horizontal lines represent energy components, and the vertical lines inductive components of £.4/./. For the sake of uniformity the lamp voltage is multiplied by 10, the ratio of transformation making 1000 volts, which is represented by the horizontal line AB, since incandescent lamps are practically non-inductive. Assuming the secondary wiring to have an energy loss of 3 per cent, BC is laid off as 30 volts, and an inductance component CD, of the same amount. (Both of these are rather high values.) Take the resist- ance in the transformers at 1 per cent, and the inductance loss at Fig. 181. Graphical Calculation of Alternating Current Circuit. one-half load as 12.5 per cent of AD, or about 13 per cent. Hence we lay off DF = 10 volts to represent resistance, and FG = 130 volts to represent inductive loss in the transformers. If there were no losses in the transformers, the current received by them would be 28.75 amperes, but with 5 per cent iron loss at half-load this becomes 30:19 amperes. The resistance cf four miles of No. 2 wire from the table on page 8 is .156 x 5.28 x 4 = 3.3 ohms, and the reactance at 125 cycles is .249 x 5.28 x 4 = 5.25 ohms. To this we add 15 per cent for distortion of current waves, making 6.04 ohms. The resistance drop is 30.19 x .83 = 99 volts, and the reactance drop is 30.19 x 6.04 = 1.82 volts, which are laid off as GHand HK. Hence the line AK represents the EMF. re- quired at the generator terminals, being 1188 volts. Extending Kf to /, we have A/ the total energy component of 1139 volts, therefore the real power in the circuit is 11.39 x 30.19 = 34,300 236 ELECTRIC LIGHTING. watts, while the volt-amperes at the generator are 35,800, so that an alternator of 36 k.w. capacity will be required ; but the power to drive it, assuming its efficiency at 90 per cent, will be 34.3 + .90 = 88 k.w., or 51 h.p. The generator should be overcom- pounded about 19 per cent to be self-regulating. Arithmetical Determination. — The same result that has been obtained graphically may be found arithmetically, as follows : ENERGY INDUCTANCE CURRENT IN COMPONENT COMPONENT AMPERES. IN VOLTs. IN VOLTS. Lamps brought to basis of 1000 volts . . . 1000 28.75 Secondary wiring with 8 per cent resistance and 3 percent inductanceloss . . : . . 30 30 1030 30 Transformer resistance loss of 1 per cent. . 10 5s inductance loss of 12.6 per cent of volts at secondary terminals. . . . 130 Primary current increased 6 per cent by core loss at. balt-load gas @ (0 ihe =.) #na eee 1.44 Line Current os 0 in © che Ls ae or ten 30.19 Line resistance loss 3.5°X 3019.3 7) 3. G.0,. 99 «< Inductance-loss 6.04 x S0;199) . 7. a 182 1139 042 Taking the square root of the sum of the squares of 1139 and 342, we find 1188 volts to be the £.1/-F. pressure at the generator terminals, being the same as obtained by measurement in Fig. 181. OVERHEAD CONDUCTORS. . 237 GelreASB lb Rex hI: OVERHEAD CONDUCTORS. THE term overhead conductors is applied to aerial electrical wires or cables carried on poles or upon brackets or other supports attached to buildings. The same general construction is used for telegraph and telephone, as well as for electric light and power lines ; but the parts are usually heavier, and the insulation higher, Hmet eecasced! teu woe lattcr,specause ot) thes larver size of the conductors and the more powerful character of the currents. Materials for Overhead Conductors.— For electric light and power purposes copper is generally employed, and has now largely supplanted iron wire even for telegraph and telephone lines. The fact that hard-drawn copper has a tensile strength of 60,000 to 70,000 lbs. per square inch, compared with 25,000 to 35,000 for soft or annealed copper, makes it especially suitable for overhead construction. On the other hand, the specific resistance of hard copper is from 2 to 4 per cent greater, and it is much more brittle, so that it is not used for underground conductors or interior wiring where its greater tensile strength is not of much advantage. The fact that it is far less flexible makes it awkward to handle; conse- quently, hard copper is not convenient even for overhead con- ductors when they are of large size, or are covered with insulation. The resistances, weights, and other data of copper wires are given on pages 8 and 15, and in Chapter XI. Aluminum has a specific resistance about .6 that of copper, both being of pure commercial quality, therefore the sectional area of equivalent conductors would be about 1.67 times and the diameter 1.3 times greater for aluminum. The specific gravity of aluminum is about 2.7, and of copper 8.89 (page 8), so that an equal volume of the latter weighs 3.3 times as much. A copper wire would be (3.3 + 1.67 = 2) about twice as heavy as an alumi- num wire of the same length and resistance. This is a great 238 ELECTRIC LIGHTING. advantage for overhead conductors, since it reduces the weight on poles, cross-arms, insulators, etc., by one-half. The tensile strength of aluminum wires is about 20,000 to 30,000 lbs. per square inch; but the addition of a small percentage of copper increases this considerably, and alloyed with 24 per cent of copper it becomes nearly 40,000 lbs. per square inch. On the other hand, the resistance is increased about 20 per cent, so that the advantage is doubtful. Furthermore, it has been found in practice that wires.made of these alloys are likely to break, even when the tests of a sample show an ample tensile strength. ‘This is due to the difficulty of making perfect alloys of aluminum, because the light metal does not readily form a thorough and homogeneous mixture with the copper, which has a density 3.3 times greater. The result is that flaws seem to exist at certain points on the wire, and a break may occur without excessive strain. In a case cited by Mr. P. N. Nunn * an average of one break per span occurred on a long transmission line composed of aluminum alloy. The following data for commercially pure aluminum wire are taken from the paper itself, and agree closely with those already given : Diameter of aluminum wire 92.0). 9.5 .ecvo.9 mis: Wtisper mile®.' 42) i. Nye issue eed ety Less Resistance; per mil foot.) 7. ware ee LO Onis ane Ce Resistance, per mile at 26° Co. a a eee E007 73 ohms: Conductivity compared with copper . . . 59.9% by dimension. Tensilestrength of wire. 5 7) .48 3) ee LO OUDs: No. of twists in six inches for fracture... . 17.9. Tensile strength per squareinch. . . . 32898. Comparing this with copper, it is seen that this wire is approx- imately the same as copper in the following sizes : — Size of aluminum wire = No. 1 B. & S. copper. 9 Resistance of 3 SINK, ue cc a“ id o Tensile strength“ “ =No.5 “* ‘6 Weight of te =a INIOnO Therefore on the basis of the same conductivity the aluminum compares with copper as follows : — * Discussion of a paper “On the Use of Aluminum Line Wire,” by Perrine and Baum, Z7rans. Amer. Inst. Elec. Eng. May, 1900. OVERHEAD CONDUCTORS. 239 Diameter for the same conductivity 1.27 times copper. Area I ‘“ 164 « & Tensile strength ‘“ &“ 629 « « Weight <“ “ ‘501 & ‘“ The number of twists necessary for fracture varies considerably, although the ductility test of wrapping six times around its own diameter, unwrapping and wrapping again, is well sustained. This irregularity in the twisting-test is generally a mark of impurity in wire; but we know so little as yet of the exact characteristics of aluminum in particular, and the twisting-test is in general so unreliable, that it is unsafe to base any exact statement on this one test, particularly as the wire after erection proved reliable. In carefully performing the test for tensile strength, no exact point could be assigned for the elastic limit, as the metal seemed to take a permanent set almost from the first; but at a stress of from 14,500 lbs. to 17,000 lbs. per square inch, there is a marked increase in the permanent set which indicates that the safe working-load lies somewhere in this region. In this the characteristics of alumi- num do not differ materially from those of copper or other similar metals; and while this is a disadvantage, it is not a singularity. The fact that the wire will permanently elongate if seriously strained makes it necessary to use the utmost care in the erection of lines, and also the known high coefficient of expansion with tem- perature changes taken in conjunction with this property renders care in line-stringing especially important and difficult. The greatest care must be taken against kinking or scarring the wire; wherever the wire is accidentally kinked or scarred, it must be cut and spliced. One of the most serious problems in connection with the use of aluminum is in the choice of a proper joint. This metal is so highly electro-positive that it is unsafe to expose it to the elements in contact with any other material, as electrolytic corrosion is almost sure to follow such construction. Many of the failures which have been reported of this metal have been due to a neglect of this fact. Whenever this metal is soldered, or used in contact with any other metal, the joint should be thoroughly waterproofed to prevent such action. Without such protection the joints may be made by slipping the ends of the wire into an oval aluminum tube about nine inches long, which is then twisted about two and 240 ELECTRIC LIGHTING. a half turns, with a pair of clamps similar to those employed in twisting the McIntire connector. The joint produced is practically equal to the original wire in both tensile strength and electrical conductivity. Tests made at the Columbia University showed the fusing points of pure aluminum wires suspended horizontally in the open air to be 180 amperes for No. 8, 185 amperes for No. 10, and 60 amperes for No. 14 A. W.G. For aluminum alloyed with J per cent copper, the fusing-points were 163 amperes for No. 8, and 64 amperes for No, 14 wire. The use of aluminum as an electrical conductor may be summed up as follows: It is especially advantageous for bare overhead lines, because it weighs only one-half as much as copper for the same resistance and length, thus reducing to one-half the weight to be carried by insulators, cross-arms, and poles. Its ten- sile strength is about one-half as great as that of copper; but its specific gravity is less than one-third (.3) as much, so that it has an advantage in this respect also. On the other hand, its diameter is 1.5 times that of an equivalent copper wire, so that it exposes cor- respondingly greater surface to wind surface and to the accumulation of ice. The electrostatic capacity of an aluminum line is higher than for copper of the same resistance and length on account of its greater diameter, as is evident from the formulae on pages 137 and 188. But the capacity being a logarithmic function of the diameter would not be much augmented by increasing the latter by 80 per cent. For example, the diameter of No. 1 wire is 42 per cent greater than that of No. 4 wire; but the capacity of a circuit com- posed of two of the former, placed 18 inches apart, is only 74 per cent greater than if the latter were used. For overhead lines the electrostatic capacity of an aluminum conductor would not be more than about 5 per cent higher than that of an equivalent copper wire. Moreover, capacity does not play an important part except in very long transmission lines. Aluminum is also a very suitable material for bus bars or other conductors that do not require to be covered with insulation ; or in other words, dave conductors that are carried upon insulating SUup- ports, which applies to overhead lines as well. In such cases the fact that aluminum would have about 30 per cent more surface is an advantage in dissipating heat. OVERHEAD CONDUCTORS. 241 On the other hand, if aluminum conductors are to be covered with insulating material, as in the case of ordinary wiring in build- ings, or especially with underground and submarine cables, then the fact that 30 per cent greater diameter and circumference are required is a disadvantage, since it increases the cost of insulation in about the same proportion. The lead covering or iron armor of. cables would also be correspondingly augmented in weight and cost, and the space occupied would be greater to the extent of about 67 per cent in cross-section. Sag and Stress in Overhead Conductors. — A wire suspended freely between two supports hangs in a curve called a catenary. The exact determination of the sag and other facts is somewhat difficult ; but for electrical lines in which the sag is usually small compared with the span, very closely approximate results may be obtained by assuming the curvesto beraaparapola, A’ A C B wire stretched between the points A & may be repre- sented by the parabolic curve AE &. (Vheshorzontal distance A C 2 is called the-span # in feet, the vertical distance JP is the deflection or sag of the lowest point in feet, Z is the actual length of the wire measured along the curve ; 7 is the tension in pounds in the wire at its lowest point, and W is the weight of the wire in pounds per foot. We have the following approximate relations : — Fig. 182. Sag of Overhead Wires. FH? W FT? W ae a ; (82) ONS With a given span // the tension 7 is a minimum when the sag D is one-third of H. In practice, the sag is made much less than this, being usually one to two per cent of the span, in order to avoid the strains and chances of making contact with other wires due to excessive swinging. Expansion and contraction by changes of temperature produce considerable effect upon the sag and tension of overhead wires. 2492 ELECTRIC LIGHTING. - For this reason a greater sag should be allowed for wires laid in warm weather, in order to allow for the contraction in winter. The actual length Z, of a copper wire at a given temperature ¢ in centigrade degrees compared with its length at 20° C. is given by the following expression : — es L,=L, [1 + .000017 (¢ — 20)] (84) The sag with the increased or decreased length may be found by solving (88) for JY, which gives: / D = Vas Cir) (85) The following table may also be used for determining the vari- ations in sag, due to temperature changes. The sag in inches is given for every 10° between 80° and 100° F., being the limits between which lines are likely to be laid. TEMPERATURE EFFECTS IN SPANS. e TEMPERATURE IN DEGREES FAHRENHEIT. FEET. SPANS IN 30° | 40° 50° 80° | 90° | 100" DEFLECTIONS IN INCHES. 10 12 14 i 16 18 20 1.2 1.6 1.9 10 TID goto bo “1b © Hard-drawn copper wire, 60,000 pounds strength per square inch. Stress at — 10° F., 30,000 pounds per square inch. At —10° F. the sag is reduced, by the contraction, to the very small values shown in the table; and the tension in the wire is raised to 30,000 lbs. per square inch, which is rather too near the breaking stress, assumed to be 60,000 Ibs. per square inch. OVERHEAD CONDUCTORS. Ay Hence it appears that the sag of about 1.7 per cent at 70° F., upon which the table is based, gives excessive tension if an over- head line, even of hard-drawn copper, is exposed to temperatures of —10° F. or less. The stretch which occurs in wires considerably modifies the results obtained by calculations, using the ordinary formulae. This is particularly true of soft-drawn copper and aluminum, which show some permanent elongation with any considerable tension applied to them, and do not seem to have a definite elastic limit, like steel. Poles. — In most cases wooden poles are employed to support overhead electrical conductors. But in some countries, notably in India, zvoz poles are used almost exclusively for telegraph and other electrical lines, because wood is rapidly destroyed by white ants. This is true of most other tropical regions. ‘The form of iron pole generally adopted is hollow and tapering, being similar in its general size and proportions to the natural wooden pole, but some- what smaller in diameter compared with length. It consists of sheet iron riveted together, and may be made in convenient lengths, the ends of which are fitted into each other. ‘These set into a cast-iron base or sole plate, which is buried in the ground. In order to protect the iron, it should be galvanized inside and out, and should also be treated with some resinous material inside and outside, as far as it is buried in the earth. The insulators are carried on iron brackets, which are bolted to the pole, making a very strong and neat construction. In this country iron poles are made of sections of wrought iron pipe, with the joints either “swaged’”’ or rusted. Sometimes for use as anchor poles, iron lattice construction is used. Iron bases or sockets are often employed with wooden poles, enabling the latter to be made smaller in diameter and straighter. This also overcomes the objection to iron poles, due to the fact that they offer a ground connection to the wires or to the work- men, which in the case of the latter is very dangerous with high voltages. ‘Wooden Poles. — Chestnut is a very good material for this pur- pose, especially sawed or hewn for smaller poles. For large poles, pine is suitable on account of size and straightness ; but pine, partic- ularly southern pine and spruce, are not as durable as chestnut or cedar. The latter has long life, but is rather too crooked and Q44 ELECTRIC LIGHEING. knotty for first-class work, where appearance is important. In California sawed redwood is recommended. Preservation of Timber. — Wooden poles for electrical lines or other exposed timber is liable to be destroyed more or less rapidly by decay, or by the ravages of various small forms of animal life. The chief cause of decay is the fermentation of the sap. When located continually under water, wood is hardly affected by decay, but may be attacked by the ¢Zevedo navalis, or other animal ene- mies. But when alternately dried and wet, or when buried in the earth, it is especially liable to decay. ‘Io prevent it various things have been tried. 1. Kyanizing consists in soaking in a solution of about three per cent corrosive sublimate (Hg Cl,). 2. Burnettizing consists in impregnating timber with a 1 to 8 per cent solution of zinc chloride (Zn Cl,), formerly by soaking, but now by forcing solution into the pores under pressure. Oak absorbs about 10 and pine about 20 per cent of its volume. The trouble with the above processes is the dissolving out of the antiseptic salt, and various means have been devised to prevent it, such as the Thilmany process, in which zinc or copper sulphate solution was first forced into the pores and then barium chloride solution to form insoluble barium sulphate (Zn SO, + Ba Cl, = Zn Cl, + Ba SO,). The Wellhouse process employed glue and tannin, and the Hagen process used gypsum to retain the salt in the wood. 3. Creosoting consists in placing the timber separated by laths on cars which are run into a large cylinder closed by heavy iron doors. Live steam at 225° to 250° F. is turned on until the tim- ber is heated through, and the albumen of the sap coagulated. A vacuum is then formed to extract the sap, and finally the cylin- der is pumped full of dead oil of coal-tar,a measured quantity being introduced under a pressure of about 100 lbs. per square inch. The amount of oil is generally from 10 to 20 lbs. per cubic foot of timber, the oil weighing 8.8 lbs. per gallon. Besides pos- sessing antiseptic qualities, the oil is insoluble in water, and is not washed out or displaced by it. The oil usually only penetrates a little below the surface, hence this skin should not be removed by subsequent work upon the timber. Creosoted telegraph poles in England showed no sign of decay OVERHEAD CONDUCTORS. 245 after 85 years.* In this country creosoted railway ties last about 20 years on the average. Cresoting also protects timber from the attacks of the ¢eredo navalis and the “imnoria terebrans. 4, Carbolining consists in treating timber at a temperature of 250° F. with an oil called carbolineum avenarius (invented by Captain Avenarius). 5. Vulcanizing is accomplished by heating timber in closed cylinders from 8 to 12 hours at 300° to 500° F., and under a pressure of 150 to 200 lbs. per square inch. A circulation of heated and dried compressed air removes moisture and any water that does not take part in the chemical reaction, and combine with the woody constituents. This process changes the character of the sap so that it does not ferment, and seals up the pores. Tests at Columbia University showed an average increase in strength of 18.9 per cent, in addition to preservative effect. 6. Applying fztch or tar to the butt of a pole may do more harm than good, as it confines the sap, hastening fermentation and decay. But, after the pole has been standing two or three years, it might be treated in this way, by digging around it. N rene en and oe a ee SS oe J ee Ses Figs. 183 and 184, Poles are 85 to 60 feet long, but are sometimes 100 feet or even longer. Those of 50 feet or more are usually set about one-tenth of their length in the ground, but for shorter poles or in soft earth they are sometimes buried to the extent of one-eighth or one-sixth of their total length. In soft ground they should be * N, W, L. Brown in Elec. Railway Gazette, October 19, 1895. 246 ELECTRICE/ICOIING: surrounded with a grouting of Portland cement, sand, and broken stone, tamped around the bottom of the pole, or the butt of the pole may be set in a barrel filled with sand or firm earth. The standard practice is to put from 40 to 50 poles per mile, making spans from 132 to 106 feet each. About every tenth pole should more 1 a Figs. 185 and 786. be guyed laterally, to prevent wind pressure from overthrowing them. This is quite likely to happen; and if one pole falls it is likely to drag down the next one, and so on fora long distance, unless they are supported by side guys at reasonably frequent intervals. The guys usually consist of several strands of No. 6 or Y 4; LL. / a Y y Yy Figs. 187 and 188. Arrangement of Guys for Turning a Corner. 8 iron or steel wire, which is more easily handled than the larger wire or rods that are sometimes used. They may be made simple, as in Fig. 183, or for high poles they have the Y form (Fig. 184). Fig. 185 shows wire guy and pole brace. When a pole is to be made very secure it is guyed in two directions, or double guyed. This adds greatly to the stability of the pole. See Fig. 186. OVERHEAD CONDUCTORS. ay On curves or at corners the guys should be more frequent and > stronger bene» placed’ on the= outer Side of, the? curve. Methods of guying suitable for lines that turn a right angle at street corners are shown in Figs. 187 and 188. In such cases, or where lines come to an end, as in front of an electric light station, A B ¢ Fig. 189. Guying of Terminal or Corner Pole. the last two or three poles should be stronger and more firmly set than the others, and may be guyed as indicated in Fig. 189. It is also well if the last one or two spans, A Band, B C, are left somewhat more slack than usual, in order not to bring too much strain on the terminal pole. Cross-Arms are of yellow pine or oak, being usually about 34 x 44, or 83 x 44 inches for smaller sizes, and as much as 4% x 53 inches for the Niagara transmission line. A cross-arm about 3 feet long is used for two insulators, about 5 or 6 feet for 4 insulators, and so on. The spacing of the pins is about 4 to 6 inches from the ends, 24 to 30 inches in the middle, and 12 to 18 inches for the. rest, de- pending upon the size of insulators and other conditions. (ies: cans. Jor, flat 'spots on which the cross-arms are placed should be cut in the pole before it ts setup, Ordinarily these are placed about 24 inches, center to Fig. 190. Bracing a Cross-Arm. center. The cross-arms should be fastened to the pole by two bolts or lag screws, placed diagonally in order not to split the wood, and are braced by two iron strap braces also attached by bolts or lag screws, as represented in Fig. 190. The cross-arms should be put alternately on opposite sides of the poles so that they cannot be pulled off successively. Guard Wires. — Where one set of overhead electrical wires pass 248 ELECTRIC LIGHTING. - under another set, the former should be protected by guard wires, An arrangement of this kind is represented in Fig. 191, A4C being the galvanized iron or steel guard wires attached directly without insulation to a cross-arm or to the top of the pole. These guard wires serve to prevent any wire that may fall from coming in con- tact with the electrical conductors carried on the insulators DE. Guard Hooks. — A hook of stout iron wire or a hoop, as indicated in Fig. 192, is often attached to the cross-arm to catch an overhead conductor, and prevent it from falling in case the insulator, insula- tor pin, or tie-wire should happen to break. They are required especially on the inside of curves or angles in the line. In this connection it may be stated that electric light or other conductors carrying high voltage or heavy current should, if pos- sible, be put over telegraph and telephone wires, because the latter are more likely to fall, not being so well laid or so carefully watched, B Fig. 191. Guard Wires. Fig. 192. Guard Hooks. and being more numerous. Another reason for this is the risk of requiring telegraph and telephone linemen to pass up through the more dangerous wires with which they may not be familiar ; whereas, electric light linemen would not be injured by telegraph or tele- phone wires. Insulators. — The problem of supporting overhead wires is some- what difficult, since those materials having sufficiently high insulat- ing qualities are not very strong mechanically. Glass and porcelain are employed almost universally for the purpose, but neither is possessed of the great strength that is very desirable in order to enable the insulators to stand the heavy stresses to which they are subjected. Other materials, such as hard rubber and various com- positions of vegetable or mineral matter, have been tried; but they | are rarely used except that the latter are commonly employed to support overhead trolley wires. OVERHEAD CONDUCTORS. 249 The advantages of porcelain over glass are that it is less brittle and generally stronger than glass, and it is less hygroscopic. On the other hand, glass is cheaper than porcelain, and the fact that it is transparent enables an internal defect to be detected more readily. It also makes the cavities in the insulator less likely to invite the building of nests by insects. Another differ- ence, which is much more serious than it sounds, is the fact that white porcelain insu- lators more often attract the eye of a boy or hunter, and frequently are made to serve as targets for stones or bullets. Glass or porcelain insulators for electric light and power lines have been developed directly from those that are employed for tele- Fig. 193. “Deep Groove, Double Petticoat ’’; Screw- glass Insulator. graph and telephone service. In fact, there is no substantial dif- ference, the only modifications being an increase in size and strength to suit the heavier conductors, and single petticoat form. forms of glass begins. form of insulators. Fig. 194, Porcelain ‘Oil Type’’ Insulator, mounted improvement in insulation by lengthening the path for leakage of current, secured by adopt- ing the double and triple in place of the Types of Insulators. — The deep grooved double petticoat pattern of screw-glass_in- sulator is the ordinary standard, being used with insulated wires for lines of 2000 volts. his type isoshown in Pig. 193.0) For higher potentials the use of porcelain or special The present increasing employment of high voltages, and the tendency to raise the voltage still higher, has brought into use new The oil insulator, shown in Fig. 194, is mounted upon an iron pin, and provided with on Iron Pin. a recess that is filled with an insulating oil. There is a “built-up” type of porcelain insulator, being made in. parts as is shown in Fig. 195, and the parts burned together with a vitreous cement. 250 ELECTRICMYICGHIING For 20,000 volts an insulator made entirely of porcelain was designed. Fig. 196 affords a very good idea of this type. In Fig. 197 is the~porcelain pin. base employed in combination with the insulator of Fig. 196. The insulator used on the trans- mission line from Niagara to Buffalo, together with its wooden pin, and sec- tion of the cross-arm, is show in Fig, 198.9 Dhe weaves tend Wtoschermame water at two points, where it will not PORCELAIN PIN BASE drip on the cross-arm. An interesting insulator is the one used by the Telluride Power Trans- (ST Pe = Fig. 195. Built-up Porcelain Insu- lator with Iron Pin, mission Company to transmit power 80 miles, from Provo to Mercur, Utah: . The / pressure, 40,000 volts, was the highest employed up to that time (1898) for com- mercial lise: gamhio ne oo shows the Provo in- sulator. A still newer type is that shown in Fig. 200. It is of brown china ware with a glass or por- celain cone extending down around the pin, which is of wood with a porcelain sleeve and base. The idea of this sleeve is to make the striking distance Fig. 196. 20,000 Volt Porcelain Insulator. OVERHEAD CONDUCTORS. 251 greater, this being of as much importance as that the length of the path from the cross-arm to the wire be made long. This insulator is 103” in diameter, about 15” high, and weighs about 12 pounds. Fig. 197. Wood Pin with Porcelain Base. Fig. 198. The Niagara Type of Porcelain Insulator, Wood Pin and Cross-arm. Another feature is the beveled trough around the top, which catches all the water at the periphery, and carries it off to one side of the cross-arm. Insulated Wire for Overhead Lines.— For long-distance trans- ~~ 20% b ELECTRIC LIGHTING. mission bare conductors, described at the beginning of this chap- ter, are generally employed, even with very high voltages. For local distribution, especially within the limits of cities and towns, & BS Pas Fig. 199. Provo Glass Insulator. electric light and power over- head wires are covered through- out with insulating material, to reduce the danger of accidental contact with persons or with other wires or conducting bodies. The insulation of overhead One of impervious wires is in two parts. insulating material to moisture, placed next to the wire, and the other of some sub- stance fitted to resist abrasion or like mechanical injury. | The inner coating is a rubber compound, or for lower grades Before this is laid on the wire it is first some cheaper substitute. tinned to prevent the sulphur in the rubber com- pound from corroding the wire. This inner coating is then covered with a hard braid of cotton or hemp, woven on to the wire, or the wire is served with a tape and insulating compound. Where the wire is to be continu- contained ally moist, gutta percha is better than rubber, but it costly. is more In the more expensive grades of wire the coatings are greater than two in number, and they alternate, insulating com- pound and then braid or tape. In Figs. 201-203 are shown Fig. 200. Locke High Potential Insulator. the manner of application of the insulation and the braid. OVERHEAD CONDUCTORS. VAST Joints in Overhead Lines. — Whether an electrical conductor is bare or insulated it is necessary that any joint made in it shall be nearly equal in conductivity and in mechanical strength to the Fig. 207. Insulated Line Wire. rest of the conductor. The ordinary “lineman’s splice” (Fig. 204) has been the standard practice for galvanized wire iron, in telegraph lines; but the use of copper wire, both hard and soft drawn, and the necessity for better connection with heavy currents, has resulted in the Fig. 202, standard Conductor, In- sulated for Outside Work. adoption of various special forms of jomt. /Ot these the MelIntire joint illustrated in’ Fig. 205 is a prominent exarnple. , de i ) yy» Z pany yyw TREES SSS ao Fi ei ley N Sy if) Ue, if ys This joint is made by use of a “connector” which consists of two tubes drawn side by side out of one piece of copper. The internal diameter of each of these tubes corresponds to the ex- RSS} == Fig. 204, Lineman’s Splice. ternal diameter of tne wire to be spliced. ‘The two wires need not be of the same size. Fig. 205. McIntyre Wire Joint. The joint 1s made by slipping the wires inside the tubes, and then by means of special pliers, twisting the tubes one on the D54 ELECTRIC. LIGHTING. other ; thus by friction the two wires are bound firmly together. Unless required by Insurance Rules, they need not be soldered, a great advantage with hard-drawn copper wire as it avoids anneal- ing the wire, and the joint more nearly retains the full strength of the wire. | In the “lineman’s splice” the actual area of contact wire to wire is small, and winless well soldered the crevices will afford places for starting corrosion, and the resistance will be high. The McIntire joint affords plenty of contact area, giving a low resistance and being impervious to moisture. This form of joint is especially valuable with the aluminum wire that is now coming into use. All joints made in insulated wire lines should be taped and painted with an insulating compound till the insulation over the joint is as good as that on the wire of the line. Method of Attaching the Line Wire to the Insulators. — The or- dinary plan is to take a simple U-shaped tie-wire, place the curve of it around the insulator, and wrap up the projecting ends around Ue Fig. 206. Tying Wire to Insulator, Fig. 207. Tying Wire to Insulator. the line-wire. This puts a side pull on the line-wire which objec- tionable feature of this tie is indicated in Fig. 206. This might, in the case of hard-drawn copper wire, cause breakage, because it is quite brittle. The standard method now in use is shown in a completed form in Fig. 207. A soft copper tie-wire is laid in and around the insu- lator groove, in such a manner that one end comes over, and the other end under the line-wire; the ends are then wrapped around the line-wire. A method of making the tie is shown in Fig. 208. When properly made in this way the line-wire is anchored to the insulator with no side pull. Tie-wires should be the same size as, or slightly smaller than, the conductors themselves. This is true even when the line-wires GCEERHAAD CONDUCTORS: 255 are insulated, and the insulation of the tie-wire should be equal in character and thickness to the line-wire that it ties. Dead Ending. — When a line ter- minates it is dead-ended by taking a turn around the insulator and wrap- ping it about itself, or by means of a McIntire connector. Service Connections and Locps. — When it is necessary to take a tap off to give service, an extra in- sulator must be mounted on the cross-arm, in order that the strain of the service main may not put a side strain on the line-wire; for a series circuit the line is usually dead-ended to the building to be served. In this case the arrangement shown in Fig. 209 may be used. See at the nearest pole, and a loop taken me. st Fig. 208. Method of making Tie. Limitations of Voltage. — The maximum voltage that it is possi- ble to employ on overhead lines depends upon conditions. In 1890 a pressure of 5000 volts was considered to be very high, but gradu- MAIN LINE nit i Fig. 209. Method of making Loop Connection. ally the apparatus and methods have been improved until 40,000 or even 60,000 volts is now regarded as commercially practicable. 256 ELECTRIC LIGHTING. fod The electrical maintenance of such a circuit depends entirely upon the insulators, since the wires are usually bare, and the poles even if made of wood should never be depended upon for insulation, especially at such high voltages. In a paper before the American Institute of Electrical Engineers,* Mr. C. F. Scott gave the results of experiments on several lines, and pointed out that the loss between wires by leakage directly through the air rose rapidly above a certain voltage. In Fig. 210, which shows some of these data, it will be noted (curve 1) that the loss between two No. 28 wires 48 inches apart was 500 watts at 30,000 volts, each wire being 1040 feet long. This is far too great . for commercial work, since the waste would amount 2000 Losses between Wires of Different Sizes Frequency 133, Smooth Armature, Length 1040" 1800 oe aa te {Na,28 Wires, 48pare Apes eee IT tO wa DOUG Fo Ola Ke wee peimiic: 1400 eee | With larger wires the loss de- og Lae SIS eS [ea Se creases SiorMexamn plc RimmeM mye Heb EAE 8 the leakage for two No. 8 eae Te Ish tah Vio oMilcs (hes samencisianicam Dake 800 AAA tA is only one-fifth as great at oe EER EEE AE 30,000 volts, being 100 watts tH AEH for the same length. When 400 EECA No. 7 wires, rubber-covered, OTS ST te ae eaearesusedarcuiver) at bemicssmis WERMMGEEACON Ho aes r++ ~~ «practically nil at 30,000 volts, 9 ina el Coa em ee 60 7 and only becomes 50 watts at Fig. 210. Loss between Wires in Air. 60,000 volts, or .25 kw. per mile. The substitution of still larger conductors secures a further reduction in this leakage through the air, so that it can be kept within reasonable limits even at 60,000 volts. A transmission line in California, which is de- siened to operate at this pressure, employs aluminum wires one inch in diameter, which should give very little air leakage, even though they are bare. When the distance between wires is increased, the loss is diminished, as shown in Fig. 211. It is also a fact that the leakage depends upon the wave form of the pressure, being greater with peaked than with flat topped waves, since the maximum volt- * Transactions, June 30, 1898. OVERHEAD CONDUCTORS. DO age is higher in the former case. The percentage of humidity in the atmosphere, or even a fall of rain or snow, does not materially increase this loss through the air. 3000 Loss on Circuit with Wiresat Different Distances. Frequency 60, Slotted Armature, Weston Wattmeter. Wires 15,22,35and 52 ins. apart aie CEEEE ECA t+ LE ee OSE oem aH-pe eh P| | | Si at ————TTCOrLCOSCS~S OA Om cunlo w20m ct eh oe oO 400 1445 48525556 8160 Thousands of Volts. Fig. 211. Loss between Wires in Air. Lightning Arresters are required in almost all cases in connection with overhead conductors. The principal forms in use, and the manner of using them, were described quite fully in Vol. I., pages 425 to 438. ill at GENERAL SPECIFICATIONS FOR ORDINARY POLE LINE CONSTRUCTION. (2000 vo.Ts.) Poles to be of best quality cedar or chestnut, round or octagonal, as specified. Height to be approximately thirty (30) to thirty-five (35) feet. Diameter of base to be ten (10) inches. Diameter of top to be about six (6) inches. The poles to be straight and knots closely trimmed. Tops to be chamfered. Gaius to be cut square with the axis of the pole and with all other gains, and to be accurately made to fit cross-arms, so as to bring the cross-arms nearly flush with the pole. Painting. Poles which are specified to be painted to have the lower six and one-half (64) feet, including the base of the pole, either creosoted or painted with a heavy coat of tar paint or equal. (This is not to be done if the pole is “green” or sappy.) The roof of the 258 ELECTRIC LIGHTING. pole to be painted with three (3) coats of best quality white lead. All gains to receive two (2) coats of best white lead previous to the introduction of cross-arms. Lower shanks of pins to be painted with white lead before being inserted into cross-arms. Cross-arms to be thoroughly painted with two (2) good coats of mineral paint put on with a brush. After the poles are erected and wires in place all the poles specified to be painted with two (2) coats of best quality dark-green mineral paint. Guy-Stubs and Anchor Logs to be used where the pole needs side guying on a sharp bend, or wherever the country does not pro- vide a more convenient way to guy them. These anchor logs or guy-stubs to be of proper dimensions, depending on the size of the pole and weight of the line. Cross-Arms to be thoroughly sound, straight-grained timber, of southern pine, and free from knots. The arms to be of requisite length, to be sawed true and square, and up to the dimen- sions specified. The top side of the cross-arm to be chamfered throughout the whole length, with the exception of eight (8) inches at the center, where the arm fits the gain. Cross-arms to have holes bored of spacing and size for pins, as specified or shown on drawings. Cross-arms to be screwed to the pole by two (2) 2 inch galvanized iron bolts extending entirely through the arm and pole. Under the head and nut of each bolt a galvanized iron washer, not less than 24 inches in diameter, shall be placed. Bolts to be stag- gered. (This construction refers to cross-arms carrying heavy wires and large number of same. On light lines lag screws are sufficient.) Iron Fittings to be of good quality best refined wrought iron, which would conform to good bridge specifications, to be thor- oughly galvanized. Galvanizing to be subject to a test. Cross- arm braces to be used on all the cross-arms having four (4) or more pins. The braces to be secured to the pole with a lag screw, and to the cross-arm with carriage bolts of sufficient length to go through the braces and arms. Galvanized iron washers to be placed under the head of all bolts, nuts, and lag screws. Pins. All pins to be best quality, sound, clear, split locust, free from knots and sapwood. Pins to be of standard dimensions, which are governed by the size and weight of the insulators, etc. The threading and tapering shall be neatly and accurately cut, OVERHEAD CONDUCTORS. 259 showing the full thread, and shall accurately fit the insulator. Each pin to be secured to the cross-arm by a sixpenny galvanized iron wire nail driven straight through cross-arm and shank of pin. On all curves pins to be bolted by galvanized iron bolts. Insulators. Insulators as per sample to be used, to be sound, strong, free from fins, having threaded holes accurately molded and of uniform size. To be double petticoated, made of glass, and subject to a break-down test of 6000 volts, from a source capable of delivering five (5) amperes at that pressure. Guy-Rods. Anchor guys shall be attached to galvanized iron guy-rods. These rods to be from 6 to 8 feet long, 3 inch in diam- eter, provided witha galvanized iron washer 2 inch thick and 3 inches square, with 3 inch hole for reception of the rod. Wire-Rope Fittings. All wire-rope fittings, such as thimbles, guy-clamps, rings, sockets, shall be of first-class quality of wire- rope fittings, equivalent in every respect to those manufactured by the Roebling Company or Washburn & Moen. To be galvanized. Lightning Rods. Every tenth pole to be supplied with a light- ning rod made of No. 6 galvanized iron wire, carried at least one foot above top of pole, and secured to same by heavy galvanized steel staples made of No. 4 (B. and S. gauge) wire. These staples shall be 24 inches long. The wire shall be carried down the pole, and thoroughly buried in the ground at the base of the pole, with at least two hand turns. _ Guy-Ropes to be made of good flexible quality galvanized iron, and to be composed of one or more strands, depending on the stress to be borne by the guy. To conform to good specifications for elongation, twist, and breaking. Construction Details. The line shall be located by measuring off and placing stakes for pole location at distances of 120 feet as an average. Such stakes must be placed as nearly in line as possi- ble. In case of obstacles, the pole should be located as near the stakes as possible. In the distribution of the poles, the strongest and heaviest poles shall be placed on line corners, while the best- looking shall be distributed throughout the town, or in the front of residences. The length of the pole shall be proportioned to the contour of the country, so that the wires may be strung without abrupt changes in level. On straight lines all poles shall be set in the ground to a depth 260 PLECTRICOLIGHTING of at least six feet, unless otherwise specified. All poles shall be set perpendicularly on straight-line work. On curves, poles should be set with an outward rake. The holes shall be dug sufficiently large to admit the butt of the pole without hewing’; and after the pole is set, the earth shall be returned and thoroughly tamped around all the base of the pole. Tamping shall be done in the proportion of three tampers to one shoveler. Upon curves the poles must be laterally guyed. Every eighth pole to be laterally guyed on both sides, and on steep hills every pole shall be head-guyed in both directions. The two end poles of each line shall be head and laterally guyed. On long spans the poles shall be head-guyed both ways, and side-guyed in both directions. «Y”’ guying to be used in all cases. Where it is difficult to get good setting for a pole, same to be set in “sand-barrel’’,or concrete, to be approved by the engineers. In cases where poles are set in rock, pole to be hewn to fit an approved iron shoe, which is to be securely bolted to rock. Shoe to be painted inside with two coats of white lead before pole is in- serted. Outside of shoe to be smooth, and hydraulic cement to be placed on top of rock on which shoe is set. Guys to be fastened to poles by means of galvanized eye-bolt, fitted with galvanized washers under head and under nut of bolt. Placing of Cross-Arms. On straight-line work, the cross-arms to be placed on alternate sides of succeeding poles. On long spans the cross-arms of terminal poles shall be placed opposite the long section. Double cross-arms to be used on all abrupt changes in direction and also on end poles. At the end of lines the arms of at least the last two poles shall be placed on the side facing the terminal of the line. On curves the cross-arms shall face towards the middle of the curve. Long spans of 200 feet shall be head-guyed, and if possible side-cuyed in both directions. Tying of Wires. Line wires shall be tied in a manner as ap- proved by the engineers. , Joints. All joints to be made either with a McIntyre sleeve or a Western Union splice, and to be thoroughly soldered, taped, and bound with cord. Guard Hooks. To be placed on each cross-arm on sharp bends. OVERHEAD CONDUCTORS. 261 Guard Wires. To be placed wherever wires cross above or below another line. Binding Wire. Binding wire to be used to secure wires on all insulators. Wires to be of first-class insulation, solid copper wire, two gauge numbers smaller than the line wire. No binding wire smaller than No. 8 B and S. gauge to be used. Excavation. All excavating and filling for pole line to be done by contractor ; also felling of trees, bushes, and all blasting, grad- ing, etc. Trees and bushes to be trimmed so that no branch can come in contact with the wires. All removing and replacing of fences or other structures which may be found necessary for locat- ing pole line to be done by the contractor. 262 ELECTRIC LIGHTING. CEVA? PD BAR texas UNDERGROUND ELECTRICAL CONDUCTORS. Tue branch of electrical light engineering that is the greatest in magnitude, and involves the largest expenditure, is that which relates to the designing, laying, and maintaining of a large system of underground conductors. In no other department of electric lighting are the practice and results so variable. For nearly ten years after electric lighting was first introduced, the distribution of a current was effected almost entirely by overhead wires, the only important exception being the Edison Underground System, first laid in 1882, and generally employed by most of the low-tension, direct-current systems in the larger cities of this country, and in many places abroad. Since 1890 the popular objection to the use of overhead electrical wires has grown, and wherever possible has demanded the substitution for them of underground conductors. The enormous expense of making the change, as well as the almost utter lack of experience with buried high-tension circuits, made this a most formidable problem at first. As is usual in such cases, extraordinary methods were devised for overcoming the apparent difficulties. It has been found, however, that very simple con- struction, provided it is of good quality, insures the practical suc- cess and permanence of underground conductors. In fact, alternating current and arc-lighting circuits of 1000 to 7000 volts are commonly laid in underground conduits, and do not give much more trouble than low-tension wires, including those employed for telegraphic and telephonic purposes. The essential elements of an underground system of conductors are, first, the conductor itself, which is almost invariably composed of copper ; second, the insula- tion, which may be either a complete covering of non-conducting material, or simply points of support; and third, the mechanical protection, which usually takes the form of a tube or conduit, and must be particularly strong in order to withstand the severe con- ditions to which it is exposed. CONDERGROOUNDI ELECTRICAL CONDGCTORS. 263 In some instances, especially in Europe, iron-armored cables are laid directly in the earth, without any conduit to protect them. The armor, which is relied upon for mechanical protection, consists of a spiral winding of iron or steel wire, like that of a submarine cable, or a spiral wrapping of iron or steel tape, with overlapping joints. In either case there is a certain amount of flexibility which a conduit does not possess, enabling the cable to pass around obstacles and adapt itself to the various underground pipes, etc., that are often very numerous in large cities. Moreover, the iron- armored cable would occupy much less space than an equivalent conduit. In some underground conductor systems the cables are drawn in after the conduits are built, and in others the conductors are put in the sections before they are laid, or are built in at the same time. FORMS OF UNDERGROUND CONDUITS. . Drawing-[n Systems. IRON. Wrought-iron pipes. ri i vay cement lined. “wood lined. Cast-iron pipes. aes trourns. Mm RW DN EARTHEN WARE. 6. Terra-cotta pipes, single or multiple duct. 6c 66 troughs 66 66 6 CONCRETE, 8. One or more ducts formed in a mass of concrete. WOOD, g. Wooden pipes. 10. « troughs. 11. Fiber pipes. Solid or Built-In Systems. 12. Edison and other underground Tube Systems. 13. Crompton and other naked Conductor Systems Wrought-Iron or Steel Pipes, similar to gas or steam pipes with screw or other connections, is the strongest and one of the most satisfactory forms of conduit ; and it has been extensively adopted, 264 ELECTRIC LIGHTING. particularly where its rather high first cost is not a serious objec- tion. Its advantages are great strength.to resist the severe strains due to the pressure of the earth, often aggravated by unequal set- tling. It is also well adapted to withstand blows of pick-axes, shovels, etc., to which conduits are exposed during subsequent ex- cavations of the same ground. Wrought-iron or steel pipes require to be of less thickness, and therefore occupy less space than any other form of conduit. They can be joined by screw or other con- nections which are most secure, and can also be made water-tight. Such a pipe can be bent to a reasonable extent without breaking or opening the joints ; whereas with almost any other form of con- duit the unequal settling of the ground, which is almost certain to occur, is likely to crack or break it. The disadvantages of wrought-iron pipes are their somewhat high first cost, and the fact that they are made of conducting material, which will cause a ground connection if the insulation of the wire is injured. It is doubtful, however, if non-conducting con- duits, such as earthenware, are really better for underground con- struction. Ifa difficulty occurs in the insulation of the wire, the incidental insulation afforded by the conduit is hardly sufficient to enable the circuit to be worked properly. The moisture which would almost always be present would produce a_ sufficient “oround’’ to render it undesirable and probably dangerous to employ the conductor. In most cases it would be just as well, and would enable a fault to be more quickly detected and located, if a “dead ground,” ie., low-resistance ground connection were made immediately. In fact, with high-tension circuits in non-conducting conduits, it is important to have the lead sheathing of the cables that are ordinarily used well grounded throughout its entire length, otherwise a defect in the insulation, or simply the electrostatic charging and discharging which takes place with alternating cur- rents, render it dangerous to touch the lead sheathing. Wrought-Iron Pipe in Hydraulic Cement. — This is used to quite a large extent, the ordinary construction for this conduit consisting in digging a trench in the street, the size depending upon the number of pipes to be laid. The bottom of this trench, after being carefully leveled or graded, is covered with a layer of good concrete 2 to 4 inches deep, and the sides are braced with plank. A suitable mixture for this purpose is composed of UNDERGROUND ELECTRICAL CONDUCTORS. 265 two parts of Rosendale cement, three parts sand, and five parts broken stone. Broken stone to pass through a one and one-half inch mesh. ‘The concrete is well rammed into place, and a layer of wrought-iron pipe is laid upon it. The diameter of these pipes depends upon the size and number of cables that they are to re- ceive, the standard being 8 or 4 inches in diameter, and 4 inch thick. The pipes are in 20-foot lengths, and are joined by means of a tapering or vanishing screw thread coupling, forming a joint which is water- and gas-tight, and can easily be made as the pipes are laid in place. When the first layer of pipes is in place, spaces between and around them are filled with concrete, ihe distance between. the | pipes as) usually one-half to three- eet eaee aie quarters of their diam- Vinee ‘ eter, the thickness of the concrete on the sides being about the same amount. The concrete is filled in over the pipes to a depth about half of a diameter, and then / 1% APPROX 1 17g PLANKS another layer of pipes is laid and packed around with concrete as before. After the last row is in place, a covering of con- crete from 2 to 8 inches in thickness is spread over it, and a layer of yellow pine plank 2 inches thick is laid upon this. The chief object of the latter is to serve as a protection against the tools of the workmen in case of later excavations. Experience has shown that men will dig down through concrete, but will turn aside from the wood. The following construction is standard in this kind of work being used in New York City. (See Fig. 212.) The thickness of cement all around the bunch of conduits is 4 inches. The boards cn the side of the trench are 14 inches, and there are 2-inch boards on the top. Three-inch wrought-iron reamed pipe is used, which is superior to the ordinary commercial pipe, in that it is reamed after being rolled to insure that there are Ys Yi 2A BP RO et Fig. 212. Cross-Section of lron Pipe Conduit. 266 ELECTRIC LICHIING. - no blisters or rough points on the interior of the pipe. The pipe is dipped in a tar to prevent rusting. Fig. 212 shows a 12-duct construction. This is larger than the average. The pipes are laid at 41 inches between centers, both between the rows and the pipes in the rows. This adds up, for total con- duit construction, about 14 feet deep by 2 feet broad. A cement formula that has been found sufficient is, Cement 1 part, sand 2 parts, and 3 inch broken stone 3} parts. It is evident that this construction is extremely substantial and well adapted to withstand the most severe mechanical forces, being also gas- and water-tight. The iron pipes are found to last very well, the action on their external surfaces being very slight. They rust internally to a considerable extent, being exposed to moisture and air; but it would take a long time for them to be corroded away entirely. Even should this occur, a smooth hole will be left in the concrete, and would still serve as a conduit for the conduc- tors, and would last almost indefinitely. The use of asphaltic concrete instead of that containing cement has been proposed ; but it would be still more expensive, and would not seem to offer a compensating advantage. [he manholes, hand- holes, and methods of distribution employed in connection with this form of conduit, will be described later, since they are quite similar for all types. Wrought-Iron Pipes Lined with Wood have also been used; but, as already stated, it is doubtful if a non-conducting conduit is especially desirable. But the wooden lining would at least serve as a means of preventing the chafing of the cable as it is drawn in, and might in that way prove a valuable feature. On the other hand, it might tend to corrode the lead covering of cables as de- scribed later under “ Wooden Conduits.”’ Wrought-Iron Pipe, Cement Lined.— This form of ducts usually consists of eight-foot lengths, made of thin (No. 26, B. W. G.) sheet iron, riveted every two inches, as represented in Fig. 213. The pipes are lined with a layer of pure Rosendale cement, % of an inch thick, no sand being used. The internal diameter is 3 inches, making the external diameter approximately 43 inches. The out- side of the pipe is tarred to prevent rusting. The interior surface of the cement is extremely smooth ; in fact, it has a polished ap- pearance, so that there is not an excessive amount of friction to UNDERGROUND ELECTRICAL CONDUCTORS. 267 interfere with the introduction or withdrawal of the cables. Each section weighs between 40 and 950 pounds, and is therefore easily handled in laying and joining. ‘These pipes are laid in cement in a manner similar to that described in the case of plain iron pipe, but the thickness of the pipes is somewhat greater. This form of conduit can be laid rapidly. Twenty ducts with a total of 12,820 feet of pipe were put down in a single day in St. Louis. The gang of men required to do this work were 86 concreters, 52 laborers, 6 bricklayers with 6 helpers, and 6 overseers. The trench in this case was already open; but the work included the Fig. 213. Cement-lined Iron Pipe. building of 13 manholes and 4 handholes. In another case a gang of 108 laborers, 40 concreters, 5 bricklayers, and 5 helpers, with 7 overseers dug a ditch 1157 feet long, put in 5 ducts, and filled in the trench in a single day. These may be considered as ex- traordinary ‘“runs,’’ an average of 500 duct feet complete being the usual result of a day’s work. Figs. 214 and 217 show con- duits in course of construction. Cast-Iron Pipe Conduit.— These are similar to the plain wrought-iron pipe already described. In order to have equal strength the cast-iron would have to be thicker than the wrought- iron, so that the cost would be as great or greater; and since the former occupies more space, and is heavier to handle, there is no great advantage in employing it. Cast-iron, however, lasts longer in the earth than wrought-iron. Cast-Iron Trough Conduits. — Various forms of this construc- tion have been used, a prominent example being the Johnstone conduit. This consists of shallow troughs of cast-iron in lengths of about 6 feet, which may be laid directly in the earth, as repre- sented in Fig. 215. The cables are then run along in the trough, and covers of cast-iron are placed over the troughs, the two being bolted together. This construction possesses the advantages that the cables are laid directly in place without being drawn in, so that there is less liability of their being injured ; and still more impor- tant is the fact that the cables are accessible at any point for a7 rae -~ LPL SHH rH HHI Sas ELECTRIC LAGH LIANG. Fig. 215. Cast-Iron Trough Conduit. Se KK \ @ UNDERGROUND ELECTRICAL CONDUCTORS. 26° inspection, repair, or branch connection, by simply removing one of the sections of cover, which are very easily unbolted and handled. For this reason the system is particularly well adapted to distribution, in contradistinction to transmission of current on feeders or trunk lines. Unfortunately the cost is so high as to be almost prohibitive. In some cases the troughs are completely filled with an insulating compound after the conductors are laid in them, thus excluding moisture, gases, chemical agents, etc., that might otherwise leak in and injure the insulation. Such construc- tion, however, comes under the head of “built-in ’’ and not “ draw- ing-in ’’ systems. The self-induction of alternating current conductors laid in iron pipes or troughs must be overcome by twin or concentric cables. Earthenware Conduits. — Various forms known as “terra- cotta,’ “ glazed-clay,” and “hollow-brick tile”’ conduits are manu- factured and_ used. The ordinary single- duct form, illustrated in Fig. 216, consists of an earthenware pipe 18 inches long, the internal diameter Fig. 216. Glazed Clay Conduit, being usually 3 inches, but smaller and larger sizes are made also. The thickness of the walls is about 2 inch, the external form being octagonal, as shown. These are made of clay burned moderately hard, and glazed inside and out. They are laid in a trench upon a | bed of concrete from 3 to 6 inches thick, being placed side by side with spaces of 4 or } inch between them, which are filled with cement mortar. Single-duct conduit joints being self-centering, are simply sock- etedjone into thesoether, The Figkbi7). Giased Clay’ Conduite conduit is built up in layers, with the pipes breaking joints very much like the bricks in an ordinary wall. The concrete con- sists of 1 part cement, 2 parts sand, and 5 parts screened gravel, broken stone, or broken brick ; the stone to pass through 14-inch i SL oe | ZE 270 ELECTRIC. TIGHTING mesh ; the cement and sand to be first thoroughly mixed dry, then a sufficient quantity of water added to make a rather soft mortar ; the gravel, stone, or brick to be added afterwards, and thoroughly mixed. The gravel, stone, or brick should not exceed one inch in its greatest diameter. The conduit is usually protected on the sides and top by a layer of concrete at least three inches thick ; but in some cases the concrete is omitted, when two one-inch yel- low pine boards are placed over conduit. Great care should be ex- ercised in laying the ducts so that the alignment is sufficiently good to enable the conductor to be easily drawn in and without injury. This is generally secured by inserting a round stick or mandrel of wood through the ducts as they are laid. The mandrel, which fits Pos ol (a a <--9'—-» FE rap se Z ZOOL EFL GA if Lf tty fb 06: LOG PEL ESL 12.GEG Legis COLOECLELOUAE ES, VIELE Yt) LIELAS PA GLACE, hie piplippiiis as SLOLLLLGIESPIEE 3 ie DTH. Wl T FO Fig. 218. Multiple-duct Earthenware Conduits. the bore of the pipes quite closely, insures that they are in line while being filled around with mortar, and there is a disk of rubber on the end of the mandrel that acts to scrape out any mortar or dirt that may happen to get into the duct. The axis of each pipe is slightly curved, and they should be laid so that the convex side is upward, in order that the joints shall interfere as little as possible with drawing in the cables. The advantage of this form of conduit is its simplicity, cheapness, and the fact that any desired number of ducts may be put together; and to avoid obstructions under- ground, the geometrical form of the conduit may be modified by different arrangement of the separate pipes, and it is quite easy to slightly change its direction, so that it may be carried around ob- stacles. UNDERGROUND ELECTRICAL CONDUCTORS. DAA Multiple-duct earthenware conduits are similar in general form and method of laying to the single-duct construction just described. The difference lies in the fact that each unit contains two or more ducts, as represented in Fig. 218. In this way space and the labor of laying are somewhat economized. Multiple-duct conduits are centered with two dowel pins at each joint, and then wrapped with Fig. 219. Joints in Multiple-duct Conduits, a six-inch strip of asphalted burlap, or with a six-inch strip of damp cheese cloth, and then given a coating of cement mortar, as shown in Biss 219, Earthenware Trough Conduits consist of clay troughs, either simple or with partitions, as represented in Fig. 220. The usual dimensions are 3 or 4 inches square for each compartment, with Fig. 220. Earthenware Trough Conduits, walls about 1 inch thick, the sections being 2 to 4 feet long, and weighing about 85 pounds each for the 2-foot four-duct trough shown. To cover the top trough a sheet of mild steel, No. 22 gauge, is bent to fit over the sides to hold it in place, and is cov- ered over with concrete. When the latter has solidified, it acts as aroof to the top layer of ducts, even though the sheet of steel rusts entirely away. PAG ELECTRIC LIGHTING. ‘ Ducts formed in Concrete. A method of constructing a con- duit consists in partly filling a trench with concrete in which continuous longitudinal holes are formed to serve as ducts after the concrete has hardened. One plan is to use collapsible man- drels of wood or metal, which are placed where the ducts are desired and then filled around with concrete. When the concrete has solidified the mandrels are made to collapse, and taken out in pieces. Another means of producing a similar result is to employ tubes made of thin sheet zinc or iron, which are placed in the concrete as it is filled in, and are just strong enough to stand the pressure to which they are sub- jected It istexpected that the thin metal will soon corrode away, but the ducts will remain in the mass of concrete. = SSS — SL ° e nod ZZ =a Wooden Pipe Conduit. — A —S—__ simple and cheap form of. con- Fig. 221. Wooden Pipe Conduit. duit consists of pieces of wood 3 to 6 feet long and 43 inches square, through which a round hole 3 inches in diameter is bored longitudinally. These are laid side by side in layers, as shown in Fig. 221, to form a conduit with any desired number of ducts. At the bottom and top a layer of plank is laid to protect and hold in place the separate pieces. There is a projection at one end of each section which fits into a_ corresponding recess in the next sec- {100 ease lidreatec: sin Fig. 222. This conduit : is often called p ump 4 Fig. 222. Wooden Pipe Conduit. log conduit. Wooden Trough or Box Conduit. — Ducts about 3 inches square are made of horizontal boards and vertical partitions of yellow pine one inch thick. This may be laid in convenient lengths of about 12 feet or may be built along continuously. The wood should = ‘ have been previously treated with creosote or dead oil to preserve it, as described on page 244, and the whole exterior of the conduit is coated with tar. The objection to this or any form of wooden conduit is the UNDERGROUND ELECTRICAL CONDUCTORS. 273 fact that the decay of the wood tends to form acetic acid which attacks the lead sheathing that is usually applied to underground conductors. This produces a white scale, irregular pits, or a white efflorescence on the lead, and is likely to cause much more damage than the dark brown uniform coating which sometimes forms on lead but stops further action. The lead acetate resulting from the first-described action is often decomposed by carbonic oxide chan- ging it to lead carbonate, and setting free the acetic acid which again attacks the lead and so on. The decay of the wood and formation of acetic acid are intended to be prevented by the treatment with creosote or dead oil, but this may not be entirely effective. Wood is good for temporary work, for it will last about 10 or 15 years, and can be easily cut into for changes and repairs or side connections. Fiber Conduit consists of pipes made of wood pulp, having about the same thickness as cast-iron pipe. S/zp joint conduit for electrical subways is 3 inches inside diameter, and has short sock- ets on the ends, one to fit inside the other, keeping the lengths centered, and making it much easier to lay than a mere butt joint. It is laid in cement lke iron pipe. The screw joint pipe will form a tight line, and is used for running underneath the lawns of private houses, or underneath the streets of villages, the importance of which will not warrant the cost of building electric subways. Being used in this manner like iron pipe, it can be cut with a saw or lathe tools. It is said not to corrode nor change in dimensions with varying temperature. Ordinary conduit construction is illustrated in Fig. 223, the conduits in this case being terra-cotta, but may consist of other kinds of pipe. It is hardly necessary to lay conduits below the frost line as they are not likely to be injured by frost. When laid in concrete they should be at least 2 feet below the surface, and when clay pipes are laid bare, 3 feet, to avoid crushing by the weight of the heavy vehicles above. : Edison Tube System is a very good one for distribution, and for extending to new territory section by section, being much more convenient in this respect than conduit and cable. The 7ude consists of one or more conductors contained in and insulated from an iron pipe. In the three-wire system which is in OCs ELECTRICALICHIING - general use, three copper rods are placed in each tube. The system is a sectional one, and each tube is as complete when it leaves the factory as is a rail from the rolling-mill. Like a rail it only needs to be joined to other similar units to become part of a continuous line. These tubes may make as many bends as cir- cumstances call for, while a conduit must run practically straight from manhole to manhole. In the three-wire system of distribution the conductors, whether overhead or underground, are divided into two classes. Feeders Fig. 223. Terra-Cotta Conduit under Construction. which run from the stations to the centers of distribution con- stitute the first class. Jazus radiate from centers of distribution, and loop the ends of the feeders together, constituting the second class. All taps to supply customers with current are taken from the mains. Tubes are therefore divided into Feeders and Mains. A main has three insulated conductors of the same size. A feeder has two principal conductors and a smaller conductor to serve as a neutral wire. A feeder also has three insulated cables of 7 strands UNDERGROUND ELECTRICAL CONDUCTORS. AGS of No. 19 B. W. G. wires each. These small cables form inde- pendent circuits from the station to the point of distribu- tion, and enable the voltage at the outside end of the feeder to be read in the station. Hence these lines are called pressure wires. ce. Mipgaa The conductors are all copper rods, 20 feet, 4 inches long, and project from 2 to 34 inches from each end of the pipe. The pipes are lap-welded steam pipe, of full weight. In’makine Uipeastubesthe ends of the» copper rod are first chamfered and tinned. The pipe is thoroughly cleansed on the inside. Each rod is wound separately with a prepared rope, and the three rods so wound are made into a triangular bundle and - Fig. 224, Construction of Edison Tube. wrapped with a fourth rope. See Fig. 224. This bundle of rods bound with rope is slipped into the pipe. When the rods are in position the pipe is placed on end, and a melted special compound is forced in from the lower end. As the compound rises it dis- places the air, and thus prevents air bubbles. The ends of the pipe are closed with a rubber plug. As soon as the insulating material is cooled the completed section of the pipe is carefully tested, the tube is then painted to preserve the iron from rust, and is ready for shipment. In order to complete the system, there is needed a means of joining the ends of the conductors in the consecutive tubes, and of insulating and protecting such a joint when made. 276 ELE CTRICGUIOATING: In Fig. 225 it will be seen that the two ends of the pipe enter an egg-shaped casting through two water-tight sleeves at either end of the oval. The copper rods forming the conductors are joined by coupling joints consisting of short pieces of flexible cable with sockets cast on each end. ‘These sockets are drilled to fit easily over the rods which the joint is to connect. After the connectors are in place they are thoroughly soldered to the ends of the conductors, thus making an electrical joint. The covering of the egg-shaped casting is bolted down on the lower half, and by means of a small hole on the top of the casting, the whole of the box is filled with melted insulating compound, which surrounds and insulates the copper rods, the joints and the tube ends. This Zo\N IN Fig. 225. poate ae Edison Tubes. compound does not grow brittle on cooling or with age, but remains somewhat plastic even at freezing temperature. The hole in the casting is closed with a cast-iron cap. Coupling-boxes are made also in the form of elbows and tees, to provide for all possible conditions, such as service wires, turns in the line, etc. The ball end that may be attached to the end of the tube, and the socket that it fits into, permit a considerable variation in direction if desired, there being a range of 18 degrees on either side of the mean position. Branches to the consumers’ premises, or as they are commonly called the servvzces, are short lengths of tube which tap the main line by means of a three-way or service box. These boxes are made for right and 45 degree angles. A four-way box readily per- UNDERGROUND ELECTRICAL CONDUCTORS. OTT mits of two services being taken from one joint. A form of box is shown in Fig. 226. Mains are so placed in the ground that the positive and nega- tive conductors are on one side of the vertical plane through the Fig. 226. Branch Connection of Edison Tubes. center of the tube, while the neutral or balancing wire is on the other wmocem ip 221.) Ihesside of (the tube which. the neutral is on is called the inside, because the main tube is so placed that che; neutral ‘copper is nearer to the curb line: ~ The feeders are laid symmetrically, with the right hand conductor as the tube leaves the sta- tion being the posi- tive. Services are never taken from the; fecder-lines: This requires that at some center of distribution the feeder be split up and branched out, or what is the same thing that it be connected up to one or more mains. The feeder enters a distributing-box, and the con- ductors are connected to three copper rings. From these rings, as a source of potential, mains are led out through fuses to supply, by FEEDERS MAINS Fig. 227. Cross-Section of Edison Tubes. 278 ELECTRIC CHTING. means of the service connections, the surrounding district. From the rings of this box the pressure wires return to the station. This box not only serves as a center of distribution, but also as center of equalization of pressure between the different parts of the sys- tem. Fig. 228 shows the arrangement of the distributing-box. In some cases the fuses are replaced by heavy copper strips. Installation of the Tubes. ‘Yhe trench in which the tubes are to be laid should be as a rule 30 inches deep, and 20 inches wide SS SSS srt mh te SoS — Fig. 228. Distributing Box. Edison Tube System. at the bottom, this giving a trench wide enough for two tubes. As is shown in Fig. 229, the main is laid nearer the curb to facili- tate the taking off of taps or services to the houses; such ser- vices running under the sidewalk, and entering the cellars of the houses. If there is only one line of main on a street, it is laid about six inches higher than the feeder, should there be one, in order that services may be taken off for both sides of the street. The system may seem burdened with details; but when it is UNDERGROUND ELECTRICAL CONDUCTORS. 279 considered that the system takes the current from the dynamo, and delivers it to the consumer, that the tube is the equivalent of three ducts in a con- duit and three insu- Z777.X lated cables,andthat 7/7) its flexibility is great, 7 : j we see that it pos- / yy sesses advantages. jj / For many years it was the most impor- tant underground system. Naked Conductor Systems.— There areat least two meth- ods by which bare wires or rods may be operated in underground conduits. One in which the wires are supported on insulators of oO © Fig. 229. Edison Underground Tubes. glass, porcelain, soapstone, etc., in the conduits ; and the other in which the conduit itself is of an insulating material. The former method is used somewhat extensively in Europe, entirely, however, for low pressure electric lighting service. In this use of bare conductors care must be taken to prevent the undue access of moisture into the ducts, and especially to prevent the flooding of the conduits. The Crompton System of bare copper strips has been exten- sively used in England. The conduit, or culvert for a three wire system is shown in Fig. 230, and will be seen to be a trench lined with concrete, and covered with a layer of flag stone. It is usually built under the sidewalks. FP eo80 © Cramnront Conduite The conductors are copper strips 1 to 14 inch wide, and 1} to } inch thick. These strips rest in notches on the top of the porcelain or glass in- sulators, which are carried on an oak timber built into the concrete walls. To prevent the leakage from moisture it is necessary that the number of the supports of the strip be reduced toa minimum. To accomplish this, about every 300 feet there is an enlarged hand- hole for a straining device which takes all the sag out of the CONCRETE 280 ELECTRIC LIGHTING. strips, and makes it possible for them to place the insulators about 50 feet apart, instead of every 10 feet. This device is shown in Fig. 281. The conductors are, when stretched, clamped by the set screws W. The ends of the strips are then joined by the Glan pac. SS SN SX XAQX®GC N Y \RAVEMENT\ J, WX QQ Ww y eT | oa ES S B Es Fists mee ie eer en eine ine el He sere SUCONCRETE 15:22" se eps Se era MSE Sore ENS oy Eke Sass Sar DISS eer nen eee oe CoN Age oy et as - Suet poner. tees Fig. 231. Stretching Device in Crompton System. In order that the insulators may be reached for inspection and for cleansing, a hand-hole is placed over each set of insulators, — that is, about every 50 feet, — and these boxes are utilized for the service connections. Rubber-covered cable is generally used for ae this service, and it is attached to the copper strips. by a clamp like Ceinsiiows2) ls The Kennedy System is a modification of the Crompton, using solid porcelain bench-like sup- ports set directly on the bottom of the conduit, as shown in Fig. 252. The Callender Solid System. In the Callender solid system a series of cast-iron troughs are laid along the bottom of a trench excavated in the street. In the trough the requisite number of Li Fig. 232, Kennedy Conduit. UNDERGROUND ELECTRICAL CONDUCTORS. 281 cables are strung, supported at intervals by insulating pieces fixed in the troughs. This protection is found to be necessary, from the fact that the insulating compound with which the trough is to be filled is never absolutely hard, but behaves like a viscous fluid ; and the cables would otherwise gradually set- tle. The cables are insulated, and with the melted asphalt that is poured into the troughs, makes an expensive in- sulation. A cast-iron cover is placed over the top of the trough. A section of this system is shown in Fig. 2338. Electric Light Cables for use in con- duits are of two classes, according as the insulation is or is not moisture proof. In the first class the insulation is rubber, or bitu- men, and the lead covering is for the protection from chemical and mechanical injuries. The second class is insulated with jute, hemp, or paper impregnated with oil, wax, or resinous compound. Fig. 233. Callender Conduit. The lead covering of this cable is absolutely necessary for its electrical integrity, on account of the hygroscopic nature of the in- sulation. The latter types are much cheaper, but need the test of time to demonstrate their value. Rubber is made durable and the cost reduced by being com- pounded with litharge, French chalk, barytes, etc., which strengthen it mechanically, and render it less liable to decompose. | Vulcanized rubber is now generally used, it being mechanically stronger, more flexible, and capable of standing higher temperatures than pure rubber. The process consists in mixing a small amount of sulphur with the rubber, and subjecting it to a temperature of 250 to 200 degrees F., while keeping it under pressure. Fearing the action of the excess of uncombined sulphur on the copper, the conductor is tinned before the insulation is applied. It will be seen that for a cable of many strands the wires themselves must be tinned. The Hooper process may be applied, consisting in first covering the cable with a layer of pure rubber, and then with a layer of rubber highly pigmented with oxide of zinc, and then to put on the vulcanized rubber. The requisite amount of sulphur can be determined so closely that the excess may be very small, so that this separating layer may not be necessary. 282 ELECTRIC OYGHUIING. The general method of insulating a cable is to first wrap round it one or more layers of pure rubber tape, which are put on spirally ; the direction of the spiral being reversed for each successive layer. On top of this, rubber compound is applied in two or more separate coatings. cach coat being put on by passing the partially formed core with two strips of rubber compound, one above and one below it, between a pair of rollers which fold each strip half around the core, and press the edges of the two strips together so as to make a Fig. 234, Lead-covered Stranded Cable. Fig. 235. Duplex Cable with Fibrous Insulation. UKE DIGG C Ene Sk i) Coe Rubber Compound sear 4 AAs OY, Fig. 2386. Underground Cable. (lip N72 Layers mee,* of Braid agi Rubber Fig. 237. Underg,sund Cable. Fig, 288. Underground Cable. good joint along each side. When a sufficient number of layers of rubber compound have been put on to give the requisite thickness, the core is tightly bound with a spiral wrapping of prepared rubber tape, and then vulcanized. After this it is tested; and if it proves satisfactory then it is taken to the taping and braiding machines, where the external covering of tapes and braiding is put on. Lead covering. These cables are generally incased for their mechanical protection. This may be done by drawing the cable into a lead tube, which is then drawn through a die and made UNDERGROUND ELECTRICAL CONDUCTORS. 283 to fit the core tightly; or the lead cover may be put on in a hydraulic press, the hot lead being forced out through an annular die around the cable. The Siemens cable is one of the second class, the conductor being wrapped with jute, and impregnated with a special bituminous compound mixed with heavy oil, and is then covered with lead. Paper cable. A similar cable is of paper wound on in strips spirally over the conductor; and as each strip is applied, it is passed through a die which presses it into a compact mass. The Gore sis athene diicdpate ae temperatures of 250° FE. to expels the moisture from the paper, and immersed in a bath of compound, from which it passes directly to the lead-covering press. With either of the methods of lead covering by a press, it is difficult to test the soundness of the lead, unless the cable is im- mersed in water for a long time. For this reason some makers prefer to use a manufactured lead tube, which can be tested under pressure to see if it is sound, and then the cable is drawn into it. Figs. 254 to 238 represent the general form of cables. As one type of special cable, Fig. 289 shows a lead-covered, three-conductor cable, such as is being used for three-phase trans- mission of current at 6600 volts, from the 96th-street station to various sub-stations in New York City. The con- ductors are each equivalent to a No. 0000 A. W. G. wire, and are composed of thirty-seven strands of tinned Lake copper. Around each conductor is a Hooper core containing no sulphur ; the total imsulating INSULATION’ —JUTE FILLING INSULATION : te STRANDED wall around each conductor is conbucTGR #3 of an inch. Together with Fig. 239. Three-phase 6600 Volt Cable. the jute fillers these conduc- tors are twisted with a lay of about 20 inches.. The whole is then covered with a second insulating wall of 34; inches’ thickness. Lead covering 1 inch thick is then put on. The covering is an alloy of lead and tin, the percentage of tin being 21 to 8. The total diameter of this cable is 23 inches. 284 ELECTRIC LIGHTING. This torm of insulation is popularly called sp/zt znsulation for the reason that it is substituted for a cable having 4$ of an inch around each conductor, and none around the bunch of three. Should the latter form of cable be used, it would perhaps be one quarter inch greater in diameter, and would cost at least 25 per cent more. The “split insulation,” it will be seen, offers the thickness (4$) of insu- lation needed between conductors, and the same thickness between conductors and the lead sheath as in the old style. Each individual conductor before assembling is tested, after d 24 hours’ immersion in water, with a break-down test of 15,000 volts, sustained for an hour, after which the insulation resistance must measure 500 megohms per mile. When the cable is com- pleted, and laid in the ground, a break-down test of 20,000 volts is applied for an hour, and at the expiration of this test the insulation resistance must measure 1000 megohms per mile. Concentric Cables. The Ferranti mains are concentric cables made in rigid lengths of 20 feet, and a cone sleeve is used to make the connections. The main consists of two copper tubes, one en- tirely within the other; they are insulated from each other by brown paper steeped in black wax, the outer tube being covered with the same material, and the whole inclosed in an iron tube. A common form is that of an inner stranded con- ductor, and an outer con- ductor formed by a layer of spirally-wound wires. The jointing of these cables is made by upsetting the outer wires and clamping them between washers, and connecting the washers by a strip of sheet copper ; Fig. 240. Concentric Cable. the inner conductors can be soldered, or clamped and then soldered. This kind of cable is shown in Fig. 240. Even if a straight soldered inner and outer joint is made, the joint is so bulky that a coupling-box is clamped on. It is provided ONDEXRGKOGND ELECTRICAL ‘CONDUCTORS. 285 with rubber washers at a, or these chambers are filled with asphalt, to make them water-tight, and the main chamber filled with compound. Such a box and joint are shown in Fig. 241. There is this difficulty in the use of concentric cables, that it is not possible to ascertain the condition of the insulation between the two conductors without cutting out the dynamo, transformer, Fig. 241. Concentric Cable Joints. or other apparatus in the circuit ; and therefore the insulation can- not be tested while the circuit is working. This is a serious dis- advantage, as with separate cables a continual test may be kept on the circuit, which will often give warning before the fault is suff- ciently developed to prevent the circuit being worked. Thus an opportunity is given to localize the fault and repair before any inter- ruption of the lighting takes place. ———Joints in Cables. — Joints in solid conductors are generally made by scarfing the ends and soldering; then the joint is tightly wrapped with a serving of copper wire, or a split sleeve put on, the joint being again soldered. The flux must contain no acid. Stranded cables are made solid by dipping the ends in solder, and treated as above. 286 ELECTRIC. TIGHTING. In multi-conductor cables the jointing. is more difficult on account of the lack of space, and the necessity of getting the insulation between the conductors. The joint of each conductor is insulated by rubber and com- pound tape to the desired thickness. The lead covering is replaced by a piece of pipe previously slipped on the cable, being soldered to the lead sheath, or a wiped joint may be applied. The object in both cases is to get a water- tight joint. In some cables, after this lead is in place, holes are punched in it, and hot compound poured into the interstices which have been left. The holes are then soldered up. Manholes. — The various systems in which the conductors are drawn into iron, earthenware, or wooden conduits usually require manholes to be located not more than 200 or ee eile om sere ae - an ats 300 feet apart, the cables Z£—____ SS - ITELLEEELEEEEZEZEEE__TTEZ_E 3 PAY ul being pulled from one to , | | | I Wk) ihe vother=by aieatis= Olt Se = za | | | wl rope. These manholes ordinarily consist of Mi aT chambers of brickwork | il il il of the general form rep- ) ea in Fig. 242. . y= a @ They should be provided “|| Go a9 with two covers,- the lower one being screwed | Mi i Tun Ht ih A | | — down on a rubber gasket Ke so as to be water-tight, and the upper one rest- ing hee a its place, itl Mild Zi V/ U/l BiH being, Aretheanthithe Fig. 242, Cross-section of Manhole. surface of the street, as indicated. According to the kind and sizes of the duct, the form of the manhole will change slightly. Fig. 243 shows a manhole used in connection with vitrified multiduct conduit. Fig. 244 will give a good idea of the interior of a manhole looking down into it from the stecet: {n dry or porous soils the bottom of the manhole is some times UNDARGROCND ELECTRICAL CONDUCTORS. 231 = wy Pry RELI EPL LLM = me LLL LL MLL ne a Geers mena era os WHtuW TAIN YZ. ANZ WA \] Ea Gs TT NENA ZEN NYA. YO: WAAL ee dd ee SE A Vid elegans [ee] oe eee) CT UP UT Kx. VEL ONC) eA ee ee ed BZ oto YHAPARADTOI ADT SR Lae See ai BAZ MLL. LL, Wh LLL Ae ae | ae | a | | a | | ee eek | ee [eee | eee [eae | eel eel | 7 LAH r LMI Whee Qa Oe DES eS ee eee ee eee enue) y 8 a I. AN OL Hi ee OLA L111 peo OOOOlE A 7/7 oo ///// EK Gavooooooooooo = HIS LITE roaeemes ea | oOwook€ [CO eres SSS ———— = En . Soe Se re Bommmncee|| i OoIWooIe i Sais Pana es a ae a ) aaoaare Ss ——————— Sse GY ey VA ZAG ieee eee ee ee TWA Y ea Ao “is Wom t+ —T PIES Z2 Yj ~ ay Woon Eig. 243. Brick and Iron Manhole, 288 “ FLECTRIC DIGHIING. - left out so as to drain it; when this is not feasible a sewer con- nection is make to accomplish the same result. The typical manhole adopted for the New York subways is shown in Fig. 245, being drained by a pipe P leading to the sewer. COE ps Hy AM Fig. 244, View into a Manhole. Fig. 246 is a view of the iron-pipe conduit used extensively in the city, showing a service box in place, being practically a small manhole with side connections for the buildings. Fig. 247 shows such a box. When the number of the ducts is not large, LIGA ‘© O asin local distribution, Z St Ne ES ei QU: the manholes and ser- vice. boxes become mere hand-holes. Drawing in the Conductors. — Since a duct is entirely empty when built, and is usually from 200 to 800 feet long between manholes, it is necessary to employ some special means of getting ESSAY Fig. 245. New York Subway Manhole, yy y /; Uy Wy ven DB L{ EZ: A if Z, IRN z, is 2; AS UNDERGROUND ELECTRICAL CONDUCTORS nay Va i t f A Yi Wy Fig. 246. Iron Pipe Conduit. Ne \\ = i) ZA Te. BS e . Sars Wags Se ig ’ p - Hi s - - . 7 7 - a BILE 7 RS é 2 eS RNY é Lg SS = ore GL wees AAS na Lies oy = ge (ea el eee ‘7 RN 4S, 7 25S SS e LOI SS RN AS “Le SS Ke, 4 ‘ RNY ea Z _ ese ea PER RN : A Fig. 247. Service Box for Terra Cotta Conduit. x y/ Ul his yy, Uiiy Yi 289 290 ELECTRIC LIGHTING. a rope through it in order to draw in the cable. One plan is simply to push through a steel tape or wire, which is provided with a rounded metal head to prevent the end from catching in the joints between the pipes. By means of this tape or wire, a small and then a large rope may be pulled through the duct. Another method is the so-called “rodding” of the ducts, a 20-minute ope- ration, consisting in inserting one after another into a duct, short rods of wood or steel, about 3 or 4 feet long and ? inch in diameter, which are connected together by screw or bayonet joints,as indicated in Fig. 248. When a sufficient number have been joined to reach from, onesmanhole to the next 4 small rope is attached to a ring in the last one, and the rods are then pulled through from the other end, being unjointed as they come out. A larger rope may then be drawn through, after which a steel scraper and a brush should be pulled through the duct in order to clean it, and remove any stones, tools, etc., that are often found in it and would be very likely to injure the cable. These are illustrated in Fig. 249. After the duct has been cleaned prop- erly, the end'of the cablevig atrached ito qu isa mee tt aula heavy rope which has been drawn through Fé the duct by attaching it to the cleaning implements. In attaching the cable to the rope, care should be exercised to avoid bringing undue strain on the copper conductor or its insulation during the operation of drawing in the cable. This may be done by putting a conical metallic head on the end of the cable, or by winding several iron wires spirally around the last foot or two of the cable, and form- ing these into a loop to which the rope is attached. For hard pulls and curved pipes the end of the cable is served, after removing about 18 inches of the lead and the insulation. The strands of the cable are then fanned out, and divided into four groups, and passed through a shackle as shown in Fig. 250; they are bent back on themselves, and bound tightly with spun yarn or wire. If the pull is to be extra hard, an iron wire may be also put through the ONDERGCRKOOUND LLECTRICAL CONDUCTORS. 291 shackle, and driven through the lead sheath. This gives an excel- lent hold on the cable, distributing the strain over all the conduc- tors, as well as to the lead covering. SHEATH ‘YARN SERVING Fig. 250. Attachment for Cable to Rope. Having attached the cable through a swivel to a strong rope, it is drawn into the duct by means of an apparatus shown in Fig. 251. The cable unwinds from the drum D, as it is drawn by the winch W. It is evident that the cable should be somewhat smaller than ®0000000 Fig. 251. Winch for “ Drawing in” Cable. the duct through which it is drawn, but the margin need not be great. For example,.cables 23 inch in diameter can be drawn through the standard 38-inch ducts. When several cables are to be put in the same duct it is much better to draw them all in at the same time; but it is possible to draw one into a duct already containing others, provided there is space enough. It is also pos- sible to withdraw one or several cables from a duct without serious injury to them, in case repair or change becomes necessary. Methods of Distribution from underground conduits constitute one of the most serious problems in connection with them. The main or trunk lines may be provided for by the various forms of conduit that have been described, but these do not readily allow branch connections to be made at frequent intervals to supply indi- vidual buildings. It is decidedly objectionable to complicate and 292 ELECTRICWLIGEHTING: - weaken the construction of the main conduits by having side con- nections, so that the best plan is to keep those intact, and provide a subsidiary duct or conduit into which the conductors required for local distribution are run at one of the manholes. Such an arrange- ment is shown in Fig. 252. ) = Ny = ee a A (NN Wi! Fig. 252, Hand-hole Distribution. In Figs. 253 and 254 are shown iron subsidiary service boxes for use in connection with terra-cotta conduits. The Edison system of distribution is well shown in Fig. 255 which illustrates also a distributing box and service box. Fig. 253. Iron Service Box for Pipe Conduit. The house vault, back yard, lamp-post, housetop, hand-hole methods of distribution are easily seen to refer to the way by which the service cable passes from the subway to the building. Street arc lamps may be supplied as indicated in Fig. 256. UNDERGROUND ELECTRICAL CONDUCTORS 293 In Baltimore there is a complete system of distribution, shown in Fig. 257, and consisting of two separate parts Fig. 254. lron Service Box for Terra-Cotta Conduit \\ ar alt |= a wank ===} fee TD be C= . =) Z: ZA i ‘€ IQ ZS py aan mM 4 ‘ Fig. 255. Edison System of Distribution For high-tension distribution there is a wrought-iron pipe duct with service boxes to transformer pits, consumers, etc., being en- tirely a subway, and entering buildings through cellars. 4 294 ELECTRICUYCGHIING For low-tension mains there is a conduit system of wrought- iron tubes, cement-lined tubes, or terra-cotta duct, leading to pole terminals situated on each block, and thence overhead to consumers. VENTILATION OF UNDERGROUND CONDUIT SYSTEMS. Considerable difficulty has been experienced in satisfactorily ventilating subways carrying electric light and power conductors. When these systems were first introduced they were made as nearly as possible air-tight and the covers of manholes hermetically QSSRQQAAYRAYA A WF yy \ A My), /} // \ QA Yj 1, RRA SY NS KY ~ Vi Yj SIL —w. ES So => SS QS \ . Ne \ \ x i \ Xs SAX WS — he Fig. 256. Lamp-post Connection. sealed. The porosity of the material forming the walls, etc., defeated this end, and allowed gas to enter the different parts of the system where it mingled with air. The blowing of a fuse or a spark due to some other cause would often ignite this mixture of air and gas, and destructive explosions sometimes resulted. To avoid this danger pumping equipments were arranged to re- move the air from the subways. This at first seemed effective, as it tended to keep the air in the subway. in motion, but it also in- duced gas to enter, due to the partial vactium caused. After abandoning this scheme a new arrangement was introduced known as the blower system. This was based on the idea that if UNDERGROUND ELECTRICAL CONDUCTORS. 295 an air pressure was maintained in the subway it would prevent out- side gas from entering. The fallacy of this hypothesis was soon evident, owing to the law that if two gases are separated by a porous diaphragm, even though greater pressure is maintained on one side SAAR je AA Fig. 257. A Complete System of Distribution. ey ea than the other, the gases will still mix. The brick walls in the subway acted in this case as the diaphragm ; and explosions still occurred after the introduction of this ventilating system, although of less frequency and violence. Tests showed, moreover, that the 296 FELEGTRICSLEIGILING - blower system produces practically no pressure except within a radius of a few blocks from the station. It was then decided to employ gangs of men who would visit various manholes, remove the covers, and let the subway at these points become ventilated. Owing to the excessive cost of these methods they were finally abandoned. The ventilation of manhole covers was then considered. The chief objection to ventilating covers was that they would allow the subway to fill up with water and dirt, and thus cause trouble. This proved, however, not to be the case in practice ; for late experiments show that the dirt and water accumulated in very small amounts, and that only a small force of men was needed to remove such accumulations. The amount of gas which collects in the conduits proves relatively much less than with the blower system, and the total saving is very considerable. The escape of gas from the gas mains still constitutes a source of much annoyance and danger, and even at the present time the precautions above mentioned have not proved sufficient to entirely eliminate the dangers of explosions. For further information regarding Underground Electrical Con- ductors reference may be made to the following works : Electric Transmission of Energy by A. V. Abbott, Second Edition, N.Y., 1899, Electric Distribution by Kilgour, Swan.and Biggs, London, 1893. Electric Light Cables by S. A. Russell, London and N. Y., 1892. Localisation of Faults in Electric Light Mains by F.C. Raphael, London andoNa ys. L898: Ligues et Transmissions Electrigues par Weiller et Vivarez, Paris, 1892. Peon RLECTRIC CRC 297 CHAPTER XIV. THE ELECTRIC ARC. Definition. — The electric arc is the phenomenon of light and heat occurring when an electric current persists in maintaining itself across an opening made in its circuit. When of short duration and of disruptive character it is known as a spark; the term arc being used to designate a continued discharge across a bridge of conducting vapor. History. — The spark was first observed by Volta in 1800, in which year, too, Sir Humphry Davy discovered the particularly bright spark between charcoal points separated in air or under liquids, and exhibited it before the Royal Institution with the aid of a battery of 150 elements. It was not until 1808 that Davy, with a battery of 2000 elements, was able to exhibit the first true arc, an extended flame nearly four inches long, before the Royal {nstitution. This discharge was maintained between horizontal charcoal points, and owing to the current of heated air which is created, assumed a bow or arch shape; hence its name of “arc.” The intense brilliancy and whiteness of this light resulted in wide-spread efforts to utilize it practically ; and numerous improve- ments followed, chief of which was Foucault’s introduction in 1843 of gas-coke carbons to replace those of charcoal hitherto used. Another early step in advance was Grove’s use of the salts of sodium and potassium to steady and increase the length of the arc. The application of the arc to practical purposes was often at- tempted, but the cost of electrical energy generated by a primary battery is so high that no commercial success was accomplished until the dynamo had been developed.* General Features of Arc.— An arc may be maintained by either direct or alternating current. Under ordinary circumstances the * Vol. L., pp. 8-17. 298 ELECTRIC TIGHIING. - two electrodes must be brought together before being separated to establish an arc, otherwise several thousand volts pressure would be required to strike across the air-gap in the first place. As soon as the separation of the terminals commences, the spark, which tends to form at any break in a circuit, vaporizes a portion of the material of the electrodes, thus establishing a bridge of conduct- ing vapor through which the current flow is maintained. The concentration of energy in a small space produces an intense heat, which vaporizes the electrodes rapidly, so that a highly refractory terminal must be employed to avoid its rapid consumption. More- over, the intensity of light given out by the arc depends on the temperature to which the electrodes can be heated without being vaporized, therefore a highly refractory substance like carbon best fulfills the requirements. Appearance. When an arc is sprung between two carbon rods placed vertically one over the other, and kept about one-eighth of an inch apart, a constant current of about 5 to 15 amperes will produce a stationary condition after a few minutes burning. If observed through smoked glass, or, better still, if the image of the arc and carbons be projected upon a white screen by a lens, it will be seen that both carbons tend to become bluntly pointed, because oxidized away by the heat ; but the positive carbon, which is usually the upper on account of its emitting more light, will have a hollow, or “crater,” at the tip. This may be .04 inch deep and 2. inch across under average conditions. This is the hottest and most luminous portion of the car- bons, attaining a temperature of approximately 8500° C. as Violle proved, breaking it off and dropping it into a water calorimeter. The in- tense heat thus generated can be realized when the melting point of platinum is considered, which is 1,775° C. The negative electrode ex- hibits no tendency to become hollowed, and remains pointed. In fact, the carbon particles burned from the tip of the positive car- bon tend to deposit in the shape of a point or nib on the negative carbon, which is much cooler and less luminous than the positive. oes yi ays 258. Appearance of Arc Light Carbons. THAME LECTRIC ARC. 299 Both carbons appear luminous some distance away from the tips, this being especially noticeable on the positive. If the carbons contain impurities, these may generally be seen in beads near the tips, to which they often work their way to be instantly volatilized. Between the carbon points is the arc stream proper, which assumes a bow shape even when the carbons are vertical, owing to the magnetic action of the earth’s lines of force on the current. The inner portion of the arc stream consists of a violet hub, probably of incandescent carbon vapor, surrounded by a thin non-luminous portion where the carbon combines with the oxygen of the atmo- sphere in dark flame to form carbon monoxide (CO). This is enveloped in turn by a layer of luminous flame in which the carbon monoxide burns to carbon dioxide (CO,). The magnified image of an arc on a screen will show occasional carbon particles flying from the positive (which on the screen seems to be the lower carbon) to the negative, while other particles are thrown off into space by the action of the heated air. Noise. Under favorable conditions the arc is perfectly quiet, but emits a hissing sound like frying if the current exceeds the proper value for the length of arc employed. Odor. A distinct odor is noticeable close to the arc, especially in damp weather, probably due to the presence in small quantities of hydrocyanic acid gas. Besides this, carbon monoxide and diox- ide as well as nitric oxide are usually present ; but none of these gases is given off in sufficient quantity to be injurious where the voltage of the arc does not rise above 505 in air. After some minutes burning, it will be observed that both carbons waste away, the positive as a rule being consumed in the open direct current arc about twice as fast as the negative, the ratio depending on various conditions. For this reason the car- bons must be fed together by hand or by some automatic device, otherwise the length of the gap would increase until its resistance exceeded the power of the generating apparatus to maintain a current through it. Physics of the Arc.— Under commercial conditions direct current open arcs usually consume about 10 amperes at 46 volts or 450 watts. Thus nearly one-half k.w. of energy is con- centrated in heating up the small extent of the crater and arc, re- sulting in the production of the very high temperature of 3500° C, 300 ELECTRICULIGH LIANG taken to be the boiling-point of carbon. That carbon is vola- tilized in the arc is undoubtedly a fact. The surface of the crater has the appearance of boiling; the hissing noise occurring with excessive current density is similar to that produced by violent boiling of water, and may result from the same cause, though carbon, like arsenic, vaporizes directly from the solid state. Car- bon consumption goes on in a vacuum, although at a slower rate than in air, and the vapor thus formed condenses on the sides of the inclosing chamber. These facts all go to show that carbon is actually evaporated. Such being the case, the temperature of the surface of the carbon would naturally remain stationary at this boiling-point, like the temperature of boiling water at atmospheric pressure, whatever the heat applied. The temperature of the negative carbon, except at its extreme point, is considerably lower than that of the positive: ” The difference 1s, due*to thesfach nat the larger part of the energy is transformed into heat at or near the surface of the positive carbon. This is evident from the rela- tive appearance of the two electrodes and is demonstrated experi- mentally by measuring the distribution of potential between the carbons. The most reliable observations show that about 40 volts drop occurs between the positive and the arc stream, with only 21 volts in the stream and 23 volts between the stream and the negative carbon. The temperature of the space between the carbons may be much higher than that of the surface in the same way that steam can be superheated above the point at which it is evaporated, there being, in fact, no limit to the possible rise in temperature. Since the current is conducted by the highly heated vapor present, it is to be expected that such a conductor will be heated by the passage of a current the same as a solid or a liquid. Theory. — It is evident that the amount of carbon vaporized at the positive crater forming the arc stream will vary with the current, therefore the resistance of the arc, which varies inversely with its cross-section, varies inversely with the current. In this respect the arc is totally unlike solid or liquid conductors, whose resistance is independent of the current, other conditions remain- ing the same. Hence Ohm’s law in its general form is inapplica- ble to the arc stream. The fact that the phenomena at the arc are more or less rever- THE ELECTRIC ARC. 301 sible, since the vaporized carbon can again be converted into the solid state by condensation, points to the existence of a counter- electro-motive force, and since the temperature of the vaporization “is constant, or nearly so, the counter electro-motive force should also be constant, which appears to be the case. Physicists have long sought to isolate and determine this experimentally, and it would seem that such a definite physical problem could easily be solved ; but there are peculiar difficulties, which up to the present time have rendered all methods and results questionable. There are great difficulties connected with retaining the arc, whose car- bons are constantly changing, at a constant condition, and a long time is required to permit the arc to assume a stationary state. Further, the depth of the crater, and consequently the true length of the arc, is very hard to measure at any given moment. Again, the resistance varies with the length of the arc and in some inverse ratio with the current. Add to this the difficulty of securing pure carbons whose density, electrical conductivity, and heat conductivity are uniform throughout, and the utter impossi- bility of retaining the counter-electro-motive force after the cur- rent which induces it has ceased to flow, and the difficulties be- come more apparent. By indirect methods an approximate value of 85 to 391 volts has been arrived at for arcs of 10 amperes and 45 volts and pure carbons. The indications point to a counter- electro-motive force at the arc, variable with the current and other conditions. In fact, it is very likely that it consists of a combina- tion of two or perhaps more separate electro-motive forces ; one ~ due to the volatilization of the carbon, another due to the thermo- electric effect at the positive carbon, and perhaps still another been said, it seems probable that whatever tends to raise the boil- ing point of carbon will likewise raise the voltage required to maintain an arc, a conclusion confirmed by experiment. Increase of atmospheric pressure, other conditions being constant, increases the arc voltage. Similarly we should be able to reduce the voltage by lowering the vaporizing point of the crater, an effect which is found to result when more volatile substances, such as the salts of the earth metals, are introduced, usually in the form of a core. Resistance. — The resistance of an arc, like that of any other 302 ELECTRIC LIGHTING. conductor, increases with its actual length, and diminishes with its cross-section. The length of an arc usually given is the apparent length ; that is, the distance from the edge of the crater to the tip of the negative. The true length is, of course, the distance from the dottom of the crater to the tip of the negative. A case may easily be imagined where, owing to the varying depth of the crater, the apparent length might be diminished yet the actual length increased. Failure to distinguish between these two is apt to result in misleading conclusions. The cross-section of the arc varies at different points between the carbons, since it has a ten- dency to spread out from electric repulsion, which causes its section to be greatest about midway between the carbon points. The arc stream tends to spread out farther as the carbons are drawn apart. The area of the crater, which is, of course, one end of the arc stream, has been found by Ayrton to vary approxi- mately according to this law: D = .128 x .15 A; where D is the diameter in inches, and A is the current expressed in amperes. The resistance of the 10-ampere arc is 74 to 4 an ohm for arc lengths from about ~, to an g of an inch in length. Houston and Kennelly give 5 ohms per inch as a rough general value. Carbons. — There are two classes of carbons used in arc light- ing, solid and cored. They may be of any diameter. For the sizes usually employed the average resistance is 0.15 ohms per foot. Solid carbons vary according to their purity, molecular struc- ture, and hardness. Coved carbons are solid except for a hole running axially through the carbon, filled with some material more soft and volatile than the remaining carbon — being usually a mix- ture of carbon and some metallic salt. Object of Core. The object of this core is first to decrease the voltage for a given length of arc, as already explained, or to increase the length for a given voltage. This has the initial effect of reducing any irregularity in carbons or the feeding mechanism to a less percentage of the whole length. Further, the core, by afford- ing a plentiful supply of vapor, tends to maintain a stable condi- tion of the arc. It also keeps the are located in one spot, and prevents the tendency to travel irregularly around the carbon due to the arc seeking the path of least resistance. When this traveling occurs it gives rise to an objectionable flicker, owing to the shadow THE ELECTRIC. ARC. 303 of the carbons being shifted in different directions, and to the vari- ations of energy which occur faster than the mechanism can follow. A core may be employed also to modify the color of the light, as for instance to produce a yellowish tinge due to the well-known sodium flame. With these facts in mind, we can explain many phenomena found in arcs as used in practice. Carbon Consumption. — With similar carbons placed vertically one over the other, the relative consumption will depend on the amount carried off by : — Volatilization and electrolytic action. Oxidation of the air. Mechanical disintegration by air currents. When carbons of different diameters are used, their life in- creases roughly in proportion to their sectional area, barring the oxidation of the air. The latter is frequently reduced, and the conductivity of the carbon increased, by plating about nine-tenths of their cylindrical surface with a thin layer of copper, leaving the tip uncoated; but the primary object of the plating is to reduce the contact resistance of the carbon. Volts and Amperes. — The volts and amperes required depend greatly upon circumstances; but for the open arcs usually employed, the amperes range from six to ten, and the volts from forty-two to fifty-two: a common value would be 47 volts and 9.6 amperes. In search-light projectors much heavier currents are frequently employed, from 50 to 150 or 200 amperes, with voltages from 48 to 53. With these heavy currents, the carbons become hotter, and are oxidized farther back from the ends, resulting in longer points. Physical Phenomena. — The positive carbon wastes away elec- trolytically inside of the crater, and by the action of the air outside of the crater, causing it to waste away about twice as rapidly as the negative in the open arc. The negative carbon is consumed by oxidation of the air alone, according as its temperature is increased by the carbon particles deposited on it, and by the heat reverber- ated from the positive crater. The closer the positive approaches the negative, the greater will be its roasting effect on the latter. With very short arcs, the deposit of graphitic carbon upon the negative accumulates faster than it wastes away, so that it forms a nib or second point on the top of the negative which finally 304 ELECTRIC LIGHTING, - crumbles away. This action may or may not be accompanied by a hissing noise. Carbon turns to Graphite. Carbon that has been exposed to the heat of the arc turns to graphite. The hard pencils of solid carbon used for high-tension lamps will not mark paper before being used. After having been burned a few minutes the tip of the negative will write black like a pencil, and even the point of the positive will show some graphite. Electrical Relations. — With both solid and cored carbons a point may be reached when the voltage will be constant if the arc length and current are kept the same. ‘This is called the stationary state. In Fig. 259 the relation of the voltage to pe eat fi H Sr itiitiiitieecitiil i [| | 5b zB nae 2'364-in ~ = ~ 1.970-in : os i 1.576_in IN _ 0:788-in! — 10.394-in Zs 0:276-in |_| 0.197in | i 4 C1 , CI 20 4 6 8 HP 121 14 761 iso feet re 24 26 ada i Coo Sige eID a SIRI RI Fig. 259. Voltage and Current with Cored Carbons. current is plotted for several apparent arc lengths with both carbons cored. It will be noted that with short arcs, less than zs inch long, the voltage rises as the current increases, due to the increased CR drop. With arcs longer than , inch, on the other hand, the voltage falls with increasing current, due to the expansion of the long arc, whose larger cross-section more than compensates for the drop caused by increasing current. This, at one time, gave rise to a theory that the arc had a negative resist- ance, an entirely unwarranted conclusion. For arcs of 45 inch, the spreading action of the stream exactly counterbalances the increase current ; so that for this arc length the voltage remains constant within wide fluctuations of current. In arcs shorter than ss inch the stream has no room to spread laterally. Another reason for CHENELECTRICOARC 305 the lower voltage with increased current shown by the curves of longer arcs is that the current increases while the cooling surface does not, so that less energy, and consequently less voltage, are required to maintain the arc at the same temperature. Effect of Cored Carbons. The same relations are shown in Figs. 260 and 261, except that the results are for a cored and | l Bl t ] a im | | |7Q_Volt TH | aI [ 5 “HPAES pecgbeaee eb ee [EESE R HH | 4 = ie] cH Pe ro Cee | am leet toHH FH mim tas 60 3 r Loar) SaaSRSen L Z, t 3 C 10> a 4 4 —— Be ae 77. | | Q oom a ‘ie | Co op = | oe | 50_ rage | HERES in eee reer rte cH EH 32192 in — i i ‘| re CCT + | [ fe r mistics rales lye [| TEA | al 2A Se cS “970 in: 1 3 OF IN T niaintal i (+9 ao a ~-Hissing Arcs E82 6[ (ae se i eS ; = se 1 vf g cA t 118 2rin. ri igh \ 0:788-in; [ a 0.394-in. } eat Beem EEE EEE - Erte 0.497-in- Lat iasavcttagsfasacaneatafazcstassfesasasoatascesfatatafotatere SI 9 a Nd a ag ag a aa 4 6 8 oe el ett ist eet tt oot aa tt 26 Tie p Fig. 260. Voltage and Current with Solid and Cored Carbons. 1185-V; & | a Ete a H ECC Eee rt u ie T 80 Ty | } a (aa) a 1 ct a 75 7 rH a - 5 t | 70- ct il t | ey 2) iit Stitt: : | i Eee roo e = 2.7581, | aa 60-1! SLE 7 2.364 c “in t t “PR Ts N 55 26047 Per 9 B 576 fi | S ] [atei be all a7 i = a : 0-788 jn. is CAC ae 894-ins N ig SS! My | | |Hissing Arcs i ia | | [| \ f | oo | am lah as [ | | | ona ue | ale HHH | 0 rp 4 10 2 14 6 181120. Pram [24 1196 28 1130 Amp. Fig, 261. Voltage and Current with Solid Carbons. 306 ELECTRIC LIGHTING. solid, and for two solid carbons respectively. The former show conditions approaching those found when using both carbons cored. But beyond a certain current strength all the arcs pass through a condition of unstable equilibrium, and no length can be found where the voltage will remain constant for all currents, plainly demonstrating one of the advantages of using cored carbons. When both carbons are solid no length of arc gives even approxi- mately constant voltage with varying current and a quiet arc. The constant voltage beyond the unstable condition is for Azssing arcs. With cored carbons the voltage is from 3 to 6 volts less than with solid carbons, owing to the greater volatility of the electrode. Resistance of the Arc. —If the current is kept constant the resistance of any arc zucreases with its /ength. With solid carbons. the ratio is a linear one as shown by Fig. 262, and nearly so for cored carbons as given in Fig. 263. | | : 4_Amp. 18-Ohms 16 ne + oe | T ih a | 14 eee at qr isle F cli ed ae = a ae 12 6. 1-0 L 8 & EB 5 Pu re 10 72 14 4 0-39 04788 1-182 17576 1:970 2:364 2:758-inches Fig. 262. Resistance for Different Arc Lengths with Solid Carbons. THEO ELECTRIC ARC. SOW As explained before, if the current is variable, the resistance of the arc stream proper varies inversely with the current. There- fore, the apparent resistance of the arc, which is the quotient of the volts and amperes, may be expressed by the formula, Aa x al, where x is some quantity varying inversely with the current and a@ is a constant. Multiplying both sides by the current / we have 10k a (PA AUE IR = E and x/ is composed of a term varying inversely with the current and one directly proportional to it, so that we may sub- + r 20-Ohms 7-8 ahaa 4 4-Amp 16 i 14. T = i! —- 3} — 7-2 | | | | 1-0 Aa, i) 7; H 1 mI ara -8: : 1 mee mt a) 6. | 0 : 15. 20 2. Pe, tls | | (4) 0.394 0-788 17182 TA ae 2.364 27758 37152-inches Fig. 263. Resistance for Different Arc Lengths with Cored Carbons. 308 ELECTRICMLICHA NG stitute a constant # for it. We may also substitute a value x for the product a/, so that the voltage E at the arc is E=m-+ uni. The most probable values of these quantities for good solid carbons seem to be those obtained by Duncan and Rowland for good pure carbons, namely, # = 40.6 and x = 40, where 7 is the length of the arc expressed in inches. Watts at Arc.—If the current is kept constant the watts increase in a linear ratio with increase of arc length as shown by Fig. 264. If the arc length is constant and the current increases, the watts will vary in a similar manner, as shown by Fig. 265. Hissing Arc. — When an arc is shortened, or its current in- creased until it hisses, the voltage drops 10 to 20 volts, and stays constant even when the current varies greatly (Figs. 260 and 261), for which no satisfactory explanation has been afforded. Photometry of the Arc. — The chief source of light in the arc is the intensely heated crater, which gives about 85 per cent of the total light. The arc proper, or flame between the electrodes, is almost non-luminous, giving only about 5 per cent, while the tip of the negative carbon gives about 10 per cent. Owing to the form and arrangement of the carbons, as shown in Fig. 258, most of the light is thrown down when the positive carbon is above, as it usually is. The exact distribution varies with the current, carbons, and other conditions ; but the general distribution of light from a con- tinuous current arc is shown in Fig. 266. The lengths of lines drawn from the arc to points on this curve represent the relative candle-power at different angles. It is evident from this diagram that it is possible to obtain various values for the candle-power of the arc according to how the measurement is made. As a matter of fact, candle-power is actually measured in four different ways : Candle-Powers. — 1. The mean horizontal candle-power, usu- ally the smallest of the four, being the average in all directions in a horizontal plane. 2. The mean hemispherical candle-power, usually greater than the last, which is the average obtained by making measurements in all directions and angles below the horizontal, showing the average value of the illumination thrown downwards. 3. The mean spherical candle-power, determined in a similar THE ELECTRIC ARC. 309 850-Watt &00 | ft Las SSeRe8ne88 [ as Nome aa) T | if Lr) | 7] i 700 [| ys | 1 A | | we, 650 | | 135 + i pity 600 ab eas t Law 550 \\ iZ = NOE 500_| Ni = T Too | LH ot we 7 oh 450 Ht ray 40 “a Tarn Cl C 1 =I a alae is 350 ame: a 7 bak 6 AL = an 300 g AME el 250 a Ame 200 g Ame : | | 150 +—t Ech Asi ela = a a eg ee af } 700__L0 0.3941 10.788 _|17.182(|1.576_|1.970\|_|.2.364_| 2.758 in. l mene & is Fig. 264. Power consumed for Different Arc Lengths. 510 ELECLRLCMLIG I Lay way, but the mean of measurements at all angles above and below the horizontal. This gives the true average candle-power of the arc in all directions. if TT | f 900_Watt i | + KEEH +H ] | HH | | | | | FH “| 7 eae 4 eaneeee 9-75 8-hn 800 ror i 1-970 .in 7002 PEPER EEE CEE eee Ht | rt aha PEEEEEE EEE : Cc ht 1-182) in SaeeuEe ! Ht HHH F a : 600-1 (2S SRSsooseaess Soe SSSES58 0:394-in 4, = }— ba bet ed Lt = | Ua SS ae = Si = ge C1 SuGeeRauRE aa t + 500-4 rH aan Seeeeee sa = saad fosidfosstosstfasrittoee seats ; I | | |_| bl _ aoe venue HH HH FEEEEH EEE td Peete H 300 A is | a +f 1] hehe | Bis FLEE reo 1001 H 1 4 i z + + 4 Seceeee PEATE ere te ae ee oe Coe og er 19 oy 814 Am, Fig. 265. Power consumed for Different Current Strengths, 180 170 160 150 140 Fig. 266. Light Distribution, Direct Current Open Are. 4, The maximum candle-power found by making observations in all directions to ascertain the greatest candle-power. This is usually found at an angle of 40 degrees below the horizontal. THE ELECTRIC ARC. SLL 5] The term “ zominal candle-power” is often employed in com- mercial work, meaning a value arbitrarily agreed upon to corre- spond to a certain consumption of energy at the arc. Thus an arc consuming 450 watts is assumed to have 2,000 nominal candle- power, and one of 300 watts to have 1,200 candle-power ; although these figures greatly exceed the true spherical candle-power, they may come somewhere near the maximum under favorable condi- tions. Asa matter of fact, the relation between watts and candle- power is quite variable, as shown later. Of the various candle-powers the mean spherical candle-power is the most absolute and important, but unfortunately is the most difficult to determine. The mean hemispherical candle-power may be properly considered where illumination is required in one direc- tion only, as in street-lighting, where the light is thrown outward and downward. The mean horizontal candle-power is of no special importance, the light given off in a horizontal plane being of no more value than that emitted in any other direction ; but it is quite easily measured, and often approximates closely to the mean spher- ical. This is, however, not always the case, and cannot be gene- rally relied upon. The formula due to Gerard is sometimes employed to find the mean hemispherical candle-power by simply measuring the horizontal and maximum candle-power, the expres- sion being : Mean hemispherical candle-power = } mean horizontal + + max- imum. This gives only approximate results, but can be used to save the trouble of making a large number of photometric deter- minations. ee Relation of Light to Current. — Investigating the factors influ- encing the amount of light given off by the arc, Violle found that the quantity emitted by a unit surface of the crater was the same, whether the arc current be 10 of 1,000 amperes. ‘This was to have been expected, since the crater cannot be heated beyond its point of volatilization and still remain in the solid state. The light is therefore roughly proportional to the area of the crater. The spherical candle-power is also approximately proportional to the total number of watts utilized in the arc, but is of course affected by anything that varies the efficiency. Efficiency. — By the efficiency of the arc is meant the ratio of the luminous flux to the total heat and light radiation. Anything ry ELECTRICVLIGHIING. that tends to dissipate the energy at any place other than the crater of the positive carbon diminishes the efficiency. The arc is the most efficient source of illumination known. The most generally accepted value for its efficiency is 18 per cent. The corresponding figures for the other sources of light, are for the candle 1} per cent, the gas-flame 1 per cent, the Welsbach light 4 per cent, and the magnesium light 12 per cent. The efficiency of the incandescent electric light is about 5 per cent. These values are comparative but probably too high. Among the causes that modify the efficiency of the arc are these: 1. The structure, density, and composition of the carbons. These affect both the volatilization point, and hence the tempera- ture, as well as the thermal conductivity upon which depends the amount of heat conducted away by the carbons, which is lost energy. Purity, softness, and evenness are desirable. 2. ‘The cross-section of the carbons. Large carbons conduct and radiate more heat than small ones for equal currents, hence for a given current the efficiency diminishes about inversely as the diameter of the carbon increases. 3. The existence of a soft core reduces the temperature of the crater, and tends to lower the efficiency. Current and Voltage. — The division of the watts at the arc into current and voltage is extremely important, depending on various factors not yet understood. Carhart found that with 450 watts at the arc, made up of 10 amperes and 40 volts, he got a maximum candle-power of 450; while with the same number of watts in 8.4 amperes and 54 volts he obtained 900 maximum candle- power, just twice as much. Blondel, however, finds the luminous flux greatest usually below 45 volts. The discrepancies are prob- ably due to differences in size and quality of carbons, because there is naturally a certain current density for each carbon, which gives the best results. Commercial Values of Voltage and Current.— The value of 45 to 47 volts at the arc, reached after years of commercial expe- rience, is probably the best. At this point the efficiency is high, and the conditions are about half-way between hissing and the flaming points. At this voltage, too, as the curves show, the voltage at the arc is only slightly affected by fluctuations in the current strength. Ordinary current values range from 6.5 to 10 RYE ME LEGCTRICVARC. 313 amperes for long arcs in air. Greater voltage is inadvisable, as it reduces the number of arcs that can be placed in one series, in- creases carbon consumption, tends to produce flaming, and intro- duces too much energy in a single-light unit. Less current than 6.5 amperes gives too little light for a unit, used under conditions suitable for a series circuit. On constant potential systems small open arcs of low current have been at- tempted, but without much success. The low current arc has a large cooling surface for the energy used. If for some of the rea- sons given later in detail, the current in the arc falls, the propor- tion of the cooling surface to thé energy is greatly increased, in fact to such an extent that the arc flickers violently or is put out by the chilling effect. Where arcs are inclosed in heat-retaining bulbs, the current may be greatly reduced before this effect takes place. Composition of Light. — The composition of the light of the arc has been determined by Meyer to be as follows, where the intensity of the yellow light is expressed by unity. Red and orange 2.09, yellow 1.00, green 0.99, blue 0.87, indigo 1.03, and violet 1.21. Taking the intensity of red as 100, Abney gives for direct sunlight ; Red 100, green 193, violet 228; while for arc light his figures are: Red 100, green 203, and violet 250. For eas-lisht he found the values.to be: Red 109, green 95, and violet 27. The composition of the light of the arc may be, however, greatly changed by the hardness of the carbon, the material of the core, and by the current and voltage. Hardness usually deter- mines the maximum temperature of the crater, while the current and voltage alter the proportions of the light fluxes coming from the yellow crater and from the violet arc stream. The vapor of the core acts to color the light as well as to determine the volatil- ization point of the crater. The color of the arc light approaches very nearly to sunlight, and it has the remarkable quality of producing a similar sunburn. Great caution is necessary on this account in avoiding exposure of the eyes to the arc at close range, otherwise a painful sunburn of the eye, producing a tedious inflammation, is apt to result. This can be avoided only by protecting the eyes oz all s¢des with leather goggles fitted with smoked glasses. 314 ELE CTRICALIGH ANG: Short Arcs. —In the earlier days of are lighting, so-called “short”? arcs were employed, taking 18 or 20 amperes at about 25 volts with an apparent arc length of ;4 to 3; of an inch. The object of such a short arc was to increase the number of arcs that could be operated in series by a given voltage. Owing to the large current, the carbon consumption was high and the line drop ex- cessive, so that these causes, combined with the frying sound and delicacy of regulation required, have brought about their abandon- ment. The long arc now employed in open arc lamps _ has roughly half the current and double the voltage. The carbon carried off the positive by electrolytic action is of course only half as great ; but the longer arc affords more opportunity for the air to oxidize the carbon, so that the carbon life in the long arc is not proportionately increased, ‘ Unstable Arcs. — Between the condition of a short arc and a long arc there lies a zone of instability for which the probable analogy is the concussive boiling of water on the dividing line between the stage of rapid evaporation and quiet ebullition. After the long arc is reached, it is necessary, in order to maintain a fixed voltage at the arc, to increase the distance between the electrodes as the current increases. The difference in the relative life of the carbons in the long arc and in the short arc is quite marked. In the short arc the positive wastes away rapidly owing to the heavy current, while the deposition of carbon on the nega- tive is almost sufficient to prevent waste; besides which the air currents have not sufficient room to form in the short arc. Blowing Out of the Arc.— A peculiar feature of all arcs is their liability to be blown by a strong gust of air unless fed by a con- stant current machine which cannot fail to maintain the current. A magnet will also blow out an arc if the pole is brought sufficiently close. This magnetic action, as previously stated, causes the bow shape characteristic of the arc, and in the case of an alternating current causes the arc stream to rapidly bend from side to side across the earth’s line of force. The blowing-out tendency of the magnet is frequently employed to direct the arc upon metals for the purpose of melting or heating them, as well as in various magnetic blow-out devices, in which the blowing-out effect rapidly extinguishes an arc formed between two contacts liable to be melted by the continued action of the current. THEGELEGCTRIC ARG 315 Arc on Constant Potential Circuits. — When arcs can be run in series on circuits furnished by constant current machines they have the great advantage of having the current maintained as long as the arc is not cut out of the circuit, so that irregularities in the arc or mechanism produce only a variation in the intensity of the light, but the illumination, good or bad, is always maintained. It is, however, often desirable to run arc lamps on constant potential mains at the usual 110 or 220 volts pressure, where the current is no longer constant unless a device is introduced to make it so. Every constant potential arc lamp has a mechanism of this kind contained in the case whose function is to separate the carbons when the current is too high, and bring them together when it is too low. If well made and adjusted, such a regulator may respond to current variations of five per cent either side of the value at which it is set, which might be expected to maintain a practically constant current. Such, however, is not the case. An arc whose carbons are fed by a mechanism of this kind, zf connected directly to a constant potential main, will behave in the most erratic manner, even if it be started by hand regulation, and allowed to warm up before the test. The mechanism adjusted to respond to five per cent curregt variation now utterly fails to keep the current anywhere nearly constant, and the arc is very unsteady. This is true even if the voltage of the mains corresponds to the voltage desired at the arc. The reason for it lies in the fact, shown by previous curves, that the resistance of an arc decreases as the current increases, ‘which results in a tendency for the current to become almost infinite if constant potential is maintained across its terminals. Similarly, if the current begins to decrease, and so lessen the cross-section of the arc, the resistance rises and further chokes off the current, until the arc goes out. The arc when directly connected to con- stant potential mains is therefore in a state of unstable equilibrium, in which the current tends to drop to zero or surge toward infin- ity. This action, depending as it does only on the instantaneous cross-section of carbon vapor at any moment, is itself instan- taneous. The mere inertia of a mechanism retards it so much that the arc is out before the regulator has perceptibly moved. The means used to counteract the instability of the arc must operate as fast as the current change can take place. Such an auxiliary 316 BLECTRIGGLIGIZ LANG. regulator, although an inefficient one, is made by the simple expedient of inserting a series resistance between either side of the arc and the mains. | The mechanism keeps the average current constant by zzcreasing or decreasing the length of the arc. The resistance overcomes the tendency toward rapid fluctuation by automatically and instan- taneously rvazsing or lowering the voltage across the arc gaps as required. As an illustration, assume an arc to be’ connected to constant potential mains of 40 volts, and the regulating magnet to be wound to pass 10 amperes with a normal length of are. If for-some redson’ the *currentssuddenly wdrops ito 8 amperes the resistance of the arc rises, though its length may not have changed, and it requires more than 40 volts to bring the current back to 10 amperes and maintain it, therefore the arc goes out. If now we connect the same lamp in series with a one-ohm resistance to a 00-volt circuit, the current again 10 amperes, the lamp will have 40 volts at the terminals as before. Now let the current fall to 8 amperes. The drop through the resistance is only 8 volts, and we have 50 — 8 = 42 volts at the arc, which is sufficient to force more than 8 amperes through it, and so restore the current to-the normal 10-ampere value. The resistance in series is sufficient when the rise of voltage at the arc, caused by less drop in the resistance, suffices to force the original current through it, in spite of its diminished cross-section. If too little resistance is used, a given decrease of current will not produce sufficient rise of voltage at the arc to maintain it, and it goes out. If too much resistance is employed, the rise of voltage at the arc is excessive for changes of current too small to move the mecha- nism, and the lamp tends to allow the current to surge beyond its proper limits. This regulating or steadying action of resistance is of course instantaneous, as it depends on electrical changes and not.on inertia or mechanical motion. To a certain extent self- induction may have a similar tendency to raise the voltage at the point of rupture or increase of resistance in an electric circuit. ‘No Resistance in Series with Series Lamps. Om series- or high-tension circuits resistance is not required, because the current is maintained, whatever the changes in the arc, by the inherent regulation of the dynamo. It is impossible, therefore, for the cur- rent to fail while the machine is in operation. THRGELECTRICVUARC Ben “ Constant Potential Lamps, Two in Series. The usual voltage of constant potential circuits being double that required for one open arc lamp with its resistance, being 110 volts or thereabouts, open arcs on these circuits are commonly connected two in series. If only one lamp is used, and the remaining excess of 60 or 70 volts taken up by resistance, the regulating action of the latter tends to make the light vary up and down slowly, as explained above. On circuits with a higher voltage than 110, more lamps. are run in series, as for instance, 10 lamps ina string across a 500- volt circuit. From what has been said, it will be evident that both current and voltage vary somewhat in the arc on constant poten- tial or incandescent circuits, while voltage alone varies on a good series circuit. A carbon that will tend to produce a steady light, such as a cored carbon, is, therefore, advisable for constant poten- tial lamps. Again, these lamps are commonly used for interior illumination, so that a better grade of carbon and one that pro- duces a softer yellowish light is desirable. Troubles in the Arc Proper. — The chief troubles found in direct current arcs not caused by the mechanism are these : — Flaming, from too long an arc, or impure carbons, or half-baked carbons containing unexpelled gases. The flame usually runs up the side of the positive, accompanied by a drop in resistance and loss of light. Hissing, due to too short an arc or too vigorous vaporization or too coarse-grained carbons. ‘This is attended with loss of light, low resistance, and an objectionable hissing noise. Sputtering, from impurities in the carbon, or loose-grained carbons. Whistling, occasioned, as in a Chicago installation, by electro- static induction between the underground conductors and _ their metal sheaths. These current vibrations reproduce themselves in variations in the volume of the arc stream, producing a shrill noise. Traveling of the arc around the carbons producing unequal illumination and flicker. This arises from the tendency of the arc to continually seek the path of least resistance, which wan- dering increases with the area of the carbon over which the arc may travel, in other words, the area of the end. This may be remedied by the use of smaller carbons or of cored carbons, in 318 ELECTRIC LIGHTING. a» which latter case the soft core vaporizes first, and the arc is con- fined to the inner surface of the crater thus produced. Bucking of arcs connected in series, owing to the mechanism of all the lamps endeavoring to correct a change of current due to the improper working of one particular lamp. This is frequently very marked on incandescent circuits, where only two lamps are in series. If one sticks, it frequently consumes all of the energy, leaving the other nearly dark. To overcome this, both proper design of the mechanism and proper adjustment are required. Alternating Arcs. — When arcs are fed by alternating current the arc is no longer a continuous flame, but is lighted and ex- tinguished at every reversal of the current. When these follow one another faster than 100 per second, corresponding to a fre- quency of 50 periods, the flicker is not apparent to most eyes. Owing to the reversal of the current each carbon acts as a positive at every other alternation. There is, therefore, no crater, both carbons remaining pointed, but the upper one wastes away 8 or 10 per cent faster than the negative, due to receiving the ascend- ing heat. Voltage and Current. Under commercial conditions, using cored carbons, open alternating arcs consume about 15 amperes at 30 to 35 volts. This would seem to be unaccountably less than that required for a continuous current arc using the same carbons ; but it must be borne in mind that an effective alternating voltage of 85 has a maximum potential of about 50 at the top of the wave. Function of Core and Object of Heavy Current. When the carbons are separated, it would appear that the first extinguishment of the arc as the current passed through zero would put out the light ; but a continuous path is provided for the current by the bridge of incandescent carbon vapor that persists until the voltage acquires a substantial value in an opposite direction. To obtain this effect the current used in alternating arcs must be larger than in continuous current arcs, and the carbons are always cored, to insure a sufficient supply of carbon vapor. Power Factor. If the current and £.//.F. are in phase the power at the arc in watts is the procuct of the volts and amperes. Steinmetz, however, has shown that since the apparent resistance varies with the current there must be a lag of current behind the electromotive force. Experiments show that the true power in THR ELEGTRICHARC. 319 the open alternating arc is about 85 per cent of the apparent watts. ) ; Wave Form. The efficiency of the alternating arc increases slightly with the number of alternations, and is considerably affected by the form of current wave, which is largely determined by the shape of the wave of 4.47./.; a flat-top wave producing a higher efficiency than a peaked one. This is because the flat-top wave creates less interruption in the flow of current than the sharp pointed wave, the latter allowing the carbon a considerable interval between maximum values in which to cool off. flum. A peculiarity of the alternating arc is its hum, corre- sponding in pitch to the alternations. It arises from the expan- sions and contractions of the arc stream with the current, pro- ducing corresponding vibrations of the adjacent air. This has nothing to do with the hissing sound that may occur from very short arcs as with the continuous current. When the alternating arc hisses, the voltage falls, but less abruptly than with the con- tinuous arc, while the current lag is in- creased until the true watts are only 75 per cent of the apparent watts. The hight emission of the alternating arc at any instant lags a little behind the curve of instantaneous value of the real watts. It never passes through zero, owing to the retention of heat by the carbons. Ei icteieyemew ith thesame energy and carbons the mean spherical candle- Vertical Axis of Carbons power of the alternating open arc is about one-half that of the continuous current open arc. _ Dustribution. The distribution of light is nearly equal above and below the» horizontaleas shown by Pig. <26%) . p50 325 Therefore the light going upward, which Fig. 267. Light Distribution, Alter- nating Current Open Are. 160 is nearly one-half, would be wasted were it not for the white reflector usually employed immediately above the arc to throw the light down. Focussing Mechanism. The more equal consumption of upper and lower carbons in alternating lamps necessitates a mechanism pal) ELECTRIC LIGHTING. that will feed both carbons in order to retain the arc in the same place. See Fig. 268. Such lamps are usually connected to the secondary of a_ trans- former through an induc- tive resistance, known as an auto-transformer Or economy coil (see Fig. 269), or they may be connected directly to a transformer wound to deliver a constant cur- rent (p. 171). The pos- sibility of substituting an Fig. 268. Focussing. Fig. 269. Connections of Economy - : : Mechanism. Coil. economy coil in which little energy is lost for the wasteful resistance used on constant potential direct current systems, compensates in a large measure for the low efficiency of the alternating arc. Inclosed Arcs. — The rapid consumption of the carbons in the open arc led many experimenters to attempt to devise methods for reducing it. The earlier workers devoted themselves to mak- ing a compromise between the arc and the glow lamp, the general features of which were a minute arc formed at the point of imper- © fect contact between two carbons. ‘These incandescent arcs were never successful for reasons now well understood. About 1882, and in the years following, attempts were made to inclose the arc of the constant current lamps in air-tight globes of various sizes, but without success. In 1894 commercial lamps were successfully used, in which the are was inclosed in a small globe or bulb of refractory glass protected by a larger outer one. It was found that the previous attempts to apply the inclosure to. existing high-tension lamps with high current and low arc voltage was a step in the wrong direction, but that the inclosed arc was well suited to low current and high voltage arcs, which would be desirable on constant potential systems. Inner Globe. ‘The inclosing bulb is an egg-shaped globe about 5$ inches long and 23 inches in diameter, tightly sealed below, and partially closed above. When an arc is sprung between the car- bons inclosed in this manner, at first the same conditions exist as in THE tELEGERIGCARC. punk open arcs. The oxygen in the bulb is, however, rapidly consumed by combination with the carbon, and in six to ten minutes the bulb is filled with highly heated CO and CO,. These gases would soon -be replaced by air were it not for the arrangement of the cover of the bulb, which is usually ground to fit at the edges, with an opening in the center slightly larger than the carbon. This narrow annular opening, especially when corrugated internally, affords, by creating eddy currents of gases, great frictional resistance to the passage of the air, so that very little enters. The conversion of the air of the bulb into inert CO and CO, soon shortens the first long arc with 80 volts across to about 33, of an inch. The com- bustion of the carbons by the oxygen of the air is now greatly re- duced, so that the positive burns flat-ended, its loss being chiefly electrolytic. The positive is apt to have a slight tendency to become concave, while the negative tends to become convex, but both remaining approximately flat ended. Were it not for the air that seeps into the bulb the negative would not be destroyed at all, because not only is it not consumed, but it receives a deposit of carbon from the positive. It is found advisable, however, to admit just enough air to combine with the carbon vapor set free and burn it, to prevent it depositing as a black condensation on the bulb, or as a fragile nib on the negative. If pointed carbons are used in a bulb, they will become flattened after burning a short time. Volts, Amperes and Watts. Yhe usual energy at the arc in direct current inclosed arcs is about 400 watts (5 amperes at 80 volts). It is found that when less than 78 volts is used the car- bon does not traverse a long enough arc to have its combustion completed, and with 50 to 60 volts, a deposit of this unconsumed carbon tends to form on the bulb, cutting off the light. When more than 80 volts is used, the carbon consumption is too high, the violet light from the arc becomes too prominent, and the arc has not sufficient resistance interposed in circuit with the usual 110 volts to keep it steady. The voltage between the electrodes being therefore fixed at 80, 5 amperes give the energy usually taken in lamps on these circuits. The inclosed arc has properties similar to the arc in air, the difference of potential increasing with the distance apart of the electrodes. Hissing occurs for the same. reason, but flaming is less, and the zone of flame is absent because there is no oxygen to support it. 322 ELECTRIC LIGHTING. Efficiency. ‘The efficiency of the inclosed arc is not as high as that of the open arc owing to the fact that more energy is expended in the arc stream and less in the crater, and that the flat-ended carbons rapidly conduct away the heat, and that some 10 per cent of the light is lost in penetrating the bulb. The retention of the heat by the bulb, however, adds to the efficiency, so that the net difference in efficiency is probably not great. Size of Bulb. Evidently a large bulb will be less efficient than a small one, and will also tend to produce a carbon deposit by chilling the vapor on its cooler surface. The size of the bulb affects the interval of unsteadiness which ensues when a lamp is started. The bulb being filled with air, which has diffused in since the lamp was last extinguished, the carbons are practically burning in the open air until the oxygen in the bulb is consumed. They therefore drawa long arc. After some time, depending on the amount of oxygen in the bulb, and on the tightness of the in- closure, the arc becomes unsteady; apparently because it is im- mersed alternately in atmospheres of carbonic oxides and oxygen, which greatly affects the arc’s resistance. At such times the arc may be so unsteady as cut itself out. Wa§th small bulbs the change from the open arc to the closed arc conditions usually occurs in from three to five minutes, after which the light is steady. Gen- erally speaking, the larger the bulb the longer the time before the arc passes through the period of unsteadiness. Carbons. — The carbons employed in inclosed arcs must be straight and smooth, otherwise they will not pass freely through the opening in the gas cap. This precludes the use of molded carbons, which have an irregular seam running the whole length, and requires forced carbons, the difference being considered more fully in the beginning of the next chapter. The carbons must con- tain the minimum amount of impurity, as it is all deposited on the inside of the bulb, hence cored carbons are not suitable. Uniform- ity in diameter is essential. The consumption of carbon in an inclosed arc is different in various positions in the bulb, chiefly owing to being more or less exposed to air currents. The greatest consumption is usually found to exist near the bottom of a bulb, and the smallest some- where in the upper portion. The ratio of the consumption of the upper to that of the Jower THE ELECTRIC ARC. 323 varies curiously in different portions of the bulb and in different lamps, usually between 15 to 1 and 2 to 1. If the inclosure were perfectly air-tight, the negative would hardly be consumed at all, or might even grow larger, whereas the positive would be con- sumed electrolytically nearly as fast as usual. The ratio might easily be 100 to 1 or 1,000 to 1 in such a case. Where, however, the air has free access to the arc, the consumption by the air may be as large as by the current, and we should then have a ratio of about 2 to 1 for positive and negative carbons respectively. Rupturing the Arc. Inclosed arcs have a peculiar tendency to “cut out,” or break the arc, which is not found in open arcs. This 1s apt to occur when a gust of fresh air enters the bulb and strikes the arc, cooling it and instantly changing its resistance. _ The arc has a tendency to travel around the large flat ends of the carbons, which produces the effect of a flicker, owing to the shifting shadow of the negative. This is less noticeable in open arcs whose pointed carbons center the arc. Inclosed arcs will also operate with much higher voltage than 80 across the arc, provided that approximately the same percentage of drop is re- tained in series with it. Thus 150-volt arcs on 220-volt circuit taking 23 amperes are often used, but give a more violet light. Series Inclosed Arcs. — Considerable difficulty was first expe- rienced in applying inclosed arcs to series high-tension circuits. On such circuits the current is often already fixed by the winding of the existing dynamos, the majority of which are wound for 6.8 or 9.6 amperes. If inclosed arcs, taking not less than 70 volts, were substituted for open arcs, taking 47 volts, the central station would have the number of lights on the circuits greatly reduced without a corresponding increase in revenue. Another objection is the heavy current, which has a tendency to overheat and soften small bulbs. Still another disadvantage is the length of time required to effect the short circuiting of the lamp by an automatic cut out when the carbons fail to feed or are consumed. With open arcs, the carbons burn so rapidly that the cut-out coil shunted across the arc acts in a few minutes, owing to the rise of the voltage across the arc. If the same construction were used, it would require about twenty times as long to effect the same action in the slow burning inclosed arc, so that the high-resistance cut-out coil, which is made of very fine wire, is liable to be damaged 324 ELECTRICUAIGHTING o by the prolonged passage of the current through it. But these ob- jections have now been overcome. ‘The demand for inclosed arcs on series circuits was not at first so keen as at present, owing to the cheap carbons used and the rough quality of the light allowable for exterior illumination, in which efficiency is highly desirable and absence of glare not so much of a consideration. Advantages of Constant Potential Inclosed Arcs. — The rapid introduction of inclosed arcs on incandescent circuits in the last few years is due to the many advantages which they possess com- pared with open arcs on the same circuits. First, perhaps, is the saving effected in carbons, which last from 100 to 150 hours per pair on the average, with proper adjustment. Ordinary open arc carbons last about 8 to 10 hours. In fact, inclosed arc lamps are usually designed so that the remnant of the positive carbon may be used as a negative on the next run, if cut to the proper length. Longer life than 100 to 120 hours is probably not desirable in inclosed arcs, because it is obtained at the expense of an exces- sive deposit on the sides of the bulb, and a sacrifice of efficiency and steadiness from carbons of increased diameter. The long life of the carbons saves not only the value of the carbons themselves, but the greater labor-expense of retrimming. The nuisance of the daily visits of a lamp-trimmer, required for open arcs, especially in places where dust or a ladder is objection- able, has had a great deal to do with the favor with which the inclosed arc has been received. The ability to light or extinguish one lamp at a time is impor- tant ; because it effects an economy over the system of open arcs, in which two in series are always thrown on or off together. Quality of Light. Absence of sparks is another feature secured by the inclosing bulb. The mechanism also admits of the utmost simplicity in its construction. The color of the inclosed arc, with proper combinations of globes, approaches very closely to daylight, since it is possible to cut out the undesirable parts of the spectrum by the use of glass of the correct shade. With clear bulb and. globe the light is a violet tinge, which is not as pleasant as the modified color. The opalescent inner bulb usually employed acts also to diffuse the light, so that no violently luminous spot exists, but the light comes from the large surface of the bulb. The effect of this is to prevent sharp shadows, and to allow the pupil of the THE ELECTRIC ARGC Sa) eye to open wider without the sensation of glare; thus increasing the apparent illumination. Distribution. The distribution of the light in a vertical plane has been investigated by Messrs. Freedman, Burroughs, and Rapa- port, whose results are quoted herewith. They found that the distribution in an inclosed arc lamp is not the same as in an open acc lamp poeceutie. 2 (0, The maximum in the former is at an angle of 25 degrees below the horizontal, instead of 40 degrees. The intensity, after decreasing, reaches another high value at 40 degrees, but not as great as at 25 degrees. The probable expla- nation of this peculiar form of curve is, that at 95 degrees the Fig. 270. Light Distribution, Direct Current Inclosed Are. light comes obliquely from the crater, but is not cut off by the negative. Descending, the nega- tive cuts off more light ; but the rays emanate more perpendic- ularly from the surface of the crater until another maximum is reached at 40 degrees. The reflection from the bulb, and the position of the arc in it, would also alter this distribution. Eifictency. Tables XI. and XII. (Freedman) show the effect of a clear and an opalescent inner globe, the same being shown graph- ically in Fig. 271. The same investigators measured the loss by opalescent outer globes, which they found varied from 35 to 50 per cent, and which occasionally is as great as 60 per cent. They conclude that with currents of 5 amperes and with two clear glass globes of the best quality the watts per candle are about .5, with opalescent inner and clear outer, the watts per candle are about .6, and with both inner and outer opalescent globes the watts per candle are about .95, being mean hemispherical candle-power in all cases. Holophane globes, whose construction is explained on page 334, gave the same loss of light as clear globes. Whether the run is continuous or intermittent will make a difference in the life, although only slight. Theoretically the life should be less for the intermittent test that when the lamp is kept burning without any stops, and this is found to be the case. The stoppage allows fresh air to get into the bulb each time, thus 326 PEE CTRICH CIGHIING. increasing the consumption. When the current is thrown off a lamp, it is noticed that the carbonic oxide gas ignites with the in- rush of air, and by a series of minute explosions causes a chatter- ing of the gas-cap. Sometimes it burns with a quiet blue flame that lasts for five or ten seconds. To find theoretically the amount of carbon consumed with intermittent use we can calculate the weight of oxygen the bulb contains when filled with fresh air, and from this determine the amount of carbon burned before the clear inner globe only Sapo ere opal inner globe only —__--—_.. Opal inner +- opal oute Fig. 271. Light Distribution, Inclosed Arc with different Globes. TABLES XI. AND XII. (FREEDMAN.) CLEAR INNER GLOBE. OpaL INNER GLOBE ANGLE FROM THE HORIZONTAL, CANDLE-POWER. CANDLE-POWER. PO -ADOVE wis Tees ce eee ee 89 Et eat ce hal Be 152 10°" « Poft go ah he ee ee 82 a rs ot ee aoe ae 184 0 ey Sree ee 7 139 hae BS ee he 347 10; below 26s gee een 501 SR yeh he tee 455 20 ns US gion het SEO KOU) a) erie Fart aah RO GS a? 735 25 a hy eal, ee ee eee SO i: adie, ee tes ores. 5? 30 ‘f A te ae eae te) 5 oe A ee ar, 985 40 W Voc! esc ae emees Etal «Lage l eT a ee ee tk ® 50 ct Lo he oe eee LOGO Ae Br Se he ee 969 60 “ ee ee Oe Se 6 Ce ee ee 855 70 a a) Wee hh SW Are See ee ee eee 734 Mean Hemispherical. . . 8850 ON hk fede ee EU THE ELECTRIC ARC. ya admission of any more fresh air. Taking 4 hours as an average run, a lamp burning 140 hours would have 36 stops, equivalent in consumption to 5 hours run, and on this basis would consume carbon as if it had burned continuously for 145 hours. An air- tight outer globe will increase the life; but it has a tendency to raise the temperature of the interior sufficient to warp or interfere with the action of some of the parts, although it will not do so with proper construction. Alternating inclosed arcs have also reached a high state of perfection. In principle they are similar to the inclosed arcs for direct. current. The essential difference is in the use of one or - both cored carbons, with consequently lower voltage and greater current. With solid carbons the long arc has a tendency to be extin- guished, and the vapor supply of a core is required to maintain the conducting medium between the electrodes. Owing to the inclosure, which gives a stability and freedom from interference of air currents, it is sufficient to use one cored carbon, and it is of course indifferent whether this be the upper or the lower. It is not advisable to use two cored carbons, for reasons explained in connection with continuous current arcs, namely, the efficiency is more or less sacrificed, and the deposit in the bulb is increased. The length of arc is greater than in the direct current lamp, being about 3 of an inch. At the start the arc may be as long as } to 8” before the air in the bulb is consumed, or the resistance up to its maximum value. When hot, the usual current is 6 amperes, with a voltage at the arc of 70 to 75 volts. With 70 volts and 6 amperes ina 104 volt circuit, the apparent watts at the lamp termi- nals are 625 and at the arc 420, the actual watts being 445 and 390 respectively. The watts consumed in the inductive resist-. ance average 35 to 45. This resistance usually consists of a coil in series with the arc wound on a laminated iron core, and mounted in the trimming of the lamp. By connecting the terminals to different portions of this coil, the reactance may be greatly varied, so that the lamp is capable of a wide range of adjustment for various circuits. As a rule the reactive coil can be adjusted to maintain 75 volts at the arc for circuits varying in voltage from 100 to 125, and in frequency from 60 to 153 cycles per second. 328 ELECTRIC! TIGHTING A striking advantage of the inclosed alternating arc is its free- dom from the hum that characterizes open alternating arcs. This is due to two causes. In the first place, the mere inclosure in a fairly well-sealed bulb reduces the noise, but the action of the bulb in keeping the gases hot is the more potent factor. It will be recalled that in the case of the open alternating arc the hum was produced by the rapid expansions and contractions of the arc stream following the waves of current. When, however, the arc is surrounded by a heat-retaining envelope of glass, the gases at the arc do not contract as violently with its instantaneous extinguish- ment, hence the amplitude of the vibrations, and the consequent hum, are much reduced. Another source of noise in alternating lamps was the vibration of the laminated iron of which all magnetic parts are constructed. The thin sheets alternately repelling each other, and losing the repulsive force, are sent into violent vibration, which readily com- municates itself to the whole lamp, with an effect like that of a sounding-board. Since by the modern method of inclosure the noise of the arc itself has been nearly eliminated, corresponding efforts have been made to reduce the hum of the iron. By clamping the core of the reactance coil and magnet cores at a great many points, the iron is held too firmly to vibrate. The iron parts are then supported entirely on springs and rubber, both in light compression, so that the vibrations are not communicated to the lamp frame. Tight inclosure of the whole lamp completes the deadening effect, so that modern alternating arcs are made nearly noiseless. The life of the alternating arc as usually constructed is much less than that of its continuous current congenitor. Owing to the complication in the mechanism caused by feeding both carbons simultaneously, and the difficulty of feeding through both ends of a bulb, the alternating inclosed lamp is usually constructed so as to feed only the upper one. But in an alternating lamp both carbons are equally consumed, and it becomes necessary either to make the lower carbon excessively long or to shorten the life. The latter is considered preferable, and the average life is about 80 hours with ordinary inclosure. For this an upper carbon of 9} x }, and a lower one of 6 x } inches are usually employed. ARC LAMPS. $29 Girit A Pier Rae xo ve ARC LAMPS. Carbons. — Manufacture. The performance of the arc light is so largely dependent on the quality of the carbons employed that some knowledge of their method of manufacture is of great assistance. Many of the discrepancies that have been found in laboratory experiments and commercial work are due to the fact that different kinds of carbons were employed. Carbons are of two kinds, according to their mode of manu- facture, molded or forced. “The molded carbon, as its name implies, is shaped in a steel mold. The forced carbon is squeezed while plastic through a circular orifice. The preliminary stages of treatment being similar, a single description will suffice for both. Various matertals have been employed; but the most promi- nent is petroleum coke, which is a product obtained in the distilla- tion of paraffin. Other materials, such as gas-coke, lamp-black, are also utilized for this purpose. The material is first crushed, then placed in retorts heated toa high temperature for 10 to 50 hours according to the result desired, thereby driving out moisture, and imparting the quality of conductivity. The carbon is next ground to a fine flour in mills, and then bolted. The carbon flour thus produced is put in mixing kettles or pans combined with the “binding material” consisting of pitch which has previously been crushed. These pans are kept warm, and the entire mixture is constantly stirred by hoes or other means for a period of fifteen minutes to an hour. The heat causes the particles of pitch to attach themselves to the particles of carbon. The mixture is then cooled, and again crushed, ground, and bolted, so that a flour of uniform grain is produced. Molded Carbons. From this point the treatment of the material depends upon whether. molded or forced carbons are to be produced. If the former, the material is brought to men 330 ELECTRIC LIGHTING. working at benches, and provided with steel molds. These are split in halves, being grooved according to the length and diameter of the carbon cylinders to be made. The molder weighs the flour in a scale, distributes it evenly over the surface of the mold, and places the steel cap upon it. The mold is then slowly heated in an oven, which causes the particles of combined pitch and carbon to become pasty. When the proper degree of heat is reached, the mold is taken from the oven, and placed under a hydraulic press, the pressure employed varying between 100 and 400 tons. From the press the molds are taken back ‘to the benches, the cover and sides removed, and the “card” of carbons carefully lifted out. When they become cool they are separated from each other ; and the little “fins”’ that have held them to their neigh- bors are scraped off each side, so that each carbon is left a fairly perfect cylinder. For lamps fed by a constant direct current, car- bons are usually made by the molded process, to which they seem best adapted. Forced Carbons.— Arc lamps for constant potential or alter- nating currents require a carbon whose particles are arranged dif- ferently from those in the molded process, and also in many instances a core of less dense material to insure steadiness of light. The flour for carbons to be made by this process is treated somewhat differently from that of the molded variety. It is usually shaped into cylindrical “plugs” about 6 inches in length, and from 2 to 6 inches in diameter. ‘These are placed in front of the plunger of a hydraulic press whose action is horizontal, and are forced through its jaws, taking any desired cross-section from the outline of the die at the mouth. As fast as the carbons issue from the die, they are received upon a table, and cut to desired length. In order to make them “cored,” a hole about 1 of an inch in diameter is left in the center of the carbon as it passes through the die, by the action of a “tongue,” projecting into the orifice of the die from the inside. ‘There are various combinations for the mixture that is used to fill the core, and the secret of its composition is usually guarded by manufacturers. This point in either process is called the “green carbon” stage. They appear shiny black in color, are quite heavy, break easily, and when held in the fingers, and tapped together, give only a dull sound. Both molded and forced carbons are next taken te ARC LAMPS. Hol the furnace-room where the volatile matter contained in them is driven off. [his is a process requiring great care. If they are baked too rapidly, they warp, and are hard to adjust in the lamps. If they are not baked sufficiently, they are too low in ~ conductivity, and give a very poor light. In some cases the baking is performed in fire-clay pots, this being the process employed by many foreign manufacturers. In this country it is customary to lay the carbons in a large rectangular furnace, layer upon layer, separated by beds of sand, the entire mass protected by a covering of sand several inches in thickness, and subjected to heat until every carbon has reached a high temperature. The total time occupied is very considerable, being often one or two weeks from the time the charging begins until the process is completed] sw lromm the {furnace the carbons, are «carried to: the sorting-tables, where they are tested by rolling them on steel plates of true surface in order to separate the straight from the crooked ones. Some of the latter are sold as seconds, others are cut into short lengths, and the worst ones are rejected. Even in the best imported forced carbons there are often found from 2 to 5 per cent of badly warped carbons. Molded carbons differ from forced carbons in many ways. They have a loose granular structure that runs lengthwise through the carbon, at right angles to the line of pressure. They also have the remnant of the web that holds a card of carbons together ; and even if this is ground off, the surface is not perfectly cylindrical. Impurities are more likely to be found in molded than in forced carbons, and they are not as uniform as the forced article. They are used in series constant current lighting chiefly, where cheapness is the greatest consideration. Copper plating these carbons is often resorted to, with the objects of increasing their conductivity, especially at the point of contact with the clamp, and prolonging their life. The copper sheathing protects the carbon near the arc from oxidizing so rapidly, and a 12” x 8” coppered carbon in a 10-ampere lamp will burn about 14 hours, whereas the plain carbon of the same make ‘vill not last more than 12 hours. The forced carbon is usually a higher grade of carbon than the molded, especially those imported from Germany and Austria. The texture is finer, and the material softer, than in the molded Bog ELECTRIC LIGHTING. form, while the grain runs transversely or at right angles to the line of pressure. Owing to the method of manufacture such carbons are more easily made to a given diameter, and are more uniform in diameter, structure, and straightness than the molded carbon. They have a comparatively high conductivity and are not copper plated. They are used for cored carbons particularly. The high grade and pure forced carbon more nearly resembles lampblack, and will make a mark on paper like a pencil, whereas the hard forced carbons will not. Where carbons are held by a small clamp far from the active end, and must fit closely but freely into an opening little larger than the carbon itself, too much stress cannot be laid on the necessity of securing straightness and uniformity. All carbons contain impurities, chiefly silica, iron, and smaller quantities of other substances. In the highest grade of imported carbons the silica is the chief impurity, with little else, but chemically pure carbons have not been produced by any manufacturer. Carbons may be cut to any desired length by nicking them all around and breaking them as one would a glass tube. In inclosed arcs and in alternating arcs both carbons are the same size, whereas in focussing high-tension lamps and in open arc low- tension lamps, the upper carbon is usually about }” larger in diameter than the lower. The only carbons that are copper-plated are those used in high-tension, series, constant current lamps. Globes. — The glassware used to inclose the arc has received little scientific attention heretofore, and it is not at all unusual to attempt a five or even one per cent saving in generating the cur- rent, and allow a 30 per cent loss in its utilization to go neglected. Globes are made of three materials, clear glass, opal (or opaline or opalescent) glass, and a combination of the two called alabaster. Arc lamp globes are either blown or molded. If the former they will vary in regularity, some being thicker, more or less curved, etc., than others. Often the mark of the tool used by the glass- blower to shape the globe as he turns it will produce streaks. Molded globes are quite regular, but frequently show the joints running vertically down the side of the mold. Clear glass globes when clean, thin, and of good quality transmit 90 to 95 per cent of the light. When dusty, thick, or of poor glass the loss is easily doubled. The most common defects in these globes are bubbles, ARC LAMPS. 333 ribs, and other inequalities which cast shadows, and render the illumination very uneven. Opal globes are made of a glass into which some substance, frequently iron, has been introduced, making it translucent, but partly destroying its transparency. This is done to diffuse the light, and to cut off certain undesirable colors. The globes will. vary from one through which outlines can be readily distinguished, to those whose appearance resembles a china plate. The denser globes effect a greater diffusion, but frequently cut off the greater part of the light.” A light opal globe: may cut off 20 to 35 per cent and a heavier one from 35 to 50 per cent. Alabaster globes are made of two layers of glass, one clear and the other translucent. They are usually very dense, and cut off 40 to 60 per cent of the light. In order to combine the high trans- missive power of a clear globe with the diffusion of an opal one, it is customary to grind clear globes, dividing the surface into equal portions. The dividing line may be either vertical or horizontal. The former is usually employed where the light is to be thrown in a direction away from the spectator, as, for instance, into a show window from a lamp hung in front. The horizontally half-ground globes are often used for the illumination of large interiors where an intense light is to be thrown on the ceiling and upper walls for diffusion and a less glaring light to the floor below. The effect of the grinding, which is usually done by a sand-blast applied to the outer surface of the globe, is to diffuse the direct rays of the arc, and form a brilliant scintillating surface. When the roughening is caused by acid the effect is less marked. When applied to both outside and inside of a globe the diffusion approaches that of light opal. A very successful effect for interiors is produced by grind- ing the lower two-thirds of the surface of the globes, thus cutting off the direct arc rays, even to those standing some distance away. The general shape of arc lamp globes is such as to protect the arc from side winds, with less attention to inclosure from rain or snow. With open arcs the shape should be such that the globe is easily cleaned without removal from the lamp, not apt to crack from. changes in temperature, and of such curvature that dust, insects, etc., fall into the cup usually placed beneath. With inclosed arcs a shape which will not show a long, dark portion in the shadow of the negative carbon should be chosen. 334 ELECTRIC LIGHTING. The distribution of the light as well as its intensity is greatly effected by the globe used, largely owing to internal reflection. The curves in Figs. 271 and 272 show approximately how the vertical distribution varies with different globes. oD! 334° 22% B3 «313 90° 78% 67K 56% 45° 3378 Fig. 272. Distribution of Light, D. C. Inclosed Are. Recently a new form of globe, the holophane (wholly lumi- nous), made of clear glass after the designs of Blondel and Psarov- daki, has come into use. This globe diffuses the light perfectly, so that every part of the globe sparkles equally brightly, redistrib- utes the light so as to throw downward many of those rays that would go upward or otherwise be lost, and is capable of varying the distribution to suit the purpose to which it is to be applied. The distribution of the light is effected by prisms, whose section is somewhat like the teeth of a circular saw, molded in horizontal rings on the outside of the globe. The vertical section of a holophane, Fig. 273, shows that each tooth differs somewhat from its neighbor. The prisms on the uppermost portion of the globe are so designed that the rays striking them will be totally ARC LAMPS. Oo reflected back through or nearly through the source of light, and emerge from the lower part of the globe on the opposite side. About 45 degrees from the top, the prisms deflect the up-going rays, so that they emerge horizontally and somewhat below the horizontal. From there down to the level of the arc the prisms. all refract and reflect the light down toward the floor or street.. Below the horizontal the function of the prismatic ribbings is to distribute the light so that the objects below the arc are uni- Fig. 273. Vertical Section of Holophane Globe. formly illuminated, weakening the intensely bright zone due to the maximum candle-power about 40 degrees below the horizontal, and lightening the darker circle that tends to exist immediately below the lamp. To secure still more perfect diffusion the globe is ribbed verti- cally inside, as shown by the horizontal section in Fig. 274. The effect of the diffusion is to make the outer edges of the globe, viewed from the side, appear as luminous as the center. Holophanes have the disadvantage of requiring a stationary source of light to work to best advantage. This calls for a focusing open arc, or a fairly stationary inclosed one, two inches “ 336 ELECTRIC WHGATIING. variation in a 12-inch globe not being excessive. They have the disadvantages of being somewhat expensive, heavy, and harder to clean, but are a marked improvement over the old style in diffu- sion and economy of light. To show the care exercised in the design of the prisms, the manufacturers of holophanes in this country state: “The large holophanes have as many as 400 calcu- lated faces, each designed for a special duty. The profile of each one of these prisms is calculated by the laws of optics, and drawn on a very much enlarged scale to secure accuracy. The drawing is then reduced and transferred, by a photographic process, to a Fig. 274. Horizontal Section of Holophane Globe, steel plate, and the profiles cut out with the accuracy of engravers’ work. A tempered steel tool is then made corresponding to this template, accurate to the one thousandth part of an inch ; and this tool is used in cutting the grooves in the mold in which the glass is pressed, after which it is annealed.” The cuts 275 and 276 show the effect of clear and opal globes on the distribution of light. Lamp Mechanisms and Constructions. — The functions of an arc-lamp mechanism may be described as follows : — ARC LAMPS. aves 1. To separate the carbons after having brought them into contact, if they were not together previously. 2. Maintain the distance between the carbons such that the energy at the arc is constant. 3. Feed one or both carbons together as they are consumed. Fig. 275. Illumination on Street Surface from Open Are. BRR Sa 00 ee SE eee | \ } | | | 1 So TEE PERO SBE MULTE ARES Ne Fig. 276. Illumination on Street Surface from Opal Globe. 4, Leave the lamp in such condition when the current is turned off that it will resume operation when the current is re- newed. To these must be added another duty, dependent on the class 338 ELECTRIGCJIIGHUING ° of circuit on which the lamp operates, usually effected by the “cut out ” in one of the following ways : 5. On constant current circuits, to maintain the continuity of the circuit through another path, if that through the carbons is broken by failure to feed or by reason of their having been used up or broken. On constant potential circuits to Open the circuit under the same conditions if the lamps run in parallel, or to substitute an equivalent resistance if two or more lamps run in series. Regulation may be by hand or automatic. Hand regulation is used where the operator is always present, as, for instance, with projection lanterns, searchlights, etc. For arcs employed for gen- eral illumination automatic regulation is invariably used. The general principle employed is the balance effected between the pull of a spring, gravity, or both against the pull of one or more solenoids or magnets. ‘The variations in mechanical details are endless. The balance is preserved when the arc is in its normal condition. The mechanism is so arranged that too great length of arc will weaken the solenoidal pull, and too short length increase it or vice versa. In lamps intended for series circuits, these func- tions are performed by two types of mechanisms, known as shunt and adzfferentzal. [un shunt lamps a circuit is led to the solenoid from opposite sides of the arc, so that the normal voltage across the coil, whose resistance may be 400 or 500 ohms, is 47 volts for an open arc. When the current is off, the carbons are held apart by the retractile force of a spring, gravity, etc. On turning on the cur- rent, a high voltage exists across the gap between the electrodes, and the solenoid overcomes the retractile force, feeding the carbons together. When the carbons touch, the voltage instantly drops, and the retractile force, overcoming the weakened solenoid, pulls the carbons apart, and springs the arc. When the voltage rises too high, the shunt coil again feeds the carbons enough to restore a balance. Two points are worthy of special attention in connection with shunt lamps. The first is that the carbons are apart at the start, introducing a very high resistance (450 ohms for each lamp in the series), unless there is an auxiliary cut-out circuit. This exceedingly high resistance introduces a difficulty in starting the average arc dynamo, and gives rise to potentials. ARC LAMPS. 339 exceeding the line voltage, and possibly dangerous. The other feature is that the mechanism is entirely independent of current strength, and will maintain a given voltage across the arc what- ever the current. Therefore such lamps will operate on circuits of various current values without any additional adjustment. They have also the property of varying the energy, and therefore the light at the arc no more nor less than the percentage that the cur- rent varies from normal. On the other hand, when the current of a line abnormally increases, decreasing the resistance of the arcs, and tending to grow still larger, these lamps do not assist the dynamo to regain its equilibrium. This gives rise to a ten- dency to unstable equilibrium of the current in the line manifested by surging of the lamps or jumping arcs. The differential lamp when at rest has its carbons 27 contact. They are separated by the pull of a series coil opposing gravity ora spring which the shunt coil asszs¢s. In this, as in the shunt lamp, the pull of the shunt coil tends to feed the carbons together. Obvi- ously this lamp has a low resistance before the current is turned on, which is an advantage. The current passing through the carbons and series coil energizes the latter to pull the carbons apart, against the action of gravity, because the shunt coil is inert when the carbons are in contact with no voltage across them. This insures a rapid and positive opening of the arc. As soon as the arc is sprung, the shunt coil begins to act, but does not effect a balance until the series coil has pulled the arc long enough for the normal voltage. When the voltage rises, the shunt coil feeds the carbons exactly as in the shunt lamp. These lamps will work properly only within small limits on either side of the current to which they are adjusted. An increase in current will result in a stronger pull of the series coil, which will draw the carbons apart, until the increased voltage, acting through the shunt coil, again effects a balance ; less current will weaken the series, and allow the carbons to approach. Such lamps therefore increase the apparent resistance of the arc as the current rises, and corre- spondingly assist the dynamo in maintaining the current value constant. For a given variation in current, however, they show a greater variation in light, since they increase the arc voltage with the current, so that the watts rise faster than the current strength. One point in favor of the differential lamp is that the striking of 540 ELECTRIC LICH IING ~ the arc is a positive action effected by the current. Both styles of lamps give satisfaction, the most notable example of the shunt lamp being the Thomson-Rice, while the most prominent type of the differential is the Brush. In addition to the elementary features above described, shunt lamps usually have an auxiliary shunt winding of coarse wire that acts in striking the arc. If the fine-wire shunt coil only were employed, the retractile mechanism of the first lamp to operate on starting up would separate the carbons without drawing an arc, since only a very small current can pass through the shunt coils of the other lamps in the series. The carbons will tend to vibrate, like the hammer of an electric bell, until all the carbons are down together, when they can all pick up their arcs. The coarse-wire shunt passes sufficient current around the carbons when apart to maintain an arc between the carbons of any other lamp in the series: As. soon as the are is struck, the circuit, of the coarse wire shunt is opened by the armature or an auxiliary magnet or some such device. Cut-out.—In all sevzes lamps it is absolutely essential to insert a device by which a continuous path is provided for the current in case the carbons fail to feed, or if they are totally consumed, or when the lamp is to be rendered harmless for inspection. Ina circuit fed by a powerful arc machine the ten- dency to maintain the current is enormous; and an arc a yard long may be drawn at a potential of two thousand volts or more, unless some positive and reliable means exists for short circuiting it. These devices, known as cut-outs, are contacts connected to the poles of the lamps, and so arranged that they are brought together as required. Cut-outs should operate — When the carbons fail to feed. When the carbons are consumed or broken off. When operated by hand by the trimmer. The surfaces of the cut-out that make contact should be made of a metal not easily oxidized, such as silver, and with the sur- faces vertical so as not to collect dust. A common form of cut- out is a silver button on the armature making contact with a fixed button, when the armature is pulled all the way over by the shunt coil. This practice is not to be commended, since it does not operate if the armature itself should stick. A better ARC LAMPS. 341 arrangement is an auxiliary cut-out in addition to the armature cut-out, which will operate even if the armature sticks. The dis- advantage of such a device is that it is liable to operate when not wanted, but this may be overcome by adjusting the auxiliary cut- out to act only at considerable increase over normal potential. The cut-out that comes into play when the carbons are con- sumed consists usually of a contact attached to the carbon rod or parts moving with it, making connection with another contact on a fixed part of the lamp. To cut out the lamp by hand, a lever is frequently provided that will have the same effect as the descent of the carbon. In any case, the effect of a series cut-out is to dead short circuit the lamp, making it safe to handle. On lamps that are run two in series on constant potential circuits, the cut-out must be set to introduce a resistance that will produce the same drop as the lamp itself, thus disturbing the other lamp or lamps in the same string as little as possible. Iuclosed arcs when run in series require a similar cut-out, but have none at all-when burning singly in parallel on incandescent circuits. In this case the arc breaks when the carbons are exhausted, and the lamp goes out. Long-range pull of magnets. It is evident that the magnets of all lamps must exert a pull on their armatures just sufficient to maintain a balance, whether the armature be close to or far from the pole. - Where the force to be overcome by the armature pull remains constant, the magnet or solenoid must be constructed so - as to have long range, that is, an even pull throughout the travel of the armature. Where, however, the attraction of the coil for the armature is very unequal through the limit of ‘motion, as it is apt to be unless specially provided for, the force of the armature is equal- ‘ized over the range of the carbon motion by some mechanical device, called an equalizer, such as a pair of cams. Temperature Correction. Another essential feature of mod- ern lamps is an arrangement whereby the energy at the arc is maintained constant, independent of variations in the temperature, and hence current in the shunt coil. In shunt lamps this is usually effected by the somewhat expensive recourse to a special metal, with a low temperature coefficient, for the wire-of the shunt. In differential lamps it is usual to shunt the series coil S42 ELECTRIC TIGHTING: itself with a small piece of low coefficient , metal. When the shunt resistance increases through heat, weakening its current and pull, it would be overpowered by the series coil were it not for the fact that the copper series coil has also risen in tempera- ture, shunting more current around itself through the low coeffi- cient wire, and thereby weakening its own pull. By proper adjustment of this “temperature” shunt the lamp, whether hot or cold, may be made to maintain constant energy at the arc. Magnetic Circuits. — The magnetic circuits of the shunt and series coils have an important bearing on the sensitiveness of the lamp. If the shunt coil is wound over or under the series coil, but in opposition to it, so that it has no separate magnetic flux of its own, increase in shunt current will weaken the pull of the series coil by a certain number of ampere turns. When, how- ever, the series and shunt have separate magnetic circuits, and pull against each other, an increase in shunt current will draw the armature or core toward the shunt coil, shortening and strength- ening its own magnetic circuit, and lengthening and weakening that of the series. This strengthens the pull of the shunt, while it weakens that of the series; the result being that the actions of the two coils are stronger and more positive when each has its own magnetic circuit. The arrangement for feeding the carbons in arc lamps is usually of the non-focusing type when intended for service where the utmost simplicity of mechanism is essential. All street lamps were formerly non-focusing ; lately, however, the care of electrical apparatus has become better understood, so that focusing lamps are quite practicable. The focusing arc lamp has several advan- tages over the non-focusing type. One of these is that the shadows of the side rods and bottom part of the lamp stay in the same place, and do not increase in size, as occurs when the arc travels downward on the negative carbon. Another advantage is that the heat is generated at the same point, and the globe is not unevenly heated rendering it liable to crack. The focusing arc also permits the use of a very efficient reflector, since the reflector can be placed and maintained close to the arc. (See Fig. 268.) In continuous current lamps this is not of such great importance, because most of the light is thrown downward anyway, but in alternating lamps it is quite essential. For use with holophane ARC LAMPS. 343 globes a focusing lamp is important, since the holophane is de- signed to diffuse and distribute the light properly when coming from a certain specified point, preferably an inch or two above the center of the globe. If this point of light should travel down- ward, as it does in the non-focusing lamps as the negative carbon is consumed, the action of the holophane would be much less reg- ular and satisfactory. The disadvantage of focusing lamps is the complication in mechanism required to feed the lower carbon, and the added difficulty of trimming. The lower carbon is usually drawn up by a chain passing around a wheel in the upper part of the lamp to the carbon-holder carrying the upper carbon. As the upper carbon descends, the lower one is drawn up, the action being regulated by a clutch working on some part of the wheel or chain mechanism. Another disadvantage of this type of lamp is the necessity for using carbons of different sizes. Thus a common combination is a 2 inch upper carbon 14 inches long, and } inch lower carbon 12 inches long, with a life of about twelve hours. If the lamps burn only seven or eight hours, as they do during a large part of the year, the remainder of the carbon is wasted. Whereas, in double carbon non-focusing lamps the same length of time would consume practically all of one pair of carbons. If it consumed more or less than an even pair of carbons, the remaining portion would be used to trim one side of the lamp, and a full set of carbons would be used in the other. Thus, no carbon is wasted, unless the lamps burn so long that more than one and one-half pairs of carbons are required ; in this case of course it would be necessary to fully renew both pairs. Again, carrying two sizes of carbons in stock is somewhat of a nuisance for arc-light stations and lamp-trimmers. Double Carbon Lamps. — Originally, in order to gain life, car- bons were made very long, and were therefore expensive and liable to break. A pair of carbons some 12 or 14 inches long, of a size that will give good efficiency and steadiness, such as ¥ inch, will not last longer than eight hours; and this was found too short for all-night service during the American winter. To meet this need, Brush invented the double-carbon lamp. In this lamp there are two independent sets of carbons, both fed by one mechanism. The Brush clutch, which is shown in Fig. 277, is the device originally used to feed one pair until it was consumed, and then the other. 344 FELECTRIC{LICAH TING. As will be seen, it consists essentially of a clamp holding two washers, each one encircling one of the carbon rods. The carbon rod is gripped by the tilting of the washer when the clamp is raised, and the clamp is so shaped (one of the jaws being wider and one washer having a larger hole than the other) that one washer is tilted more than the other. This causes one washer to grip its rod before the other. As the clamp is raised, the rod first gripped Carbon Rod Carbon Rod Washer IN Fig. 277. Original Brush Clutch. lifts its carbon from the negative, and the arc follows between the next carbon and its negative. In order to keep feeding the carbon last raised, and not the other one, a stop, or release, is so placed that on the descent of the clamp it will come in contact with the washer that has been most tilted, and release its grip on the rod before the other washer has struck the detent. When this carbon rod has descended its full length, the clamp in trying to feed falls still more, finally touching the release, allowing the second carbon rod to slip down and strike the arc. ‘The extreme simplicity of this mechanism found immediate favor, and the short carbons that it was enabled to use materially lessened the cost of arc lighting. Practically the same device is applied at present in a modified form in all double-carbon lamps. Carbon Feed. — Two methods are commonly employed to feed the carbons: In one, the clutch mechanism acts directly on the carbon, and this is termed direct or carbon feed. In the other. form, the carbon is gripped by a clamp attached to a long rod, sometimes to a chain passing over a wheel, and the feeding is done by the action of a clutch gripping the rod or wheel. When the rod is used the lamp is known asa rod lamp, or in the case of a chain or band as a chain or band lamp. Rod Feed. Taking up the rod feed first: The advantage it ARC LAMBS. 345 offers is that the carbon is positively clamped in a holder making good contact between carbon and metal. The surface resistance of carbon is very high, causing a great tendency to arc unless the contacts are broad and substantial. The metallic rod usually has a smooth and polished surface on which the clutch grips, so that the action of the clutch is quite regular.. If, instead of a tilted ring, a modified form of clutch is used, having a longer gripping surface, such as shown in Fig. 278, being the new Brush clutch, nicks or Fig. 278. New Brush Clutch. dirt on the carbon rod will not affect the evenness of the feed. The weight of the metallic rod makes its descent sure, and the current is fed to it easily by means of contact brushes or springs bearing on it. The disadvantage of the rod is that it requires to be polished or kept bright ; that it is apt to warp by heat, or be bent by carelessness in handling; and that the length of the lamp is greatly increased since the carbon rod must be considerably longer than the carbon itself. The chain-and-wheel feed is a modified rod feed in which the clutch grips the wheel or chain instead of the carbon. This avoids the chance of bending the rod; and, as the wheel may be inclosed in the lamp, does away with the necessity of polishing the working surfaces that are tarnished by exposure to the atmosphere, as in the case of the rod. But the wlement of friction is introduced in all lamps having chain feed, which is apt to cause irregularity in a lamp after it has been in service some time and the parts have become oxidized. Direct or Carbon Feed. Passing to lamps in which the clutch grips the carbon directly, it will be noticed that they have the advantage of minimum length, since the whole lamp need be but very little longer than the added lengths of the carbons used. The difficulty with these lamps is that the carbon has little weight, and will not descend with a positive action unless it is weighted by 546 ELECTRIC LIGHTING. some device at the top. This lengthens the lamp. Again, what is probably the greatest difficulty is the feeding of the current to the carbon. If this is accomplished by means of rings, or any sliding contact device, there will always be a tendency to arc if the parts become oxidized or covered with dust, since the contact between the. carbon and the ring or brush cannot be very good without introducing too much friction. Ifa flexible cord is used to convey the current to the carbon, it requires a form of carbon holder which will take up considerable room, and needs a guide to direct the upper end of the carbon ; furthermore a flexible cord is not a very reliable thing to employ, because continual bending of the wire and insulation will ultimately break the wire and fray the insulation. Clutches. — A clutch is a device intended to move freely over a surface in one direction, and grip it when the movement is reversed. The usual function of a clutch in an arc lamp is to grip the carbon or rod so that it cannot slip until the clutch has been opened by descending far enough to touch a release. There are any number of different types of clutch, but as a rule the simplest are the best. Clutches are frequently constructed like an ice-tongs, which grip when the supporting weight is on them, but release as soon as pressed down. (See Fig. 295.) Any cam sur- face will act as a clutch when the pivot around which the cam moves is fixed, and the object is gripped between the edge of the cam and another fixed surface. There is a critical angle at whicha clutch of this kind fails to work, varying according to the material of which the cam and the surface that it grips are made. A com- mon value for this angle would be fifteen degrees above the hori- zontal. A good clutch should release the surface that it grips with the minimum effort. -by the teleasexie lhe eitectsonma clutch sticking is to cause most of its weight to rest on the release, therefore varying the weight supported by the mag- net coils, and causing irregular lengths of arc. Ifa clutch sticks badly the arc will lengthen until it goes out, in the case of constant potential lamps. he srollerasclotch minh ig Meio aiseseit- evident in its manner of working. Such clutches as are not Fig. 279. Roller Clutch. ARC LAMES. 347 specially shown will be found, in their many forms, on the cuts of lamp mechanism in this chapter. Other forms of mechanism that are not so much used in modern practice are the rack or escape- ment feed and the hot-wire mechanism. The name of the former is sufficiently descriptive, and all lamps operating on this principle give the same trouble that a clock would if exposed to heat, dust, and atmospheric influences. When in good condition these clockwork lamps give fine results, but quickly get out of order. The hot-wire lamp, shown in Fig. 280, is the other extreme, since it has practically no mechanism at all. One end of a wire made of an alloy with a high coefficient of expansion is fastened to the frame of the lamp. The other end of the wire, which usually passes several times up and down through the lamp, is fastened to a clutch, gripping the carbon. This clutch is held up by a spring working against the pull of the wire. For an inclosed arc lamp, the wire is in series with the arc. When the lamp is cold, the wire contracts, and draws the clutch and carbon down. When current is turned on, the wire heats rapidly, and expanding allows the spring to draw up the; carbon, -springine the;arc. If the current in- creases, the wire expands further, allowing the spring to lengthen the arc and vice versa. Despite its apparent simplicity and positive action, there are several difficulties with this type of lamp. One is the lack of sensitiveness in the wire ; because small current variations have little effect upon the tem- perature, and a mechanism of this kind is slow to re- spond, compared with a solenoid or magnet. If the arc is accidentally extinguished, the length of time required to cool the wire so that the carbons touch again, is considerable. Such lamps are also af- - Fig. 280. Jones fected by the surrounding temperature unless com- — ot-wire age pensating wires are introduced, which destroy the simplicity. For alternating lamps the hot wire has the great ad- vantage of being free from magnetic vibration. It is essentially a cheap and simple construction capable of greater development. 348 ELECTRIC LIGHTING. Inverted Arcs. — In order to obtain diffused illumination the arc is sometimes inverted (Fig. 281), so that most of the light is thrown on the ceiling to be reflected downward. In such cases Fig. 281. Inverted Are Lamp. it is usual to feed the negative carbon as it is then the upper one. It may be a trifle larger and the positive a little smaller than usual, since the rising heat of the arc adds to the rate of consumption of the negative in- stead of the *positivew, Horea similar reason inverted arcs are _almosta third shorter fora given voltage than ordinary arcs, and the efficiency is diminished. This results because it requires more energy to maintain the heat sot thes crater jwhen. itis underneath, which is equivalent to a less length for the same energy. Ordinary lamps are also used, with upturned reflectors underneath. Lamp Mechanism. Fig. 282. The variations of open-arc mechanisms The Under Reflector. are almost endless, but with the previously explained principles any may be mastered with a little study. ARC LAMPS. 349 In Fig. 283 we have the Thomson-Houston lamp mechanism, showing the electric circuits. It consists of a seesaw lever, LL, pivoted at QO, and provided with a long tail, 7, the motion of which is modified by an air dash-pot. Below is an electro-magnet, J7/, in the main (circuit) and above isa second, S, which is connected as apenunt. seul hespoles pieces are. of conoidial shape, protruding through apertures in the armatures, aa, and 66, to give a longer range of pull. The lower and the upper arms of the clutch, marked’ R and £, close together when the tail, JZ, rises, gripping the carbon rod, C, and raising it. . The current enters the lamp through an insulated terminal at + P, flows first around JZ, and then goes to the frame of the lamp. Thence it divides, the main current finding its way to the upper carbon-holder, and so through the arc to the lower carbon- holder, whence it returns (by a route not shown) to the insulated Fig. 283. Circuits of Thomson-Houston Lamp. negative terminal, — P. A smaller portion of the current flows up around the shunt electro-magnet to — P. The arc is struck by the preponderating main current in // attracting the lever end of the seesaw lever, and raising the clutch. The feeding is accomplished by the increased pull of the shunt magnet if the arc tends to become too long owing to the carbons burning away. The resistance wire & from + FP to ¢ constitutes a cut-out cir- cuit, which is brought into operation by the augumented current in Son any failure of the main current. The small coil connected across from + / to the lamp frame is a mere adjustment to regu- late once for all the power of the series coil, JZ, relatively to the shunt coil, SS. Among the best-known open-arc lamps are the Brush (differ- ential), Fig. 284 ; Thomson-Rice (shunt), Fig. 285 ; Adams-Bagnal (differential-focusing), Fig. 286; Bergmann (constant potential), and Westinghouse (alternating); and many other successful forms. 350 ELECTRIC LIGHTING. - Mechanism of Inclosed Arc Lamps. In inclosed arc lamps a simpler mechanical construction has been reached, by reason of the greater length of arc permissible, and the introduction of inde- pendent units on constant potential mains. The general construc- tion of d.c. and a.c. lamps is similar, the essential difference being the lamination of the magnetic parts of the a.c. lamps. In a d.c. inclosed arc, the lower carbon is usually fixed in the bulb, and the upper carbon slides down upon it by gravity. To spring a quiet arc the upper carbon must be lifted about % inch, Fig. 284. Brush Lamp Mechanism. and remain there, when the current is, say 5 amperes. The cur- rent should stay at 5 amperes whether the armature of the sup- porting magnet is near the upper or the lower part of its travel, and whether it carries the full weight of a new carbon or half the weight of one near the end of its run. The carbon must feed down a little each time the armature drops to the feeding-point, without greatly varying the current. In any case the current must be fed to the moving carbon by some positive device, as the contact resistance of carbon is high and variable. ARC LAMPS. Son Effect of Weight of Moving Parts. The weight of a carbon 12 inches long and $ inch in diameter is 2 ounces. After a run about 6 inches will be left in the holder. Thus the weight of the moving parts has decreased by about one ounce. If originally the total weight of carbon, clutch, armature, etc., was 5 ounces, and it required a current of 5 amperes to hold them in balance, then 4 amperes will balance the weight at the end of the run, and the Chimney Base: : Divided Insulation -Rocker | for Tension Frame _ # Spring Ae Chitch AdjuSe~ Tension. | able | Spring CIuUECH 2. a Bridge ee ~Contack. Clutch oe Adjusting Dust-proof Screw Cylinder 7 Fig. 285. Thomson-Rice Lamp Mechanism. lamp gives less light. Unfortunately to this is added the effect of the dust which has collected in the bulb during the run. It is, therefore, very desirable to keep the current as nearly the same as possible, but this necessitates heavy moving parts like a carbon rod or weighted carbon-holder. With a total weight of 10 to 20 ounces, the decrease of current with carbon consumption is usually too small to be objectionable. Having settled on the allowable 302 ETE CTRICGALIGH TING: current variation from beginning to end of run, the next consider- ation is the current variation caused by the position of the armature. Effect of Position of Armature. The magnet or solenoid with 5 amperes of current should exactly balance the weight of the e oq sp / = TS ORO So V/ i r Cc disc) 0 G0 ap oo 2 Be fe} Y Fig. 286, Adams-Bagnal!l Lamp. Fig. 287. Carbon-Feed, Constant Potential, Direct Current Lamp. moving parts anywhere within their limits of travel, usually 2 to of an inch. The best procedure to ascertain the accuracy of a mechanism is to disconnect the are gap, and substitute for it an adjustable resistance. Adjusting the current to 5 amperes, the armature will usually be found to travel upward when near the ARC LAMPS. BOS solenoid, but fall when some distance down. This would be a departure from perfect design, and should be avoided as much as possible. Magnetic parts can be constructed to balance at any part of a §” travel within one per cent of the normal current. It may occur that with an armature in the upper position only 41 amperes will be required to hold it there, while at the lower limit of travel 5} may benecessary. Such alamp would have a variation of at least 14 amperes, depend- ing on the position of its arma- ture. The latter varies any- where from top to bottom, according to how delicately the clutch feeds the carbons and according to the voltage. If the clutch allows only a very small length of carbon to slip down when it feeds, the armature will tend always to hang near the. position of clutch release unless the line voltage rises accident- ally. Effect of Levers and Com- plications. Some lamps contain combinations of levers or link “motion by which the balance of the moving parts is affected. For instance, the carbon and holder may be partially counter- balanced by weights, or the clutch or solenoid may act through a lever or at an angle. Fig, 288. Electrical Circuits in Fig. 287. In such cases it is important to see that the introduction of the levers, etc., does not vary the weight upon the solenoid by reason of the leverage changing with the position of the armature. It is also necessary that the net weight, after deducting for any counter-balancing, is large com- pared to the weight of the carbon alone. Effect of Friction. In all lamps the working parts and carbons move with more or less friction in the guides, dashpots, gas-cap, Aya ELECTRICOLICHTING. etc. This introduces a variation in current equal to double the current equivalent of the friction. This may be detected as fol- lows: stroke the moving parts gently in a downward direction, and release them gradually. They will rise until the solenoid has overcome the weight plus the friction; then note the current, which may be 54 amperes. In the same manner raise the moving parts slightly, and allow them to descend gently, when the solenoid over- comes the weight minus the friction, and the current will be, say 43 amperes. This would bea frictional variation of one-half ampere. Causes of Variation of Arc Voltage. It is, of course, under- stood that variations of current produce corresponding changes of voltage at the arc, where the arc is in series with a resistance as it is in constant potential direct current work. A further change arises from the increase of the resistances in series with the arc due to heat. A resistance drop of perhaps one volt occurs in the average carbon, possibly two volts in the magnet winding, but the larger part in the series resistance. When the lamp has become thoroughly heated, the drop may have increased nearly one volt in the magnet, and several volts in the resistance, unless it has been made of material having a low temperature coefficient. If the total increase in drop amounts to more than three or four volts it becomes quite noticeable in the arc. We must also take into consideration the fluctuation of line voltage superimposed on the resistance variation, which may be anything from two to twelve volts or more. If the lamp is perfectly adjusted to maintain con- stant current, the drop in the resistance being the same, the arc voltage varies exactly with the line voltage; if out of adjustment, the arc voltage is still more unsteady. Cumulative Effect of Disturbances. The several disturbing factors are sometimes cumulative, and tests on the same lamp at different hours of the day, with varying positions of the armature, different periods of the run, etc, often show an astonishing variation. For example, take the following data for a lamp, not worse than many found in everyday practice : — Current balancing a 12” carbon in a given position of armature . . . . 5’ amp. 6“ “ 6” 6c “ same 9 6 “ oe eg ee a5 amp. (74 ° h . tb h h . . . . Tide | with a given carbon when mechanism is nising .... . . . 5: amp. ARC LAMPS. SH Current with a given carbon when mechanism is falling . .. .. . . 42amp. ue balancing a given carbon (armature up) ee Sane eel Bees TTI: : 4 aS = ( come OW! Meier mene mts... \./ OR aTnp, mesistance (magnetand sesistance wite) cold’; 377s. 7... . \Gobms. HOGRe se hal alert ye te Tee col OMS. Mains at different hours vary from 117 to 112 volts. With the lamp hot after the first few hours of its run, armature low, and being raised against friction of carbon in gas-cap, etc., the current and voltage will be : — Current = 53 amp. due to long carbon + 3 amp. due to position of arma- putea zai pere Cueto NCuOn, ames ka gh-8 os bes os) oa. G2 amperes Dropuntresistance Hot (762) 0. | aramen P- We i es 4a. D.VOS With mains at 112 volts, we have arc voltage (112 SOs OMe hy ie) see UO.2eey OLS with 6% amperes of current. With lamp cold, started after 140 hours previous burning, armature high and descending, the other extreme condition will be : — (Current = 5 amp. due to short carbon—?dueto friction . . . . . 42 amperes Resistance (maser anurresiatanceywite) COld * 1. i SM is 6 ohms Drop in resistance (6X42) . . . Pie ott) he apoyo ZO TT YOltS With mains at 117 volts arc voltage is (117283 ) Sue ts Awe le” iL eas Goe-NOltS with 43 amperes of current. This lamp might vary from 88} volts at 42 amperes to 68} volts at 6 amperes, without considering the variation introduced by the clutch release supporting more or less of the weight of the moving parts at the feeding-point. When it is remembered that the same lamps with the same adjustment are used in different parts of a building with an isolated plant, or in various parts of central-station territory where the extremes of voltages at different hours are much larger than those given above, and the lamps them- selves worse, it is not surprising that some lamps are unsteady, give irregular life, and show blackened bulbs. High arc voltage produces short carbon life and clean bulbs, low voltage vice versa. High current tends to burn the dust into the bulb, deposits uncon- sumed carbon on it, and may even soften it. Poor inclosure at gas- cap edges, center, or at the base of the bulb, reduces the life. Too perfect inclosure results in blackened bulbs, due to unconsumed carbon. Gas-caps often warp after being greatly heated, asbestos washers for bulbs often chip, dashpots stick, etc., so that when difficulty is experienced in the operation of a lamp it must be 356 FLECTRICHMOE ENG patiently ferreted out, whether due to the lamp construction, the carelessness of the trimmer, or the fluctuations of the circuit. The curves below illustrate the result of weeding out difficulties in lamp mechanism one by one. Fig. 289 shows the lamp current, and Fig. 290 the line voltage of a poor lamp under unfavorable conditions. Fig. 291 is current for the same lamp, its faults cor- rected on the same circuit, the voltage card being the same as LESS PEE PBS SSS WN SN = Direct Current OBSERVED AT aoe tees a Sy <> Were a ee mat ee gt Tt ae ey i tami ae =a 8 EPL10:50 a.m. %7 — 1:05 p.m. 940=25shst S 60 hrs. to 85% hrs, 6%" and 2'%5 £7 Ly rs alia oe fos Hu te, ee ae = j e\\ isnt Wie SSS ui S ii RU \\ ‘Mi NY i it wu wh RUAN me \\\ LT HTT rreLet aan \\\ Fig. 289. Current Variations in Badly Adjusted Lamp. before (Fig. 290). As modified, it shows constant current under violently fluctuating line voltage. . Function of Dash-pots. Air dash-pots are used in most lamps to overcome the tendency of the moving parts to overshoot on account of their inertia. When constructed so.as not to become corroded, clogged with dust, nor warped by heat, they offer very little resistance to slow motion, but effectually check the too rapid or excessive motion due to inertia. ARC LAMPS. Bot Usual Current and Voltage. In incloseda arc lamps for con- stant potential direct current circuits, the’ arc is usually run with 5 amperes, at about 79 volts. The voltage may be as high as 95 volts if the mechanism is sufficiently sensitive. It-is usual to interpose resistance alone between the mains and the,arc, but it is claimed that a considerable amount of self-induction can be used to replace part ot the resistance. As a rule, the higher ry \ | oa i it KL \\ ACOA awe Sea Ny dela 8 8% AH au 2:30 p.m. %3q — 3:30 p.m. 31 tH ey, HA tH KT TAU SSek | sh SS A ane ax | | yry TL “y K \S aw WY RSS ORS me i [Nee N/ Pa ie WA \\A Fig. 290. Variable Voltage of Supply Circuit. the line voltage, the higher the permissible voltage at the arc, but only within narrow limits. High-voltage arcs tend to waver and cut out. Roughly speaking, the most economical line voltage for d. c. constant potential inclosed lamps is about 105 volts. Constant Potential Alternating Inclosed Arcs. For constant potential alternating circuits the inclosed arc has come into exten- sive use, with a voltage of 70 to 73 at the arc and 6 amperes. 358 ELECTRIGCHLIGITING. Such lamps (Fig. 292), are similar to direct current lamps except for the lamination of the magnetic parts, the use of an adjustable reactance coil in place of a resistance, and the cushioning or spring suspension of the parts that vibrate and tend to produce humming. The reactance coil is usually adjustable to compensate for secondary voltages between 100 and 120, and cycles from 60 to 140. Since the lower carbon is consumed nearly as fast as the ea ea an eueasl 7 Ear RD SQ pee a <2 Direct Current OBSERVED AT aot AS s eS HH Chart ( JOCASIB LTS ou as Oe . — LE Se I, EEN AK? oe ee SE Ly aueN SES (SR f ay Fig. 291. Current in Readjusted Lamp. upper one, a. c., inclosed arcs are provided with long bulbs to per- mit of a long lower carbon being used. As already mentioned, one cored and one solid carbon give the highest voltage arc that will be stable. The upper is usually 9} inches long for a six-inch lower, this difference being necessary because the upper carbon is consumed somewhat faster, and because it must project from its holder down through the gas-cap far enough into the bulb so : ARC LAMPS. 359 that the arc will not be sufficiently near the gas-cap to injure it. The life being limited by the length of the lower carbon, which, in turn, is limited by the length of the bulb, these lamps burn only 80 to 100 hours. Of 625 apparent watts a good lamp con- Fig. 292. Carbon-Feed, Con- Fig. 293. Electrical Circuits in Fig. 294, Carbon-Feed, Con- stant Potential, Alterna- Fig. 292. stant Direct Current, ting Current Lamp. Series Lamp. sumes only 430 to 450 true watts, of which 35 are lost in the reactance coil. Direct Current Inclosed Arcs for Series Circuits. Inclosed arcs for d. c. series circuits are either shunt or differential the same as open arcs. (Figs. 294, and 295.) They are used on cir- 360 LOL LEGA Deter alee 10 : cuits of 5 to 6.8 amperes, with 68 to 75 volts at the arc, and burn from 100 to 140 hours. The differential lamps require some temperature correction to compensate for the decrease of current in the shunt winding of the magnet, as its resistance rises with heat. They may also be readjusted to circuits of various current —Shunt Magnet Starting Resistance Fig. 295. Electrical Circuits in Fig, 294, strengths by shunting more or less current through a small auxiliary resistance, and like open arcs are usually provided with a resistance in series with the automatic cut-out. When the cut- out operates, it short circuits the lamp terminals through this resistance, thus maintaining sufficient difference of potential: ARC LAMPS. 364 across the magnets to enable the lamp to start up again if nothing is wrong with it. Inclosed Arcs for Series Alternating Ctrcutts. Many alter- nating stations have street-lighting circuits, most of which have been operated by series direct current machines and lamps, thus employing two kinds of generating machinery. Series circuits are particularly adapted to widely scattered lamps outside of the regular lighting service, on account of the low cost of the single-wire series line. Furthermore, it is usually desir- able to have the street lamps on one or more independent circuits, so that they may be lighted or extinguished from the station. By using the series alter- nating arc system this can be effected without separate machinery. It con- sists of the lamp circuit, and a device connected to the regular constant po- tential alternating mains, which will maintain a constant alternating current in the independent lines. One method is to use a transformer already described (p. 171) having ex- cessive magnetic leakage, whereby the secondary is made to deliver constant current. Another device is an auto- matically varying reactance coil con- nected directly to the primary mains, “which adjusts the amount of self-induc- tion in accordance with the instanta- Fig. 296. Carbon-Feed, Constant Alternating Current, Series Lamp. neous impedance of the series circuit in such degree as to maintain constant current. Such devices are capable of keeping the current at 6 amperes, within one-tenth ampere either way. Of the two, the variable self-induction has the advantage of low first cost and repair, small space, and greater simplicity. When of the proper size for the number of lamps to be governed, its power factor and efficiency are higher than those 362 ELECTRICALIGHTING: of the transformer, and it permits of simple adjustment to greater current strength, if more light is demanded by the municipal authorities. It has the disadvantage of connecting the primary mains directly to an external circuit. Fig. 135 shows the internal arrangements of a 100-light General Electric series transformer which was fully described on page 172, and Fig. 298 the appear- ance of a Manhattan regulator. Series Magnet ee Starting esic cance | Ik Me aa | Fig. 297. Electrical Circuits in Fig. 296. In the regulator a reactance coil in series with the lamps is balanced against a weight in such a manner that it incloses more or less of the central leg of a W-shaped core. When the current rises, the coil is magnetically drawn in by the core, thus increasing the self-induction, and reducing the current to normal. Fig. 299 shows the mechanism of a Manhattan series alter- nating lamp, which is of the shunt type. The cut-out includes a small magnet in its circuit, which holds the cut-out contact closed when it has once operated after the carbons are consumed. ARC LAMPS. 363 The number of series a.c. lamps permissible on a circuit. with a regulator is approximately the line voltage divided by 80. More than this tends to cause simultaneous jumping of all the lamps. In either system the watt loss in the regulating device is only the iron loss and the small resistance loss, but the regulator has some- what less than the transformer. The shunt lamp has the same voltage at the arc as at the lamp terminals, while the differential Fig. 298. Constant Alternating Current Regulator. lamp has 4 or 5 volts less. The arc, in either case, runs at about 70 volts with 64 amperes. The power factor of an entire system of regulator and lamps may be as high as 88 per cent. Light Effictency. The luminous efficiency of the series alter- nating lamp is considerably lower than that of a corresponding direct current series open arc, and half of the light would be directed upward were it not for the reflector, which may send downward again nearly 90 per cent of the up-going rays. Despite its lower efficiency, it gives satisfactory and high apparent illumination, since the pupil of the eye is not contracted by the lower light intensity from the large surface of bulb and reflector, 364 ELECTRIC LIGHTING. nearly as much as from the small and intense crater of the open arc. The pavement below the lamp is more free from the vio- lently contrasting bands of light and shade, characteristic of the open arc, thus producing the appearance of more uniform bright- ness. For these reasons, largely, municipal authorities often prefer the series alternating lamp with its lower efficiency to open direct current lamps consuming the same energy. Fig. 299. Series Alternating Current Lamp. Fig. 300. Mounting of Hood upon Pole. Street Arc Lamps must have additional features, such as hoods, pulleys, mast arms, suspension hooks. Hoods should be so arranged and set on the pole tops as to cast the minimum shadow. (See Fig. 300.) Where a lamp is suspended it is well to have some arrange- ment which cuts out the lamp when lowered to trim. That the lamp is held independent of lowering rope, and hence ~ will not fall if rope should be broken, is a valuable feature of any arc suspension. (See Fig. 301.) These pulley suspensions hold the lamp when raised, and release it when about to be lowered. ARC LAMPS. 365 On raising the lamp a knob is engaged by ridges on the sides of the pulley, and takes all strain off the rope. A pull at the rope guides the knob out, so that the lamp can readily be lowered. Mast arm and pole arrangement is shown in Fig 802. A suspen- sion canopy is shown in Fig. 303, and side bracket suspension in Fig. 304. An arc light cut-out switch is represented in Fig. 306. Fig. 301. Cut-out Pulley. Fig. 302. Fig. 303. Hood for Suspension. Projection Apparatus. Special forms of arc lamps are used in electric projection lanterns, photo-engravers’ lamps, stage projectors, locomotive headlights, etc. The life of the carbons in such appa- ratus is of minor importance, and they are usually of the open-arc type. The carbons are generally inclined away from the object so that the maximum rays at an angle of about 45° (Fig. 307) from the axis of the positive carbon will be directed nearly horizontally at the point to be illuminated. Besides being tilted, the upper 366 ELECTRIC LIGHTING. carbon is often set back somewhat out of line with the negative, which brings the crater at an angle without requiring the tilting of the carbons, as represented in Fig. 306. These lamps are Fig. 305. Series Cut-out Switch. Figs. 306 and 307. .Position of Carbons in Projector Lamps. frequently made with hand or clock work feed, partly because of the diffi- culty of feeding inclined carbons by gravity, and because they are under the care of: an operator. A reflector é F ‘ Fig. 808. Search Lamp with with a polished or dead-white surface [Ror ast is placed dehznd the arc. In searchlight projectors, on the contrary, the arc is directed toward the reflector and away from the object to be illuminated. This is done in order that all the emerging rays shall be parallel, in which condition their intensity is theoretically the same at any ARC LAMPS. 367 distance but practically not, owing to unavoidable dispersion due to the size of the light-emitting surface, aberration in the reflector, and the refraction and absorption of the atmosphere. With the crater turned toward the front of the search-light all of its rays that did not strike the reflector would be divergent instead of parallel. In search-lights the carbons may be either inclined or horizontal, in which latter case the positive is in front. The Fig. 309. Search Lamp. carbons may both be solid if of high-grade soft carbon, but frequently the positive is cored. To keep the arc stationary the carbons must either be fed at different rates of speed, or may be suitably proportioned, the positive being cored and larger in diameter than the negative. “Any variation of the arc from the focal position may be corrected by hand, using the colored-glass windows provided for that purpose. The feeding’ mechanism is 368 ELECTRICOLIGHTING usually motor-actuated so as to be positive. At the sides the light is surrounded by a cylindrical casing, supporting at the rear a reflector far enough away not to be damaged by the heat of the arc. These reflectors, in cheap search-lights for mining and contractor’s work, are sometimes made of silvered copper, but are preferably of glass. The latter are made in two styles, aplanatic and parabolic. The aplanatic or Mangin mirror has two spherical but not concen- tric surfaces, as shown in Fig. 8308. Owing to its unequal thick- as Lom TTT —— — ne 7; i = . = a * SSS Fig. 310. Search Lamp. ness it is somewhat more liable to crack than the parabolic. The latter is a truly parabolic piece of silvered glass about 4 inch to % inch thick throughout. The front of the search-light is closed by plate-glass strips (Fig. 309) instead of one piece, to avoid breakage by heat, and to allow easy renewal of a broken section. In case the light is intended to cover a preater area, that 1s “to diverge, a diverging front of lens strips, usually for a 20° diver- gence, may be swung into place instead of the plane strips, although ARC: LAMPS. 369 the same effect may be produced by moving the arc out of its correct position. Average values for the current in search-light projectors of various sizes are about :— 45 amperes for an 18-inch light SOY Ha ia DAN | ata ce Lobe gs B05 cates TEO Ee 5 Se, SGU ek 200 «§ « & 4B « « The commercial ratings of candle-power of search-lights are mis- leading. The average light intensity of the beam is multiplied by Fig. 871. Projection Lantern. its area to get the total flux. The rated candle-power is then obtained by dividing this figure by the area of the crater, which gives relatively but absurdly high values to the candle-power. Thus, supposing that an arc with a mean hemispherical candle- power of 10,000 were combined with a Mangin reflector two feet distant, the average illumination on the mirror would be 10000 (HF = 2500 candle-feet ; and if we neglect small losses, this will 310 ELECTRIC LIGHTING. be the intensity of the beam. To multiply this value by the ratio area of beam area of crater denominator a figure much greater than 10,000, the true candle- power of the arc. A more rational method would be the product of the light intensity of the beam in candle-feet, by its area. The movements of the beam of light are produced indepen- dently of the arc mechanism, by hand or by distant motor control. In the latter instance the projector may be provided with a vertical wheel and chain actuated by a motor, which turns the barrel around a horizontal axis, as well as with a motor-driven revolving base, to swing the whole lamp in either direction. These motors may be operated by a distant controller, and the lamp suitably fitted with a two-joint receptable for the lighting cables and usually a five-point socket for the motor cables , gives for any possible values of the numerator and 2NTERIOR WIRING. Sik CHAPTER XVI. INTERIOR WIRING. Tue laying of electrical wires does not appear to be as impor- tant from the engineering point of view as the construction of overhead and underground conductors ; nevertheless, an additional and most important consideration is involved, this being the fire hazard. When electric lighting was first introduced this difficulty was so great, being naturally magnified by prejudice against the new method of illumination, that insurance and municipal fire department authorities were often strongly opposed to the intro- duction of electrical conductors into buildings. But improvements in methods of construction have gradually reduced the risk, until now insurance companies and fire departments consider electric lighting less dangerous than any other form of artificial illumina- tion. This is undoubtedly a fact; but electrical wires are still the cause of many fires, the consequences of which are often very serious. Hence, it behooves those who are responsible for the installation of electrical apparatus and conductors inside of build- ings, to exercise the greatest possible care. This is the more neces- sary, in view of the cenditions under which electric. light wiring must be installed to meet the varied requirements. In a large class of installations no small amount of judgment, ability, and ingenuity is often required to overcome the difficulties met with, to adapt the material at hand to the purposes, or to devise new methods to secure unusual results. Slaughter houses, dye houses, chemical works, bleacheries, and breweries offer many peculiar difficulties to proper wiring. Before the actual interior wiring can be of use it must be connected with the service wires, and this necessitates in most cases that at some point the wires enter the building. In order that the moisture may not travel along the wires from outside to the interior installation, there is at the service entrance a drip loop outside ; and the hole through which the conductors must pass is SZ ELECTRIC TIGHIING. bushed with a drip tube, which must slant up towards tke inside. (See Fig. 8312.) The wire entering these tubes should have solid rubber insulation, at least ¢; of an inch thick, and covered with a sub- stantial braid. The space between the wires as they enter the building bn, 4g should be at least one foot, and ar- Sete Oe Diet rits oe ranged so that no cross connection can be made by water. The wires should never come in contact with anything but their insulators. Running them along the face of the building should be avoided, and they must be fastened to the wall near the entrance tubes by insulators mounted on special brackets having two coats of water- proof paint to prevent the absorption of moisture. Automatic cut-outs such as circuit breakers or fuses should be placed on each of all service wires as near as- possible to the point where they enter the building, on inside of the walls, and arranged to cut off the entire current from the building. The wires then run to the service switch, which should be capable of opening the circuit when carrying the entire current of the building. This is a knife switch, and should be installed so that the handle will be up when the circuit is closed, so that gravity will tend to open the switch, rather than accidentally to close it. | With alternating systems the best place for the transformer is on the pole away from the building. The transformer, when placed on the outside wall of the building, must be hung on well- insulated supports, the construction being as shown in Fig. 318. Where it is impossible to exclude it from the building, the proper place is a vault or room with brick walls containing nothing else but transformers. As a last resort it may be put in a part of the cellar where it is well ventilated and dry, being carefully insulated from the walls and the ground. The next piece of apparatus in the building is the switch board or in small installations the distributing panel board. This wili carry the meters, the knife switches and the fuses for the feeders. If electric power is to be used besides the lighting the separation of the two kinds of circuits should be made at this point. The principal methods which have been, or now are, used io carry the wires from the entrance devices to the ‘lamps are as follows : wv lh ill INTERIOR WIRING. 3190 (1) Wires inclosed in molding. (2) Wires carried by wooden cleats. - (Obsolete.) (3) Wires carried by porcelain cleats or knobs in open work. (4) Wires carried by porcelain knobs and tubes concealed. (5) Wires concealed in plaster. (Obsolete.) (6) Wires concealed in tubes, interior conduits. (7) Wires laid on some cornice, wainscoting, or other architectu- ral feature adapted to the purpose. (8) Fished wires. (Not desirable.) ee y pe 7 aed Ypp | WW PRIMARY |} CUT-OUT. |f <2 S32 Ni (FZ poe ee LT ] | pragma A ie 1 A Y : : a i k— W) : | TRANSFORMER. AL L y (eat Ey al ee Pele ripih | DETAISUF OC Yj \ \ eR RE \ G USUI at ee 199) = ee is ite. | za ! rm 184, WATER LOOP, v WATER LOOP, 7 a i _ edb ie tes i ra 3 DIA 15/6 DETAIL OF A ech From Standard Wiring by HC. Cushing. jr; CONSTRUCTION WORK INSTALLING TRANSFORMERS Fig. 318. 374 ELECERIC TAGHIING Three of these —i.e., the second, fifth, and eighth— are no longer considered good practice, in fact they are forbidden by the National Electric Code. In order, however, to fully appreciate the difficulties in this important branch of electrical engineering, it will be well to consider all of the above methods in the order given. Wooden Molding. — The advantages of this construction are simplicity, cheapness, and accessibility. It is particularly appli- cable to buildings in which no provision has previously been made for electrical conductors, the wires being laid after the building is completed. At first this was the general condition, and a very large proportion of the wiring laid during the early history of elec- tric lighting was installed in this way. At present the use of electric light is decided upon, or at least contemplated, before the Lh 4 LLL AL Miedo tii Yt Fig. 315. Two-wire Molding. building is erected, and the plans ‘provide specially for it. In such cases, particularly where expense is not a prime consideration, the use of the so-called “interior conduit,” laid in the walls, is the standard practice for low tension (below 450 volts circuit). For high-tension wires, the only approved plan is to carry them upon porcelain knobs or cleats. These two methods will be considered later, in their proper order. P Wooden molding usually has the cross-section represented in Fig. 815, consisting of a strip or “backing,” in which are cut grooves corresponding to the number of wires to be laid, only one conductor being placed in each groove. The backing is fastened to the wall by thin wire nails or brads, being made continuous as far as possible. Angles and branches are formed by fitting pieces together, as indicated in Fig. 817. The wires are then laid in the grooves, being also preferably continuous, although joints are INTERIOR WIRING. 315 allowed, provided they are securely made mechanically by splicing and soldering, and provided the insulation is made equal to that of the rest of the wire by careful wrapping of tape. The capping is then put in place, and fastened by small tacks or brads. Molding has been used in which the grooves were formed in the capping without any backing. This, however, is bad practice, and should not be adopted even where the wires are laid against a wooden wall or ceiling. The chief disadvan- tages of wooden molding are the facts that it is not sufficiently impervi- Yiiyy ous to moisture, is liable ++ Ty tH: tiégzy to be crushed or punc- WY tog tured mechanically, and is combustible. These difficulties are overcome as far as possible by coating the molding, both inside and out, with water-proof paint, or by impregnating it with moisture repellent. It is also recommended that only hard- wood molding be used. But soft-wood molding is often laid be cause it follows the wall line better. In the standard forms, tne backing is at least three-eighths of an inch thick under the grooves, and one-half an inch between them. The capping should ti iffy GVH. Vs : eens 816. Three-wire Molding. SetmillarOnec-cix eclt incor alulinch WoL more, into the backing, and should lap over the grooves not less than one- eighth inch on each side. These min- imum dimensions are represented in Figs. 815 and 316, but much larger sizes of molding are used for heavier Fig. 317. Right Angle Joint in molding. wires. Rats gnawing through a molding may destroy the insulation of the wire, and bring the copper in contact with the wood. Wires for use in molding must have rubber insulation, at least @ of an inch thick; and as the size of the wire increases from No. 14 to No. 0000 the thickness of the rubber changes to>,2, inch. In molding where one of the wires must cross over, it is a her ELECTHICANGHIING. brought out through the capping and across it, so that a certain thickness of wood is between the two conductors. Wooden Cleats. — These may be looked upon as a discontinu- ous molding. In fact, their cross-section is practically the same. Their use is rarely to be tolerated at present, cheapness being their only recommendation. Experience has shown that it is a great mistake to attempt extreme economy in the laying of electrical conductors. The serious difficulties which arise in the shape of damage by fire and interruption of service are far more expensive in the long run than a considerable increase in first cost. Wooden cleats have all the disadvantages stated for wooden molding, and are open to two additional objections. One of these is the fact that the wires are left exposed fora large portion of their length, and are therefore liable to be injured or to form a short circuit or ground connection by coming in contact with each other or with some pipe, nail, or other conducting body. Wooden cleats are also likely to have small splinters projecting from them which cut through the insulation of the wires, and have been found to be a source of much trouble. Porcelain Knobs or Cleats. —In open work various forms of these devices are used. (Figs. 818 to 321.) This construction Figs. 3818-821. Porcelain Knobs. seems to be open to the same objection as the use of wooden cleats, the wires being entirely exposed. between the points of support. Nevertheless, as already stated, this construction is prac- tically the only one allowed for high-tension circuits (over 450 9 INTERIOR WIRING. ales volts) inside of buildings. The explanation of this apparent anom- aly is found in the fact that high-tension circuits are very carefully treated when brought within buildings. For example, the primary circuit of the alternating current system is rarely allowed to run more than a few feet after it enters a building, the potential being immediately transformed down to a safe value of about 100 or 200 volts. Even in such cases the high-tension wires are only per- mitted in the cellar or other portion of the building not generally used; in fact, the transformer is usually placed outside of the building wherever possible. The series arc lighting circuits which are also high tension (2000 to 5000 volts) are most carefully laid when brought into buildings, the path being as short and direct as possible, and located where the wires are not likely to be touched by persons or to come in contact with anything but the insulators. They must be rigidly supported on glass or porcelain insula- tors, which raise the wire at least one inch from the surface wired (Lia a baal wu pl Oh =F eon Figs. 322-824, Porcelain Cleats. over, and must be kept apart at least four inches for voltages up to 750 and at least eight inches for voltages over 750. Rigid supporting requires under ordinary conditions, where wiring along flat surfaces, supports at least about every four and one-half feet. If the wires are unusually lable to be disturbed, the distance between supports should be shortened. Such circuits are never introduced into buildings to anything like the same extent as low-tension wires, which run in great num- bers to all parts of most modern structures. The porcelain cleat is, moreover, free from the splinters which constitute a serious objection to wooden cleats. Glass insulators may be used instead of porcelain, but the latter is usually to be preferred because it is stronger, tougher, and less hygroscopic. The statement is often made that “any material which is non-conducting, incombustible and non-absorptive”’ may be used, for this or other similar purpose. In point of fact, porce- 378 ELECTRIC LIGHTING. lain and glass are practically the only available substances which fulfill these requirements ; but if any other equivalent material can be found, its use would be permitted. For concealed “knob and tube”’ work: the wires are run on the timbers and studding by means of porcelain knobs, and the wires tied to them by tie wires of equal insulation to the main wire. The wires are carried through the beams by WA Ny AN i} 4 means of porcelain tubes. These tubes are (TAN. AAI set in the beams by forcing into auger-holes, and are kept in place by the friction and by : x | ? | the head formed on one end of the tube. hy, Inhale : (Fig. 825.) All the porcelain devices must Fig. 326. Porcelain Tube) een tthe wire (One Inc nek Oniatir mel ace wired over, and the wires must be kept ten inches apart. They are preferably run on separate beams. They must be stretched so as to have no sag, and are to be supported every four feet, or even closer when necessary. This style of work is much used in coun- try houses, where an installation for a ten-room house costs only about forty dollars. The outlets are protected by a canvas jacket called circular loom, or by a curved porcelain tube, or even one of the beam tubes may be used for the purpose. Mill construction: in buildings of this character mains of No. 8 wire or over, where not liable to be disturbed, may be separated 4 inches, and run from timber to timber, not breaking around, and may be supported at each timber only, otherwise, the construction in Fig. 826 or the plan of running through the timbers in Fig. 327, which cut also shows a boring-tool for this work. Unless some special tool is used, the holes will not be in line, and unsightliness as well as waste of wire is the consequence. Wires in Plaster. — Another method of concealed wiring which at one time was considered to be an ideal one, consisted in embed- ding the conductors in the plastering of the walls and ceilings of a building. This method could be adopted either during the original construction of the building, or in case of repairs or re- plastering. It was employed in many fine structures where the best construction was desired, regardless of the expense; but it was soon found that the detrimental effect of the lime in the plaster upon the insulating material rapidly injured or destroyed it. INTERIOR WIRING. 379 Furthermore, the changing or repairing of a wire was rendered very difficult, necessitating the tearing away of the plastering, the trouble being aggravated by the fact that the exact location of Fig. 326. Factory Wiring. the wire was hard to determine. The result is that at the present time the National Code distinctly states “that wires must not be laid [directly] in plaster, cement, or similar finish.”’ Fig. 827. Wiring through Timbers. Interior Conduits. — The most approved method of low-tension electrical wiring consists in providing tubes, usually laid in the floors, walls, or ceilings of a building while it is being erected, in 380 ELECTRIC LIGHTING. the same general manner that gas-pipes are put in. The wires are drawn into these conduits when the building is nearly com- pleted. It is interesting to follow the development of this stan- dard form of construction. At first interior conduits consisted of tubes of insulating material, that is, vegetable fiber impregnated with resinous matter. Experience showed that this insulating tube was likely to be crushed or perforated by nails, either during the construction of a building or afterwards. Hence the next step was to protect the insulating tube with a thin sheathing of brass, giving the so-called “ brass-armored conduit.’’ Even this was not found to be an adequate protection against mechanical injury ; so that an iron or steel pipe, about equal in strength to an ordi- nary gas-pipe of the same diameter, was substituted for the thin brass sheathing, producing the well known “iron-armored con- duit.” The final stage of the development was reached when the National Electrical Code of 1897 allowed the use of plain iron. or steel pipes as conduits, “ provided their interior surfaces are smooth and free from burrs; pipe to be galvanized, or the interior surfaces coated or enameled, to prevent oxidation, with some substance which will not soften, so as to become sticky, and pre- vent wire from being withdrawn from the pipe.” This evolution clearly shows that the object of such a conduit is to facilitate the insertion or extraction of the conductors, to protect them from mechanical injury, and as far as possible from moisture. These tubes or conduits are to be considered merely as raceways, and are not to be relied upon for insulation between wire and wire or between the wire and the ground. On the other hand, the presence of a lining of insulating material is undoubtedly an advantage in most cases, and it would probably be worth the extra expense that it involves. The permission of the National Code to use plain iron or steel tubes in no way implies that they are better than insulated conduits. It simply means that the general use of some form of conduit is to be encouraged, and to this end restrictions are removed as far as possible. | The various styles of house conduit, such as brass-armored, paper-lined tube, etc., have been gradually discarded; and the standard conduit of the present time is either iron pipe with an inner insulating lining, or iron pipe with an enamel finish inside and out. The objection to rubber compound insulating lining is INTERIOR WIRING. 38 the fact that if the pipe is subjected to any heat, owing to its being in a hot boiler-room or other location where there is an unusually high temperature, the insulation somewhat deteriorates, and renders it very difficult to pull out old wires and replace them with new ones. The enamel conduit is rapidly growing in favor, owing to the fact that it forms a good raceway for the wires, and it is not subject to the above-mentioned disadvantage of the insulated conduit. Electro-duct, Armor-duct and Loricated Pipe conduits are iron pipes with the inner and outer surfaces covered with enamel, whose service is to render the conduit rust-proof, as rust is highly injurious to rubber, which is: the insulation used on wiring in interior conduits. There are many conduits having an insulated lining, such as Armorite and [ron-armored conduit (Fig. 3828). These. insulations are of paper, wood- fiber impregnated with a moisture repellent, or are of some bitu- Fig. 828. Insulated lron-armored Conduit. minous compound. There is also a form of conduit which is lined with vitrified clay pipe. ‘¢ Fished Wiring.’’ — In order to avoid the unsightliness of ex- posed wiring, various methods of concealed wiring have been in vogue. The obvious plan of running conductors through the spaces in walls, floors, etc., of buildings, has been followed from the first. In one method of doing this, the wire is pushed or drawn by hooks from point to point, trusting largely to chance. Hence the process of introducing it is called “fishing.” This hap- hazard method of laying wires is not to be recommended, since it is evident that the wires may come in contact with nails, steam and gas pipes, sharp edges of beams, etc., which might cause serious difficulties. In some cases, however, it may be the only practicable way to carry a conductor from one point to another. In this case the attitude of insurance authorities on the matter may best be shown by quoting from the National Electrical Code, Rule 24, Cand S. “Wires must not be fished for any great dis- tance, and only in places where the inspector can satisfy himself that the rules have been complied with. When from the nature of the case it is impossible to place concealed wiring [in conduits 382 ELEGTRIC WIGHT. or in the regular way] on non-combustible insulating supports of glass or porcelain, the wires, if not exposed to moisture, may be fished on the loop system if incased throughout in approved con- tinuous flexible tubing or. conduit.” The method of “fishing ” wires should never be attempted for high-tension conductors. Fig. 330, vJunction or Outlet Box for Flexible Conduit. This flexible conduit may bea flexible steel tubing composed of convex and concave steel ribbons wound in a spiral around a mandrel so as to interlock. Its use is permitted by the under- writers for either finished or new houses. ‘This tubing is not water-proof ; and some difficulty is experienced in getting the wire through it, owing to the fact that it is heavy, and sags somewhat between the points of support. The tubing is made in lengths of 100 feet, and is intended to be installed without joints between outlets. An outlet box and portions of the tube are shown in Fig. 330, and an iron clamp for turning a right angle in Fig. 3381. Circular loom, or canvas Fig. 831. Flexible Metallic Conduit. jacket, is a tubular woven fabric treated with compound, and rolled in mica dust while yet soft. This is sometimes used, one tube for each wire, in finished houses where the wires are fished. Wires unprotected and fished between walls are not allowed under any circumstances. Flextble Tron-armored conductors have come into the market, INTERIOR WIRING. 383 and are successfully used for repair-work, and in places where the conductors are exposed, as in “drop cords’’ for example. Installation of Interior Conduit. — All conduits should be con- tinuous from one junction box to another, or to the fixtures, and the conduit tube should fit properly into all the fittings, else the conductors are not properly protected, and water is much more likely to enter the conduit. The entire conduit system of a build- ing should be completely installed, and the mechanical work on the building finished before the wires are drawn in. In the houses which are not fire-proof, tubing is generally supported from the un- derside of the floor beams, while in buildings of fire-proof construc- tion they run on top of floor beams and under the finished floor. The tubing of houses is generally put up as soon as the parti- tions have been erected; and when the tubing and outlet boxes have been placed, the lathing or plastering is proceeded with. On the completion of the plastering, the wire is pulled in, and switches, receptacles, etc., put in position. After all the conductors have been drawn in place, the outlets should be plugged up with a wood or fiber plug, made in parts to fit around the wire, and the outlet painted with some compound. The aim should be to make the whole system air-tight and mois- ture-proof. incaminaleatcsh eis) then madertto, sce that. there. are no grounds on the different parts of the wiring, and that the insula- tion resistance is sufficiently high to conform with the underwriters’ requirements. | The metal of all conduits should be effectually and perma- nently grounded. It is impossible to prevent the conduit from being partially grounded, and hence it should be purposely and completely connected with the earth. Conduit Wiring. — Standard rubber-covered wire should be used, because there is always the possibility of dampness getting into the conduit ; and the insulating lining of the conduit, if there is one, may be defective in places. The insulating lning of all conduits may be said to be defective, in that it is not continuous, but must be cut at each of the couplings. Conduit work is made a complete system by the use of outlets, junction boxes, and panel boxes with doors and locks, thus thor- oughly protecting the circuits at all places. 384 ELECTRIC CLIGH TING. There are two types of outlet box. One where the box is made for a given position and number of outlets (Fig. 332), and the other where the number of outlets to the box is variable. In this Fig. 332. Outlet Box. latter type the sides are made in such a manner that a blow of a hammer will knock out a disk of metal and make an outlet. These are called Universal boxes. .—~ Control. — In connection with the systems of wiring that have \. been explained, there is the system of control, that is the same for all. } From the switch board, which may, according to the size and character of the installation, vary from a combination of marble panels to an asbestos-lined, hardwood box, the feeders run to the channels provided, and are.carried to the various parts of the build- ing. " As a rule one pair of feeders for each floor is sufficient. feeders are the conductors that carry the main currents to feed the branch circuits and mains. They also preserve the regulation by bringing various points of the installation to a certain predeter- mined potential. The feeders must be designed to carry any load that may be legitimately put upon them. A usual method of cal- culating their size is to assume the load to be 80% of the total load that can be brought on them, and then, with that current, design for about 2% loss of voltage. This figure can in small plants, due to the short length of the runs, be reduced to 1.5%. Each feeder will run directly to a panel board. These boards INTERIOR WIRING. 385 vary from marbleized slate to asbestos-covered wood. The better kinds are inclosed in a panel box lined with slate, and furnished with a door and a lock. Double-pole “baby” knife switches and fuses are placed on the panel board. The space between the lining of the box and the panel board proper is known as the “ gutter.’”’ The con- duits of the various tap-circuits enter this gutter, and the wires are protected from the ends of the conduit by bushings. All wires in the gutter are inclosed in flexible tubes, and connections are made on the panel-boards by means of the switches between the feeders and the various pairs of mains supplied by them. Mains and Taps.— From the panel board the mains run to cut- out boxes; and there the mains branch out into the taps, going to the outlets and supplying the lights. These circuits are designed for a 1.5% loss, and in small installations may be brought down to 1%. This gives in large plants with long runs a total voltage loss of about 3.5%, and for smaller plants about 2.5%. The lights will be supported on a fixture or a drop cord, and will usually be controlled by a-key socket. From this it will be seen that there may be from the service to the lamp the following succession of conductors and safety devices : Service connection, service wire, double-pole cut-out, double-pole knife switch, trunk-line to switch board, double-pole knife switch, fuses to protect the feeder, feeder to distributing panel box, double- pole. baby knife switch, fuse to protect: the main, main to the cut-out box, plug or inclosed fuse to protect the branch, taps to the outlets through single-pole snap switch to control light or group of lights, wire to the fixture or rosette, fixture wire to socket, key switch in the socket, and finally the lamp, as indicated in Fig. 333. The number of tap-circuits will be determined by the under- writers’ rule that no group of lights requiring more than 660 watts shall depend on one cut-out. Fuses and cut-outs must not be con- cealed in the canopies or shells of fixtures. Another rule of the underwriters requires a fuse to be placed at every point where a change is made in the size of the wire, unless the cut-out in the larger wire will protect the smaller. It is well, therefore, to lay out the wiring so that while obeying this rule there will not be too many fuses in series located at different points, which always cause delay in case of trouble. 386 é ‘ON YaGas4 ee) asna qavidvL SdWY1‘d 0 9b XIS NVHL SYOW LON HOLIMS 310d-J19NIS ELECTRIC VL IG (VG. SERVICE LINE asna SNM MNNYL ae HOLIMS i SWITCH BOARD "A OLE ‘d’O OL SdWV7 Z syaisnio L3aN00S Alay 91d nc 1no Fig. 335. / eee [e; KEY / SOCKET | "ON ¥5dG334 PANEL| BOX g Vt HONVYS SYyIM SYNLXI4 AGWVT ‘A OLE “dO 9L-ZE C "3 ‘1 LLVM 099 AlddNS NVO HONVYE HOVA L4ax908 Adam FLEXIBLE CORD “4 PENDANT OR DROP LIGHT 3113809 TAP 1AAB Scheme cf Interior Wiring. VV HONVYUa The number of fuses in series must be kept down to the minimum, and they should be centralized as far as pos- sible to aid the rapid location and replacement of the fuse, in case it blows. Inthe calculation of wiring the following simple formulze will be all that is necessary for the direct current system and for the secondary wiring of alternating systems. TOS SON Wx ae m= : v v FS ——— ZA Kee c.m. = circular mils. d=the distance, i.e. the length of wire in feet on one side of the circuit between the points in question. c = load carried by the wire in amperes. ¥ = Volts to abe lost im the line. T = Lesistanices pels toOtman the wire to be used. 10.8 = resistance of one mil- foot of commercial copper at Doe When the circular mils or the resistance per foot of the wire has been determined, find the size of the wire to be used by reference to the table on page 8, taking the nearest size larger than the wire as calcu- lated. Then refer to the table INTERIOR WIRING. 387 on page 15 to see if the current that is to be carried is below the allowable value. Safety Devices in interior wiring are of special importance, since the inconvenience of repairing circuits is great, and the con- sequences of overheating wires by excessive current are likely to be very serious. On sevzes circuits some form of czt-out (p. 25 and p. 340) is applied to each lamp to short-circuzt it if by accident it tends to interrupt the circuit. In this case the continuzty of the circuit must be preserved since the voltage is usually very high, and a long, dangerous arc would be formed if attempt is made to open it at any point. Furthermore, there is little risk of the cur- rent (in amperes) varying much from its normal value, since a special regulator is provided (p. 171, p. 186 and p. 189) to main- tain a nearly constant current. On the other hand, faralle/ distribution, usually operating at practically constant potential, requires the main conductors as well as each branch to be protected by a device that will ofex the par- ticular circuit whenever the current in it tends to become exces- sive. In this case the number of amperes is inversely proportional to the resistance of each branch circuit. If this resistance is reduced abnormally by a ijeak or short cir- cuit between the + and — conductors, the current may rise to many times its safe value, and the wires may be greatly over- heated, even to the fusing point, since the heating effect increases as the square of the current. For protection against this danger, fuses and circuit-breakers are employed in large numbers, there being hundreds of the former in any parallel system of distribu- tion of even moderate size. These two devices are discussed in Vol. I. Chap. XXII., but it will be well to supplement that general discussion by considering the particular forms used in connection with interior wiring. In principle, safety fuses are weak links purposely introduced into the: circuit; and intended to mélt and open it if the current tends to exceed a certain safe limit. Lack of confidence in fuses is often expressed by those who have had much experience with them ; but on account of their simplicity and cheapness they are used almost universally except on switchboards or in connection with motors, or in other special cases where the importance of the circuit or the likelihood of its being overloaded are sufficient to 388 ELECTRIC LIGHTING. warrant the use of circuit-breakers. For the numerous branch circuits in electric-lighting, carrying only a few amperes each, it is practically necessary to adopt fuses. Furthermore, it is a fact that the use of fuses of standard lengths and cross-sections under defi- nite conditions will give uniform and reliable results. Formerly the length of the fuse wire was not defined, and it was sometimes open and sometimes inclosed so that it might carry much more current in one case than in another. The principal forms are “ink, plug and inclosed fuses. A standard type of link fuse block made of porcelain is illustrated in Fig. 334, and the fuse itself in Fig. 835. These are suitable for Fig. 384. circuits having a potential not exceeding 125 volts across one fuse link and a maximum capacity of 75 amperes. Larger and heavier blocks are made for greater capacities, and are provided with mica sheets as covers. This form requires the use of a screw-driver in replacing a fuse, and this in unskilled hands, or even in skilled ones, may by accident cause a short circuit. In Fig. 836 is shown the Edison porcelain plug cut-out. These have a maximum current capacity of 830 amperes and a maximum po- tential of 125 volts across one fuse. The fuse is contained in a screw plug, shown in Fig. 887. When a fuse has blown, the plug is unscrewed and a new one put in its place. This is a very simple and safe operation, even if the short cir- cuit still exists when the new plug is screwed in. Fig. 337. Edison Plug Cnt-Oat, INTERIOR WIRING. 389 The zzclosed fuse represented in Fig. 358 possesses the same advantage in convenience and safety of renewal. It is also claimed to be more definite in its action, since the conditions are more nearly uniform than in the case of open or partly open fuses. Still another advantage is the fact that a fuse closely surrounded.with solid, f- brous, or powdered material, has a greater capacity for heat than if it were isolated. Consequently it re- quires a little longer time to reach Fig. 338. the fusing point, and an excess of current must be very great in amount, or must flow for an appreci- able time, in order to “blow” the fuse. A current that only mo- mentarily exceeds the normal will not injure the wires or apparatus, and it is simply a nuisance to have it opensiheicireult, “Lhesform «of fuse illustrated in Fig. 339 is very conven- iently replaced, and is also capable of being used as a sort of switch to open and close the various circuits. It is pro- vided with a copper blade at either end, which fits into clips just as in an ordi- nary knife switch. A similar arrangement is shown in Fig. 340, Fig. 339. the fuses being carried in por- celain boxes with projecting blades that are pressed into the clips in the cut-out cabinet. Circuit- Breakers. — These devices are shown and described in Vol. I. Chapter XXII., and belong more to the generating plant and switchboard than to interior wiring. Essentially they are switches that are con- trolled by an electromagnetic device so as to open automatic- ally when the current exceeds a certain limit. They are much 390 BLECTRICMUCHIING more accurate in this respect than a fuse, and can be adjusted to act at any given current within a considerable range. Their accu- racy and adjustability are important, but their great advantage over the fuse is the facility of resetting them compared with the trouble and delay of renewing a fuse. The fact that a circuit-breaker flies open the instant that the current rises above the value for which it is set, may be a doubtful advantage. This might happen very often when the excess of current does not last long enough to cause any harm, as explained in connection with fuses on the preced- ing page. The main circuit-breaker may open at the same time as one in a branch circuit, thus shutting off other parts of the system needlessly. In some forms the so-called time factor is purposely introduced, a device being added to prevent action unless the overload is of a certain duration. An example of circuit-breaker illustrated in Fig. 341, differs from most types in the fact that the arms for the two sides of the circuit are in- dependent, so that one may open automatically if it is attempted to close the other while a short circuit still remains. Switches. — The general facts concerning these important de- vices are set-forth in Vol. I. Chapter XXII., but there are certain additional points to be noted in connection with interior wiring. The rules for switches as laid down by the National Electrical Code will be found on pages 9, 10 and 23 of the Appendix to the present volume. Ordinarily the type used on switchboards is the simple lever £uzfe-switch. Vhey are sometimes made in the guwzck- break form in order to reduce the duration and therefore the burn- ing effect of the arc produced on opening the circuit. It is a fact, Fig. 341. Automatic Circuit-Breaker. however, that switches are used to c/ose the main circuits or those carrying heavy currents, but should not be used to open them except in emergency. In starting up an electrical plant, or in closing the service switch of an installation, the switches controlling the various circuits may be closed before or after the current is INTERIOR WIRING. 391 turned on, and a switch may be left open if no energy is required in the corresponding circuit. In case of overload, a circuit-breaker or fuse should be provided in each main or branch circuit to open it when the current exceeds a safe value, but the switch itself when carrying heavy current is to be opened only as a last resort. Under ordinary circumstances the lamps are disconnected in comparatively small groups by means of snap switches, one of which is shown in Fig. 342. In fact, the current allowed in each branch cir- cuit is limited by the insurance rules, being a maxi fig, 342,— mum of 660 watts (equivalent to 12 lamps of 16 c.p. S"aP-Switeh. each at 110 volts). Even in a theater or other place where many lamps are turned on or off at about the same time, it is customary to control them in groups, each having a separate switch. If the current does not exceed 8 amperes at 110 volts a single-pole switch is allowed, but for currents greater than this it must be double-pole. Lreak-down Switch. A building having its own generating plant may be provided with a connection to the street circuit of some electric lighting company. In case OleaccicentetO,. Or repain of, the isolated plant, current may be obtained from the central station. The switch which enables this to be done is called a “break-down ”’ Switch, since it is used in case the local machinery is broken down. It is usually of the double-throw type (Fig, 343); and since street circuits are often three-wire Fig. 343. Break-down Suicch. systems, and isolated generating plants are operated in most cases on the two-wire plan, the switch is generally arranged to convert from the former to the latter, as described on page 82. ‘Multi-Control Switches. Ut often happens that it is desired to light or extinguish a given lamp or group of lamps from two or more different points. A common case is that of a lamp which may be turned on at the foot of a staircase and turned off at the top, or vice versa. This may be accomplished in various ways: one plan indicated in Fig. 344 requires a three-way key socket at the lamp, a three-way switch at the other point of operation, and an 392 ELECTRIC TIGHLING. - extra wire between them in addition to the two mains that supply the current. With this arrangement the lamp may be lighted or extinguished by either switch. TRree -Way Switch Three-way Socket Fig. 344, Panel Boards and Cut-out Cabinets are miniature switch-boards or sub-centers of distribution which afford means of splitting up the mains into branches and of grouping the cut-outs. The various forms used differ mainly in the styles of switches or cut-outs and Fig. 345. Panel Board. in their arrangement, as illustrated in Figs. 340, 345 and 846. The two or the three main conductors are represented by parallel wires or bars of metal, from which the branch circuits are led out through switches and cut-outs. ) Fixtures and Sockets. — The endless variety of fixtures used for supporting arc and incandescent lamps may be classed as furniture INTERIOR WIRING. ' 393. or ornament, as they are not of a character to be included in a technical treatise ; the only technical features they contain being the fixture wiring and the insulated joint interposed between the fixture and the gas-pipe to which it may be attached. The sockets are described in the next chapter, on Incandescent Lamps. In a general treatise it is impossible to go into the details of interior wiring, as they depend largely upon the conditions in each particu- Fig. 346, Cut-Out Cabinet. lar case. The principles involved and the chief elements of con- struction have been set forth, and for further information refer- ence may be made to the National Electrical Code printed in full in the Appendix and to the following publications : — Cushing, H. C., Jr., Standard Wiring for Incandescent Light and Power, pp. 116, N. Y., 1900. Emmet, W. L. R., Alternating Current Wiring and Mestribu- LOA; DDO aN aN oh 898. Leaf, H. M., [utertor Wiring of Buildings, pp. 195. London, 1899 (gives English Practice). 394 ELECTISACILIGHIING Noll, A., How to Wire Buildings, pp. 162, N. Y., 1899. Pierce and Richardson, Vational Electric Code (Explanation of), DpM22cjgN anes Robb, R., Electric Wiring, pp. 183, N. Y., 1896. A series of articles on /utertor Wiring, by Charles E. Knox, in the American Electrician, N. Y., 1898 to 1900. INCANDESCENT (LAMPS. ogo C@.EeAshel PR Vell, INCANDESCENT LAMPS. AN incandescent electric lamp is one in which light is pro- duced by the passage through a solid conductor of a current sufficiently strong to raise it to a temperature of incandescence. In this case the conductor is solid and continuous, while in an arc lamp, the other important type described in Chapters XIV. and XV., light is produced at a gap in the circuit across which the current is carried by the heated vapor present. The ordinary type of incandescent lamp, enormous numbers of which are now used, consists essentially of a high resistance carbon filament hermet- ically sealed in a nearly perfect vacuum. In fact, these words are substantially the same as the patent claims of Edison,* who de- veloped the incandescent lamp as well as the necessary generator method of distributing current and the various auxiliary devices to a condition of commercial success. Many forms of incandescent lamp have been devised employ- ing filaments composed of materials other than carbon, and not requiring a vacuum; but these are special types that will be described later. The present chapter is confined to the ordinary incandescent lamp, as already defined, which has been used to the practical exclusion of any other form since the introduction of incandescent lighting in 1880. Materials Used for Filaments. —In the earlier lamps made by Edison, the filaments consisted of platinum wire, but that metal soon lost its strength, even at normal working temperature ; and if accidentally raised above this point it was likely to be melted. The high cost of platinum is also a serious objection to its use for this purpose. Consequently Edison soon substituted carbon for platinum in his lamps. After trying many materials, carbonized * US. Patents, 1570. 396 ELECTRIC LIGHTING. bamboo was adopted, and generally used in the Edison lamps made for about fifteen years. Other manufacturers employed | thread, thin strips of cardboard, or some special compound in place of bamboo. In practically all cases some organic substance carbonized by heat has been used. For several years the tendency has been to adopt almost universally the so-called “squirted filaments.” They are usually made by dissolving cotton in a solution of zinc chloride producing a viscous, semi-transparent liquid in which the appearance and fibrous character of the cotton are entirely lost. This gelatinous material is forced or squirted through a small hole, and received in a vessel containing alcohol, which causes it to set and harden sufficiently to be handled afterwards. After washing, the material, having the appearance and consistency of cooked vermicelli, is wound upon a large drum and dried, after which it possesses considerable strength, and looks much lke a cat-gut string such as is used on a violin. It is then cut into lengths suitable for filaments, and carbonized at a high temperature. The advantage of using this product in place of some solid substance, such as bamboo, is the fact that it is perfectly homo- geneous, and can be made readily and accurately of any desired cross-section or length. On the other hand, there is considerable difficulty in eliminating entirely bubbles of air from the viscous solution of cotton. If they are present, even though very small, they will cause a flaw in the filament at any point where a bubble may happen to exist. In order to get rid of them the solution is filtered and heated: under a vacuum. ‘To avoid the presence of impurities and to insure a perfectly homogeneous product the best quality of cotton wool should be used, the specially high grade employed by surgeons as absorbent cotton being adopted by the best manufacturers. Great skill and care are required in making the mixture, the exact density and temperature of the zinc chloride solution as well as the proportion of cotton dissolved in it being matters of particular importance. The formation of lumps in the jelly-like mass is likely to occur, and should be prevented by con- stant stirring, otherwise the resulting filaments will not be of uniform cross-section. Measuring and Sorting the Filaments. — After being carbon- ized, the filaments are carefully measured and sorted according to INCANDESCENT LAMPS. 397 length and diameter. The latter is reduced very greatly by the processes of drying and carbonizing, so that it must be determined very exactly by means of a micrometer. A filament made from “< squirted ” cellulose solution is somewhat elliptical in cross-section owing to its having been wound, while soft, upon the drying-drum. For this reason it is necessary to measure both maximum and minimum diameters in order to determine its true cross-section. The filaments suitable for the various types and sizes of lamps are thus selected. Ina general way the length is proportional to the voltage, and the surface is proportional to the candle-power for which the lamp is intended. Flashing or Treating the Filaments. — The object of this pro- cess is to render the filaments stronger and more uniform. — For- merly, when they were made from bamboo, thread, and similar materials, the filaments obtained were far from uniform throughout their length. The present forms of “squirted” filaments are better in this respect, being more uniform in diameter and more homogeneous ; but even these require to be treated after being carbonized. The treatment consists in raising the filament to incandescence by passing through it an electric current in an atmosphere of hydrocarbon vapor or gas. The high temperature of the filament decomposes the hydro- carbon, and causes carbon to be deposited upon it. This deposit occurs over the entire surface of the filament, but is greater at any point where the electrical resistance may be abnormally high, because the temperature there will also be higher. Hence the tendency is to produce a filament of uniform resistance throughout its length. In the same way the strength is made more uniform, because any part thinner or weaker than the rest is likely to have a higher electrical resistance, so that it will be reinforced by receiv- ing a heavier deposit of carbon. On the other hand the deposited carbon is graphitic in character, and has a lower specific resistance of 10 to 15 per cent that of the original filament, which is undesir- able especially for high voltage lamps. | The filaments are treated after they have been carbonized, but before they have been mounted, the process being performed in a jar containing hydrocarbon vapor. The stopper of the jar carries metallic holders into which the. ends of a filament are inserted, the latter being then introduced into the jar. By means of the 398 ELECTRIC LIGHTING. metallic holders which serve also as electrical connections, a cur- rent is caused to flow through the filament in order to bring it to incandescence. The resulting deposit of carbon reduces the resist- ance until a certain value is reached, when the current is inter- rupted, and the filament is taken out, to be followed by another and soon. The proper resistance is predetermined by experience or calculation for each type of lamp. It may be measured during the process of treatment by disconnecting the filament from the cur- rent supply, and connecting it to some resistance measuring device, such as an ohmmeter, a double-throw switch being used to make the change. In this case the measurement is made while the fila- ment is cold, and it is generally assumed that at working tempera- ture the resistance is reduced one-half. It is also an easy matter to determine the resistance when the filament is incandescent, and the carbon is being deposited upon it. By measuring the voltage across the terminals of the filament and the current flowing in it, we know from Ohm’s law that ohms = volts + amperes. In another method the filament is made one arm of a Wheatstone bridge, and the other three resistances are so adjusted that no current flows in the galvanometer circuit when the filament reaches the proper resist- ance. A relay put in place of the galvanometer will release its armature at that moment, and may be arranged to stop automati- cally the current through the filament. The increase in diameter resulting from the deposit of carbon is about 10 per cent, but varies in different sizes and makes of filament. Mounting the Filaments. — In order to mount the filaments, that is, connect them to the “leading-in” wires (CC in Fig. 352), that are to supply them with current, various methods have been devised and used. One plan consists in electroplating a sleeve of copper around the end of the filament and of the wire, thereby mechanically binding and electrically connecting them together. In lamps formerly made from carbonized cardboard, the ends of the filaments were enlarged so that they could be attached to the ends of the wire by very small bolts. Another method consists in forming a socket at the end of the wire into which the end of the filament is inserted, and held in place by squeezing the socket around it. These means of connection have been in most cases abandoned for the simpler and cheaper joint, made by pasting together the ends of the filament and wire, using a INCANDESCENT LAMBS. 399 mixture of powdered carbon and molasses, or other similar sticky material. Still another form of joint is made by heating the junction of the filament and the wires by an electric current in an atmosphere of hydrocarbon gas or under a hydrocarbon liquid. In this way a deposit of solid carbon is formed around the filament and wire, which binds them together. The deposit takes place more rapidly in the liquid, but the latter is objection- able because it adheres to the filament and wires. ‘This so-called deposited carbon joint is a very good one, but is more troublesome and expensive to make than the pasted joint. Platinum “ Leading-[n ”' Wires. —‘To insure a perfectly air- tight seal where the “leading-in” wires pass through the glass, they are made of platinum, because its coefficient of expansion by heat agrees with that of the glass which is used. If the two coefficients of expansion differed materially, it is obvious that there would be a tendency either to crack the glass or to let in air when temperature changes occurred. Platinum being a very expensive metal, even the small amount required in an incandescent lamp is a considerable item in the cost, so that many attempts have been made to substitute some cheaper metal or alloy. While alloys having about the same coefficient of expansion as that of glass can be made, they are open to the objection of not being able to stand the high tempera- sure of melting glass or the action of the blowpipe flame without melting or burning while being sealed in, so that there is likely to be a leak owing to the imperfect fusing of the glass to the wire. In order to economize as much as possible in the cost of plati- puinetorecacilainp,. ine oteatcr portion) of sthe wlength, ofthe leading-in wires is composed of copper, platinum being used only where the wire passes through the glass. [or example, in Fig. 302, the longer parts, D C, of the leading-in wires are of copper, and the shorter parts, D £, are of platinum, the joints between the two being made by electrical welding. Glass Portions of Incandescent Lamps. — The several steps in their manufacture are indicated in Figs. 847-353, a standard Edison 16. c¢. g. lamp being represented on a half scale. To Yee Nemannside. parigas an Plass tubers 1s) used,” beings first softened by heat and flared out at one end 4. ‘The other end is then softened, the leading-in wires C & introduced, and the plastic 400 Manufacture of Incandescent Lamps. ELECTRICALICH ING: Figs. 347-358, INCANDESCENT LAMPS. 401 glass is pinched around the wires at YD so as to form a her- metical seal. If the filament is to be of a form to require it, an anchor / is introduced at the same time. The next step is the making of the bulb G, which is simply blown on the end of a glass tube H7/, in the ordinary way. The extreme end of the bulb is then heated by a blow-pipe, and a small glass tube ZL J is attached to it at that point. The bulb is now disconnected from the tube / by melting the latter around at A and pulling the two apart. The inside part 4 upon which the treated filament /V has been mounted by means of pasted joints at & £& F, is next introduced into the bulb G, and the two are united by fusing together the circular edges L B and K Kk. The partially completed lamp is now ready to be exhausted of air through’the tube Z JZ, The Objects of the Vacuum produced in the bulb of an incan- descent lamp are: 1. To avoid the combustion of the carbon filament. 2. To reduce wear on the filament due to “air-washing.” 3. To diminish the loss of heat from the filament. | 4. To decrease the flow of current in the space around the filament. At various times, it has been attempted to attain the first of these objects by using an atmosphere of some gas or vapor, such as nitrogen or bromine, which it is expected will not com- bine with the carbon. But even a small quantity of any gas left in the bulb may tend to consume the carbon, partly by ~ chemical combination, and partly by a mechanical action called air-washing that wears away the filament. The presence of any gas or vapor also causes a more rapid transfer of heat, by conduc- tion and by convection from the filament to the bulb. In a vacuum, on the other hand, the filament loses heat by radia- tion alone, so that a smaller quantity of energy is required to maintain it at a certain temperature and candle-power. Hence the efficiency is improved, being inversely proportional to the energy consumed, other things being equal. or the same reason the bulb of a vacuum lamp is cool enough to touch with the hand, even while burning, and will not ignite anything that may come in contact with it unless the heat is allowed to accumulate by leaving it for some time partly or completely surrounded by an 402 ELECTRIGQIICHTING. inflammable material, such as cloth or wood. The bulb of a lamp containing some gas becomes considerably hotter, and is therefore less convenient to handle, as well as more likely to start a fire. The ‘air-washing ” effect is not considered to be as important as formerly, the wearing out of the filament being due chiefly to pro- ) jection of particles from its surface, and chemical action upon it if any active gas is present. The flow of current through the space around the filament is called the ELdzson effect, having been first observed by him. It is a loss of energy, since the pale bluish light that it produces adds little or nothing to the candle-power. This flow is greatly reduced when a nearly perfect vacuum is reached. In fact, lamps. are tested to see if the vacuum is sufficiently high, by connecting them to an induction coil; those showing the pale glow through- out the bulb being rejected. On the other hand, the presence of any considerable quantity of gas would also stop the wasteful cur- rent, so that for this reason alone either a very high or a compara- tively low vacuum is desirable. ‘The flow of current by the Edison effect may take place without the blue glow, but Mr. J. W. How- ell has shown * that the two often go together. | The blackening of the bulb which gradually occurs while the lamp is burning was found by Prof. W. A. Anthony + to be con- siderably less in lamps containing a slight atmosphere of bromine than in ordinary high vacuum lamps. The transfer of carbon from the filament to the bulb seems to occur as a sort of projection of particles along straight lines in a manner similar to the Crooke’s effect. Hence it is quite natural that the presence of even a small quantity of vapor would interfere with the deposit by reducing the “mean free rath” of the particles. The blackening of the bulb by the deposit of carbon upon its inner surface is one of the impor- tant causes in the falling off in candle-power of lamps. This matter will be considered further under the head of “ Relation b] between Candle-Power and Age” on page 416. The methods of exhausting bulbs used singly or in combination are as follows: 1. By means of mercury pumps. * Trans. Amer. Inst. Elec. Eng., vol. xiv. p. 27, Feb., 1897. + bid. vol. xi. p. 132, March, 1894. INCANDESCENT LAMEBPS. 403 2. By means of mechanical pumps. 3. By the so-called chemical process. The first of these consists in connecting the tube LJ7/ (Fig. 352) to a Sprengel or other suitable form of mercury pump capa- ble of producing the very high vacuum required. At first the quantity of bubbles in the tube of the mercury pump show that the air is being rapidly removed, but later the bubbles become fewer and smaller, until finally none are visible. This indicates that no more air can be drawn out under the existing circum- stances, but there is still considerable gas clinging to the glass, filament and leading-in wires. The lamp is now heated by passing current through the filament or by external heat in order to drive off these gases and allow them to be removed by the pump. When the vacuum is sufficiently high, the tube /J7/ is softened close to the bulb by a blow-pipe flame and drawn out to form the tip Z (Fig. 353), thus hermetically sealing the lamp. The second plan, employing a mechanical pump, is now cap- able of producing sufficiently perfect exhaustion for high vacuum lamps, being also used to save time and expense in removing the greater part of the air at first, when the final vacuum is obtained by a mercury pump or by the chemical process. In the chemical process the lamp is nearly exhausted by a mechanical or mercury pump, and some substance previously introduced into the bulb is then caused to combine with the small ‘emaining quantity of gas. In most cases a small quantity of phosphorus is put in the tube ZJZ/, and ignited by heat applied to the outside. It combines with the residue of gas present to form solids or non-conducting gases which are practically equiva- lent to a perfect vacuum. It is found that red phosphorus, which is comparatively harmless, can be used instead of the yellow form, that would be injurious to the employees who handled it. Bases and Sockets. — The sealed lamp is now ready to receive the base which supports it, and at the same time makes the ne- cessary electrical connections that supply it with current. Many forms of base have been used, the most prominent being the Aazsoz standard type shown in Fig. 354. This consists of a brass shell formed into a screw-thread, to which one leading-in wire is soldered at P, and a brass button to which the other leading-in wire is soldered at R. To hold the parts together and insulate them from 404 ELECTRIC LIGHTING. each other, the spaces between are filled in with soft plaster of paris as indicated by dots in Fig. 808. ‘This is allowed to harden and is then dried, otherwise the moisture would short-circuit the terminals. At present porcelain pieces are generally used instead of plaster. - The corresponding Edison socket, which is the same as that used with plug cut-outs (Fig. 8386), is made with a screw-thread and contact point to receive the base of the lamp and make elec- trical connections to it. Simplicity and cheapness are the chief Edison. Sawyer-Man or United States or Thomson-Houston. Westinghouse. Weston. AMM ult . HI il i { Fill Mt or English Brush-Swan. Mathei-Perkins. Schaefer. Ediswan Figs. 354-861, Typical Lamp Bases. Two-thirds Size. advantages of this form; and it is gradually displacing other types in this country. The Sawyer-Man or Westinghouse lamp base, illustrated in Fig. 355, is provided with electrical contacts similar to those of the Edison type; but the lamp is held in place by inserting it in a socket consisting of spring clips or fingers which grasp it on all sides. The Thomson-Houston base has a central hole in which a thread is cut, so that it may be screwed down upon a projecting screw in the socket. The Swan, and similar bases represented in Figs. 8358-360, are of the bayonet type, having small pins on the sides which fit into slots in the socket, being inserted and then turned slightly in order to lock them. All of these may be classified under the three heads of the screw, clip, and bayonet INCANDESCENT LAMPS. 405 types. The first class possesses the advantages that have been given for the Edison base, and the lamp may be lighted or ex- tinguished by screwing it in or out about one turn. This is con- venient in case the switch is not easily reached or is out of order. The clip or bayonet bases are not adapted to be used in this way; on the other hand, they are not so likely to work loose as the screw forms. For very large lamps, special types of socket, as in Fig. 368, are often employed. It has been attempted to secure the general adoption of a standard lamp base and socket ; but owing to patent questions, the jealousy of manufacturers, and the fact that large numbers of the different types have been installed, the effort has not been very successful in this country or abroad. Now that the original patents have expired, it would seem that this uniformity might be attained in order to save makers and dealers the trouble and ex- pense of carrying in stock so many styles of lamps and sockets. The great variety in voltage, candle-power, form and color of bulb, and type of base, makes almost innumerable combinations that may be called for. Forms of Filament. — The ordinary 16 candle-power lamp at 110 volts consumes about } ampere, consequently its resistance must be about 220 ohms when burning. A filament having this Single Single Curl Double Double Double Curl Curl Anchored U Curl Anchored Single U Figs. 862-367. Different Forms of Filament. resistance, and sufficient cross-section to give mechanical strength and the required illumination, should be about 7 to 9 inches long. The single U shape (Fig. 362) was generally adopted in incan- descent lamps for many years, but the excessive length of the U, and its tendency to droop, demand a large bulb. Furthermore, its distribution of light is poor, as explained later. To avoid these objections, the curled forms of filament are now being used almost 406 ELECTRICOLIGHIING. universally. The single curl, the single curl anchored, and the double curl (Figs. 368, 364, and 366), are common forms in lamps for 100 to 125 volts, and from 8 to 50 candle-power. Fig. 363. Three Hundred-Candle-Power Lamp. One-third Size. The Filament of a 220-Volt Lamp should be about twice as long as and about one-half the cross-section of a standard 110-volt filament giving the same candle-power, because the former takes INCANDESCENT LAMES. 407 one-half as much current at twice the voltage, in order to consume the same number of watts. Asa matter of fact, a 220-volt lamp requires a larger number of watts per candle-power ; consequently its current is about six-tenths instead of one-half as great. The 220-volt filament, with its increased length, is usually made in the double UY or double curl forms (Figs. 865-7), in order not to require a large bulb. Large Lamps of 100 to 3800 candle- power are usually made with the single VY filament, as represented in Fig. 368, or with the double UV form (Fig. 865). In this case the cross-section is much greater, the current in a 100 candle-power lamp | being about six times that in a 16 candle- power lamp of the same voltage. Anchored Filaments are used when their length or the form of the bulb is such that there is danger of their touch- ing the glass and cracking it so as to let in the air and burn up the filament. This occurs either from excessive vibration or from the gradual drooping or bending of the filament, which is likely to take place, especially when the lamp is not vertical and pointing downward. Two arrange- ments have already been described (Figs. 303 and 867), in which the anchors are el tachedetOsstiess Sinner part’ through which the leading-in wires pass. Another common form of anchor is sealed in the tip of the bulb, being a necessity in the tubu- lar lamp shown in Fig. 369 to support the - extreme end of the filament, and prevent it from touching the Fig. 8369. Tubular Lamp. glass. The Distribution of Light differs greatly in the various types of lamp, depending almost wholly upon the shape of the filament. The straight single V form (Fig. 362), giving 16 mean horizontal 408 ELECTRIC LIGHTING. candle-power, emits only 5.7 candle-power in the direction of the tip. A “long curl anchored” filament (Fig. 3864), having the same mean horizontal candle-power, gives 7.05 candle-power from the tip; and a double curl filament (Fig. 366), of the same mean horizontal candle-power, gives 10.1 candle-power from the tip, showing great variations in illuminating power in different direc- tions. The horizontal and vertical distribution of light is shown by curves in Figs. 370 and 871 for long curl anchored and double 4, anit Mf, Ya ei HIT | [/ Hh Vi igers oe l Il Curve of Vertical Distribution of Candle Power NG CUR -ANCKORED FILAMENT NY i, \S \ (\ Nill i) waist itm: GLE MiE eae | i fi | MT t ll a-b posi m ion of lament shanks 75 go° Cure of Horizontal Distribution Curve of Vertical Distribution. of Candle Power DOUBLE COIL Candle Power Figs. 870, 871. Distribution of Light. curl filaments respectively. The results for the five principal forms as ordinarily proportioned are given in the table on page 409. A comparison of these figures shows that the candle-power measured in line with the tip 1s much less in the single or double U-shaped filament than in the single or double curl form, the mean INCANDESCENT. LAMPS. 409 horizontal candle-power being the same in all cases. The obvious reason for the difference is the fact that the two former expose less surface in that direction than the two latter. By slightly twisting a U filament so that it does not lie in one plane, the candle-power from the tip may be increased, and by shaping it with one or two curls the distribution of light will evidently be still more uniform. It is also a fact that the tip itself intercepts, or rather reflects and refracts, some of the light tending to pass through it, so that the CANDLE-POWER OF LAMPS WITH DIFFERENT FORMS OF FILAMENTS. CANDLE-POWER. CANDLE-POWER TAKEN. Single U-shaped Filament. Fig. 362. Single Curl Filament. Scheme Recs Long Curl Anchored Filament. Fig. 364. Double U-shaped Filament. Fig. 365, Double Curl Filament. Fig. 366. 16 ' 16.72 15.6 13.8 14.0 14.5 10.9 LOL (a) Mean horizontal at 180 revolutions (4) Mean horizontal (standard method) (c) Mean horizontal (from curves). . (7) Mean spherical (standard method) (¢) Mean with axis at 45 degrees (7) Mean hemispherical : (¢) Mean within 380 degrees from tip . (2) From the tip . Sat 16 candle-power in that particular direction is still further diminished. For this reason “tipless lamps’’ are made, the end of the bulb being made perfectly smooth. On the other hand, the table shows that the mean spherical and mean hemi-spherical candle-power are very nearly the same for all five forms of filament. Asa matter of fact, either of these is far more important practically and scientifically than the candle- power in any one direction, except for some special purpose, in which case a reflector may be used to throw in the direction re- quired nearly all of the light emitted by the lamp. The facts given in Figs. 870-371 and in the table are obtained from a paper by Professor A. J. Rowland before the Franklin Institute,* in which the great importance of the light from the tip is insisted upon. In fact, he calls this the “useful light,” because lamps are usually placed with the tip downward. incandescent But in many * Electrical World and Engineer, Oct. 18, 1900. 410 ELECTRIGUIHGHTING cases they are arranged the other way; and even when they are not, there are reflections from the ceilings and walls, and from the globes or shades with which lamps are generally provided, so that a fairly uniform distribution of light results. Resistance of Filaments. — The only electrical property that an incandescent lamp possesses is its resistance. The ordinary carbon filament has about twice as much resistance when cold as it has when raised to the working temperature. Obviously the latter is the important value, and absolutely determines the current and the power that the lamp will consume at a given voltage. Calling the latter V, the resistance in ohms of the hot filament ,, the current in amperes (Gand the power in watts, WV, we have> +O — ae and Ve h Ordinarily lamps of almost any size take from 3 to 4 watts per candle-power. Assuming an average value of 3.5, and that the voltage Vis 110, we have from the second equation: 3.5 X c. p. = 12100 + R, or R, = 3457 +c. p. Hence the resistance of a 46 candle-power lamp using 3.5 watts per candle-power is 3457 -+- 16 = 216 ohms, the» current ‘is 110-216 ==.51 ampere,sand the power is 12100 + 216 = 56 watts. ‘A lamp of 32 candle-power consuming the same number of watts per candle-power has a resist- ance of 3457 + 32 = 108 ohms, or exactly one-half as much as before, the current is 110 + 108 = 1.02 ampere, and the power is 12100 + 108 = 112 watts, the two latter being twice as great as for the 16 candle-power lamp. In short, for a given efficiency and voltage, the resistances of lamps are inversely proportional to their candle-power, and the current and power increase directly with the candle-power. | Specific Resistance of Filaments. —The completed filament con- sists partly of the original cellulose or other material carbonized, and partly of the carbon deposited upon it when it is “flashed ” or “treated.” The specific resistance of the former differs greatly according to the material used, but is ordinarily between .0022 and .0035 ohms per cubic centimeter when hot, and about 1} to 2 times greater when cold. The deposited carbon has a specific resistance about .12 to .16 as large as that of the untreated filament when both are hot, but the resistance of the former is increased about 2 to 23 times at ordinary temperature. In almost all lamps the INCANDESCENT LAMBS. 411 proportion of the two kinds of carbon is such that the resistance of the treated filament is about twice as great when cold as when hot. The variation in resistance is not great at or near the working , temperature; in fact, most of the reduction in resistance occurs before the filament becomes red hot. This is fortunate, because a lamp is exceedingly sensitive to variations in voltage, and a de- creased resistance with increased temperature would aggravate this difficulty. It would be desirable, in fact, to have the resistance of the filament increase with temperature, as in the case of metals, tending to keep the current constant if the voltage happens to rise or fall. Mr. J. W. Howell * has shown that this effect is obtained when the proportion of deposited carbon is large. This form of carbon is graphitic in character ; and its resistance falls rapidly until the voltage is 30 or 40 per cent of its rated value, above which the resistance increases steadily even at 60 per cent excess over the normal voltage, which is the limit of the experiments, as the lamps burn out very quickly at this high temperature. The resistance of the untreated filament does not fall so rapidly at first as that of the deposited carbon, but it continues to diminish even when the voltage is raised to 60 per cent above the normal. Hence it is possible, by varying the proportion of original and deposited carbon, to have a positive, zero, or negative temperature coefficient at working voltage. In most cases it is practically zero. The original filament of Edison lamps has a specific resistance of 1.726 ohms per cubic mil cold, and .88 ohms at a temperature corre- sponding to 3.1 watts per c. p. The figures for the deposited carbon are .26 and .12 ohms respectively. Size of Filaments. — The dimensions of a filament must fulfill two conditions: first, the resistance must be such that the lamp shall take the proper current and power at the voltage for which it is intended, as explained on p. 410; and second, the power lost by the filament as heat at the working temperature must exactly balance the electrical power supplied. The loss of heat from a body may take place in three ways: (1) by conduction through the bodies with which it is in contact; (2) by convection currents in the gas or liquid surrounding it; (8) by radiation. The fila- ment of an incandescent lamp loses a small amount of heat by * Trans. Amer. Inst. Elec. Eng., vol. xiv,, p. 80, 1897. 412 ELECTRIC LIGHTING. - conduction through the leading-in wires; and since it is usually situated in a nearly perfect vacuum, it loses practically nothing by convection, hence the loss occurs almost entirely by radiation. The rate at which a body radiates heat is proportional to its surface, other things being equal, so that this surface must have a certain value for a given number of watts supplied. According to New- ton’s law of cooling, the loss of heat is also proportional to the elevation in temperature, and finally it depends upon the character or emissivity of the surface, that is, the number of heat units emitted from a unit surface per degree of temperature above that of the surrounding bodies. Above a red heat the illumination from the filament increases much more rapidly than the emission of heat, consequently the efficiency or candle-power per watt is greater the higher the temperature. There is a practical limit, however, to the temperature, above which the filament is too rapidly destroyed, so that there must be a compromise between the life of a lamp and its efficiency. The actual working temperature of filaments is very difficult to meéasure.. According to Prof: Hie aW eberait 15 159 Gan 3.1 watts and 1560° at 4 watts per candle-power. In practice this temperature is indirectly determined by the color of the light, the efficiency and the life of lamps, all of which depend upon it. The filament may be rectangular in cross-section when cut out of cardboard or sheets of other material, and it has sometimes been made hollow; but ordinarily it is solid and circular, or slightly elliptical in section. Ina filament having a diameter J and length Z in centimeters, surface S in square cm., resistance when hot £&,, and carrying a current C, the heat produced must be proportional to the surface, since it is lost almost entirely by radiation. If is this loss measured in watts per square cm., and ¢ the specific resistance per cubic cm. : 4 - Hence by substitution Geary, Ae SORE we ,3/ 4r ieee een S77) ) ee OE) Fyr2? PLY Cage) Dene \ Tie The quantity under the radical sign is constant for a given material at the working temperature, consequently J, the diame- INCANDESCENT LAMPS. 413 ter of the filament, must be made proportional to the 3 power of the current. The length Z disappears, hence for a given tempera- ture (about the same for lamps using the same watts per candle- power) the diameter depends solely upon the current to be carried. The resistance A; increases directly with the length Z, hence C #,, or the voltage required between the terminals of a filament, is directly proportional to its length, other things being equal. For example, a 220-volt lamp may be made by doubling the length of a 110-voltefilament, a common plan being to use two of the latter in series. The current would be the same in both cases, hence the watts are twice as great for the 220-volt lamp, and the candle-power, being nearly proportional to the watts, would also be doubled. Two 16 candle-power filaments would give 32 candle-power; so in order to make a 16 candle-power, 220-volt lamp, it is necessary to reduce the diameter of the filament, making it the same as for 8 candle-power at 110 volts. Unfortunately the weakness, due to diminished diameter, requires the lamp to be run at a somewhat lower temperature, and therefore lower efficiency. For example, a 220-volt lamp may take 3.8 compared with 3.1 watts per candle-power for 110-volt lamps, or about 20% more power. The actual sizes of filaments depend largely upon the propor- tion of deposited carbon, the specific resistance of the latter being about .12 to .16 that of the untreated filament, as already stated. Ordinarily the diameter is increased about 10 or 20% by the de- position of carbon. \ Relations Between Voltage, Candle-Power, Efficiency, and Life. | — The voltage of a lamp is the potential difference measured across | its terminals. Candle-power may be defined in the various ways/ stated on page 308, the mean spherical candle-power being the complete measure, but the most difficult to determine. The mean horizontal candle-power may be easily measured, while the lamp is rotated 180 to 220 times per minute with its axis vertical, and it is that by which lamps are rated by their manufacturers; but the mean spherical is usually 15 to 20% less, as shown in the table on page 409. It has been recommended by the National Electric Light Association to measure the candle-power while the lamp is rotated with its axis inclined 45° to the photometer. This usually gives results approximating the mean spherical candle- power, but does not necessarily do so. In what follows, the mean 414 BLECTRIGALIGHIING: horizontal candle-power is used, since lamps are generally rated by it. The effictency of a lamp is measured by the number of candle- power per watt. It is usually stated as the number of “ watts per candle-power ;” but of course this is the inverse of efficiency, since it is larger with poorer lamps. ‘The /zfe of a lamp means either the total number of hours it gives light before burning out, or the number of hours it burns, until its candle-power has fallen to a certain fraction — usually 80% of its rated value. The former might be called the ¢o¢a/, and the latter the wsefu/ life, beyond which it is not economical, and should be replaced by a new lamp, even if it is capable of burning much longer. Unless otherwise stated, all data are given for lamps up to 125 volts, and are sub- stantially true for 220 volts, but the latter are more sensitive. The candle-power, etc., of lamps vary in much greater propor- tion than the voltage supplied, as shown in the following table: ” ate VARIATION IN CANDLE-POWER, EFFICIENCY, AND LIFE. In the following table is shown the variation in candle-power, efficiency, and useful life of General Electric standard 100 to 125 volt 3.1 and 3.5 watt lamps, due to variation of voltage supplied to them. EFFICIENCY IN E FF Per Cent PER CENT Watts Ae Wie, RELATIVE N oF NorMAL Per CANDLE rae x LIFE oF NorRMAL Canoe ? 3.1-Watr PER CANDLE, Br Waris VOLTAGE. 3.5 WATTS. ‘ * On PAINTER OOH a DOSMARAD Co NAH Pe RO C0: COO C2 G2) COO Ce a Or ON WOSHNWADUDSHNRADOW ARTIoDIdS BOWNSARAUSS 4.6 4.4 4.2 4.1 3.9 3.7 3.6 3.4 3.3 3.2 3.1 2.9 2.9 2.8 2.7 2.6 2.5 me BS For example, a lamp of 16 candle-power, 105 volts, and 3.1 watts, if burned at 103 % of normal voltage, or about 108 volts, INCANDESCENT LAMPS. Wy: will give 118% of 16 candle-power, or 17.9 candle-power, the efficiency will be 2.8 watts per candle, but the life is reduced nearly one-half, being .562 of the normal. In other words an increase of only 3% in voltage raises the candle-power 18 %, the explanation being that up to a red heat no light is given, but after that increases rapidly with the tempera- ture. Since the resistance of the filament is almost constant near the working temperature the current rises directly with the volt- age, so that the watts for this case are 103 x 103 = 106 % of their normal value ; that is, 6 % more power produces 18 % more candle-power, the watts per candle-power being reduced from 3.1 to 2.8, an improvement of 10%. Unfortunately this very desir- able gain is offset by the decrease in life resulting from the higher temperature and the rapid falling off in candle-power and efficiency. Long experience has shown that filaments for 125 volts or less should be designed to run at a temperature that gives an efficiency of 8.1 to 3.5 watts per candle-power. Above this, the trouble and expense of renewing the lamps more than counterbalance the saving in power. It is also important to note that the rated watts per candle-power usually represent the zzzzzal efficiency and filaments that burn at too high temperature, soon show much poorer results, so that the average efficiency may not be improved. In the case of lamps for about 220 volts, it has not been found practicable to do better than 3.4 to 4 watts per candle- power (initial) owing to the greater length and smaller diameter of the filament as explained on p. 406. Another table, issued by the makers of the Packard lamp, is . given below, showing results somewhat different from those on p. 414, the average life being less affected by raising or lowering the voltage. For example, 3.1 watt lamps having 580 hours _ average life at normal voltage, are stated to have a life of 280 hours at 6 % increased voltage. In the previous table the rela- tive life is given at .31, that is, 381 x 580 = 179.8 hours, which is considerably less. The results of actual practice probably approxi- mate more closely those contained in the former table. The usefus life at normal or excessive voltage is actually less than here stated with the usual limit of 80 of the initial candle- power. For lives as long as those given in the table, the average candle-power would fall to 60 or 70 4% of its initial value. A416 ELECTRIC 17GHIING: EFFICIENCY AND AVERAGE LIFE OF LAMPS AT VARIOUS VOLTAGES. 98 PER Cent 100 PER CENT 102 Per CENT 104 PER CENT 106 Per CEentT oF NorRMAL oF NorRMAL oF NORMAL oF NorMAL oF NorMAL VOLTAGE. VOLTAGE. VOLTAGE. VOLTAGE. VOLTAGE. Actual | Actual Actual Actual Actual Actual Actual Actual Actual | Actual Watts Life Watts Life Watts Life Watts Life Watts Life per in per in per in per in per in CoEPs Hours. Cee. Hours. ‘Opa 2 Hours. Gye: Hours. Geir. Hours. 4.85 | 3500 4.5 2400 4.21 | 1830 3.92 | 1400 Dail, 1120 4.51 | 2000 4. 1500 on14 | LL6O 3.48 880 3.28 710 Sri t 113200 3.5 900 oat oy 100 3.05 550 2.87 440 3.04 760 3.1 580 2.9 460 2.7 360 2.54 280 2.69 350 2.5 260 2.54 210 2.18 170 2.05 140 Individual Performance of Lamps. The discrepancies just pointed out illustrate the great differences in rating and in re- sults. This is partly due to different methods of manufacture and testing, and partly to wide variations in individual perform- ance. The figures are supposed to represent averages ; but lamps, even when rated exactly the same, differ greatly from each other. The results of tests on a number of lamps are plotted upon what are called “target diagrams,” as illustrated in Figs. 872 and 373. The former shows that the watts required to produce a given candle-power vary about 10%, and the candle-power runs from 16 to 21, with watts from 63 to 71, although the lamps were all supposed to be the same. These are bad results; but Fig. 373 is considered very satisfactory, since only 2 lamps out of 50 are out- side of the limiting target, which permits 1.5 candle-power, and 3 watts range above or below the normal. The manufacturer attempts to hit the center, that is, obtain uniform results ; and in this case has done fairly well with most of the lamps. Neverthe- less, the individual differences are considerable, and show the diff- culty of rating incandescent lamps exactly. . The Relation Between Candle-Power and Age in Fig. 374 also illustrates these differences, lamps of approximately equal initial candle-power giving from 11} to 174 candle-power at the end of 100 hours run. In some cases the candle-power rises at first, in others later, and in some not at all; but the general tendency is downward for all. The average shown by the dotted line is fairly good ; being 14.1 candle-power, or 88 % of 16 candle- power at 600 hours, and apparently would fall to 80% at about 417 66 G7 _ 6G LNGCANDESCENT LAMES. Watts oo 5S 8 GO %O 7 57 GA 65 0) FREESE EEE Figs. 372, 373. Relation between Initial Candle-power and Watts Consumed. 0 2 =o Cy Ot Oe a ee % S samod-s}pueD, the “wemod-e|puey rg 418 ELECTRIG#OLI GETING. 850 hours, a satisfactory result for a 3,5 watt lamp. The decline in candle-power of an incandescent lamp, which continues until finally the filament burns out, is due to the following causes: 1. The filament wears away by evaporating or projecting particles of carbon from its surface. flours O lOO. 200 S00 A400 500 GOO is Gt A CECE EERE [ 7 ee RSS m7 ) PECE Eee SESSSeSSE800058 = Perri Coo HH 1 aa Ss ce in coo S ies aS an ag CCC SI] BERUSABEB PS RwSES we ¢ a a a _ RSS Oo CI PSOE ‘aE CESS N CeoCCeso 1 ELE | \ “ root 1] re) PS PSEC CPPS Can poets coors is SECC 0) b~@n EER CS — SSE ; CPAs O Serer era a Bs m() Denna 2aneES O CCC SCOCCH COCAINE PSS ACOA Coes CNET CCC BEEP> oa cor = Cert t 4 Pere ce Saseeneee seecene SOL apy OFZ Sanaa ae aide seuaesn de H we - (re 3874. Relation vo PRR and Age. 2. The interior surface of the bulb becomes blackened and less transparent, owing to the carbon deposited upon it. 3. The emissivity (for heat) of the filament increases so that its temperature is lowered. This falling candle-power is the most serious trouble with incandescent lamps, since it is not shown by a test in the begin- ning, and being so gradual escapes definite attention. It is, how- ever, a common cause for dissatisfaction, producing the very INCANDESCENT LAMPS. 419 general impression that a so-called 16 candle-power incandescent lamp does not give as much light as the ordinary gas-burner (using 5 cu. feet per hour). This idea is well founded, because we have seen that lamps are rated by their mean horizontal candle-power, which is usually 15 to 20% less than the mean spherical. Furthermore, the former falls to 80 % of its initial value at the end of the useful life, and the average is 90 %, assuming a uniform de- cline, which is close enough for practical purposes; so that the actual candle-power is about 25 % less than the nominal, or about 12 candle-power, compared with about 20 to 30 for the gas- burner. This trouble is aggravated by the fact that lamps are rarely renewed when they have fallen 20% in candle-power, as the almost universal custom is to let them run until they burn out, which may take 1500 hours or more. During the latter half of this period, their average candle-power is often not more than 60 or 70 % of the initial candle-power; and the watts per candle-power have risen from 38.1 or 3.5 to 5 or 6, a loss of 30 or 40 % in actual light, and an increase in cost of 60 or 70 % per candle-power. The obvious conclusion is that it is uneconomical to burn lamps more than about 500 or 600 hours; and it would be a positive saving if one should break them at the end of this period ; in fact, this limit has been aptly called the “smashing-point ’ * It is safe to say that hundreds of thousands of dollars are wasted annually in the world by not observing this important fact. A further exaggeration of this bad situation arises from the common tendency to supply lamps with less than their normal voltage. A deficiency of at least 1 or 2 volts is the rule and not the exception, and it often amounts to 4 or 5 volts. This custom is the result of attempting to prolong the life of lamps as much as possible. It is, however, a “penny-wise and pound-foolish ” prac- tice, since the lamp itself costs about 20 cents, while the energy that it consumes costs $3.00 during a normal life of 400 hours at an ordinary price of .75 cent per 16 candle-power lamp-hour. A lamp using 38.1 watts per candle-power at normal voltage has an average life of 400 hours. If this lamp were burned at 98 % of normal voltage its life would be increased to 584 hours, but its * Electrica World, Dec. 24, 1892. Trans. Amer. Inst. Elec. Eng., vol. x., p. 65, 1898. 420 ELECTRIC LIGHTING. consumption of energy is raised to 3.54 watts per candle-power or 7.4% more. At .75 cent per lamp-hour and for 584 hours’ life the energy costs $4.38, on which an increase of 7.4% amounts to 32 cents. The lamp burns 584 — 400 = 184 hours longer, but this effects a saving of only (184+ 400) x 20=9 cents, which is :28 as’ much “as the 7extrarcost sor fenerey,) thewnet weses bem 32 —9= 28 cents. The sole advantage gained is the fact that the lamps are renewed every 548 instead -of 400 hours, but this saving is only a small fraction of a cent per renewal. This false economy is almost universal, because a man in charge of a lighting-plant notices each time that a lamp burns out, and considers it an actual loss of 20 cents. He forgets that in most cases the lamp has given light for 600 hours or more, and its cost per hour of service is only 20 + 600 = .03 cent compared with .7 cent for energy. The importance of the latter item is only found by calculations based upon electrical and photometric tests, hence it is rarely appreciated by the ordinary user. In the comparisons made above, the price of electrical energy has been taken at the rate charged by central stations, which varies from .5 to 1 cent per hour for a 16 candle-power lamp consuming 50 to 55 watts, an average rate being .75 cent. For isolated plants, in which the energy is generated on the spot, its cost may be only .15 to .25 cent per lamp-hour; but even then the extra energy required at 98% of normal voltage is one-fifth to one-third as much as before, or 82 + 5 to 82 + 3 = 64 to 10.7 cents for 554 hours, compared with a saving in lamps of 9 cents. Further- more the price of lamps may be less than 20 cents. It is custom- ary to regard the rated voltage of a lamp as a maximum value never to be exceeded, consequently the inevitable variations that occur produce an average at least 1 or 2% below the normal pres- sure, and often it is 4 or 5% too low, resulting in very poor economy. While it is injurious to run lamps above their normal voltage, nevertheless the average pressure should equal that for which they are rated, and the regulation (i.e., uniformity of voltage) ’ should be good enough to avoid any serious shortening of life. Aside from any direct question of dollars and cents, the dissatisfac- tion and the depressing effect produced by running lamps beyond (their normal life or below their rated pressure are sufficient reasons ; to demand very careful attention to this point. ) INCANDESCENT LAMPS. 421 Approximate Formule for Relations between Voltage, Effictency, Life, and Candle-Power may be used within a limited range of about 5 % above or below the normal values, which is enough for any practical purpose. Calling V the voltage supplied at the lamp terminals, C the current in amperes, R its resistance (hot) in ohms assumed to be constant within the range named, W the power in watts consumed by it, D its candle-power, A the so-called efficiency “in watts per candle-power, and L the normal life or average number of hours to reach 80 % of the rated candle-power, we have: — | C=V+R(QA) R=V+CQ) WH=V?+R=CRE Corals since # is nearly constant, (4) Hee OC Ties Bi het ‘s - (5) Ao - ore 2 ore =e a a“ «<6 ‘< (6) DO Fas Ode Om ers ote ke a i (7) 1 ih 1 jae LD rm Gm © Tar’ Edison 3.1 Watt Lamps, p. 414, (8) To apply the above expressions, we ‘nay find R from (2) by measuring V and C with volt- and ampere-meters while a lamp is burning. From (4) we know that a certain increase or decrease in voltage, say 2 %, produces the same change in the current and (5) shows that the watts are proportional to the square of either volt- age or current. From (6) we find that a lamp consuming 3.1 - watts per candle-power, at rated voltage, requires at 98 % of that voltage 3.1 + .98+ = 3.1 + .922* = 3.386 watts per candle-power. This agrees closely with the value 3.34 given in the table on p. 414. From (7) its candle-power is found to be .98° = .886 of the normal compared with .89 in the table and from (9) its life is 1 + .9829 =1.5 times normal compared with 1.46 in the table. These formule, as already stated, are merely approximate, and do not apply to individual lamps, but they bring out very important and interesting facts that are practically true when a number of results are averaged. Special Lamps. — Almost innumerable varieties of lamps are made for different purposes. The standard voltages are 50 to 60, 100 to 120, and 200 to 240 for central station or isolated plant 422, ELEGLIRICMIGH LING: lighting ; but many others between and below these limits are also adopted. Prominent among these special types are the low-voltage lamps for use with storage or primary batteries, and ranging from 3 to 12 volts. In the larger sizes these are made like the standard lamps; but for the smaller sizes they are given special forms, as, for example, the bicycle lamp in Fig. 875, giving ? candle-power, and consuming .0 ampere at 4 volts. The surgical lamp, shown actual size in Fig. . 376, represents one of the very smallest Fig. 376. forms, requiring 38 volts and about 1 am- eds ia ae ae pere, and giving } candle-power. Another important class of low-voltage lamps includes the so-called series lamps. They are used in a candelabra or sign in order to subdivide the light and also simplify the wiring. They may be connected in series across an ordinary 110 volt or other constant potential circuit. It is necessary that the members of each series should be designed for the same current within .03 ampere. The lamps may differ in voltage, but the sum of the voltages in any series must equal that of the circuit within 3 volts. The lamp represented full size in Fig. 877 gives 1 candle-power, and consumes .33 ampere at 12.5 to 15 volts, being run 8 in series on 100 to 120 volts. Series lamps are also made for constant current circuits as described on pp. 24, 25, and in Chapter X. For a 10 ampere series arc Fig. 877. Series Lamp. circuit they require about 4 volt per candle- power, and for the 8 or 8.5 ampere alternating or direct current circuits they use about 1 volt per candle-power ; in either case the filaments are made sufficiently large to carry the current. In addition to the many different voltages for which lamps are made, several different sizes are supplied for each voltage, the standards being 8, 16, 24, 32, 50, and 100 candle-power, but others are often required. Various shapes of lamps are manufactured for special purposes, as, for example, the tubular lamp in Fig. 369, and some ornamental forms. Finally lamps are made in many INCANDESCENT LAMPS. aU colors, such as red, blue, green, amber, opal, frosted, etc., besides the ordinary clear glass bulbs. The result is that the total number of different styles of lamp that are made is many thousand. Renewal of Filaments. — In most cases whena lamp burns out or is reduced in candle-power and efficiency below the economical limit, it is the filament alone that is worn out. The bulb, base, leading-in wires, etc., are usually intact and capable of being used again. The renewal of the filament and vacuum is now carried on success- fully, the process being as follows: A hole is made in the bulb by removing the tip, and the carbonaceous deposit on the inner sur- face is burned off by the application of heat to the outside. The old filament is taken out through the aperture in the bulb and a new one introduced, being then connected to the leading-in wires by a pasted joint as described on p. 398. The joint is set, and the gases driven off by a blow-pipe inserted through the hole in the bulb. A tube similar to L M on p. 400 is attached to the bulb at the point where the hole was made, the parts being fused together, and the bulb is exhausted and hermetically sealed in the usual manner as described on p. 401. The brass base and the outer sur- face of the bulb are also cleaned if they require it, and the lamp then has the appearance and useful qualities of a new one. The so-called stopper-lamps are easily renewable, since the inner part (A B in Fig. 349) that carries the filament fits into the bulb with a tapering ground joint similar to that of a glass stopper for a bottle. It is cemented in place in order to hold it and also make the seal more nearly air-tight. This type is more expensive to -make than the ordinary lamp with fused joints, and does not main- tain the vacuum so well. ‘¢Turning Down’’ Incandescent Lamps. — It is often urged as an objection against these lamps that they cannot readily be made to give more or less light, as in the case of gas or oil lamps. In the majority of instances when the latter are turned down it is to save the trouble of relighting them, which does not apply at all to the incandescent lamp. Furthermore the light of the last-named may be diminished temporarily or permanently in several ways. One plan is to substitute a lower candle-power lamp in the same socket, which can easily be done in a few seconds. Another method is to insert resistance or, for alternating currents, induct- ance in the circuit. The resistance or inductance is placed ina 424 ELECTRIC-LMGHIING special socket and controlled by a key. A third arrangement em- ploys two filaments which are connected in parallel or singly for full light and in series for reduced effect. A form of light is made in which a 16 and a 1 candle-power filament are put in the same bulb, and either may be lighted by turning the socket slightly. An obvious way to dim a light is to put a shade ora less translucent globe around it. This fails to save any energy when less illumination is required, but is simple and effective. A combustible shade, such as cloth, paper, or wood, should never be put in contact with or close around a lamp, as the heat will accumulate and may start a fire. Specifications for Lamps.— In buying or making contracts for any considerable quantity of lamps it is customary to specify certain requirements. The initial candle-power at the rated voltage should not be more than 9% above or below the value called for. This margin amounts to 1} candle-power for a 16 candle-power lamp, as shown in Fig. 378, the “target’’ (within which the lamps, must hit) extending from 14} to 17} candle-power. This limit applies to the individual performance of every lamp, and any that exceed it may be rejected. The avexage initial candle-power of a certain lot of lamps should be within 6 % of the rated value (1 candle-power for a 16 candle-power lamp). It is not desirable for lamps to be either above or below their rated candle-power, since their life is shortened in the former case, and their efficiency reduced in the latter, and either interferes with uniformity of results. The candle- power is usually measured when the lamp is mounted vertically in the photometer and rotated at about 180 r. p. m., the result being the mean horizontal candle-power. ‘The relations between this and the mean spherical candle-power and candle-power from the tip are shown in Figs. 870-1. As already stated, the mean horizontal candle-power, being easily measured, and the one by which candle, oil, and gas lights are rated, is generally adopted. In some cases, it is specified that lamps shall not give less than 7 candle-power from the tip. The unit of light commonly accepted in this country is the British ‘Parliamentary Standard”’ candle-power. From this as a przmary standard a number of incandescent lamps are carefully rated, and serve as excellent secondary standards, being burned only a minute or so at a time to check other incan- descent lamps that are used as working standards. INCANDESCENT LAMPS. 425 Life of Lamps.— Since all of the above statements refer to entttal candle-power, it is necessary to specify the useful life of a lamp, or the time it will burn before falling to a certain candle- power, usually 80 % of the initial candle-power. For lamps having an initial efficiency of 3.1 watts per candle-power, the useful life is about 400 to 450 hours. At 3.5 watts it is about 800, and at 4 watts about 1600 hours. Candle-hours —The true measure of a lamp’s value is the product of its useful life in hours and its average candle-power during that time. The latter is usually about 90 % of the initial, hence a 3.1 watt 16 candle-power lamp should give at least 400 x 16 x .90=5760 candle-hours. | Liffictency. — The number of watts consumed per candle-power is another important point in lamp specifications. It refers usually to initial values, the specified useful life being a check upon the fall in candle-power and indirectly upon the efficiency. The standard efficiencies are 3.1, 8.5, and 4 watts per candle-power. Each lamp at rated voltage should take within 6 % of the watts specified, and the average for a large number should be within 4 % of the specified figure. If the efficiency is high (i.e., small con- sumption of power) the life is shortened, and vice versa, a fair ‘ compromise being adopted in practice, as explained on pp. 415 and 418. If the cost of energy is low, as for example in some water-power plants, a lower. efficiency lamp may be used, but it is seldom economical to use 4 instead of 3.5 watt lamps. The useful life of the former is about 1600 hours compared with 800 for the latter, which would save one lamp costing about 20 cents every 1600 hours. The energy consumed in 1600 hours costs } to 3 cent per lamp-hour at ordinary central station rates, or $8 to $12; and a 3.0-watt lamp would. use one-eighth less energy than a 4-watt lamp, the saving being $1 to $1.50, which is 5 to 74 times the cost of a lamp. Isolated electric-lighting plants in hotels, factories, etc., involve very little extra expense for engineers and other labor, or for coal when the exhaust steam is used for heating ; hence the electrical energy may be produced at 15 to .25 cent per lamp-hour. For 1600 hours it amounts to $2.40 to $4.00, and one-eighth of this is 30 to 50 cents, which is also greater than the cost of a lamp, so that even then 4-watt lamps are less economical than those using 3.5-watts per candle-power. It may happen that 426 ELECTRICMIIGHUANG lamps are located in some inaccessible place, such as the ceiling of a large hall or railway station, and in that case it might be better to use the long-lived 4-watt lamps to save the trouble of frequent renewals. Where the regulation is poor (i.e., voltage varies con- siderably) the life is shortened, and it may be desirable to use 4- watt lamps. Bulbs and Bases. —‘Vhe former are specified to be uniform in size and of best quality glass, clean and free from flaws or blem- ishes. The metallic parts of the base should be of good quality brass, uniformly and accurately fitted to the bulb so as to be im- pervious to moisture. When placed in the socket no live metallic part (i.e., connected to the circuit) should be exposed. Vacuum. — All lamps must have a practically perfect vacuum, and show no glow when tested with an induction coil giving a half- inch spark. For further information regarding Incandescent Lamps, refer- ence may be made to the following : — Lhe Incandescent Lamp and Its Manufacture, by Gilbert S. Ram, pp. 218, London, 1898. A Life and Effictency Test of Incandescent Lamps, by Pro- fessor B. F. Thomas and Messrs. Martin and Hassler, 7vansactions of the American Institute of Electrical Engineers, vol. ix., p. 271, 1892. The Most Economical Age of Incandescent Lamps, by Carl Hering, zdzd@., vol. x., p. 65, 1893. Conductivity of Incandescent Carbon Filaments and of the Space Surrounding Them, by John W. Howell, zdzd., vol. xiv., p. 27, 1897. The Incandescent Lamp (Manufacture), by Manning K. Eyre, The Electrical World, Jan. 5, 1895. Incandescent Lamps, by Francis W. Willcox, Journal of the Franklin Institute, April, 1900. LAMES NOL HUPLOYVING CARBON. 427 GArieAs Rly EaRgexe vel: LAMPS NOT EMPLOYING CARBON. Av forms of electric lamp in successful use prior to 1900 employed carbon as the light-giving body. This applies to are lamps, which in all cases are provided with carbon electrodes, and to incandescent lamps, which employ carbon filaments. There are, however, two other interesting classes of lamps which do not use carbon: one includes the so-called vacuum tubes, in which all the light is emitted by a gas or vapor; and the other comprises incandescent lamps, in which the filament is composed of some material other than carbon, the ernst lamp being a prominent example. The use of vacuum tubes as sources of light is a very old idea, being described by Hauksbee in a treatise published about two hundred years ago.* He employed glass vessels con- taining rarefied air, made luminous by frictional electricity, and to quote his own words, the light was “so great that large print, without much difficulty, could be read by it.”’ Similar, but not much more successful, attempts have been made repeatedly during the succeeding two centuries. The devel- opment of the Geissler and other improved forms of vacuum tube, and of the induction coil, during the past fifty years or more, has facilitated and encouraged such investigations. Mr. Nikola Tesla, in a paper on “Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial [lumination,’ + gave prominence to this subject, and has since’ investigated and written further in con- nection with it, but has not yet advanced beyond the experimental stage. A paper on “Recent Developments in Vacuum Tube Lighting,” { by Mr. D. McFarlan Moore, describes the methods * Physico mechanical Experiments, etc., London, 1709. + Transact. Amer, Inst. Elec. Eng., vol. viii., p. 267, May, 1891. t J/édzd., vol. xiii., p. 85, April, 1896. ‘ 428 ELECTRIGSLIGH GING. employed and results obtained by him. In his laboratory and at the New York Electrical Exhibition of 1896 he showed a room of considerable size lighted fairly well in this way, but no com- mercial applications have yet been made. In another series of | investigations, Mr. Cooper Hewitt of New York City has suc- ceeded in making a vacuum tube lamp of several hundred candle- power, and having a very high efficiency of about 4 watt per candle- power ; but these very promising results have not been published, and his methods up to the present time have not been applied commercially. | The chief advantages to be expected from vacuum tube lamps are high efficiency, long life, and distribution of light. The last is due to the large volume from which the light is given off; for example, a tube one foot long and an inch in diameter, or even larger, is lumincus throughout. In the ordinary incandescent lamp the light is emitted from a filament six to ten inches long and a few thousandths of an inch in diameter. This is practically a line, and produces too sharp an image upon the retina, as shown by the fact that it persists after the eye is shut or turned away from the light. The vacuum tube should have a long life, since the light- giving body being a gas, and not a solid, is not worn away. On the other hand, the degree of vacuum may rise or fall owing to absorption of the gas or leakage of air, in either case changing the resistance of the tube and interfering with constancy of action. The high efficiency of a vacuum tube results from the fact that a gas or vapor may be raised to a much higher temperature than a solid. The consequence is, the quantity of light emitted is increased in comparison with the emission of heat. In fact, such sources are often said to give “light without heat,” but in most cases heat is given off with the light. Nevertheless, it is true that a glow-worm, for example, or some phosphorescent body, radiates a large part of its energy within the visible spectrum, the proportion of the longer, non-visible waves, called radiant heat, being far less than with ordinary sources of light. There appears to be a discrepancy between the statements that the temperature in a vacuum tube is high, and yet the heat given off is small, but these are easily reconciled. If a 110-volt, 16 candle-power lamp is supplied with about 125 volts, it will give LAMPS NOT EMPLOYING CARBON. 429 32 candle-power. The power consumed is increased in about the ratio 110? : 125? = 12100 : 15625, or about 30 per cent, as shown on page 421. Hence the rate of the total emission of energy is raised 30 per cent, but the light emitted is doubled. Thus the quantity of heat for the same amount of light would be only 130 + 2 = 65 per cent as great as before. By carrying this still further, the proportion of heat to light can be reduced very greatly, and what is called “light without heat’ may be produced. It is also a fact that the temperature of the filament is increased at the same time, but in order to give the same candle-power its mass may be diminished. This applies exactly toa vacuum tube lamp in which the mass is very small, but the temperature of the indi- - vidual particles is raised to a high point by the passage of electric current or discharge. It is possible that the electrical effect upon the atoms or ions may be somewhat different from what is ordi- narily called high temperature ; but it amounts to the same thing, since high rates of vibration or short wave lengths are produced. In the experiments of Tesla, luminous discharges were created in vacuum tubes or even in the open air by a high frequency generator (10,000 to 20,000 periods per second) connected to the primary of an induction coil, the secondary of which gave very high voltage. He also employed a form of induction coil in the primary of which electrical oscillations are set up by sudden breaking of the circuit, producing a much higher frequency (100,000 or more periods per second), and therefore giving an _ extremely high voltage with only a few turns of wire. In this way vacuum tubes were made to glow by holding them near the terminals, but without any electrical connection to them. Such forms of apparatus are hardly suitable for practical use, and they involve considerable losses from leakage. Moore employed induction tubes with connections made to them in the usual manner and operating at comparatively low voltage obtained from a self-induction coil with an electromagnetic make-and-break in the circuit. The latter was placed in a vacuum in order to give a sudden break and to avoid burning the contact points, but even with this precaution such a device is likely to give trouble. The Wehnheldt interrupter may be substituted, but it is doubtful if any form of break yet devised can be relied upon to act for the long periods of time demanded in lighting service. 430 ELECTRIC LIGHTING. The tubes developed by Hewitt are of sufficiently low resist- ance to operate at ordinary pressures. They may be connected directly to the present 110-volt circuits without requiring any step-up transformer or make-and-break device, which is a great advantage from the practical standpoint. Unfortunately it requires about 1000 volts to start the discharge, after which it is maintained by 110 volts. The Nernst Lamp. — The type of lamp invented by Professor Nernst * of Géttingen, employs, in place of the long carbon fila- ment of the ordinary incandescent lamp, a shorter “strip of mate- rial which is an insulator at ordinary temperatures, but becomes a good conductor and luminant at high temperatures.’’ Usually it is composed of a mixture of metallic oxides, such as magnesia, yttria, zirconia, thoria, or ceria. Another feature of the Nernst lamp is the fact that the incandescent material is not burned by exposure to the air, consequently it need not be inclosed in a vacuum. Since the filament does not become a conductor until heated, some means must be provided to raise its temperature so that the current may flow through it. Two methods are employed, one consisting simply in applying the flame of a match or alcohol lamp directly to the filament after it is connected to the circuit. When its temperature is raised sufficiently the current passes through it, bringing it up to and maintaining it at a white heat. The other method is automatic, the current being passed through a spiral 4A which surrounds Fig. 378. Automatic Nernst Lamp. the filament / (Fig. 3878), and heats it until the current flows through it. This causes the magnet J/ in series with / to attract its arma- ture A and break contact with the screw P, thus disconnecting the heating spiral A H which is in parallel with the filament F. When the lamp is turned out by opening the circuit, the spring S brings the armature A back into contact with the screw P, and the automatic device is ready to act again. The heating device H{ff is of porcelain, which, before being baked, is wound with a * U.S. Patent No. 623,811, April 25, 1899, LAMPS NOT EMPLOYING CARBONS. 431 great many turns of fine platinum wire. During the baking this wire becomes embedded in the porcelain, and is thus held firmly, only the outer surface being visible. The resistance # consists of iron wire placed in series with the filament F, so that the increase in resistance of the former compensates for the decreasing resistance of the latter, when the temperature rises. The non-automatic lamp is provided with an open globe to permit lighting by a match; and the automatic form is contained in a closed globe, but it need not be air-tight. These lamps may be connected in parallel to the ordinary 110 or 220 volt circuits,. and are claimed to have a high efficiency of 1.5 to 1.75 watts per candle-power, being one-half the power required by a carbon fila- ment giving the same light. On the other hand the hfe is shorter, the average being about 200 to 300 hours, after which the filament loses its strength and increases in resistance. This, however, is the only part that is used up, and may be readily renewed, since the lamp is not hermetically sealed. The light is whiter than that of ordinary incandescent lamps, and the filament being much shorter, + and giving 25, 50, or 100 candle-power, produces a dazzling effect on the eye unless a ground glass or equivalent globe- is used. At the Paris Exposition of 1900 the Allgemeine Elektricitats Gesell- schaft of Berlin exhibited a room brilliantly illuminated by Nernst lamps, being the first important public application. 432 FLUE CTRICHTAGIOIING OE Agr at Rael Ne | METERS. THE general name meter may be applied to any device for measuring electrical quantities, and we have many forms of ampere- meter, voltmeter, wattmeter, etc. Ordinarily, however, the term meter or electric meter, unless combined with another word, means an instrument to vecord, register, or integrate current in ampere- hours or energy in watt-hours. They are commonly used in stations or in the service connections of the various consumers to take account of the amount of current or energy supplied. Classification of Meters. — Various electrical effects have been utilized in connection with meters, and the latter may be classified from that point of view, as follows : Principle of action. Example. Electrochemical effects. . . Edison meter. Electrical heating effects. . . Forbes meter. Electromagnetic effects . . American (Marks) meter. Electrodynamic effects. . . Thomson meter. Alternating current effects . Shallenberger meter. Qualities Required in Meters. — Few devices are called upon to fulfill so many and such difficult conditions as those under which an electric meter is likely to work. For this reason its de- velopment has been one of the most serious problems that elec- trical engineers have had to solve. The chief qualities that are required or desired in meters are the following : 1. Accuracy. Under any reasonable conditions a meter should be at least commercially accurate, that is, its errors should not ex- ceed 2 or 8 per cent. ; | 2. Range. A meter should measure with commercial accu- racy for any load from the maximum down to the smallest that may exist. This is probably the most difficult condition METERS. yey to meet. For example, a meter that will record correctly for 100 lamps is not generally capable of acting at all when only one lamp is burning. Even if it takes some account of a single lamp, the record would be very inaccurate. To be sure, a certain percentage of error with a few lamps is less serious than for many, but it often happens that a small number may burn nearly all the time, in which case the aggregate error becomes large. 3. Consumption of Energy. Practically all forms of meter consume some energy,.and if this loss goes on continually in a great many of them it may amount to a large item in the course of a year. Hence a meter should waste less than one per cent of the energy that it measures, and this loss should decrease some- what in proportion to the load, which is usually the case. 4. Drop in Voltage. Besides the mere consumption of power, it is even more objectionable to have a drop in voltage on a con- stant potential system, especially for incandescent lighting. — If the current C passes through any resistance R a drop C R is pro- duced, hence the resistance introduced into the circuit by the meter should be as small as possible, so that the drop at full load shall not exceed + per cent of the working voltage. In a watt- meter the series resistance produces such a drop as well as loss of energy, but the shunt coil merely uses a very small portion of the current, which is less objectionable. 0. Durability. It is very important that none of the parts should be likely to wear rapidly or get out of order. 6. Aztentzon. The care and attention required should be small, and frequent inspection or testing unnecessary. 1. Registration. The meter should record or register in a clear manner, so that the consumer can read it at any time and check its accuracy. 8. Testing. It should be an easy matter to test the meter and verify it. : 9. Cheating. The meter should be so constructed and pro- tected that it is not liable to be tampered with in order to change its reading. 10. Cost. The price should be sufficiently low, so that a large deposit or rental need not be charged. 11. Alternating and Direct Currents. It is desirable that a meter may be used for either kind of current; but it is generally 434 ELECTRIC LACH TING: bought for one or the other, and this point is not so very impor- tant. 12. Frequency. It is desirable also that -variatiors in fre- quency should have no effect ; but the latter being fixed in most cases, it is sufficient to adjust for it in the first place. 13. Portability. A meter should be strong enough so with moderate care it may be carried about without injury. It cannot be expected that any meter will fulfill all of the above conditions, but there are several types in use which do so reasonably well. Methods of Charging for Electrical Energy. — If the demand upon an electrical generating plant were uniform at all times, a simple charge of a certain rate per k. w. hour would be sufficient, possibly giving the larger consumers a lower rate, as is customary in other branches of business. In electric lighting, however, the demand varies widely at different hours of the day and night, which introduces serious difficulties in technical as well as business management. For example, the load between 5 and 6 P.M. in winter may be many times the average load. It is customary to call the ratio of the average load to the maximum the “ load factor.” This is often as low as 10% and is rarely higher than 25 % in ‘electric. lighting.s ~The (use of senersy for motors, heatersmecies tends to make the demand more uniform, and therefore raises the load factor. It is evident that the capacity of machinery, etc., in an electric lighting plant must be somewhat greater than the maximum demand, in order to give a margin in case of break- down of part of the apparatus. Hence an increase in the load at its maximum point requires a corresponding increase in capacity. On the other hand, the demand upon the system during hours in the day when the load is light can be taken care of without any increase in plant. In other words the station can afford to sell energy at a much lower rate during those hours. A striking illustration of the importance of this point is the fact that about one-quarter of the generating machinery in electric lighting stations is used only 50 to 100 hours per year, and may be prac- tically idle during all the rest of the time. These hours are usually between 5 and 7 p.m. during December and January. It is quite evident that this machinery cannot possibly earn its interest and depreciation charge durings these few hours at ordi- METERS. 435 nary rates. It is necessary, however, to install it in order that the business may be held for the rest of the year. Various attempts have been made to take account of these points in charging for energy, and also to encourage its use at those hours when it can be delivered more economically. The several plans for selling electrical energy, some of which take account of these conditions, are as follows : — METHODS OF CHARGING FOR ELECTRICAL ENERGY. Ist, Contract to supply a certain number of lamps at a fixed price per month, whether they are used or not. 2d, Meter with “ flat” (i.e., uniform) rate. 3d, Meter taking account of maximum demand. 4th, Meter, with two or more rates of charge for different periods of the day. oth, /ixed charge for energy plus a graded charge for the maximum capacity. 6th, Prepayment meter, which only allows energy to be delivered for a certain coin deposited. The contract system of charging a fixed amount for a certain number of lamps was commonly adopted in the early days of electric lighting, except in Edison systems using the chemical meter. For street lighting, and other service requiring lights for a definite time, this system is satisfactory, but for residence light- ing it is quite unsatisfactory, because the consumer is likely to burn the lamps for the full time when they are not needed. This wastes a large amount of energy, which must ultimately be paid for by the users. In such cases, and in fact for general use, some form of meter should be adopted. The common plan is to charge a cer- tain price per lamp-hour or k. w. hour, which is’ graded according to the amount used. For example, a common practice is to charge one cent -per lamp-hour, if the consumption is equivalent to the full number of lamps burning for one hour per day; 4 cent if equivalent to two hours, and so on. This accomplishes its pur- pose fairly well, but fails, however, to take into account the par- ticular time at which the lamps are burned. The latter point may be covered by using a two-rate meter, which separates the energy consumed during certain hours from that used during the rest of the time, a higher rate being charged for the former. One way of accomplishing this is to use two separate meters which are switched in or out of the circuit by clock-work. Another plan is to cause the meter to run faster during the time that a higher 436 ELECTERICALICLHIING: charge is to be made, thus arriving at the same result. The prepayment meter is not intended to accomplish any of these ends, but is simply to avoid the necessity for giving credit. The arbitrary method of charging a certain price per unit for one quantity, and 4 as much for a greater quantity, is objection- able, because it leads to the absurd result that one may reduce the amount of his bill by using a little more current. The sliding- scale, in which the reduction is a certain percentage of the amount used, would avoid this difficulty. It might not be quite so easily understood as a certain price per lamp-hour, but customers would soon understand this plan. It would seem to be practically im- possible to make a perfectly fair arrangement between producers and consumers; but a reasonable approximation can be reached, which is close enough for ordinary business purposes. | The Edison Chemical Meter was the first successful type, and many thousands of them were in regular and satisfactory service fora number of years. For reasons, given later, they have been replaced by other forms operating mechanically. In principle, this meter is based upon Faraday’s law, according to which the amount of electrochemical action, for example, the weight of metal de- posited or dissolved in an electrolytic cell, 1s directly proportional to the current, the chemical equivalent, and the time. The ampere, as legalized in all important countries, being defined in terms of the weight of silver deposited, this principle is funda- mentally correct. For reasons of cheapness and as a result of numerous experiments, Edison adopted zinc as the best metal for the purpose. The meter consists of a cell C, containing a solution of zinc sulphate having a density of 1.11, in which two zinc electrodes, A and 2B, are immersed. These are kept parallel’ and at a fixed distance apart by hard-rubber bolts. Connection is made to them by copper rods inserted in their upper ends, as indicated. In one of the main conductors, + or — which supply the lamps Z, whose current is to be measured, a german-silver shunt S is introduced, having a certain resistance that is practically constant for ordinary ‘temperature changes. The electrolyte in the cell has a certain resistance, which decreases with rise of temperature ; and in series with it is a coil R of copper wire whose resistance increases with temperature, the METERS. A3T two being proportioned so that they compensate each other and keep the resistance of the cell circuit almost perfectly constant for ordinary changes in temperature, The resistance of the coil & is about 4 times that of the bottle, and the two together have many times the resistance of the shunt S, so that a certain small fraction of the total current passes through the cell, dissolving zinc from the anode A and depositing it upon the cathode C. Once each month the electrodes are removed from the cell, being replaced by others, and. the loss in weight of the anode is carefully weighed by a chemical balance. This loss in grams multiplied by the ratio of resistances of the two branches of the circuit and divided by .000337, the electrochemical equivalent of zinc, gives the total number of ampere-seconds during the month. The possible sources of error are due to temperature changes, which are almost perfectly compensated in the cell circuit as already explained, and only vary the Shunbes anoutmeiperecentstor. 15°C above or below the normal. The cell + has a certain counter E.M.F. of .001 to .003 volt, which introduces an error at loads less than 3 per cent of the maximum. Oxzzdation of the plates © also occurs and is allowed for, other- wise the loss in weight would appear i, 379. Edison Chemical Meter. too low. The drop produced in the - main circuit is small even at full load, being only about 4 %. With reasonable care this meter is fairly accurate; but the trouble of collecting and weighing the plates, and the fact that the consumer cannot read the record himself, has lead to the substi- — tution of more convenient forms. Thomson Recording Wattmeter.— This type, developed by Professor Elihu Thomson, and used in very large numbers in this country and abroad, is essentially an electric motor. The general appearance of the standard two-wire form for direct or alternating currents is shown in Fig. 380, and the connections in Fig. 381. The field magnet of the motor consists of two stationary coils of’ heavy wire directly in series with one of the main supply con- ductors, so that the entire current to be measured passes through them. An armature provided with a winding of many turns of 438 ELECTRIC LIGHTING. fine wire and having a resistance in series with it is connected across the circuit between the main conductors, and is mounted to rotate on a vertical axis between the two field coils. The armature 1s equipped with a miniature silver commutator and with brushes similar to those of a direct current motor, but, like the field coils, does not contain any iron core. Since the magnetic circuit passes through air and other non-magnetic materials, the flux through the armature, though small, is directly proportional to the main current. The armature circuit having a constant high Fig. 380. Thomson Recording Wattmeter. resistance connected across the two supply conductors, takes a current exactly proportional to the voltage between them. Hence the torque of the motor is in proportion to the product of these two currents or to the number of watts supplied at any instant. A copper disc mounted upon the shaft of the motor revolves between the poles of permanent magnets, as shown in Fig. 380, and acts as a brake, owing to the Foucault currents generated in it. These currents being directly proportional to the speed, the armature will rotate twice as fast with twice the METERS. 439 torque; consequently the revolutions per minute are directly pro- portional to the power supplied in watts. The total number of revolutions in any given time represents the energy in watt-hours. A train of wheels operates a series of five dials representing 1,111,100 units, usually watt-hours, and readings taken _periodi- cally show the energy consumed during the intervals. It is evident that a motor meter requires a certain current to overcome friction, and would fail to record any current below this limit. In order to overcome this difficulty, an auxiliary field coil of fine wire marked “shunt” in Fig. 381 is put in series with the armature. Since the latter is connected across the main con- ductors, the current through it depends solely upon the voltage whether any lamps are burning or not. The resistance of the armature circuit 1s so adjusted that this current passing also through the shunt coil develops a torque almost sufficient to over- come friction; hence any current flowing in the main circuit and field coils will produce its full effect in rotating the armature. If the armature current is too strong it will cause slow rotation, even when no lamps are burning, Fig. 381. Connections of Two-wire Meter. and this ‘creeping’ should be stopped by reducing the number of turns in the shunt field coil until the armature is not quite able to turn when no current for lighting or other purposes is being used. The accuracy of the meter also requires that a certain number of watts supplied shall produce the proper number of revolutions per minute. Ordinarily 60 watts should cause the armature to rotate once per minute, and so on for other loads. Measuring the power with a wattmeter, or putting on a known load of lamps, enables the accuracy of the meter to be tested by counting the revolutions per minute. If found incorrect, the speed may be Jowered by setting the permanent magnets farther out, or vice versa, an adjustment of about 16% being possible. Since a reversal of current in both field and armature of a motor does not change the direction of rotation, an alternating current may be measured by this same instrument. The absence of iron cores 440 ELECTRIC LIGHTING. avoids any troubles from hysteresis or eddy current; but it is necessary that the reactance of the armature circuit should be very small compared with its resistance, a condition which is fulfilled by inserting resistance, as explained in relation to Fig. 381. A lag in the main current due to the load itself, as in the case of arc lamps or motors, does not introduce any error, since the instrument properly measures and integrates the true energy in watt-hours. Special Forms of Thomson Meter are made for various pur- poses. The type whose connections are shown in Fig. 882 is designed for direct or alternating current three-wire circuits. In this case one main field coil is connected in series with each of the outer conductors, the armature circuit, including the shunt field, being connected between the neutral and one of the outer Generator Fig. 382. Thomson Three-wire Meter. Fig. 383. Meter for Large Currents. conductors, assuming that the two sides of the system are per- fectly balanced. . It may also be connected across both sides, thus measuring the total voltage; but the armature will then rotate twice as rapidly, for which fact adjustment or allowance should be made. The currents on either or both sides of the system pro- duce their full effect in the main field coils. ' The meters already described may be used for secondary or other low-voltage alternating circuits, but for primary or high- voltage circuits a modified form is applicable. The modification consists in inserting a small meter-transformer, which reduces the high voltage to a moderate value proportional to the original, the instrument being calibrated accordingly. For measuring very large direct or alternating currents, as in generating plants, the field magnetism is produced by a single METERS. 441 bar of copper which passes between two armatures, as represented in 383, the arrangement in other respects being similar to that already described. Two or three phase currents may be measured by means of a separate meter connected in each phase. If the energy of the two or three phases is kept balanced, one meter in one phase is sufficient, its readings being multiplied by two or three as the case may be. For unbalanced circuits the two or three separate instruments may be combined in one, as illustrated in Fig. 384. Fig. 884. Thomson Polyphase Meter. A single instrument of this kind is capable of recording cor- rectly the total load on balanced or unbalanced two-phase, three- phase, or monocyclic circuits, and saves space, expense, as well as the trouble of reading, and keeping account of two or more sets of dials. In many cases, however, separate meters are em- ployed for the different phases of current. The arrangement for unbalanced three-phase or monocyclic circuits is shown in Fig. 385. One meter is connected to one branch of the circuit, and a second to another branch ; but none is required in the third branch, since no current can flow in it without passing through one or both of the other branches. The algebraic sum of the readings 442, BELECTRICRLIGCH LING. of the two instruments gives the total power and is independent of the balance or lag of the currents. If the latter is less than 60°, giving a power factor greater than .50, the arithmetical sum of the readings is taken; but with a lag greater than 60°, the relation between the currents in the series and shunt coils of one CONNECTIONS OF THOMSON RECORDING WATT-METER UNBALANCED THREE-PHASE CIRCUITS OR MONOCYCLIC SECONDARY CIRCUITS S TO ISO AMPERES t if ' ‘0 ' H i ' HL ' Resistance Q) Resistance Line In monocyclic circuit this is the common connection of the transformers. Line fig. 385. Polyphase Cireuit with Two Meters. of the wattmeters causes it to have a negative reading, hence the difference between the two readings is equal to the actual power. In electric lighting the power factor is always greater than .50, and even with motors on the circuit it should be kept above that value. This arrangement is adapted to secondary or other low- voltage circuits ; for primary or other high-voltage circuits, the METERS. 445 connections are similar, except that the pressure is reduced by meter-transformers as already explained. For three-phase circuits with Y connection (p. 144), three sep- arate meters may be used, one in each branch, but the single or the two-instrument arrangements are generally preferred. Sertes or Arc-Circuit Meter. — The Thomson instrument is adapted also to constant current circuits for series arc or incan- descent lighting, the connections Fig. 386. Series Circuit Meter. being represented in Fig. 386. The field coils of coarse wire are Fig. 387. Shallenberger Meter. connected in series with one main conductor carry- meethe winllicurrent,. the same as for constant po- tential circuits. In this Case, however, 7a’) cut-out is provided which short- circuits, the lines at =that point in case. the circuit is opened beyond. The cut- out magnet is operated by coils in series with a high resistance and connected across the line wires. The armature 1s shunted across a portion of this resistance, as shown, and therefore re- ceives a certain small frac- tion of the line voltage, so that with proper calibra- tion the meter registers the watt-hours consumed in that portion of the circuit. The Shallenberger Meter illustrated in Fig. 387 is a promt- nent form, having been made for many years by the Westinghouse 444 ELECTRIC LIGHTING. Company. It is of the motor type, but is applicable only to alter- nating currents and records ampere-hours but not watt-hours. It consists of large fixed coil having a few turns of heavy wire through which passes the entire current to be measured. Inside of this, and at an angle to it, is placed a closed copper coil. Within the latter a thin metallic disc is mounted to rotate upon a vertical spindle connected at its upper end with a train of recording gears and equipped below with four aluminium fan blades. When an alternating current passes through the large coil, it acts as a primary, and induces a current in the closed or secondary coil. The magnetic field produced by the secondary is at an angle to that of the primary, and the two combine to form a resultant field; but as. their alternations do not coincide in time the direc- tion of this resultant is continually shifting and produces a rotating field,. as explained on page 145. The metallic disk tends to. rotate in unison with the field, but is retarded by the fan blades. “This de- vice being in principle an induction motor, its. torque increases as the Fig. 388. square of the primary current; but since the resistance to rotation rises as the square of the speed (at moderate values), the number of revolutions per minute are directly proportional to the current, which is the condition required. The Westinghouse Integrating Wattmeter (Fig. 388) is also of the induction motor type, but differs from the Shallenberger instru- ment, in having in addition to the series coils a shunt winding, the effect of which is proportional to the voltage, so that watt-hours. are recorded. Since a rotating aluminium disk is acted upon by induction, it can be used only with alternating currents. . The moving parts being very light require only 1.25 watts in the shunt winding, and even less in the series coils. The former advantage METERS. 445 is important because that loss occurs all the time, even when lamps are not burning, and any drop in the series coil is equally objection- able, since it reduces the pressure at the lamps. For 400 volts or less and currents up to 80 amperes, the meter is connected directly Fig. 389. Connections for Two-phase Westinghouse Meters. tothe circuit. With higher voltages or heavier currents a potential transformer is used to reduce the pressure for the shunt winding, and a series transformer is inserted in one of the main conductors to obtain a smaller but proportionate current for the series coil. The connections for a two-phase circuit are shown in Fig. 389, a 446 ELECTRIG\LIGHTING. potential transformer for each phase being placed below and a smaller series transformer for each phase above, with ail four secondary wires leading to a single meter in the center. The energy is brought from two-phase generators by four wires from the right, and after being measured is carried to the lamps by four wires on the left. The dials record the total energy supplied in all the branches of a polyphase circuit into which the wattmeter is connected, no multiplier being necessary. The Duncan Integrating Wattmeter, represented in Fig. 390, is another well-known form of the induction motor type for alternating currents only. It comprises a series field core of laminated iron with inwardly projecting poles carrying the series coils, between which the armature — an inverted aluminium cup — rotates upon a vertical spindle, as shown. The shunt coil placed inside of the arma- ture, with its axis at right angles to that of the series coils, is also wound upon a laminated iron core, being stationary and supported from below by a brass arm. The upper end of the arm- ature spindle carries a gear driving the train of dials, and on the lower end is mounted the retarding disk of aluminium which revolves between the poles of two permanent magnets. The so-called compensator is a copper ring with an iron core shown in front of the armature and supported by a movable arm. By adjusting its position, friction may be overcome and at the same time “creeping” avoided. The lower bearing of the spindle is of sapphire and the spindle point of hardened steel, both of which are easily renewable. Fig. 390. Duncan Meter. METERS. AAT The Gutmann Integrating Wattmeter is similar in principle to the Westinghouse and Duncan instruments, being of the induction motor type with series and shunt windings, and is adapted only to alternating currents. The arrangement of the dials, retarding disk and other parts is also similar, but the armature or rotating member is a diagonally slotted aluminium cylinder. The American Integrating Amperemeter, illustrated in Fig. 391, differs radically from those already described, both in principle and in construction. It consists of a solenoid and core placed above aself-starting pendulum, actuated by the electric current. The pendulum, by means of a cam, raises a pawl on a ratchet Fig. 891. The American Integrating Amperemeter. wheel to a uniform height each stroke. The solenoid, by means of its core, shifts the angular position of a pendent arch attached — to its axIs so as to permit this pawl to drop along the ratchet wheel a number of teeth proportional to the current passing through the meter; thus at each stroke of the pendulum the load in amperes passing to the consumer is, by means of the ratchet wheel and the counter register, measured and added up in ampere- hours. The pendulum ceases to swing when no lamps are burning ; but as soon as any are turned on, and current flows in the main con- ductor, the pendulum is started automatically, being actuated by a A4S ELECTRICOGIGH TING shunt circuit across the mains. The form illustrated is for use on a three-wire system, both of the outer conductors being carried through the solenoid, as indicated, so that its action upon the core, and therefore the position of the latter, depends upon the com- bined effects of the two currents, thus measuring the total load on both sides of the system. The core of soft iron is magnetized to saturation by a winding in circuit with the coil that drives the pendulum, the object being to avoid variations due to hysteresis. Having this construction, the instrument measures ampere-hours simply, since ordinary changes in voltage would produce no ap- preciable effect. The Ferranti Meter used in England is based upon the princi- ple that a conductor carrying a current in a magnetic field tends to move in a direction perpendicular to the current and to the lines of force. In this case the conductor consists of mercury con- tained in a shallow circular chamber placed between the poles of a magnet excited by a coil through which the main current passes. This current also flows radially through the mercury being intro- duced at the center by a pin, and taken off at the periphery by a metallic rim. The retarding force is that due to fluid friction of the mercury against the inner surface of the chamber, in which radial grooves are formed to increase the effect and make it as nearly as possible proportional to the square of the speed. Since the current flows in both the magnet and the mercury, the driving force 1s proportional to the square of the current, and the speed, therefore, increases directly with the current. A vane dipping in the mercury transmits the motion of the latter to the counting dials by means of'a small spindle. The driving force due to residual magnetism is designed to overcome the retarding force due to the solid friction of the-parts. . It 1s evident that this instrument measures ampere-hours and not watt-hours. The Aron Meter consisted originally -of an ordinary clock having a permanent magnet for the bob of the pendulum, below which was placed a coil with its axis vertical. The current to be measured passed through the coil-in the direction to repell the magnet, thus neutralizing a part of its gravity which caused the clock to lose time. The loss of time in any period enabled the ampere-hours during that period to be determined. A later form comprised two separate clocks, one acting normally and the other METERS. 449 being influenced by the current. The two trains of wheels were connected by a differential gear to a third train with dials and pointers which indicated the difference in action of the two clocks, being calibrated in ampere-hours. This type involves two obvious difficulties : one 1s the trouble of winding the clocks, and the other is the practical impossibility of keeping correct a large number of clocks of reasonable cost. “These objections are overcome in a still more recent form which is made self-winding by the action of the current, and each pendulum is acted upon by a coil so that one is accelerated and the other retarded, thus doubling the effect. Furthermore, at frequent intervals the connections of the coils are reversed, so that first one and then the other pendulum is the faster, the difference between the two being always registered on the dials. In this way even a considerable deviation from accuracy in one or both of the clocks is eliminated by the constant reversal . of their relations. Another feature in the improved Aron instru- ment is the substitution of a shunt coil for the permanent magnet on each pendulum, thus converting it into an integrating watt- meter. It is adapted to either direct or alternating currents, and is made in forms suitable for two-wire, three-wire, and other circuits. Lhe Terms [ntegrating, Recording, and Registering Meter are all used for designating the various devices described in this chap- ter. The first is certainly correct, since in most cases the instru- ment merely integrates or sums up the total number of ampere- or watt-hours, without making any record of the almost constant vari- ations in load which usually occur. There are also instruments commonly called recording volt-, ampere-, or wattmeters, in which a line is traced out on paper showing the number of volts, etc., at any time during the entire twenty-four hours. By using an am- pere- or wattmeter of this kind and properly integrating the record obtained, the number of ampere- or watt-hours may be determined. This is much less convenient, however, than an instrument which automatically performs the integration and gives the result on a dial. On the other hand, the maximum demand and other values would all be shown, so that a charge could be made taking them into account, as explained in the beginning of this chapter. Asa matter of fact, such instruments are rarely used except to ascertain the uniformity of voltage, etc., when it is desired to have it con- 450 ELECTRIC LIGHTING stant, and are used merely as a check on the regulation. It has been proposed to distinguish between this class and those which integrate, by calling the former recording and the latter register- ing; but common usage is the other way, and the term integrating meter is more distinctive for the latter. In most cases the simple y word “meter” is understood to mean the integrating instrument, whether used for measuring gas, water, or electrical quantities. JaNel ed vod SND “NATIONAL ELECTRICAL CODE” RULES AND REQUIREMENTS OF THE NATIONAL BOARD of FIRE UNDERWRITERS FOR THE INSTALLATION OF WIRING AND APPARATUS BORBEEEG ERI GUMIGHIASHEAT, AND ROWER AS RECOMMENDED BY THE UNDERWRITERS’ NATIONAL ELECTRIC ASSOCIATION EDITION OF 1o01 The National Electrical Code, as it is here presented, is the result of the united efforts of the various Electrical, Insurance, Architectural, and allied interests which have, through the Nationa) Conference on Standard Electrical Rules, composed of delegates from various National Associations, unanimously voted to recommend it-to their respective Associations for approval or adoption. The following is a list of the Associations represented in the Conference, all of which have approved of the Code: AMERICAN INSTITUTE OF ARCHITECTS AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS AMERICAN SocIETY OF MECHANICAL ENGINEERS AMERICAN STREET RAILWAY ASSOCIA TION Facrory Mutua Fire INSURANCE COMPANIES Nationa ASSOCIATION OF FIRE ENGINEERS NATIONAL BOARD OF FIRE UNDERWRITERS NaTIONAL Evectric Light ASSOCIATION UNDERWRITERS’ NATIONAL ELECTRIC ASSOCIATION GENERAL PLAN GOVERNING THE ARRANGEMENT OF RULES CLASS A.—Central Stations, Dynamo, Motor, and Storage-Battery Rooms, Transformer Substations, etc. Rules 1 torr. CLASS B.— Outside Work, all systems and voltages. Rules 12 and 13. 451 452 ELECTRIC LIGHTING. CLASS C.—Inside Work. Rules 14 to 39. Subdivided as follows: General Rules, applying to all systems and voltages. Rules 14 to 17. Constant-Current systems. Rules 18 to 20. Constant-Potential systems. All voltages. Rules 21 to 23. Voltage not over 550. Rules 24 to 31. Voltage between 550 and 3,500. Rules 32 to 37. Voltage over 3,500. Rules 38 and 39. CLASS D.— Specifications for Wires and Fittings. Rules 40 to 62. CLASS E.—Miscellaneous. Rules 64 to 67. CLASS F. — Marine Wiring. Rules 68 to 80. CLiass A STATIONS AND DYNAMO ROOMS INCLUDES CENTRAL STATIONS, DYNAMO, MOTOR, AND STORAGE-BATTERY ROOMS, TRANSFORMER SUBSTATIONS, ETC. 1. Generators — a. Must be located in a dry place. 6. Must never be placed in a room where any hazardous process is carried on, nor in places where they would be exposed to inflammable gases or flyings of combustible materials. c. Must be insulated on floors or base frames, which must be kept filled to prevent absorption of moisture, and also kept clean and dry. Where frame insulation is impracticable, the Inspection Department having jurisdiction may, in writing, permit its omission, in which case the frame must be per- manently and effectively grounded. A high-potential machine which, on account of great weight or for other reasons, cannot have its frame insulated from the ground, should be surrounded with an insulated platform. This may be made of wood, mounted on insulating supports, and so arranged that a man must always stand upon it in order to touch any part of the machine. In case of a machine having an insulated frame, if there is trouble from static electricity due to belt friction, it should be overcome by placing near the belt a metallic comb connected with the earth, or by grounding the frame through a very high resistance of not less than 200 ohms per volt generated by the machine. ad. Every constant-potential generator must be protected from excessive current by a safety fuse, or equivalent device, of approved design in each lead wire. These devices should be placed on the machine or as near it as possible. Where the needs of the service make these devices impracticable, the Inspection Department having jurisdiction may, in writing, modify the requirements. e. Must each be provided with a waterproof cover. J. Must each be provided with a name-plate, giving the maker’s name, the capacity in volts and amperes, and the normal speed in revolutions per minute. 2. Conductors — v From generators to switchboards, rheostats, or other instruments, and thence to outside lines. APPENDIX I. 453 a. Must be in plain sight or readily accessible. 6. Must have an approved insulating covering as called for by rules in Class “C” for similar work, except that in central stations, on exposed circuits, the wire which is used must have a heavy braided non-combustible outer covering. Bus bars may be made of bare metal. c. Must be kept so rigidly in place that they cannot come in contact. ad. Must in all other respects be installed under the same precautions as required by rules in Class “C” for wires carrying a current of the same volume and potential. 3. Switchboards — a. Must be so placed as to reduce to a minimum the danger of communi- cating fire to adjacent combustible material. Special attention is called to the fact that switchboards should not be built down to the floor, nor up to the ceiling, but a space of at least ten or twelve inches should be left between the floor and the board, and from eighteen to twenty-four inches between the ceiling and the board in order to prevent fire from communicating from the switchboard to the floor or ceiling, and also to prevent the forming of a partially concealed space very liable to be used for storage of rubbish and oily waste. 6. Must be made of non-combustible material or of hardwood in skeleton form filled to prevent absorption of moisture. c. Must be accessible from all sides when the connections are on the back, but may be placed against a brick or stone wall when the wiring is entirely on the face. a. Must be kept free from moisture. -e. Bus bars must be equipped in accordance with rules for placing conductors. 4. Resistance Boxes and Equalizers — (Lor construction rules, see Vo, 60.) a. Must be placed on a switchboard or, if not thereon, at a distance of a a foot from combustible material, or separated therefrom by a non-inflammable, ‘non-absorptive, insulating material. 5. Lightning Arresters — (For construction rules, see No. 63.) a. Must be attached to each side of every overhead circuit connected with the station. It is recommended to all electric light and power companies that arresters be connected at intervals over systems in such numbers and so located as to prevent ordinary discharges entering (over the wires) ‘buildings connected to the lines. 6. Must be located in readily accessible places away from combustible materials, and as near as practicable to the point where the wires enter the building. Station arresters should generally be placed in plain sight on the switch- board. In all cases, kinks, coils, and sharp bends in the wires between the arresters and the outdoor lines must be avoided as far as possible. 454 ELECTRICCLIGHI ING: c. Must be connected with a thoroughly good and permanent ground con- nection by metallic strips or wires having a conductivity not less than that of a No. 6 B. & S. copper wire, which must be run as nearly in a straight line as possible from the arresters to the earth connection. Ground wires for lightning arresters must not be attached to gas-pipes within the buildings. It is often desirable to introduce a choke coil in circuit between the arresters and the dynamo. In no case should the ground wire from a lightning arrester be put into iron pipes, as these would tend to impede the discharge. 6. Care and Attendance — a. A competent man must be kept on duty where generators are operating. 6. Oily waste must be kept in approved metal cans and removed daily. Approved waste cans shall be made of metal, with legs raising can three inches from the floor, and with self-closing covers. 7. Testing of Insulation Resistance — a. All circuits, except such as are permanently grounded in accordance with Rule 13 A, must be provided with reliable ground detectors. Detectors which indicate continuously, and give an instant and permanent indication of a ground, are preferable. Ground wires from detectors must not be attached to gas-pipes within the building. 6. Where continuously indicating detectors are not feasible, the circuits should be tested at least once per day, and preferably oftener. c. Data obtained from all tests must be preserved for examination by the Inspection Department having jurisdiction. These rules on testing to be applied at such places as may be designated by the Inspection Depart- ment having jurisdiction. 8. Motors — a. Must be insulated on floors or base frames, which must be kept filled to prevent absorption of moisture; and must be kept clean and dry. Where frame insulation is impracticable the Inspection Department having jurisdic- tion may, in writing, permit its omission, in which case the frame must be per- manently and effectively grounded. A high-potential machine which, on account of great weight or for other reasons, cannot have its frame insulated, should be surrounded with an insulated platform. This may be made of wood mounted on insulating supports, and so arranged that a man must stand upon it in order to touch any part of the machine. e Ane In case of a machine having an insulated frame, if there is trouble from static electricity due to belt friction, it should be overcome by placing near the belt a metallic comb connected to the earth, or by grounding the frame through a very high resistance of not less than 200 ohms per volt generated by the machine. b. Must be wired under the same precautions as required by rules in class “C”, for wires carrying a current of the same volume and potential. The leads or branch circuits should be designed to carry a current at least fifty per cent greater than that required by the rated capacity of the motor to provide for the inevitable overloading of the motor at times without overfusing the wires. c. The motor and resistance box must be protected by a cutout and con- trolled by a switch (see No. 17 a), said switch plainly indicating whether “on i or “off” Where one-fourth horse-power or less is used on low-tension circuits APPENDIX TT. 455 a single-pole switch will be accepted. The switch and rheostat must be located within sight of the motor, except in such cases where special permission to locate them elsewhere is given in writing by the Inspection Department hav- ing jurisdiction. @. Must have their rheostats or starting-boxes located as to conform to the requirements of No. 4. In connection with motors the use of circuit-breakers, automatic starting-boxes and automatic under-load switches is recommended, and they west be used when required. ée. Must not be run in series-multiple or multiple-series, except on constant- potential systems, and then only by special permission of the Inspection De- partment having jurisdiction. J. Must be covered with a waterproof cover when not in use, and, if deemed necessary by the Inspection Department having jurisdiction, must be inclosed in an approved case. From the nature of the question the decision as to what is an approved case must be left to the Inspection Department having jurisdiction to determine in each instance. g. Must, when combined with ceiling fans, be hung from insulated hooks, or else there must be an insulator interposed between the motor and its support. hk. Must each be provided with a name-plate, giving the maker’s name, the capacity in volts and amperes, and the normal speed in revolutions per minute. 9. Railway Power Plants — a. Must be equipped in each feed wire before it leaves the station with an approved automatic circuit-breaker (see No. 52) or other device, which will immediately cut off the current in case of an accidental ground. This device must be mounted on a fireproof base, and in full view and reach of the attendant. 10. Storage or Primary Batteries — a. When current for light and power is taken from primary or secondary _ batteries, the same general regulations must be observed as applied to similar apparatus fed from dynamo generators developing the same difference of potential. 6. Storage battery rooms must be thoroughly ventilated. c. Special attention is directed to the rules for rooms where acid fumes exist (see No. 24, 7 and &). d, All secondary batteries must be mounted on non-absorptive, non- combustible insulators, such as glass or thoroughly vitrified and glazed porcelain. é. The use of any metal liable to corrosion must be avoided in cell connec- tions of secondary batteries. 11, Transformers — (For construction rules, see Vo. 62.) a. In central or substations the transformers must be so placed that smoke from the burning out of the coils or the boiling over of the oil (where oil-filled cases are used) could do no harm, 456 ELECTRIC LIGHTING, (SEAS ei, OUTSIDE WORK. ALL SYSTEMS AND VOLTAGES. 12, Wires — a. Service wires must have an approved rubber insulating covering (see No, 41). Line wires, other than services, must have an approved weatherproof, or rubber insulating covering (Nos. 41 and 44). All the wires must have an insulation equal to that of the conductors they confine. 6. Must be so placed that moisture cannot form a cross connection be- tween them, not less than a foot apart, and not in contact with any substance other than their insulating supports. Service blocks must be covered over their entire surface with at least two coats of waterproof paint. c. Must be at least seven feet above the highest point of flat roofs, and at least one foot above the ridge of pitched roofs over which they pass or to which they are attached. d. Must be protected by dead insulated guard iron or wires from possibility of contact with other conducting wires or substances to which current may leak. Special precautions of this kind must be taken where sharp angles occur, or where any wires might possibly come in contact with electric light or power wires. é. Must be provided with petticoat insulators of glass or porcelain. Por- celain knobs or cleats and rubber hooks will not be approved. J. Must beso spliced or joined as to be both mechanically and electrically secure without solder. The joints must then be soldered, to insure preserva- tion, and covered with an insulation equal to that on the conductors. All joints must be soldered, even if made with some form of patent splicing device. ‘This ruling applies to joints and splices in all classes of wiring covered by these rules. g. Must, where they enter buildings, have drip loops outside, and the holes through which the conductors must be bushed with non-combustible, non- absorptive insulating tubes slanting upward toward the inside. hi. Telegraph, telephone, and similar wires must not be placed on the same cross-arm with electric light or power wires ; and when placed on the same pole with such wires the distance between the two inside pins of each cross-arm must not be less than twenty-six inches. z. The metallic sheaths to cables must be permanently and effectively connected to “earth.” TROLLEY WIRES: 7. Must not be smaller than No. 0 B. & S. copper or No. 4 B. & S. silicon bronze, and must readily stand the strain put upon them when in use. k. Must have a double insulation from the ground. In wooden-pole con- struction the pole will be considered as one insulation. 7. Must be capable of being disconnected at the power plant, or of being divided into sections, so that, in case of fire on the railway route, the current may be shut off from the particular section and not interfere with the work of the firemen. This rule also applies to feeders. 4 —_—- APPENDIX I. 457 m. Must be safely protected against accidental contact where crossed by other conductors. ’ Guard wires should be insulated from the ground, and should be electrically disconnected in sections of not more than 300 feet in length. GROUND RETURN WIRES. mz. For the diminution of electrolytic corrosion of underground metal work, ground return wires must be so arranged that the difference of potential be- tween the grounded dynamo terminal and any point on the return circuit will not exceed twenty-five volts. It is suggested that the positive pole of the dynamo be connected to the trolley line, and that whenever pipes or other underground metal work are found to be electrically positive to the rails or surrounding earth, that they be connected by conductors arranged so as to prevent as far as possible current flow from the pipes into the ground. 13. Transformers — (Lor construction rules, see Vo. 62.) a. Must not be placed inside of any building, excepting central stations, unless by special permission of the Inspection Department having jurisdiction. 6. Must not be attached to the outside walls of buildings, unless separated therefrom by substantial supports. 13 A. Grounding Low Potential Circuits. The grounding of low potential circuits under the following regulations is only allowed when so arranged that under normal conditions there will be no flow of current through the ground wire. Direct Current 3-Wire Systems. a. Neutral wire may be grounded, and when grounded the following rules must be complied with: — 1. Must be grounded at the Central Station on a metal plate buried in coke beneath permanent moisture level, and also through all available underground water and gas-pipe systems. 2. In underground systems the neutral wire must also be grounded at each distributing-box through the box. yp . 3. In overhead systems the neutral wire must be grounded every 500 feet, as provided in Sections ¢, ¢, and-/. The Inspection Department having jurisdiction may require troundine if they deem it necessary. Two-wire direct current systems having no accessible neutral point are not to be grounded. Alternating Current Secondary Systems. 6. The neutral point of transformers, or the neutral wire of distributing systems, may be grounded, and when grounded the following rules must be complied with : — 1. Transformers feeding 2-wire systems must be grounded at the center of the secondary coils. 2 Transformers feeding systems with a neutral wire must have the neutral wire grounded at the transformer and at least every 250 feet beyond. Inspection Department having jurisdiction may regzire grounding if they deem it necessary. Ground Connections. c. The ground wire in D. C. 3-wire systems must not at Central Stations be smaller than the neutral wire and not smaller than No. 6 B. & S. elsewhere. 458 ELECTRIC LIGHTING. ad. The ground wire in A. C. systems must never be less than No. 6 B. & S., and must always have equal carrying capacity to the secondary lead of the transformer, or the combined leads where transformers are banked. e. The ground wire must be kept outside of buildings, but may be di- rectly attached to the building or pole. The wire must be carried in as nearly a straight line as possible, and kinks, coils and sharp bends must be avoided. fy. The ground connection for Central Stations, transformer sub-stations, and banks of transformers must be made through metal plates buried in coke below permanent moisture level, and connection should also be made to all available underground piping systems. For individual transformers and building ser- vices the ground connection may be made as above, or may be made to water or other piping systems running into the buildings. This connection may be made by carrying the ground wire into the cellar and connecting on the street side of meters, main clocks, etc. In connecting ground wires to piping systems, where possible the wires should be soldered into one or more brass plugs and the plugs forcibly screwed into a pipe-fitting, or where the pipes are cast iron into a hole tapped to the pipe itself. For large stations, where connecting to underground pipes with bell and spigot joints, it is well to connect to several lengths, as the pipe joints may be of rather high resistance. Where such plugs cannot be used the surface of the pipe may be filed or scraped bright, the wire wound around it, and a strong clamp put over the wire and firmly bolted together. Where ground plates are useda No. 16 copper plate, about 3 x 6 feet in size, with about two feet of crushed coke or charcoal about pea size both under and over it, would make a ground of sufficient capacity for a moderate size station, and would probably answer for the ordinary sub-station or bank of transformers. For a large Central Station considerable more area might be necessary, depending upon the other underground connections available. The ground wire should be riveted to such a plate ina number of places, and soldered for its whole length. Perhaps even better than a copperplate is a cast- iron plate with projecting forks, the idea of the fork being to distribute the connection to the ground over a fairly broad area, and to give a large surface contact. The ground wire can probably best be connected to such a cast-iron plate by brass plugs screwed into the plate to which the wire is soldered. In al] cases the joint between the plate and the ground wire should be thoroughly protected against corrosion by suitable painting with waterproof paint or some equivalent. G@rass--C. INSIDE WORK ALL SYSTEMS AND VOLTAGES. GENERAL RULES — ALL SYSTEMS AND VOLTAGES. 14. Wires — (For special rules See Nos. 18, 24, 32, 38, and 89.) a. Must not be of smaller size than No. 14 B. & S., except as allowed under Rules 24 ¢ and 454, 6. Tie wires must have an insulation equal to that of the conductors they confine. c. Must be so spliced or joined as to be both mechanically and electrically secure without solder; they must be then soldered to insure preservation, and the joint covered with an insulation equal to that on the conductors. Stranded wires must be soldered before being fastened under clamps or binding screws ; and, when they have a conductivity greater than No. 10 B. & S. copper wire, they will be soldered into lugs. APPENDIX I. 459 All joints must be soldered, even if made with some form of patent splicing device. This ruling applies to joints and splices in all classes of wiring covered by these rules. dad. Must be separated from contact with walls, floors, timbers, or partitions through which they may pass by non-combustible, non-absorptive insulating tubes, such as glass or porcelain. Bushings must be long enough to bush the entire length of the hole in one continuous piece, or els? the hole must first be bushed by a continuous waterproof tube, which may be a conductor, such as iron pipe; the tube then is to have a non-conducting bushing pushed in at each end so as to keep the wire absolutely out of contact with the conducting pipe. é. Must be kept free from contact with gas, water, or other metallic piping, or any other conductors or conducting material which they may cross, by some continuous and firmly fixed non-conductor, creating a separation of at least one inch. Deviations from this rule may sometimes be allowed by special permission. F. Must be so placed in wet places that an air space will be left between conductors and pipes in crossing, and the former must be run in such a way that they cannot come in contact with the pipe accidentally. Wires should be run over, rather than under, pipes upon which moisture is likely to gather or which, by leaking, might cause trouble on a circuit. 15. Underground Conductors — a. Must be protected, when brought into a building, against moisture and mechanical injury, and all combustible material must be kept removed from the immediate vicinity. 6. Must not be so arranged as to shunt the current through a building around any catch-box. 16. Table Carrying Capacity of Wires — Below is a table which must be followed in placing interior conductors, showing the allowable carrying capacity of wires and cables of ninety-eight per cent conductivity, according to the standard adopted by the American Institute of Electrical Engineers. TABLE A. TABLE B. TABLE A. TABLE B. RUBBER- WEATHER- RUBBER- W EATHER- CovERED PROOF | 1 *CrRcoLar Gircuian CovERED WIRES. WIRES. Mits. MILs. WIRES. See No. 41, SEE Sze No, 41. No. 42 To 44. AMPERES. AMPERES, AMPERES. - AMPERES. 1,624 200,000 200 300 2,583 300,000 270 400 4,107 400,000 330 500 6,530 500.000 390 590 10,380 600,000 450 680 16,510 700,000 500 760 26,250 800,000 550 840 33,100 900,000 600 920 41,740 1,000,000 650 1,000 52,630 1,100,000 690 1,080 66,370 1,200,000 730 1,150 83,690 1,300,000 770 1,220 105,500 1,400,000 810 1,290 133,100 1,500,000 850 1,360 167,800 1,600,000 890 1,430 211,600 1,700,000 930 1,490 1,800,000 970 1,550 1,900,000 1,010 1,610 2,000,000 1,050 1,670 18 16 14 12 10 8 6 5 4 3 2 1 0 00 000 S S SC 460 ELECTRIC. LIGHTING. The lower limit is specified for rubber-covered wires to prevent gradual deterioration of the high insulations by the heat of the wires, but not from fear of igniting the insulation. The question of drop is not taken into consideration in the above tables. The carrying capacity of sixteen and eighteen wire is given, but no smaller than fourteen is to be used, except as allowed under Rules 24 ¢ and 45 6. 17. Switches, Cutouts, Circuit-Breakers, etc. (For construction rules, see Vos, 51, 52, and 58.) a. Must, whenever called for, unless otherwise provided (for exceptions, see No. 8c and No. 22 c), be so arranged that the cutouts will protect, and the opening of the switch or circuit-breaker will disconnect, all of the wires; that is, in a two-wire system the two wires, and in a three-wire system the three wires, must be protected by the cutout, and disconnected by the operation of the switch or circuit-breaker. 6. Must not be placed in the immediate vicinity of easily ignitible stuff or where exposed to inflammable gases or dust or to flyings of combustible material. c. Must, when exposed to dampness, either be inclosed ina waterproof box or mounted on porcelain knobs. CONSTANT CURRENT SYSTEMS. Principally Series Arc Lighting. (See also (Vos. 14, 15, and 16.) a. Must have an approved rubber insulating covering (see No. 41). 6. Must be arranged to enter and leave the building through an approved double-contact service switch (see No. 51), mounted in a non-combustible case, kept free from moisture, and easy of access to police or firemen. So-called ‘snap switches’ must not be used on high-potential circuits. c. Must always be in plain sight, and never incased, except when required by the Inspection Department having jurisdiction. @d. Must be supported on glass or porcelain insulators, which separate the wire at least one inch from the surface wired over, and must be kept vzezdly at least eight inches from each other, except within the structure of lamps, on hanger-boards, in cutout boxes, or like places, where a less distance is necessary, é. Must, on side walls, be protected from mechanical injury by a sub- stantial boxing, retaining an air space of one inch arounc the conductors, closed at the top (the wires passing through bushed holes), and extending not less than seven feet from the floor. When crossing floor-timbers in cellars or in rooms, where they might be exposed to injury, wires must be attached by their insulating supports to the under side of a wooden strip not less than one- half an inch in thickness. 18. Wires — 19. Arc Lamps — (For construction rules, see No. 57.) a. Must be carefully isolated from inflammable material. 6. Must be provided at all times with a glass globe surrounding the arc, securely fastened upon a closed base. No broken or cracked globes to be used. c. Must be provided with a wire netting (having a mesh not exceeding one and one-fourth inches) around the globe, and an approved spark arrester (see No. 58), when readily inflammable material is in the vicinity of the lamps, to APPENDIX I. 461 prevent escape of sparks, melted copper, or carbon. It is recommended that plain carbons, not copper-plated, be used for lamps in such places. Arc lamps, when used in places where they are exposed to flyings of easily inflammable material, should have the carbons inclosed completely in a giobe in such manner as to avoid the necessity for spark arresters. For the present, globes and spark arresters will not be required on so-called ‘‘ inverted arc” lamps, but this type of lamp must not be used where exposed to flyings of easily inflammable materials. d. Where hanger-boards (see No. 56) are not used, lamps must be hung from insulating supports other than their conductors. 20. Incandescent Lamps in Series Circuits — a. Must have the conductors installed as provided in No. 18, and each lamp must be provided with an automatic cutout. 6, Must have each lamp suspended from a hanger-board by means of rigid tube. c. No electro-magnetic device for switches and no system of multiple- series or series-multiple lighting will be approved. @. Under no circumstances can they be attached to gas fixtures. CONSTANT POTENTIAL SYSTEMS. GENERAL RULES FALL. VOLTAGES, 21. Automatic Cutouts (Fuses and Circuit-Breakers). (See Vo. 17, and for construction Nos. 52 and 538.) a. Must be placed on all service wires, either overhead or underground, as near as possible to the point where they enter the building and inside the walls, and arranged to cut off the entire current from the building. Where the switch required by rule No. 22 is inside the building, the cutout required by this section must be placed so as to protect it. 6. Must be placed at every point where a change is made in the size of wire [unless the cutout in the larger wire will protect the smaller (see No. 16)]. c. Must be in plain sight, or inclosed in an approved box (see No. 54) and readily accessible. They must not be placed in the canopies or shells of fixtures. d. Must be so placed that no set of incandescent lamps, whether grouped on one fixture or several fixtures or pendants, requiring more than 660 watts, shall be dependent upon one cutout. Special permission may be given in writing by the Inspection Department having jurisdiction for departure from this rule in case of large chandeliers, stage borders, and illuminated signs. é. Must be provided with fuses, the rated capacity of which does not ex- ceed the allowable carrying capacity of the wire; and, when circuit-breakers are used, they must not be set more than about thirty per cent above the allow- able carrying capacity of the wire, unless a fusible cutout is also installed in the circuit (see No. 16). 22. Switches — . (See Wo. 17, and for construction, No. &1.) a. Must be placed on all service wires, either overhead or underground in a readily accessible place, as near as possible to the point where the wires enter the building, and arranged to cut off the entire current. 6. Must always be placed in dry, accessible places, and be grouped as far as possible. Knife switches must be so placed that gravity will tend to open rather than close the switch. 462 ELECTRICLLIOGH LNG c. Must not be single-pole, except when the circuits which they control supply not more than six 16-candle power lamps or their equivalent. ad. Where flush switches are used, whether with conduit systems or not, the switches must be inclosed in boxes constructed of or lined with fire-resisting material. No push-buttons for bells, gas-lighting circuits or the like shall be placed in the same wall-plate with switches controlling electric light or | power wiring. 23. Electric Heaters — a. Must, if stationary, be placed in a safe situation, isolated from inflam- mable materials, and be treated as sources of heat. 6. Must each have a cutout and zzadzcating-switch (see No. 17a). c. Must have the attachments of feed wires to the heaters in plain sight, easily accessible, and protected from interference, accidental or otherwise. ad. The flexible conductors for portable apparatus, such as irons, etc., must have an approved insulating covering (see No. 45). ée. Must each be provided with name-plate, giving the maker’s name and the normal capacity in volts and amperes. LOW-POTENTIAL SYSTEMS. 550 Volts or less. Any circuit attached to any machine, or combination of machines, which develops a difference of potential, between any two wztres, of over ten volts and less than 550 volts, shall be considered as a low-potential circuit, and as coming under thts class, unless an approved transform- ing device is used, which cuts the difference of potential down to ten volts or less. The primary circuit not to exceed a potential of 3,500 volts. 24, Wires — GENERAL RULES. (See also Nos. 14, 15, and 16.) a. Must not be laid in plaster, cement, or similar finish. 6. Must never be fastened with staples. c. Must not be fished for any great distance, and only in places where the inspector can satisfy himself that the rules have been complied with. @d. Twin wires must never be used, except in conduits, or where flexible conductors are necessary. é. Must be protected on side walls from mechanical injury. When cross- ing floor-timbers in cellars or in rooms, where they might be exposed to injury, wires must be attached by their insulating supports to the under side of a wooden strip, not less than one-half inch in thickness, and not less than three inches in width. Suitable pretection on side walls may be secured by a substantial boxing, retaining an air space of one inch around the conductor, closed at the top (the wires passing through bushed holes), and extend- ing not less than five feet from the floor; or by an iron-armored or metal-sheathed insulating conduit sufficiently strong to withstand the strain it will be subjected to; or plain metal pipe, lined with insu- lating tubing, which must extend one-half inch beyond the end of the metal tube. The pipe must extend not less than five feet above the floor, and may extend through the floor in place of a floor bushing. If iron pipes are used with alternating currents, the two or more wires of a circuit ust be placed APPENDIX I. 465 in the same conduit. In this case the insulation of each wire must be reinforced by a tough conduit tubing projecting beyond the ends of the iron pipe at least two inches. Ff. When run immediately under roofs, or in proximity to water tanks or pipes, will be considered as exposed to moisture. SEE CIn be RULES: For open work: Ln ary places: g. Must have an approved rubber or “slow-burning " waterproof insula- tion (see Nos, 41 and 42). hi. Must be rigidly supported on non-combustible, non-absorptive insula- tors, which separate the wires from each other and from the surface wired over in accordance with following table : OLTAGE. DISTANCE FROM SURFACE, DISTANCE BETWEEN WIRES, @ to 225 4 inch. 2% inches. 225, ** 550 Ly 4 “ Rigid supporting requires under ordinary conditions, where wiring along flat surfaces, supports at least every four and one-half feet. If the wires are liable to be disturbed, the distance between supports should be shortened. In buildings of mill construction, mains of No. 8 B. & S, wire or over, where not liable to be disturbed, may be separated about four inches, and run from timber to timber, not breaking around, and may be supported at each timber only. This rule will not be interpreted to forbid the placing of the neutral of a three-wire system in the center of a three-wire cleat, provided the outside wires are separated in accordance with above table. Ln damp places, such as Breweries, Sugar Houses, Packing Houses, Stables, Dye Houses, Paper or Pulp Mills, or buildings specially liable to moisture or acta or other fumes liable toinjure the wires or their insulation, except where used for pendants : z. Must have an approved rubber insulating covering (see No. 41). j. Must be rigidly supported on non-combustible, non-absorptive insula- tors, which separate the wire at least one inch from the surface wired over, and they must be kept apart at least two and one-half inches. Rigid supporting requires under ordinary conditions, where wiring over flat surfaces, supports at least every four and one-half feet. If the wires are liable to be disturbed, the distance between sup- ports should be shortened. In buildings of mill construction, mains of No. 8 B. & S. wire or over, where not liable to be disturbed, may be separated about four inches, and run from timber to timber, not breaking around, and may be supported at each timber only, k. Must have no joints or splices. For molding work : 7. Must have approved rubber insulating covering (see No, 41). mz. Must never be placed in molding in concealed or damp places. For conduit work: z. Must have an approved rubber insulating covering (see No. 47). o. Must not be drawn in until all mechanical work on the building has been, as far as possible, completed. p. Must, for alternating systems, have the two or more wires of a circuit drawn in the same conduit. It is advised that this be done for direct-current systems also, so that they may be changed to alternating systems at any time, induction troubles preventing such a change unless this construction is followed. For concealed ‘‘ knob and tube’’ work: g. Must have an approved rubber insulating covering (see No. 41). 464 ELECTRIC LIGHTING yr. Must be rigidly supported on non-combustible, non-absorptive insula- tors which separate the wire at least one inch from the surface wired over, and must be kept at least ten inches apart, and, when possible, should be run singly on separate timbers or studding. Rigid supporting requires under ordinary conditions, where wiring along flat surfaces, supports at least every four and one-half feet. If the wires are liable to be disturbed, the distance between supports should be shortened. s. When, from the nature of the case, it is impossible to place concealed wiring on non-combustible, insulating supports of glass or porcelain, an a@f- proved armored cable with single or twin conductors (see No. 48) may be used where the difference of potential between wires is not over 300 volts, provided it is installed without joints between outlets, and the cable armor properly enters all fittings and is rigidly secured in place; or, if the difference of potential be- tween wires is not over 300 volts, and if wires are not exposed to moisture, they may be fished on the loop system if separately incased throughout in approved flexible tubing or conduits. For fixture work: ¢t. Must have an approved rubber insulating covering (see No. 46), and shall not be less in size than No. 18 B. & S. u. Supply conductors, and especially the splices to fixture wires, must be kept clear of the grounded part of gas-pipes; and. where shells are used, the latter must be constructed in a manner affording sufficient area to allow this requirement. v. Must, when fixtures are wired outside, be so secured as not to be cut or abraded by the pressure of the fastenings or motion of the fixture. 25, Interior Conduits — (See also Nos. 24 n to p, and 49.) The object of a tube or conduit is to facilitate the insertion or extraction of the conductors to pro- tect them from mechanical injury and, as far as possible, from moisture. Tubes or conduits are to be considered merely as raceways, and are not to be relied upon for insulation between wire and wire, or between the wire and the ground. a. No conduit tube having an internal diameter of less than five-eighths of an inch shall be used. (If conduit is lined, measurement to be taken inside of lining.) 4. Must be continuous from one junction box to another or to fixtures, and the conduit tube must properly enter all fittings. c. Must be first installed as a complete conduit system, without the con- ductors. dad. Must be equipped at every outlet with an approved outlet box. é. Metal conduits, where they enter junction boxes, and at all other out- lets, etc., must be fitted with a capping of approved insulating material, fitted so as to protect wire from abrasion. f. Must have the metal of the conduit permanently and effectively grounded. 26, Fixtures — (See also Vo. 24 zt to v.) a. Must, when supported from the gas-piping of a building, be insulated from the gas-pipe system by means of approved insulating joints (see No. 59) placed as close as possible to the ceiling. It is recommended that the gas outlet pipe be protected above the insulating joint by a non-com- bustible, non-absorptive insulating tube, having a flange at the lower end where it comes in contact with APPENDIX I. 465 the insulating joint ; and that, where outlet tubes are used, they be of sufficient length to extend below the insulating joint, and that they be so secured that they will not be pushed back when the canopy is put in place. Where iron ceilings are used, care must be taken to see that the canopy is thoroughly and permanently insulated from the ceiling. 6. Must have all burs, or fins, removed before the conductors are drawn into the fixture. c. The tendency to condensation within the pipes should be guarded against by sealing the upper end of the fixture. ad. No combination fixture in which the conductors are concealed in a space less than one-fourth inch between the inside pipe and the outside casing will be approved. é. Must be tested for “contacts” between conductors and fixture, for ‘short circuits,” and for ground connections before it is connected to its supply conductors. f. Ceiling blocks for fixtures should be made of insulating material; if not the wires in passing through the plate must be surrounded with non-combustible non-absorptive, insulating material, such as glass or porcelain. g. Under no conditions shall there be a difference of potential of more than 300 volts between wires contained in or attached to the same fixture 27. Sockets — (For construction rules, see No. 55.) a. In rooms where inflammable gases may exist the incandescent lamp and socket must be inclosed in a vapor-tight globe, and supported on a pipe hanger, wired with approved rubber-covered wire (see No. 41) soldered directly to the circuit. 6. In damp or wet places, or over specially inflammable stuff, waterproof sockets must be used. When waterproof sockets are used, they should be hung by separate stranded rubber-covered wires, not smaller than No. 14 B. & S., which should preferably be twisted together when the drop is over three feet. These wires should be soldered direct to the circuit wires, but supported independently of them. 28. Flexible Cord — a. Must have an approved insulation and covering (see No. 45). 6. Must not be used where the difference of potential between the two wires is over 300 volts. c. Must not be used as a support for clusters. ad. Must not be used except for pendants, wiring of fixtures, and portable lamps or motors. e. Must not be used in show windows. jf. Must be protected by insulating bushings where the cord enters the socket. g. Must be so suspended that the entire weight of the socket and lamp will be borne by knots under the bushing in the socket, and above the point where the cord comes through the ceiling block or rosette, in order that the strain may be taken from the joints and binding screws. 29. Arc Lights on Low-Potential Circuits — a. Must have a cutout (see No. 17a) for each lamp or each series of amps. 466 ELECTRIC LIGHTING The branch conductors should have a carrying capacity about fifty per cent in excess of the normal current required by the lamp to provide for heavy current required when lamp is started or when carbons become stuck without overfusing the wires. 6. Must only be furnished with such resistances or regulators as are in- closed in non-combustible material, such resistances being treated as sources of heat. Incandescent lamps must not be used for resistance devices. c. Must be supplied with globes and protected by spark arresters and wire netting around globe, as in the case of arc lights on high-potential circuits (see Nos. 19 and 58). 30. Economy Coils — a. Economy and compensator coils for arc lamps must be mounted on non-combustible, non-absorptive insulating supports, such as glass or porcelain, allowing an air space of at least one inch between frame and support, and in general to be treated like sources of heat. 31. Decorative Series Lamps — a. Incandescent lamps run in series shall not be used for decorative pur- poses inside of buildings, except by special permission in writing from the Inspection Department having jurisdiction. 32, Car Wiring — a. Must be always run out of reach of the passengers, and must have an approved rubber-insulating covering (see No. 41) 33. Car Houses — a. Must have the trolley wires securely supported on insulating hangers. 6. Must have the trolley hangers placed at such distance apart that, in case of a break in the trolley wire, contact cannot be made with the floor. c. Must have cutout switch located at a proper place outside of the build- ing, so that all trolley circuits in the building can be cut out at one point, and line circuit-breakers must be installed, so that when this cutout switch is open the trolley wire will be dead at all points within 100 feet of the building. The current must be cut out of the building whenever the same is not in use or the road not in operation. ad. Must have all lamps and stationary motors installed in such a way that one main switch can control the whole of each installation —lighting or power —independently of main feeder-switch. No portable incandescent lamps or twin wire allowed, except that portable incandescent lamps may be used in the pits, connections to be made by two approved rubber-covered flex- ible wires (see No. 41), properly protected against mechanical injury; the cir- cuit to be controlled by a switch placed outside of the pit. e. Must have all wiring and apparatus installed in accordance with rules under Class “C” for constant potential systems. f. Must not have any system of feeder distribution centering in the building. g. Must have the rails bonded at each joint with no less than No.2 B. &S. annealed copper wire, also a supplementary wire to be run for each track. h. Must not have cars left with trolley in electrical connection with the trolley wire. APPENDIX I. 467 34, Lighting and Power from Railway Wires — a. Must not be permitted, under any pretense, in the same circuit with trolley wires with a ground return, except in electric railway cars, electric car houses and their power stations; nor shall the same dynamo be used for both purposes. HIGH-POTENTIAL SYSTEMS. 550 TO 3,500 VoLTs. Any circuit attached to any machine, or combination of machines, which de- velops a aifference of potential, between any two wrres, of over 300 volts and less than 3,500 volts, shall be considered as a high-potential circuit, and as coming under that class, unless an approved transforming device 7s used, which cuts the difference of potential down to 300 volts or less. 35. Wires — (See also Nos. 14, 15, and 16.) a. Must have an approved rubber-insulating covering (see No. 41). 6. Must be always in plain sight and never incased, except where required by the Inspection Department having jurisdiction. c. Must be rigidly supported on glass or porcelain insulators, which raise the wire at least one inch from the surface wired over, and must be kept apart at least four inches for voltages up to 750 and at least eight inches for voltages over 750, Rigid supporting requires under ordinary conditions, where wiring along flat surfaces, supports at least about every four and one-half feet. If the wires are unusually liable to be disturbed, the dis- tance between supports should be shortened. In buildings of mill construction, mains of No. 8 B. & S. wire or over, where not liable to be dis- turbed, may be separated about six inches for voltages up to 750 and about ten inches for voltages above 750 ; and run from timber to timber, not breaking around, and may be supported at each timber only. @. Must be protected on side walls from mechanical injury by a substantial boxing, retaining an air space of one inch around the conductors, closed at the top (the wires passing through bushed holes) and extending not less than seven feet from the floor. When crossing floor-timbers, in cellars or in rooms, where they might be exposed to injury, wires must be attached by their insulating supports to the under side of a wooden strip not less than one-half an inch in - thickness. 36, Transformers (when permitted inside buildings, see No. 13) — (For construction rules, see No. 62.) a. Must be located ata point as near as possible to that at which the pri- mary wires enter the building. 6. Must be placed in an inclosure constructed of or lined with fire-resisting material: the inclosure to be used only for this purpose, and to be kept securely locked, and access to the same allowed only to responsible persons. c. Must be effectually insulated from the ground, and the inclosure in which they are placed must be practically air-tight, except that it shall be thoroughly ventilated to the outdoor air, if possible, through a chimney or flue. There should be at least six inches air space on all sides of the transformer. o7. Series Lamps — a. No system of multiple-series or series-multiple for light or power will be approved. 6. Under no circumstances can lamps be attached to gas fixtures. 468 ELECT RICQLAGHIING. EXTRA HIGH-POTENTIAL SYSTEMS. OVER 3,500 VOLTS. Any circuit attached to any machine or combination of machines, which de. velops a difference of potential, between any two wires, of over 3,500 volts, shall be considered as an extra high-potential circuit, and as coming under that class, unless an approved transforming device ts used, which cuts the difference of potential down to 3,500 volts or less. 38, Primary Wires — a. Must not be brought into or over building, except power and substations. 39. Secondary Wires — a. Must be installed under rules for high-potential systems, when their immediate primary wires carry a current of over 3,500 volts, unless the primary wires are entirely underground, within city and village limits. The presence of wires carrying a current with a potential of over 3,500 volts in the streets of cities, towns, and villages is considered to increase the fire hazard. Extra high potential circuits are also objectionable in any location where telephone, telegraph, and similar circuits run in proximity to them. As the underwriters have no jurisdiction over streets and roads they can only take this indirect way of discouraging such systems ; but further, it is strongly urged that municipal authorities absolutely refuse to grant any franchise for right of way for overhead wires carrying a current of extra high potential through streets or roads which are used to any great extent for public travel or for trunk-line, telephone, or telegraph circuits. Crass D. FITTINGS, MATERIALS, AND DETAILS OF CONSTRUCTION. All Systems and Voltages. Insulated Wires— Rules 4o to 48. 40. General Rules — . a. Copper for insulated conductors must never vary in diameter so as to be more than two one-thousandths of an inch less than the specified size. 6. Wires and cables of all kinds designed to meet the following specifica- tions must be plainly tagged or marked as follows: 1, The maximum voltage at which the wire is designed to be used. 2. The words “ National Electrical Code Standard.” 3. Name of the manufacturing company, and, if desired, trade-name of the wire. 4, Month and year when manufactured. 41. Rubber-Covered — a. Copper for conductors must be thoroughly tinned. APPENDIX 1. 469 Insulation for voltages between o and 600: 6. Must be of rubber or other approved substance, and be of a thickness not less than that given in the following table for B. & S. gauge sizes: From 18 to 16, inclusive, 3%” th 14 to 8, “ da” a Weta 2. S re” “ 1 to 0000, « fe’ es 0000 to 500,000, C. M. oa” “ 500,000 to 1,000,000, “ da’ Larger than 1,000,000, y’ Measurements of insulating wall are to be made at the thinnest portion of the dielectric. . c. The completed coverings must show an insulation resistance of at least 100 megohms per mile during thirty days’ immersion in water at seventy degrees Fahrenheit. @. Each foot of the completed covering must show a dielectric strength sufficient to resist throughout five minutes the application of an electro-motive force of 3,000 volts per one-sixty-fourth of an inch thickness of insulation under the following conditions : The source of alternating electro-motive force shall be a transformer of at least one kilowatt capacity. The application of the electro-motive force shall first be made at 4,000 volts for five minutes and then the voltage increased by steps of not over 3,000 volts, each held for five minutes, until the rupture of the insulation occurs. The tests for dielectric strength shall be made on a sample of wire which has been immersed for seventy-two hours in water, one foot of which is submerged in a conducting liquid held in a metal trough, one of the transformer terminals being connected to the copper of the wire and the other to the metal of the trough. Insulations for voltages between 600 and 3,500: e. The thickness of the insulating walls must not be less than those given in the following table for B. & S. gauge sizes: From 14 to 1, inclusive, 2” From 0 to 500,000, C. M., ss” covered by a tape or a braid. Larger than 500,000, C. M., s:” covered by a tape or a braid. f. The requirements as to insulation and break-down resistance for wires for low-potential systems shall apply, with the exception that an insulation re- sistance of not less than 300 megohms per mile shall be required. g. Wire for arc-light circuits exceeding 3,500 volts potential shall have an insulating wall not less than six-thirty-seconds of an inch in thickness, and shall withstand a break-down test of at least 30,000 volts and have an insula- tion of at least 500 megohms per mile. The tests on this wire to be made under the same conditions as for low- potential wires. Specifications for insulations for alternating currents exceeding 3,500 volts have been considered, but on account of the somewhat complex conditions in such work, it has so far been deemed inexpedient to specify general insulations for this use. 470 ELECTRICWIGHILNVG, h. All of the above insulations must be protected by a substantial braided covering properly saturated with a preservative compound and sufficiently strong to withstand all the abrasion likely to be met with in practice, and sufficiently elastic to permit all wires smaller than No.7 B. & S. gage to be bent around a cylinder with twice the diameter of the wire, without injury to the braid. 42, Slow-burning Weatherproof — a. The insulation shall consist of two coatings, the inner one to be fire- proof in character, the outer to be weatherproof. The inner fireproof coating must comprise at least six-tenths of the total thickness of the wall. The com- pleted covering must be of a thickness not less than that given in the following table for B. & S. gauge sizes: From 14 to 8, inclusive, &” + 7 to Ze Tea cs 2 to 0000, ex” es 0000'to 500,000, C.M., =” 4 500,000 to 1,000,000, “ oa” Larger than 1,000,000,“ 4’ Measurements of insulating wall are to be made at the thinnest portion of the dielectric. 6. The inner fireproof coating shall be layers of cotton or other thread, the outer one of which must be braided. All the interstices of these layers are to be filled with the fireproofing compound. This is to be material whose solid constituent is not susceptible to moisture, and which will not burn even when ground in an oxidizable oil, making a compound which, while proof against fire and moisture, at the same time has considerable elasticity, and which when dry will suffer no change at a temperature of 250 degrees Fahrenheit, and which will not burn at even higher temperature. c. The weatherproof coating shall be a stout braid thoroughly saturated with a dense moistureproof compound thoroughly slicked down, applied in such manner as to drive any atmospheric moisture from the cotton braiding, thereby securing a covering to a great degree waterproof and of high insulat- ing power. This compound to retain its elasticity at zero Fahrenheit, and not to drip at 160 degrees Fahrenheit. This wire is not as burnable as the old ‘‘ weatherproof,’’ nor as subject to softening under heat, but still is able to repel the ordinary amount of moisture found indoors. It would not usually be used for outside work. 43. Slow-burning — a. The insulation shall be the same as the ‘‘slow-burning weatherproof,” except that the outer braiding shall be impregnated with a fireproofing com- pound similar to that required for the interior layers, and with the outer sur- face finished smooth and hard. This ‘‘ slow-burning ’’ wire shall only be used with special permission of the Inspection Depart- ment having jurisdiction. This is practically the old ‘‘ Underwriters’ ’’ insulation. It is specially useful in hot, dry places where ordinary insulations would perish, also where wires are bunched, as on the back of a large APPENDIX I. Ae switchboard or in a wire tower, so that the accumulation of rubber or weatherproof insulation would result in an objectionably large mass of highly inflammable material. Its use is restricted, as its insulating qualities are not high and are damaged by moisture. 44. Weatherproof — a. The insulating covering shall consist of at least three braids thoroughly impregnated with a dense moisture repellent, which will not drip at a temper- ature lower than 180 degrees Fahrenheit. The thickness of insulation shall be not less than that of “slow-burning weatherproof.” The outer surface shall be thoroughly slicked down.” This wire is for outdoor use where moisture is certain and where fireproof qualities are not neces- sary. 45, Flexible Cord — a. Must be made of stranded copper conductors, each strand to be not larger than No. 26 or smaller than No. 30 B. & S. gauge, and-each stranded conductor must be covered by an approved insulation and protected from mechanical injury by a tough braided outer covering. For pendent lamps: In this class is to be included all flexible cord which under usual condi- tions hangs freely in air, and which is not likely to be moved sufficiently to come in contact with surrounding objects. 6. Each stranded conductor must have a carrying capacity equivalent to not less than a No. 18 B. & S. gauge wire. c. The covering of each stranded conductor must be made up as follows : 1, A tight, close wind of fine cotton. 2. The insulation proper, which shall be either waterproof or slow- burning. 8. An outer cover of silk or cotton. The wind of cotton tends to prevent a broken strand puncturing the insulation and causing a short circuit. It also keeps the rubber from corroding the copper. d. Waterproof insulation must be solid, at least one-thirty-second of an inch thick, and must show an insulation resistance of fifty megohms per mile throughout two weeks’ immersion in water at 70 degrees Fahrenheit, and stand the tests prescribed for low-tension wires as far as they apply. é. Slow-burning insulation must be at least one-thirty-second of an inch in thickness, and composed of substantial, elastic, slow-burning materials, which will suffer no damage at a temperature of 250 degrees Fahrenheit. yf. The outer protecting braiding should be so put on and sealed in place that when cut it will not fray out, and where cotton is used, it should be im- pregnated with a flameproof paint, which will not have an injurious effect on the insulation. For portables : In this class is included all cord used on portable lamps, small portable motors, etc. g. Flexible cord for portable use must have waterproof insulation as required in section @ for pendent cord, and in addition be provided with a rein- forcing cover especially designed to withstand the abrasion it will be subject to in the uses to which it is to be put. AX? ELECTRICOLIGRIUING For portable heating apparatus: hk. Must be made up as follows: — 1, A tight, close wind of fine cotton. A thin layer of rubber about one-one-hundredth of an inch thick, or other cementing material. A layer of asbestos insulation at least three-sixty-fourths of an inch thick. 4. A stout braid of cotton. 5, An outer reinforcing cover especially designed to withstand abrasion. eo This cord is in no sense waterproof, the thin layer of rubber being specified in order that it may serve merely as a Seal to help hold in place the fine cotton and asbestos, and it should beso put on as to accomplish this. 46. Fixture Wire — a. Must have a solid insulation, with a slow-burning, tough, outer cover- ing, the whole to be at one-thirty-second of an inch in thickness, and show an insulation resistance between conductors, and between either conductor and the ground, of at least one megohm per mile, after one week’s submersion in water at seventy degrees Fahrenheit, and after three minutes’ electrification, with 550 volts. 47. Conduit Wire — Must comply with the following specifications: a. For metal conduits, having a lining of insulating material, single wires must comply with No. 41, and all duplex, twin, and concentric conductors must comply with No. 41, and must also have each conductor separately braided or taped and a substantial braid covering the whole. 6. For unlined metal conduits, conductors must conform to the specifica- tions given for lined conduits, and in addition have a second outer fibrous covering at least one-thirty-second of an inch in thickness, and sufficiently tenacious to withstand the abrasion of being hauled through the metal conduit. The braid required around each conductor in duplex, twin, and concentric cables is to hold the rubber insulation in place and prevent jamming and flattening. 48. Armored Cable — a. The armor of such cables must be at least equal in thickness and of equal strength to resist penetration by nails, etc., as the armor of metal cover- ings of metal conduits (see No. 49 0). 6. The conductors in same, single wire or twin conductors, must have an insulating covering as required by No. 41, any filler used to secure a round exterior must be impregnated with a moisture repellent, and’ the whole bunch of conductors and fillers must have a separate exterior covering of insulating material at least one-thirty-second of an inch in thickness, conforming to the insulation standard given in No. 41, and covered with a substantial braid. Very reliable insulation is specified, as such cables are liable to hard usage, and in part of their length may be subject to moisture, while they may not be easily removable, so that a breakdown of insulation is likely to be expensive. APPENDIX I. Ate 49. Interior Conduits — (For wiring rules, see Nos. 24 and 28.) a. Each length of conduit, whether insulated or uninsulated, must have the maker’s name or initials stamped in the metal or attached thereto in a satisfactory manner, so that the inspectors can readily see the same. METAL CONDUITS WITH LINING OF INSULATING MATERIAL. 6. The metal covering or pipe must be equal in strength to the ordinary commercial forms of gas-pipe of the same size, and its thickness must be not less than that of standard gas-pipe, as shown by the following table: Size. Thickness of Size. Thickness of Inches. Wall— Inches. Inches. Wall— inches. 3 109 14 .140 § 111 13 145 3 ts 2 154 1 134 An allowance of two-one-hundredths of an inch for variation in manufac- turing and loss of thickness by cleaning will be permitted. c. Must not be seriously affected externally by burning out a wire inside the tube when the iron pipe is connected to one side of the circuit. dad. Must have the insulating lining firmly secured to the pipe. e. The insulating lining must not crack or break when a length of the conduit is uniformly bent at temperature of 212 degrees Fahrenheit to an angle of ninety degrees, with a curve having a radius of fifteen inches, for pipes of one inch and less, and fifteen times the diameter of pipe for larger pipes. f. The insulating lining must not soften injuriously at a temperature below 212 degrees Fahrenheit, and must leave water in which it is boiled practically neutral. g. The insulating lining must be at least one-thirty-second of an inch in thickness, and the materials of which it is composed must be of such a nature as will not have a deteriorating effect on the insulation of the conductor, and be sufficiently tough and tenacious to withstand the abrasion test of drawing long lengths of conductors in and out of same. h. The insulating lining must not be mechanically weak after three days’ submersion in water, and when removed from the pipe entire must not absorb more than ten per cent of its weight of water during 100 hours of submersion. z. All elbows or bends must be so made that the conduit or lining of same will not be injured. The radius of the curve of the inner edge of any elbow not to be less than three and one-half inches. Must have not more than the equivalent of four quarter bends from outlet to outlet, the bends at the outlets not being counted. UNLINED METAL CONDUITS. j. Plain iron or steel pipes of equal thickness and strengths specified for lined conduits in No. 49 6 may be used as conduits, provided their interior sur- faces are smooth and free from burs; pipe to be galvanized, or the interior surfaces coated or enameled, to prevent oxidation, with some substance which will not soften so as to become sticky and prevent wire from being withdrawn from the pipe. AT4 ELECTRICOLRIGHZING. k, All elbows or bends must be so made that the conduit will not be injured. The radius of the curve of the inner edge of any elbow not to be less than three and one-half inches. Must have not more than the equivalent of four quarter bends from outlet to outlet, the bends at the outlet not being counted. 50. Wooden Moldings — (Lor wiring rules, see Vo. 24.) a. Must have, both outside and inside, at least two coats of waterproof paint, or be impregnated with a moisture repellent. 6. Must be made of two pieces, a backing and capping so constructed as to thoroughly incase the wire, and provide a one-half-inch tongue between the conductors, and a solid backing, which, under grooves, shall not be less than three-eighths of an inch in thickness, and must afford suitable protection from abrasion. It is recommended that only hardwood molding be used, 51. Switches— (See Vos. 17 and 22.) a. Must be mounted on non-combustible, non-absorptive, insulating bases, such as slate or porcelain. 6. Must have carrying capacity sufficient to prevent undue heating. c. Must, when used for service switches, indicate, on inspection, whether the*current.be on or oft.” ad. Must be plainly marked, where it will always be visible, with the name of the maker and the current and voltage for which the switch is designed. e. Must, for constant potential systems, operate successfully at fifty per cent overload in amperes, with twenty-five per cent excess voltage under the most severe conditions they are liable to meet with in practice. J. Must, for constant potential systems, have a firm and secure contact; must make and break readily, and not stop when motion has once been imparted by the handle. g. Must, for constant current systems, close the main circuit and discon- nect the branch wires when turned ‘ off”; must be so constructed that they shall be automatic in action, not stopping between points when started, and must prevent an arc between the points under all circumstances. They must indicate, upon inspection, whether the current be ‘‘on” or “ off.” 52. Cutouts and Circuit-Breakers — (For installation rules, see Nos, 17 ang 21.) a. Must be supported on bases of non-combustible, non-absorptive insu- lating material. 6. Cutouts must be provided with covers, when not arranged in approved cabinets, so as to obviate any danger of the melted fuse metal coming in con- tact with any substance which might be ignited thereby. c. Cutouts must operate successfully, under the most severe conditions they are liable to meet with in practice, on short circuits with fuses rated at fifty per cent above, and with a voltage twenty-five per cent above the current and voltage for which they are designed. APPENDIX I. 05 ad. Circuit-breakers must operate successfully, under the most severe con- ditions they are liable to meet with in practice, on short circuits when set at fifty per cent above the current, and with a voltage twenty-five per cent above that for which they are designed. é. Must be plainly marked, where it will always be visible, with the name of the maker, and current and voltage for which the device is designed. 53. Fuses — : - (for installation rules, see Nos. 17 and 21.) a. Must have contact surfaces or tips of harder metal having perfect electrical connection with the fusible part of the strip. 6. Must be stamped with about eighty per cent of the maximum current they can carry indefinitely, thus allowing about twenty-five per cent overload before fuse melts. With naked open fuses, of ordinary shapes and not over 500 amperes capacity, the maxzmum cur- rent which will melt them in about five minutes may be safely taken as the melting point, as the fuse practically reaches its maximum temperature in this time. With larger fuses a longer time is necessary. Inclosed fuses where the fuse is often in contact with substances having good conductivity to heat and often of considerable volume, require a much longer time to reach a maximum temperature, on account of the surrounding material which heats up slowly. These data are given to facilitate testing. c. Fuse terminals must be stamped with the maker’s name, initials, or some known trade-mark, 54. Cutout Cabinets — a. Must be so constructed, and cutouts so arranged, as to obviate any danger of the melted fuse metal coming in contact with any substance which might be ignited thereby. A suitable box can be made of marble, slate, or wood, strongly put together, the door to close against a rabbet so as to be perfectly dust-tight, and it should be hung on strong hinges, and held closed by a strong hook or catch. If the box is wood, the inside should be lined with sheets of asbestos board about one-sixteenth of an inch in thickness, neatly put on and firmly secured in place by shellac and tacks. The wire should enter through holes bushed with porcelain bushings; the bushings tightly fitting the holes in the box, and the wires tightly fitting the bushings (using tape to build up the wire, if necessary) so as to keep out the dust. 55. Sockets — (See Vo. 27.) Sockets of all kinds, including wall receptacles, must be constructed in accordance with the following specifications : — a. STANDARD SIZES.— The standard lamp socket shall be suitable for use on any voltage not exceeding 250 and with any size lamp up to fifty candle- power. For lamps larger than fifty candle-power a standard keyless socket may be used, or if a key is required, a special socket designed for the current to be used must be made. Any special sockets must follow the general spirit of these specifications. 6. MARKING. — The standard socket must be plainly marked fifty candle- power, 250 volts, and with either the manufacturer’s name or registered trade- 476 ELECTRIC LICH TIING, - mark. Special large sockets must be marked with the current and voltage for which they are designed. c. SHELL.— Metal used for shells must be moderately hard, but not hard enough to be brittle or so soft as to be easily dented or knocked out of place. Brass shells must be at least 0.013 inch in thickness, and shells of any other material must be thick enough to give the same stiffness and strength of brass. d. LINING.— The inside of the shells must be lined with insulating _material, which shall absolutely prevent the shell from becoming a part of the circuit, even though the wires inside the socket should start from their position under. binding screws. The material used for lining must be at least one-thirty-second of an inch in thickness, and must be tough and tenacious. It must not be injuriously affected by the heat from the largest lamp permitted in the socket, and must leave the water in which it is boiled practically neutral. It must be so firmly secured to the shell that it will not fall out with ordinary handling of the socket. It is preferable to have the lining in one piece. e. Cap.— Caps when of sheet brass must be at least 0.013 inch in thickness, and when cast or made of other metals must be of equivalent strength. The inlet piece, except for special sockets, must be tapped and threaded for ordinary one-eighth-inch pipe. It must contain sufficient metal for a full, strong thread, and, when not of the same piece as the cap, must be joined to it in a way to give the strength of a single piece. There must be sufficient room in the cap to enable the ordinary wireman to easily and quickly make a knot in the cord and push it into place in cap without crowding. All parts of the cap upon which the knot is likely to bear must be smooth and well insulated. jf. FRAME AND SCREWS.— The frame holding moving parts | must be sufficiently heavy to give ample strength and stiffness. Brass pieces containing screw threads must be at least 0.06 of an inch in thickness. Binding post screws must not be smaller than No. 5 wire and about forty threads per inch, g. SPACING.— Points of opposite polarity must everywhere be kept not less than three-sixty-fourths of an inch apart unless separated by a reliable insulation, hk. CONNECTIONS.— The connecting points for the flexible cord must be made to very securely grip a No. 16 or 18 B. & S. conductor. A turned-up lug, arranged so that the cord may be gripped between the screw and the lug in such a way that it cannot possibly come out, is strongly advised. z. Lamp HOLDER.— The socket must firmly hold the lamp in place so that it cannot be easily jarred out, and must provide a contact good enough to pre- vent undue heating with maximum current allowed. The holding pieces, springs and the like, if a part of the circuit, must not be sufficiently exposed to allow them to be brought in contact with anything outside of lamp and socket. j. BASE.— The inside parts of the socket, which are of insulating material, except the lining, must be made of porcelain. &. KeEy.— The socket key-handle must be of such a material that it will not soften from the heat of a fifty candle-power lamp hanging downwards in. APPENDIX 1. ATT air at seventy degrees Fahrenheit from the socket, and must be securely, but not necessarily rigidly, attached to the metal spindle it is designed to turn. , . 7. SEALING.—AIl screws in porcelain pieces, which can be firmly sealed in place, must be so sealed by a waterproof compound which will not melt below 200 degrees Fahrenheit. m. PUTTING TOGETHER, — The socket must, as a whole, be so put together that it will not rattle to pieces. Bayonet joints or equivalent are recommended. nz. TEST.— The socket when slowly turned “on and off,” at the rate of about two or three times per minute, must ‘‘make and break”’ the circuit 6,000 times before failing, when carrying a load of one ampere at 220 volts. o. KEYLESS SOCKETS. — Keyless sockets of all kinds must comply with requirements for key sockets as far as they apply. p. SOCKETS OF INSULATING MATERIALS.— Sockets made of porcelain or other insulating material must conform to the above requirements as far as they apply, and all parts must be strong enough to withstand a moderate amount of hard usage without breaking. g. INLET BUSHING.— When the socket is not attached to fixtures the threaded inlet must be provided with a strong insulating bushing, having a smooth hole of at least fifteen-sixty-fourths of an inch in diameter. The corners of the bushing must be rounded and all inside fins removed, so that in no place will the cord be subjected to the cutting or wearing action of a sharp edge. 56, Hanger-boards — a. Hanger-boards must be so constructed that all wires and current-carry- ing devices thereon shall be exposed to view, and thoroughly insulated by being mounted on a non-combustible, non-absorptive insulating substance. All switches attached to the same must be so constructed that they shall be automatic in their action, cutting off both poles to the lamp, not stopping between points when started, and preventing an arc between points under all circumstances. 57, Arc Lamps — (For installation rules, see No, 19.) a. Must be provided with reliable stops to prevent carbons from falling out in case the clamps become loose. 6. Must be carefully insulated from the circuit in all their exposed parts. c. Must, for constant-current systems, be provided with an approved hand switch, also an automatic switch that will shunt the current around the carbons, should they fail to feed properly. The hand switch to be approved, if placed anywhere except on the lamp itself, must comply with requirements for switches on hanger-boards as laid down in No. 56. 58. Spark Arresters — (See /Vo. 19 c.) a. Spark arresters must so close the upper orifice of the globe that it will be impossible for any sparks thrown off by the carbons to escape. A478 ELECTRIC LIGHTING. 59, Insulating Joints — (See Vo. 26 a.) a. Must be entirely made of material that will resist the action of illumi- nating gases, and will not give way or soften under the heat of an ordinary gas flame or leak under a moderate pressure. They shall be so arranged that a deposit of moisture will not destroy the insulating effect, and shall have an insulating resistance of at least 250,000 ohms between the gas-pipe attachments, and be sufficiently strong to resist the strain they will be liable to be subjected to in being installed. 6. Insulating joints having soft rubber in their construction will not be — approved. 60. Resistance Boxes and Equalizers — (For znstallation rules, see Vo. 4.) a. Must be equipped with metal, or with other non-combustible frames. The word ‘‘ frame ’’ in this section relates to the entire case and surroundings of the rheostat, and not alone to the upholding supports. 61. Reactive Coils and Condensers — a. Reactive coils must be made of non-combustible material, mounted on non-combustible bases, and treated, in general, like sources of heat. 6. Condensers must be treated like apparatus operating with equivalent voltage and currents. They must have non-combustible cases and supports, and must be isolated from all combustible materials, and, in general, treated like sources of heat. , 62. Transformers — (For installation rules, see Nos. 11, 13, and 33.) a. Must not be placed in any but metallic or other non-combustible cases. 6. Must be constructed to comply with the following tests: 1. Shalf be run for eight consecutive hours at full load in watts under conditions of service, and at the end of that time the rise in tem- perature, as measured by the increase of resistance of the primary coil, shall not exceed 135 degrees Fahrenheit. 2. The insulation of transformers when heated shall withstand continu- ously for five minutes a difference of potential of 10,000 volts (alternating) between primary and secondary coils and core, and between the primary coils and core and a no-load ‘“‘run”’ at double voltage for thirty minutes. 63. Lightning Arresters — (For installation rules, See No. 5.) a. Must be mounted on non-combustible bases, and must be so constructed as not to maintain an arc after the discharge has passed, and must have no moving parts. APPENDIX I. 479 CUASS. Lo MISCELLANEOUS. 64. Signaling Systems (governing wiring for telephone, telegraph, district messenger, and call-bell circuits, fire and burglar alarms, and all similar systems) — a. Outside wires should be run in underground ducts or strung on poles and, as far as possible, kept off of buildings, and must not be placed on the same cross-arm with electric light or power wires. 6. When outside wires are run on same pole with electric light or power wires, the distance between the two inside pins of each cross-arm must not be less than twenty-six inches. c. All aerial conductors and underground conductors which are directly connected to aerial wires must be provided with some approved protective de- vice, which shall be located as near their point of entrance to the building as possible, and not less than six inches from curtains or other inflammable material. d@. If the protector is placed inside of building, wires, from outside sup- port to binding-posts of protector, shall comply with the following require- ments: Must be of copper, and not smaller than No. 16 B. & S. gauge. Must have an approved rubber insulating covering (see No. 41). Must have drip loops in each wire immediately outside the building. ees a Must enter buildings through separate holes sloping upward from the outside ; when practicable, holes to be bushed with non-absorptive, non-combustible insulating tubes extending through their entire length. Where tubing is not practicable, the wires shall be wrapped with two layers of insulating tape. 5. Must be supported on porcelain insulators, so that they will not come in contact with anything other than their designed supports. 6. A separation between wires of at least two and one-half inches must be maintained. In case of crosses these wires may become a part of a high-voltage circuit, so that similar care to that given high-voltage circuits is needed in placing them. Reliable porcelain bushings at the entrance holes are desirable, and are only waived under adverse conditions, because the state of the art in this type of wiring makes an absolute requirement inadvisable. e. The ground wire of the protective device shall be run in accordance with the following requirements : 1, Shall be of copper, and not smaller than No. 16 B. & S. 2. Must have an approved rubber insulating covering (see No. 41). 3. Shall run in as straight a line as possible to a good permanent ground, to be made by connecting to water- or gas-pipe, preferably water-pipe. If gas-pipe is used, the connection, in all cases, must be made between the meter and service pipes. In the absence of other good ground, the ground shall be made by means of a metallic plate or bunch of wires buried in permanently moist earth. AS0 ELECTRIC LIGHTING. 4. Shall be kept at least three inches from all other conductors and sup- ported on porcelain insulators, so as not to come in contact with anything other than its designed supports. In attaching a ground wire to a pipe, it is often difficult to make a thoroughly reliable solder joint. It is better, therefore, where possible, to carefully solder the wire to a brass plug, which may then be firmly screwed into a pipe fitting. Where such joints are made under ground, they should be thoroughly painted and taped to prevent corrosion. jf. The protector to be approved must comply with the following require- ments: 1, Must be mounted on non-combustible, non-absorptive insulating bases, so designed that when the protector is in place, all parts which may be alive will be thoroughly insulated from the wall hold- ing the protector. 2. Must have the following parts: A lightning arrester which will operate with a difference of potential between wires of not over 500 volts, and so arranged that the chance of accidental grounding is reduced to a minimum. A fuse designed to open the circuit in case the wires become crossed with light or power circuits. The fuse must be able to open the circuit without arcing or serious flashing when crossed with any ordinary commercial light or power circuit. A heat coil which will operate before a sneak current can damage the instrument the protector is guarding. The heat coil is designed to warm up and melt out with a current large enough to endanger the instruments if continued for a long time, but so small that it would not blow the fuses ordinarily found necessary for such instruments. These smaller currents are often called “sneak ’’ currents. 3. The fuses must be so placed as to protect the arrester and heat coils, and the protector terminals must be plainly marked “line,” “ in- strument,” ‘ ground.” g. Wires beyond the protector, except where bunched, must be neatly arranged and securely fastened in place in any convenient, workmanlike man- ner. They must not come nearer than six inches to any electric light or power wire in the building, unless incased in approved tubing so secured as to pre- vent its slipping out of place. The wires would ordinarily be insulated, but the kind of insulation is not specified, as the pro- tector is relied upon to stop all dangerous currents. Porcelain tubing or circular loom conduit may be used for incasing wires where required as above. h. Wires connected with outside circuits, where bunched together within any building, or inside wires, where laid in conduits or ducts, with electric light or power wires, must have fire-resisting coverings, or else must be inclosed in an air-tight tube or duct. It is feared that if a burnable insulation were used, a chance spark might ignite it and cause a serious fire, for many installations contain a large amount of very readily burnable matter. 65, Electric Gas Lighting — Where electric gas lighting is to be used on the same fixture with the electric light: APPENDIX I. 481 a. No part of the gas-piping or fixture shall be in electric connection with the gas-lighting circuit. 6. The wires used with the fixtures must have a non-inflammable insula- tion, or, where concealed between the pipe and shell of the fixture, the insula- tion must be such as required for fixture wiring for the electric light. c. The whole installation must test free from ‘“ grounds.” ad. The two installations must test perfectly free from connection with each other. 66. Insulation Resistance — The wiring in any building rust test free from grounds; i.e., the com- plete installation must have an insulation between conductors and between all conductors and the ground (not including attachments, sockets, recep- ticles, etc.) of not less than the following: Up to 5 amperes. ... . . . 4,000,000 ohms. ae 10 < a Sarna ess ee 25000000 Fa we 25 SC oe SO it stem ys CO0-000 mon" WY 50 Me ees oe eey 2 £00000: 7 4° 100 ie Se eee ee oe 2000004)" fe ch 200 s Ro. igo ae Me A 100,000 ‘* ne 400 es Paes so bee eh oe Oe 25,000 ‘* ce 800 oe Rein i eee oe 25,000 ‘* 2Y 1,600 : yah LAY Werf \ Gera: 12,500 ‘ All cutouts and safety devices in place in the above. Where lamp sockets, receptacles, and electroliers, etc., are connected, one- half of the above will be required. 67. Soldering Fluid — a. The following formula for soldering fluid is suggested : Saturated solution of zinc chloride . ... . . 5 parts INGO Ge G4 ae es 8 See on ye Bae era Glycerinet waar) Be eute bet ee a ee cee part CLASsSer. MARINE WORK. 68. Generators — a. Must be located in a dry place. 6. Must have their frames insulated from their bed-plates. c. Must each be provided with a waterproof cover. d. Must each be provided with a name-plate, giving the maker’s name, the capacity in voltage and amperes and normal speed in revolutions per minute. 69. Wires — a. Must have an approved insulating covering. 482 ELECTRIC LIGHTING. The insulation for all conductors, except for portables, to be approved, must be at least one- eighth-inch in thickness and be covered with a substantial waterproof and flameproof braid. The physical characteristics shall not be affected by any change in temperature up to 200 degrees Fahrenheit. After two weeks’ submersion in salt water at seventy degrees Fahrenheit it must show an insulation resistance of one megohm per mile after three minutes’ electrification, with 550 volts. 6. Must have no single wire larger than No. 12 B. & S. Wires to be stranded when greater carrying capacity is required. No single solid wire smaller than No. 14 B. & S., except in fixture wiring, to be used. Stranded wires must be soldered before being fastened under clamps or binding screws, and when they have a conductivity greater than No. 10 B. & S. copper wire they must be soldered into lugs. c. Must be supported in approved molding, except at switch boards and portables. Special permission may be given for deviation from this rule in dynamo-rooms. d@. Must be bushed with hard-rubber tubing one-eighth of an inch in thickness when passing through beams and non-water-tight bulkheads. e. Must have, when passing through water-tight bulkheads and through all decks, a metallic stuffing tube lined with hard rubber. In case of deck tubes they shall be boxed near deck to prevent mechanical injury. fy. Splices or taps in conductors must be avoided as far as possible. Where it is necessary to make them they must be so spliced or joined as to be both mechanically and electrically secure without solder. They must then be sol- dered, to insure preservation, covered with an insulating compound equal to the insulation of the wire, and further protected by a waterproof tape. The joint must then be coated or painted with a waterproof compound. 70, Portable Conductors — a. Must be made of two stranded conductors, each having a carrying capacity equivalent to not less than No. 14 B. & S. wire, and each covered with an approved insulation and covering. Where not exposed to moisture or severe mechanical injury, each stranded conductor must have a solid insulation at least one-thirty-second of an inch in thickness, and must show an insulation resistance between conductors, and between either conductor and the ground, of at least one megohm per mile after one week’s submersion in water at seventy degrees Fahrenheit and after three minutes’ electrifica- tion, with 590 volts, and be protected by a slow-burning, tough-braided outer covering. Where exposed to moisture and mechanical injury —as for use on decks, holds, and fire-rooms — each stranded conductor shall have a solid insulation to be approved, of at least one thirty-second of an inch in thickness and protected by a tough braid. The two conductors shall then be stranded together, using a jute filling. The whole shall then be covered with a layer of flax, either woven or braided, at least one-thirty-second of an inch in thickness, and treated with a non-inflammable waterproof compound. After one week’s submersion in water at seventy degrees Fahrenheit, at 5s0 volts and a three minutes’ electrification, must show an insulation between the two conductors, or between either conductor and the ground, of one megohm per mile. 71. Bell or Other Wires — a. Shall never run in same duct with lighting or power wires. 72. Table of Capacity of Wires — APPENDIX I. 483 B. & &. G. Area Actual C. M. penis EE a es Amperes. 19 1,288 rae ae ate 18 1,624 ae ee 3 17 2,048 Hes es aie 16 2,583 es aie 6 15 3,257 io i ee 14 4,107 iA ae 12 12 6,530 < es 17 9.016 7 19 21 11,368 7 18 25 ae 14,336 e 17 30 se 18,081 7 16 35 ane 22,799 if 15 40 AC 30,856 19 18 50 ae 38,912 19 17 60 is 49,077 19 16 70 ac 60,088 37 18 85 ae 75,776 37 17 100 -, 99,064 61 18 120 ne 124,928 61 17 145 - 157,563 61 16 170 198,677 61 15 200 250,527 61 14 235 296,387 91 15 270 oe 373,737 91 14 320 ae 412,639 127 15 340 When greater conducting area than that of a single wire is required, the conductor shall be stranded in a series of 7, 19, 37, 61, 91, or 127 wires, as may be required ; the strand consisting of one cen- tral wire, the remainder laid around it concentrically, each layer to be twisted in the opposite direction from the preceding. 73. Switchboards — ~ a. Must be made of non-combustible, non-absorptive insulating material, such as marble or slate. 6. Must be kept free from moisture, and must be located so as to be acces- sible from all sides. c. Must have a main switch, main cutout and ammeter for each generator. Must also have a voltmeter and ground detector. d. Must have a cutout and switch for each side of each circuit leading from board. 74. Resistance Boxes — a. Must be made of non-combustible material. 6. Must be located on switchboard or away from combustible material. When not placed on switchboard they must be mounted on non-inflammable, non-absorptive insulating material. c. Must be so constructed as to allow sufficient ventilation for the uses to which they are put. 75. Switches — a. Must have non-combustible, non-absorptive insulating bases. 6. Must operate successfully at fifty per cent overload in amperes with twenty-five per cent excess voltage under the most severe conditions they are 484 ELECTRICLIGH TING liable to meet with in practice, and must be plainly marked, where they will always be visible, with the name of the maker and the current and voltage for which the switch is designed.. c. Must be double pole when circuits which they control supply more than six sixteen-candle-power lamps or their equivalent. d. When exposed to dampness, they must be inclosed in a water-tight case. 76. Cutouts — a. Must have non-combustible, non-absorptive insulating bases. 6. Must operate successfully, under the most severe conditions they are liable to meet with in practice, on short circuit with fuse rated at fifty per cent above, and with a voltage twenty-five per cent above the current and voltage they are designed for, and must be plainly marked, where they will always be visible, with the name of the maker and current and voltage for which the device is designed. c. Must be placed at every point where a change is made in the size of the wire (unless the cutout in the larger wire will protect the smaller). @. In places such as upper decks, holds, cargo spaces, and fire-rooms a water-tight and fireproof cutout may be used, connecting directly to mains when such cutout supplies circuits requiring not more than 660 watts energy. e. When placed anywhere except on switchboards and certain places, as cargo spaces, holds, fire-rooms, etc., where it is impossible to run from center of distribution, they shall be in a cabinet lined with fire-resisting material. Ff. Except for motors, searchlights, and diving-lamps shall be so placed that no group of lamps, requiring a current of more than six amperes, shall ultimately be dependent upon one cutout. A single-pole covered cutout may be placed in the molding when same contains conductor sup- plying circuits requiring not more than 220 watts energy. 77. Fixtures — a. Shall be mounted on blocks made from well-seasoned lumber treated with two coats of white lead or shellac. 6. Where exposed to dampness, the lamp must be surrounded by a vapor- proof globe. c. Where exposed to mechanical injury, the lamp must be surrounded by a globe protected by a stout wire guard. @. Shall be wired with same grade of insulation as portable conductors which are not exposed to moisture or mechanical injury. 78. Sockets — a. No portion of the lamp socket or lamp base exposed to contact with outside objects shall be allowed to come into electrical contact with either of the conductors. 79. Wooden Moldings — a. Must be made of well-seasoned lumber, and be treated inside and out with at least two coats of white lead or shellac. 6. Must be made of two pieces, a backing and a capping, so constructed as to thoroughly incase the wire, and provide a one-half inch tongue between the conductors, and a solid backing which, under grooves, shall not be less than three-eighths of an inch in thickness. APPENDIX 1. 485 c. Where molding is run over rivets, beams, etc.,a backing strip must first be put up and the molding secured to this. ad. Capping must be secured by brass screws. 80. Motors — a. Must be wired under the same precautions as with a current of same volume and potential for lighting. The motor and resistance box must be protected by a double-pole cutout, and controlled by a double-pole switch, except in cases where one-quarter horse-power or less is used. ‘The leads or branch circuits should be designed to carry a current at least fifty per cent greater than that required by the rated capacity of the motor to provide for the inevitable overloading of the motor at times. 6. Must be thoroughly insulated. Where possible, should be set on base frames made from filled, hard, dry wood, and raised above surrounding deck. On hoists and winches they shall be insulated from bed-plates by hard rubber, fiber, or similar insulating material. c. Shall be covered with a waterproof cover when not in use. ad. Must each be provided with a name-plate giving maker’s name, the capacity in volts and amperes, and the normal speed in revolutions per minute. GENERAL SUGGESTIONS. In all electric work conductors, however well insulated, should always be treated as bare, to the end that under no conditions, existing or likely to exist, can a grounding or short circuit occur, and so that all leakage from conductor to conductor, or between conductor and ground, may be reduced to the minimum, : In all wiring special attention must be paid to the mechanical execution of the work. Careful and neat running, connecting, soldering, taping of conduc- tors and securing and attaching of fittings, are specially conducive to security and efficiency, and will be strongly insisted on. In laying out an installation, except for constant-current systems, the work should, if possible, be started from a center of distribution, and the switches and cutouts, controlling and connected with the several branches, be grouped together in a safe and easily accessible place, where they can be readily got at for attention or repairs. The load should be divided as evenly as possible among the branches, and all complicated and unnecessary wiring avoided. The use of wire-ways for rendering concealed wiring permanently access- ible is most heartily indorsed and recommended ; and this method of accessible concealed construction is advised for general use. Architects are urged, when drawing plans and specifications, to make pro- vision for the channeling and pocketing of buildings for electric light or power wires, and in specifications for electric gas lighting to require a two-wire circuit, whether the building is to be wired for electric lighting or not, so that no part of the gas fixtures or gas-piping be allowed to be used for the gas- lighting circuit. HN ed ed in D) ic INE REPORT OF THE COMMITTEE ON STANDARDIZATION. [Accepted by the INSTITUTE, June 26, 1899.] To the Council of The AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS. Gentlemen: Your committee on Standardization begs to submit the following report, covering such subjects as have been deemed of pressing and immediate impor- tance, and which are of such a nature that general agreement may be expected upon them. While it is the opinion of the committee that many other matters might advantageously have been considered, as, for example, standard methods of testing: yet it has been deemed inexpedient to attempt to cover in a single report more than is here submitted. Yours respectfully, FRANCIS B. CROCKER, Chairman. CARY T. HUTCHINSON. A. E. KENNELLY. JOHN W. LIEB, Jr. CHARLES P. STEINMETZ. LEWIS B. STILLWELL. ELIHU THOMSON. GENERAL PLAN, Efficiency. Sections 1 to 24 (I) Commutating Machines, Sections 6 to 11 (11) Synchronous Machines, - 10 to 11 (111) Synchronous Commutating Machines, : 12 to 15 (IV) Rectifying Machines, a 16 to 17 (V) Stationary Induction Apparatus, fh 18 to 19 (VI) Rotary Induction Apparatus, m 20 to 23 (VII) Transmission Lines, : 24 Rise of Temperature. Sections 25 to 31. Insulation. Sections 32 to 41. Regulation. Sections 42 to 61. Variation and Pulsation. Sections 62 to 65, Rating. Sections 66 to 73. Classification of Voltages and Frequencies. Sections 74 to 78. Overload Capacities. Sections 79 to 82. Appendices. (1) Efficiency. (II) Apparent Efficiency. (III) Power Factor and Inductance Factor. (IV) Notation. (V) Table of Sparking Distances. 486 APPENDIX I. 487 Electrical Apparatus will be treated under the following heads : — I. Commutating Machines, which comprise a constant magnetic field, a closed-coil armature, and a multi-segmental commutator connected thereto. Under this head may be classed the following: Direct-current generators ; direct-current motors ; direct-current boosters; motor-generators ; dynamotors; converters and closed-coil arc machines. A booster is a machine inserted in series in a circuit to change its voltage, and may be driven either by an electric motor, or otherwise. In the former case it is a motor-booster. A motor-generator is a transforming device consisting of two machines; a motor and a generator, mechanically connected together. A dynamotor is a transforming device combining both motor and gene- rator action in one magnetic field, with two armatures or with an armature having two separate windings. For converters, see III. II. Synchronous Machines, which comprise a constant magnetic field, and an armature receiving or delivering alternating currents in synchronism with the motion of the machine; z. ¢., having a frequency equal to the product of the number of pairs of poles and the speed of the machine in revolutions per second. III. Synchronous Commutating Machines: — These include: 1. Synchron- ous converters: i.e., converters from alternating to direct, or from direct to alternating current, and 2. Double-current generators; i.e., generators pro- ducing both direct and and alternating currents. A converter is a rotary device transforming electric energy from one form into another without passing it through the intermediary form of mechanical energy. A converter may be either: a. A direct-current converter, converting from a direct current to a direct current or 6. A synchronous converter, formerly called a rotary converter, convert- ing from an alternating to a direct current, or vice versa. Phase converters are converters from an alternating-current system to an alternating-current system of the same frequency but different phase. Frequency converters are converters from an alternating-current system of one frequency to an alternating-current system of another frequency, with or without changes of phase. IV. Rectifying Machines, or Pulsating-Current-Generators, which produce a unidirectional current of periodically varying strength. V. Stationary Induction Apparatus: i.e., stationary apparatus changing electric energy from one form into another, without passing it through an intermediary form of energy. These comprise: a. Transformers, or stationary induction apparatus in which the primary and secondary windings are electrically insulated from each other. 6. Auto-transformers, formerly called compensators: i.e., stationary induc- tion apparatus in which part of the primary winding is used as a secondary winding, or conversely. c. Potential regulators, or stationary induction apparatus having a coil 488 ELECTRIC THICHPING. in shunt, and a coil in series with the circuit, so arranged that the ratio of transformation between them is variable at will. These may be divided into: — 1. Compensator potential-regulators, in which the number of turns of one of the coils is changed. 2. Induction potential-regulators, in which the relative positions of pri- mary and secondary coils is changed. 5. Magneto potential-regulators, in which the direction of the magnetic flux with respect to the coils is changed. ad. Reactive coils, or reactance coils, formerly called choking coils: i.e., stationary induction apparatus. used to produce impedance or phase dis- placement. VI. Rotary Induction Apparatus, which consists of primary and secondary windings rotating with respect to each other, They comprise: — a. Induction motors. 6. Induction generators. c. Frequency changers. d@. Rotary phase converters. EFFICIENCY. 1. The “efficiency ” of an apparatus is the ratio of its net power output to its gross power input.* _2. Electric power should be measured at the terminals of the apparatus. 3. In determining the efficiency of alternating-current apparatus, the elec- tric power should be measured when the current is in phase with the Z.AZF., unless otherwise specified, except when a definite phase difference is inherent in the apparatus, as in induction motors, etc. 4. Mechanical power in machines should be measured at the pulley, gearing, coupling, etc., thus excluding the loss of power in said pulley, gear- ing, or coupling, but including the bearing friction and windage. The magni- tude of bearing friction and windage may be considered as independent of the load. The loss of power in the belt and the increase of bearing friction due to belt tension, should be excluded. Where, however, a machine is mounted upon the shaft of a prime mover, in such a manner that it cannot be separated therefrom, the frictional losses in bearings and in windage, which ought, by definition, to be included in determining the efficiency, should be excluded, owing to the practical impossibility of determining them satisfac- torily. The brush friction, however, should be included. a. Where a machine has auxiliary apparatus, such as an exciter, the power lost in the auxiliary apparatus shou'd not be charged to the machine, but to the plant consisting of machine and auxiliary apparatus taken together, The plant efficiency in such cases should be distinguished from the machine efficiency. 5. The efficiency may be determined by measuring all the losses individ- ually and adding their sum to the output to derive the input, or subtract- ing their sum from the input to derive the output. All losses should be measured at, or reduced to, the temperature assumed in continuous operation, or in operation under conditions specified. (See Sections 25 to 31.) * An exception should be noted in the case of storage batteries or apparatus for storing energy, in which the efficiency, unless otherwise qualified, should be understood as the ratio of the energy output to the energy intake in a normal cycle. APPENDIX Il. 489 In order to consider the application of the foregoing rules to various machines in general use, the latter may be conveniently divided into classes as follows : — I. Commutating Machines. — 6. In commutating machines the losses are: — a. Bearing friction and windage. (See Section 4.) 6. Molecular magnetic friction, and eddy currents in iron and copper. These losses should be determined with the machine on open circuit, and at a voltage equal to the rated voltaze + 7y in a generator, and — /y in a motor, where / denotes the current strength, and 7 denotes the internal resistance of ‘the machine. They should be measured at the correct speed and voltage, since they do not usually vary in proportion to the speed or to any definite power of the voltage. c. Armature resistance losses, /? 7’, where / is the current strength in the armature, and 7’ is the resistance between armature brushes, excluding the resistance of brushes and brush contacts. @. Commutator brush friction. e. Commutator brush-coatact resistance. It is desirable to point out that with carbon brushes the losses (@) and (é) are usually considerable in low- voltage machines. fy Field excitation. With separately excited fields, the loss of power in the resistance of the field coils alone should be considered. With shunt fields or series fields, however, the loss of power in the accompanying rheostat should also be included, the said rheostat being considered as an essential part of the machine, and not as separate auxiliary apparatus. (0) and (c) are losses in the armature or ‘‘armature losses ;” (@) and (e) “commutator losses ;” (f) ‘field losses.” 7. The difference between the total losses under load and the sum of the losses above specified, should be considered as ‘“‘load losses” and are usu- ally trivial in commutating machines of small field distortion. When the field distortion is large, as is shown by the necessity for shifting the brushes between no load and full load, or with variations of load, these load losses may be considerable, and should be taken into account. In this case the efficiency may be determined either by input and output measurements, or the load losses may be estimated by the method of Section II. 8. Boosters should be considered and treated like other direct-current machines in regard to losses. 9. In motor-generators, dynamotors, or converters, the efficiency is the electric output electric input. Il. Synchronous Machines. — 10. In synchronous machines the output or input should be measured with the current tn phase with the terminal “.47./., except when otherwise expressly specified. Owing to the uncertainty necessarily involved in the approximation of load losses, it is preferable, whenever possible, to determine the efficiency of synchronous machines, by input and output tests. 11. The losses in synchronous machines are: a. Bearing friction and windage. (See Section 4.) 490 FLECTRIC LIGHTING. - 6. Molecular magnetic friction and eddy currents in iron, copper, and other metallic parts. These losses should be determined at open circuit of the machine at the rated speed and at the rated voltage, + 7/7 in a syn- chronous generator, —/7 in a synchronous motor, where / = current in arma- ture, 7 = armature resistance. It is undesirable to compute these losses from observations made at other speeds or voltages. These losses may be determined either by driving the machine by a motor, or by running it as a synchronous motor, and adjusting its fields so as to get minimum current input and measuring the input by wattmeter. The former is the preferable method, and in polyphase machines the latter method is liable to give erroneous results in consequence of unequal distribution of currents in the different circuits caused by inequalities of the impedance of connecting leads, etc. c. Armature-resistance loss, which may be expressed by # /?7; where vy =resistance of one armature circuit or branch, 7/=the current in such armature circuit or branch, and =the number of armature circuits or branches. ad. Load losses as defined in Section 7. While these losses cannot well be determined individually, they may be considerable and, therefore, their joint influence should be determined by observation. This can be done by operating the machine on short circuit and at full-load current, that is, by de- termining what may be called the ‘short-circuit core loss.’”’ With the low field intensity and great lag of current existing in this case, the load losses are usually greatly exaggerated. One-third of the short-circuit core loss may, as an approximation, and in the absence of more accurate information, be assumed as the load loss. e. Collector-ring friction and contact resistance. These are generally negligible, except in machines of extremely low voltage. fj. Field excitation. In separately-excited machines, the 7? 7 of the field coils proper should be used. In self-exciting machines, however, the loss in the field rheostat should be included. (See Section 6/.) III. Synchronous Commutating Machines. — 12. In synchronous converters, the power of the alternating-current side is to be measured with the current in phase with the terminal &.A7./., unless otherwise specified. | 13. In double-current generators, the efficiency of the machine should be determined as a direct-current generator in accordance with Section 6, and as an alternating-current generator in accordance with Section 11. The two values of efficiency may be different, and should be clearly distinguished. 14. In synchronous converters the losses should be determined when driving the machine by a motor. These losses are: — a. Bearing friction and windage. (See Section 4.) ce. Molecular magnetic friction and eddy currents in. iron, copper, and metallic parts. These losses should be determined at open circuit and at the rated terminal voltage, no allowance being made for the armature resistance, since the alternating and the direct currents flow in opposite directions. c. Armature resistance. The loss in the armature is g /27, where 7/= direct current in armature, » = armature resistance, and g a factor which is equal to 1.37 in single-phasers, 0.56 in three-phasers, 0.87 in quarter-phasers and 0.26 in six-phasers. APPENDIX II. 491 ad. Load losses. The load losses should be determined in the same manner as described in Section 11 d@., with reference to the direct-current side. e and f. Losses in commutator and collector friction and brush-contact resistances, (pee oections 6 and 11)) g. Field excitation. In separately-excited fields, the 7? 7 loss in the field coils proper should be taken, while in shunt and series fields the rheostat loss should be included, except where fields and rheostats are intentionally modified to produce effects outside of the conversion of electric power, as for producing phase displacement for voltage control. In this case 25 per cent of the 7/27 loss in the field proper at non-inductive alternating circuit should be added as proper estimated allowance for normal rheostat losses. (See Section 6/.) 15. Where two similar synchronous machines are available, their efficiency can be determined by operating one machine as a converter from direct to alternating, and the other as a converter from alternating to direct, connecting the alternating sides together, and measuring the difference between the direct- current input and the direct-current output. This process may be modified by returning the output of the second machine through two boosters into the first machine and measuring the losses. Another modification might be to supply the losses by an alternator between the two machines, using potential regu- lators. IV. Rectifying Machines or Pulsating-Current Generators. — 16. These include: Open-coil arc machines, constant-current rectifiers, constant-potential rectifiers. The losses in open-coil arc machines are essentially the same as in Sec- tions 6 to 9 (closed-coil commutating machines.) In alternating-current recti- fiers, however, the output must be measured by wattmeter and not by volt- meter and ammeter, since, owing to the pulsation of current and #.47.F.,a considerable discrepancy may exist between watts and volt amperes, amount- ing to as much as 10 or 15 per cent. 17. In constant-current rectifiers, transforming from constant-potential alternating to constant direct current by means of constant-current trans- formers and rectifying commutators, the losses in the transformers are to be included in the efficiency, and have to be measured when operating the recti- fier, since in this case the losses are generally greater than when feeding an alternating secondary circuit. In constant-current transformers the load losses are usually larger than in constant-potential transformers, and thus should not be neglected. The most satisfactory method of determining the efficiency in rectifiers is to measure electric input and electric output by wattmeter. The input is usually not non-inductive, owing to a considerable phase displacement and to wave distortion. For this reason the apparent efficiency should also be con- sidered, since it is usually much lower than the true efficiency. The power consumed by the synchronous motor or other source driving the rectifier sbould be included in the electric input. 4 V. Stationary Induction Apparatus. — 18. Since the efficiency of induction apparatus depends upon the wave shape of -.1/./., it should be referred to a sine wave of £.17.F., except 492 ELACTRIGC. LIGHHING expressly specified otherwise. The efficiency should be measured with non- inductive load, and at rated frequency, except where expressly specified other- wise. The losses are: a. Molecular magnetic friction and eddy currents measured at open cir- cuit and at rated voltage — /7, where /=rated current, y= resistance of primary circuit. 6. Resistance losses, the sum of the 727 of primary and of secondary in a transformer, or of the two sections of the coil in the compensator or auto- transformer, where / = current in the coil or section of coil, 7 = resistance. c. Load losses, i.e., eddy currents in the iron and especially in the copper conductors, caused by the current. They should be measured by short-cir- cuiting the secondary of the transformer and impressing upon the primary an £.M F., sufficient to send full-load current through the transformer. The loss in the transformer under these conditions measured by wattmeter gives the load losses + 7? 7 losses in both primary and secondary coils. d. Losses due to the methods of cooling, as power consumed by the blower in air-blast transformers, and power consumed by the motor driving pumps in oil or water cooled transformers. Where the same cooling appara- tus supplies a number of transformers, or is installed to supply future addi- tions, allowance should be made therefor. 19. In potential regulators the efficiency should be taken at the maximum voltage for which the apparatus is designed, and with non-inductive load, unless otherwise specified. VI. Rotary Induction Apparatus. — 20. Owing to the existence of load losses and since the magnetic density in the induction motor under load changes in a complex manner, the efficiency should be determined by measuring the electric input by wattmeter and the mechanical output at the pulley, gear, coupling, etc. 21. The efficiency should be determined at the rated frequency and the input measured with sine waves of impressed £.4Z.F. 22. The efficiency may be calculated from the apparent input, the power factor, and the power output. The same applies to induction generators. Since phase displacement is inherent in induction. machines, their apparent efficiency is also important. 23. In frequency changers; i.e., apparatus transforming from a polyphase system to an alternating system of different frequency, with or without a change in the number of phases, and phase converters; i.e., apparatus convert- ing from an alternating system, usually single phase, to another alternating sys- tem, usually polyphase, of the same frequency, the efficiency should also be determined by measuring both output and input. o~ VII. Transmission Lines. — 24. The efficiency of transmission lines should be measured with non- inductive load at the receiving end, with the rated receiving pressure and fre- quency, also with sinusoidal impressed /.//. /.’s., except where expressly specified otherwise, and with the exclusion of transformers or other apparatus at the ends of the line. APPENDIX II. 493 RISE, OF. TEMPERATURE, General Principles. — 25. Under regular service conditions, the temperature of electrical machinery should never be allowed to remain at a point at which permanent deterioration of its insulating material takes place. 26. The rise of temperature should be referred to the standard conditions of a room-temperature of 25° C., a barometric pressure of 760 mm. and normal conditions of ventilation; that is, the apparatus under test should neither be exposed to draught nor inclosed, except where expressly specified. 27. If the room temperature during the test differs from 25° C., the ob- served rise of temperature should be corrected by = per cent for each degree C.* Thus with a room temperature of 35° C., the observed rise of temperature has to be decreased by 4 per cent, and with a room temperature of 15° C., the observed rise of temperature has to be increased by 5 per cent. The ther- mometer indicating the room temperature should be screened from thermal radiation emitted by heated bodies, or from draughts of air. When it is im- practicable to secure normal conditions of ventilation on account of an adja- cent engine, or other sources of heat, the thermometer for measuring the air temperature should be placed so as fairly to indicate the temperature which the machine would have if it were idle, in order that the rise of temperature determined shall be that caused by the operation of the machine. 28. The temperature should be measured after a run of sufficient duration to reach practical constancy. This is usually from 6 to 18 hours, according to the size and construction of the apparatus. It is permissible, however, to shorten the time of the test by running a lesser time on an overload in current and voltage, then reducing the load to normal, and maintaining it thus until the temperature has become constant. In apparatus intended for intermittent service, as railway motors, starting rheostats, etc., the rise of temperature should be measured after a shorter time, depending upon the nature of the service, and should be specified. In apparatus which by the nature of their service may be exposed to over- load, as railway converters, and in very high voltage circuits, a smaller rise of temperature should be specified than in apparatus not liable to overloads or in low voltage apparatus. In apparatus built for conditions of limited space, as railway motors, a higher rise of temperature must be allowed. 29. In electrical conductors, the rise of temperature should be determined by their increase of resistance. For this purpose the resistance may be meas- ured either by galvanometer test, or by drop-of-potential method. A tem- perature coefficient of 0.4 per cent per degree C. may be assumed for copper.t Temperature elevations measured in this way are usually in excess of tem- perature elevations measured by thermometers. 30. It is recommended that the following maximum values of temperature elevation should not be exceeded: * This correction is also intended to compensate, as nearly as is at present practicable, for the error involved in the assumption of a constant temperature coefficient of resistivity ;i.e., o.4 per cent degree C. taken with varying initial temperatures. + By the formula R=, (1 +0.0049). Where Bs is the resistance at room temperature, A, the resistance when heated, and @ the temperature elevation (7 —7) in degrees centigrade. 494 BLRCTRICWIGHTIVG. Commutating machines, rectifying machines, and synchronous machines. Field and armature, by resistance, 50° C, Commutator and collector rings and brushes, by thermometer, 55° C, Bearings and other parts of machine, by thermometer, 40° C, Rotary induction apparatus: Electric circuits, 50° C., by resistance. Bearings and other parts of the machine, 40° C., by thermometer. In squirrel-cage or short-circuited armatures, 55° C., by thermometer, may be allowed. Transformers for continuous service — electric circuits by resistance, 50° C., other parts by thermometer, 40° C., under conditions of normal ventilation. Reactive coils, induction and magneto regulators -— electric circuits by resistance, 55° C., other parts by thermometer, 45° C. Where a thermometer, applied to a coil or winding, indicates a higher temperature elevation than that shown by resistance measurement, the ther- mometer indication should be accepted. In using the thermometer, care should be taken so to protect its bulb as to prevent radiation from it, and, at the same time, not to interfere seriously with the normal radiation from the part to which it is applied. 31. In the case of apparatus intended for intermittent service, the tem- perature elevation which is attained at the end of the period corresponding to the term of full load should not exceed -50° C., by resistance in electric cir- cuits. In the case of transformers intended for intermittent service, or not operating continuously at full load, but continuously in circuit, as in the ordi- nary case of lighting transformers, the temperature elevation above the sur- rounding air-temperature should not exceed 50°C. by resistance in electric circuits, and 40° C. by thermometer in other parts, after the period correspond- ing to the term of full load. In this instance, the test load should not be applied until the transformer has been in circuit for a sufficient time to attain the temperature elevation due to core loss. With transformers for commercial lighting, the duration of the full-load test may be taken as three hours, unless otherwise specified. In the case of railway, crane, and elevator motors, the conditions of service are necessarily so varied that no specific period cor- responding to the full-load term can be stated. INSULATION: 32. The ohmic resistance of the insulation is of secondary importance only, as compared with the dielectric strength, or resistance to rupture by high voltage. . Since the ohmic resistance of the insulation can be very greatly increased by baking, but the dielectric strength is liable to be weakened thereby, it is preferable to specify a high dielectric strength rather than a high insulation resistance. The high-voltage test for dielectric strength should always be applied. Insulation Resistance. — 33. Insulation resistance tests should, if possible, be made at the pressure for which the apparatus is designed. The insulation resistance of the complete apparatus must be such that the rated voltage of the apparatus will not send more than t0d5000 of the full-load APPENDIX II. 495 current, at the rated terminal voltage, through the insulation. Where the value found in this way exceeds | megohm, 1 megohm is sufficient. Dielectric Strength. — 34. The dielectric strength or resistance to rupture should be determined by a continued application of an alternating -£.A7./. for one minute. The source of alternating 4.4/./. should be a transformer of such size that the charging current of the apparatus as a condenser does not exceed 25 per cent of the rated capacity of the transformer. 35. The high-voltage tests should not be applied when the insulation is low, owing to dirt or moisture, and should be applied before the machine is put into commercial service. 36. It should be pointed out that tests at high voltages considerably in excess of the normal voltages are admissible on new machines, to determine whether they fulfill their specifications, but should not be made subsequently at a voltage much exceeding the normal, as the actual insulation of the machine may be weakened by such tests. 37. The test for dielectric strength should be made with the completely assembled apparatus and not with its individual parts; and the voltage should be applied as follows: — Ist. Between electric circuits and surrounding conducting material; and, 2d. Between adjacent electric circuits, where such exist, as in trans- formers. The tests should be made. with a sine wave of £./47./,, or where this is not available, at a voltage giving the same striking distance between needle points in air as a sine wave of the specified 4.4/./., except where expressly specified otherwise. As needles, new sewing-needles should be used. It is recommended to shunt the apparatus during the test by a spark gap of needle points set for a voltage exceeding the required voltage by 10 per cent. A table of approximate sparking distances is given in Appendix V. 38. The following voltages are recommended for apparatus, not including transmission lines or switchboards: Rated Terminal Voltage. Capacity Testing Voltage. Not exceedine 400 volts. 25 ae. a Undenl0- AS = 1000: volts: ee ¥ & oe ee OK eh and lovers: 1oUQ) 400 and over, but less than 800 volts. Under1l0 4%. W. . 1500 «* - se bs 1075. Wand over 2000° 800 “ « 1200 « wee Pa 1 e500". 2% 1200 ~ Pe OUDT MeV fee etan te rete UOQ » 9% 2500 De Any § Double the normal Z500 i me les Ce -y rated voltages. Synchronous motor fields and fields of converters started fromthe, alternatine current side.) san =. 5000 volts. Alternator field circuits should be tested att a Be ndanh test voltage corresponding to the rated voltage of the exciter, and referred to an output equal to the output of the alternator; i.e., the exciter should be rated for this test as having an output equal to that of the machine it excites. Condensers should be tested at twice their rated voltage and at their rated frequency. The values in the table above are effective values, or square roots of mean square reduced to a sine wave of £.A7.F. 496 ELECTRIC LIGHTING. 39. In testing insulation between different electric circuits, as between primary and secondary of transformers, the testing voltage must be chosen corresponding to the high-voltage circuit. 4). In transformers of from 10,000 volts to 20,000 volts, it should be con- sidered as sufficient to operate the transformer at twice its rated voltage, by connecting first the one, and then the other terminal of the high-voltage wind- ing to the core and to the low-voltage winding. The test of dielectric resist ance between the low-voltage winding and the core should be in accordance with the recommendation in Section 38 for similar voltages and capacities. 41. When machines or apparatus are to be operated in series, so as to employ the sum of their separate /.J/.F.’s, the voltage should be referred to this sum, except where the frames of the machine are separately insulated both from ground and from each other. REGULATION, 42. The term “regulation” should have the same meaning as the term “inherent regulation,” at present frequently used. 43. The regulation of an apparatus intended for the generation of con- stant potential, constant current, constant speed, etc., is to be measured by the maximum variation of potential, current, speed, etc., occurring within the range from full load to no load, under such constant conditions of operation as give the required full-load values, the condition of full load being considered in all cases as the normal condition of operation. 44, The regulation of an apparatus intended for the generation of a poten- tial, current, speed, etc., varying in a definite manner between full load and no load, is to be measured by the maximum variation of potential current, speed, etc., from the satisfied condition, under such constant conditions of operation as give the required full-load values. If the manner in which the variation in potential, current, speed, etc., between full load and no load, is not specified, it should be assumed to be a simple linear relation, i.e., undergoing uniform variation between full load and no load. The regulation of an apparatus may, therefore, differ according to its qualification for use. Thus the regulation of a compound-wound generator specified as a constant-potential generator will be different from that it possesses when specified as an over-compounded generator. 45. The regulation is given in percentage of the full-load value of potential, current, speed, etc.; and the apparatus should be steadily operated during the test under the same conditions as at full load. 46. The regulation of generators is to be determined at constant speed ; of alternating apparatus at constant impressed frequency. 47. The regulation of a generator-unit, consisting of a generator united with a prime-mover, should be determined at constant conditions of the prime-mover; i.e., constant steam pressure, head, etc. It would include the inherent speed variations of the prime-mover. For this reason the regulation of a generator-unit is to be distinguished from the regulation of either the prime-mover, or of the generator contained in it, when taken separately. 48. In apparatus generating, transforming, or transmitting alternating cur- APPENDIX II. 497 rents, regulation should be understood to refer to non-inductive load ; that is, to a load in which the current is in phase with the “.J47./. at the output side of the apparatus, except where expressly specified otherwise. 49, In alternating apparatus receiving electric power, regulation should refer to a sine wave of /#.iW./., except where expressly specified otherwise. 50. In commutating machines, rectifying machines, and synchronous ma- chines, as ‘direct-current generators and motors, alternating-current and poly- phase generators, the regulation is to be determined under the following conditions: a. At constant excitation in separately excited fields , 6. With constant resistance in shunt-field circuits ; and .c. With constant resistances hunting series fields; i.e., the field adjust- ment should remain constant, and should be so chosen as to give the required full-load voltage at full-load current. 51. In constant potential machines, the regulation is the ratio of the maximum difference of terminal voltage from the rated full-load value (occur- ring within the range from full load to open circuit) to the full-load terminal voltage. 52. In constant-current machines, the regulation is the ratio of the maxi- mum difference of current from the rated full-load value (occurring within the range from full load to short circuit) to the full-load current. 53. In constant-power machines, the regulation is the ratio of maximum difference of power from the rated full-load value (occurring within the range of operation specified) to the rated power. 54. In over-compounded machines, the regulation is the ratio of the maxi- mum difference in voltage from a straight line connecting the no-load and full- load values of terminal voltage as function of the current to the full-load terminal voltage. 55. In constant-speed continuous-current motors, the regulation is the ratio of the maximum variation of speed from its full-load value (occurring within the range from full load to no load) to the full-load speed. 56. In transformers, the regulation is the ratio of the rise of secondary terminal voltage from full load to no load (at constant primary impressed terminal voltage) to the secondary terminal voltage. 57. In induction motors, the regulation is the ratio of the rise of speed from full load to no load (at constant impressed voltage), to the full-load ‘speed. The regulation of an induction motor is, therefore, not identical with the slip of the motor, which is the ratio of the drop in speed from synchronism to the synchronous speed. 58. In converters, dynamotors, motor generators, and frequency changers, ~ the regulation is the ratio of the maximum difference of terminal voltage at the output side from the rated full-load voltage (at constant impressed voltage and at constant frequency) to the full-load voltage on the output side. 59. In transmission lines, feeders, etc., the regulation is the ratio of maxi- mum voltage difference at the receiving end, between no-load and full non- inductive load, to the full-load voltage at the receiving end, with constant voltage impressed upon the sending end. 60. In steam engines, the regulation is the ratio of the maximum varia- tion of speed in passing from full load to no load (at constant steam pressure at the throttle) to the full-load speed. 498 ELECTRICATMIGHIING 61, In a turbine or other water motor, the regulation is the ratio of the maximum variation of speed from full load to no load (at constant head of water ; i.e., at constant difference of level between tail race and head race) to the full-load speed. Variation and Pulsation. — 62. In prime-movers which do not give an absolutely uniform rate of rota- tion or speed, as in steam engines, the “ variation” is the maximum angular displacement in position of the revolving member expressed in degrees, from the position it would occupy with uniform rotation, and with one revolution as 360°; and the pulsation is the ratio of the maximum change of speed in an engine cycle to the average speed. 63. In alternators or alternating-current circuits in general, the variation is the maximum difference in phase of the generated wave of 4.4/7.7. from a wave of absolutely constant frequency, expressed in degrees, and is due to: the variation of the prime-mover. The pulsation is the ratio of the maximum change of frequency during an engine cycle to the average frequency. 64. If 2 = number of poles, the variation of an alternator is s times the Te : sete n : i variation of its prime-mover if direct-connected, and 3 p times the variation of the prime-mover if rigidly connected thereto in the velocity ratio p. 65. The pulsation of an alternating-current circuit is the same as the pul- sation of the prime-mover of its alternator. RATING. 66. Both electrical and mechanical power should be expressed in kilo- watts, except when otherwise specified. Alternating-current apparatus should be rated in kilowatts on the basis of non-inductive condition; i.e., with the current in phase with the terminal voltage. 67. Thus the electric power generated by an alternating-current apparatus equals its rating only at a non-inductive load; that is, when the current is in phase with the terminal voltage. 68. Apparent power should be expressed in kilovolt-amperes as distin- guished from real power in kilowatts. 69, If a power-factor other than 10 per cent is specified, the rating should be expressed in kilovolt-amperes and power-factor, at full load. 70. The full-load current of an electric generator is that current which with the rated full-load terminal voltage gives the rated kilowatts, but in alter- nating-current apparatus only at non-inductive load. 71. Thus in machines in which the full-load voltage differs from the no- load voltage, the full-load current should refer to the former. If P=rating of an electric-generator and / = full-load terminal voltage, the full-load current is: D) Y ss E in a continuous-current machine or single-phase alternator. a4 if ee in a quarter-phase alternator. 72. Constant-current machines, such as series arc-light generators, should be rated in kilowatts based on terminal volts and amperes at full load. in a three-phase alternator. APPENDIX II. 499 73. The rating of a fuse or circuit breaker should be the current strength at which it will open the circuit, and not the working-current strength. Classification of Voltages and Frequencies. — 74. In direct-current, low-tension generators, the following average termi- nal voltages are in general use and are recommended : 125 volts. 250 volts. 550 volts. 75. In direct-current, and alternating-current, low-pressure circuits, the following average terminal voltages are in general use and are recommended: 110 volts. 220 volts. In direct-current power circuits, for railway and other service, 500 volts may be considered as standard. 76. In alternating-current, high-pressure circuits at the receiving énd, the following pressures are in general use, and are recommended : 1000 volts. 2000 volts. 3000 volts. 6000 volts. 10000 volts. 15000 volts. 20000 volts. 77. In alternating-current, high-pressure generators or generating systems the following terminal voltages are in general use, and are recommended : 1150 volts. 2300 volts. 3450 volts. These pressures allow of a maximum drop in transmission of 15 per cent of the pressure at the receiving end. If the drop required is greater than 15 per cent, the generator should be considered as special. 78. In alternating-current circuits, the following approximate frequencies are recommended as desirable: 25 —~ or 30 ~ 40 ~ 60~ 120 ~ (*) These frequencies are already in extensive use, and it is deemed advisable to adhere to them as closely as possible. Overload Capacities. — 79. All guaranties on heating, regulation, sparking, etc., should apply to the rated load, except where expressly specified otherwise, and in alternating- current apparatus to the current in phase with the terminal £.A7.F., except where a phase displacement is inherent in the apparatus. 80. All apparatus should be able to carry a reasonable overload without self- destruction by heating, sparking, mechanical weakness, etc., and with an increase of temperature elevation not exceeding 15° C. above those specified for full loads. (See Sections 25 to 81.) 81. Overload guaranties should refer to normal conditions of operation regarding speed, frequency, voltage, etc., and to non-inductive conditions in alternating apparatus, except where a phase displacement is inherent in the apparatus. 82. The following overload capacities are recommended : 1st. In direct-current generators and alternating-current generators: 25 per cent for one-half hour. 2d. In direct-current motors and synchronous motors: 25 per cent for one-half hour, 50 per cent for one minute; except in railway motors and other apparatus intended for intermittent service. * The frequency of 120 “ may be considered as covering the already existing commercial fre- quencies between 120 ~ and 140 ™, and the frequency of 60 ~ as covering the already existing com- mercial frequencies between 60 ~ and ™ 70. 500 ELECTRICALIGHLIING 3d. Induction motors: 25 per cent for one-half hour, 50 per cent for one minute. 4th. Synchronous converters: 50 per cent for one-half hour. 5th. Transformers: 25 per cent for one-half hour; except in trans- formers connected to apparatus for which a different overload is guaranteed, in which case the same guaranties shall apply for the transformers as for the apparatus connected thereto. 6th. Exciters of alternators and other synchronous machines, 10 per cent more overload than is required for the excitation of the synchronous machine at its guaranteed overload and for the same period of time. APPEN DIAS: EP PEC UE IN Gye Efficiency of Phase-Displacing Apparatus. — In apparatus producing phase displacement, as, for example, synchro- nous compensators, exciters of induction generators, reactive coils, condensers, olarization cells, etc., the efficiency should be understood to be the ratio of the volt-ampere activity to the volt-ampere activity plus power loss. The efficiency may be calculated by determining the losses individually, adding to them the volt-ampere activity, and then dividing the volt-ampere activity by the sum. 1st. In synchronous compensators and exciters of induction generators, the determination of losses is the same as in other synchronous machines under Sections 10 and 11. 2d. In reactive coils the losses are molecular friction, eddy losses, and f?r loss. They should be measured by wattmeter. The efficiency of reactive coils should be determined with a sine wave of impressed £./47./., except where expressly specified otherwise. 3d. In condensers, the losses are due to dielectric hysteresis and leakage, and should be determined by wattmeter with a sine wave of 4.A47.F. 4th, In polarization cells, the losses are those due to electric resistivity and a loss in the electrolyte of the nature of chemical hysteresis, and are usually very considerable. They depend upon the frequency, voltage, and tempera- ture, and should be determined with a sine wave of impressed £.1/7.F., except where expressly specified otherwise. AEE NDT ate Apparent Efficiency. — In apparatus in which a phase displacement is inherent to their operation, apparent efficiency should be understood as the ratio of net power output to volt-ampere input. Such apparatus comprise induction motors, reactive synchronous con- verters, synchronous converters controlling the voltage of an alternating-cur- rent system, self-exciting synchronous motors, potential regulators, and open magnetic circuit transformers, etc. Since the apparent efficiency of apparatus generating electric power depends upon the power factor of the load, the apparent efficiency, unless otherwise specified, should be referred to a load power-factor of unity. APPENDIX I. 501 APRPENDI Xa LIs Power Factor and Inductance Factor. — The power factor in alternating circuits or apparatus may be defined as the ratio of the electric power, in watts, to volt-amperes. The inductance factor is to be considered as the ratio of wattless volt- amperes to total volt-amperes. Thus, if p = power factor, g = inductance factor, sige bs Seal (energy component of current or £.47.F-.) (total current or £.AZ.F. The power factor is the and the inductance factor is the (wattless component of current or &.47./. __ true power. (total current or £.4Z.F,, ~ volt amperes. Since the power-factor of apparatus supplying electric power depends upon the power-factor of the load, the power-factor of the load should be con- sidered as unity, unless otherwise specified. APPENDIX. LV. The following notation is recommended : — E, é, voltage, £.47.F., potential difference. . / 7p Current. P, power. @, magnetic flux. ®, magnetic density. Ae, v7 resistance, XG areactance: Z, #, impedance. ZL, é, inductance. Cy cycapacity- Vector quantities when used should be denoted by capital italics. APE ENDL Xa. Table of Sparking Distances in Air between Opposed Sharp Needle- Points, for Various Effective Sinusoidal Voltages, in inches and in centi- meters. DISTANCE. DISTANCE KILOVOLTS KILOVOLTS SQ. RooT oF SQ. Root or MEAN MEAN SQUARE. KR Be Os] OU COUN 0.2 0.4 0.7 1.0 1.3 1.6 2.0 2.4 2. 3. O1 60 5 & CAAA WORMS SONS HS WOOO asso SEEN S800 Or cooos = H-RRRACHR ee SUSTOH AGI LATCO Wt. ne Ge es Ageing of Transformer Iron . , Alternating Currents . .. . Fundamental Waves. . . . Harmonic Waves. ... .- Cncuits, Caleulationof. a... Powercot fieags) es Systems of Dewibution® A 6 Weave, Hormofer = sss cp Aluminum asa Conductor . . . rAngleiotiar kom, eihcs ts (ols Are; the Wlectric< % Gal See. so See On Mains, Cates ien a Pee era aL tes! Induction, Self and Mutual. . .. . 115 OmENiets Works a. ie) ese eee eee LOS Insulation . Pri Pe Bria Maree rte Be, Oho’ S09 Drawing-injof Conductors 27.) 2 eee eos Inferor Conduit ree een tcn cme mae Dynamotor 93 Wiring Wee ee 374 Dynamo, Conpound LBS Iron Losses in Transformers 5 154 Double 74 Joints in Cables . 285: Economy in Conductors . 10, 87, 226 in Line Wires °° #3 253 Edison Tube System 1 2id Eddy Current Losses . Wee 156 Kelyinis ba. W 2s ese ieas emt enon ere re Efficiency of Transformers 159, 161 Kennedyis;System™., .0> 3) 3 2. Se ne ecu: All Day, Table . . 163 [Knobmand: Dube Wirlng, 47. . ect t Effective Value of A. C. Volts aid eee . 114 Exciting Current in Transformers , . 125 Lag of Current). - 0. 24. @ se 8s 117 Lag Due to Inductance 117 Farad . . 1205 GeadiofiCurrent aes ues 121 Feeders . Lee ee ee es 884 | Leakage Current in Transformers . 158 Régulation.) Gaus hoes Y nol 198. | Leakage, Magnetic in Transformers . . . . 150 Flashing Filaments . 397 | Lightning Arresters . . . . 176 Filaments of Incandescent laine . 395 , Anchored . 407 | Magnetizing Currents in Transformers 158 Resistance of 410 | Magnetic Leakage in ‘Transformers 150: Sizes of . « . 411 Mains, Calculation of . ea 38 Fished Wiring . 381 Mainsiand apse. | iene) ieee neee ian: . 385 Five-wire Systems 86 Manholes . « 286 Fixtures for Lamps . c : 392 Matthiessen’s Standard oA Conductivity A ese Oe Flux Densities in Transformer Cc ores 157 Mershon Compensator . . he 199 Frequency Changer . 218 Mesh Connection. See Delta. Frequency . 111 Metersigemecpae site 760) ire el ae ae emcnaie 432-451 Choice of . : 224 Motor-Converter 93. Foucault Current Losses . ; 156 Motor-Dynamo . cae 93 Fuse Blocks for Transformers . . . . . - 179 Motor-Dynamos as Boosters 99 ASuGOMpPensatOrs we Eee men 99 Grounded Shield for Transformers 177 Motor,’tnduction ..% 4. '. ew lan Grounding the neutral of 3-wire system Synchronous 147 84 and App. T Transformer . 93 of Transformers 177 and App. I Municipal Lighting Grants! Sotelo Guard Wires and Hooks . 249 Mutual Induction 115, 134 Guying Pole Lines . - 246 Inductance of Cireuits : . 131 Harmonics 1mA- C. Waves i Waked Conductor Underground Systems . 279 Fenty ; a Networks of Conductors . . 103 High Voltage D. 3 ‘Distribution . a Current and Drop in . . 103 Holophane Globe es Desighoe a, pa Hysteresis Loss in Transformers ; me Neutral Conductor, Gaoindine ASS 84 and App. I TImpedancer as bate 131 of Parallel Circuits : 126 Over-Compound dynamos, regulation by 53. Due to Resistance and iddttanes : 116 Overhead Conductors. . + + + * . 237 of Series Circuits 126 Insulators for . 248 Incandescent Lamps 395 Materials for . .. + : ot Incandescent Lamp Bases and Beene : 403 PolesiOnecu).) e-Mas wes eeeeromeel ure . a Globes 399 SeieeeiNoliodacee Mg of i OF ty C ose on Arc Circuits 24 Specifications for. « 4 % 2): )*.% * 237 Light Distribution 407 er Target Diagram : oot ay eee eae Danel Board as ©.” topes see ceine iaatee Voltage, Candle- Power Efficiency anda Parallel Systems of Distribution . . . 28 Life 413 Regulation Gf 24: tows: Gu sue = teen INDEX. 505 PAGE PAGE Period and Periodicity, Alternating Current . 111 Switches, Multi-Control. . . Cee OU L Pipe’ Conduits: o apes teak ae ees es ee OG Synchronous Polyphase Motors . . .. . 147 Pole Lines . rer 243-257 Polyphase Carrents\)?s 1. oa . 141 | Tapering Conductors .. . - es SL RMAtora eet eel Leen te ok. ag Target Diagram for Incandesceut eye cere Systems of Transmission . . . . . . 200 Temperature Coefficient of Copper . ... 6 Regulation of) va. uct pate ae 204 WDlaveSS jor WINING Go 6 in go 6 65 & AE Transtormers) oo mee ETSA Whree-phases Currents ieee om outs nce La Potential Regulator, Indbetion Type gol Fon LOE Three-wire and Five-wire Systems 10-92 Transtormiers* <. wee poy | SIRS Three-wire System, Auxiliary Generator . . 78 Power Factors... ee 1 90) Bridgvetarrangvement of (ae) ys en (4: Power, Real] and eee of Ny C. Aad naw) with Three-brush Dynamo .... . 76 Preservation*of Timber. 2. se 244 of A.C. distribution. . . . . . . . 191 Projectionglcanter nie cimtedins sen fee eed kei on SOD Transfer Bus bars . . - . . «1 + + + 65 Properties of Conductors . . Ae Rist, OMe Transformers: . 44% 3 + “2 « es, 5 149 Protective Devices for ‘Transformers ee eel T4 Agemg of Iron in. «4 ee + 5» 150 ie Deivyebniclency Oty amma ied sere nen tl Oo Reactanve; EoOMeF el ays... ee tod Gonstantieurm crite sic umm leon one onl @onstructionroleuieunnmes ince meme LOO IDuetOLGapaciivar-ma sm. a Ml san sane eran 20) Wuestominductanceyes ss.) ie eee ene cO Rectifiers baat ase hn ei wast Wee ete ee eed Regulation in Constant Potential, A. C. SVStCMS Mn mee oulh Ge rete eek Parallelusystems "2 vn. .) cue chen en enrol Polyphasersystems =) mites eet cue eevee et Rotary Dranstormersie reece eee eee OD Series! S VStemIS) aetna te ce Sam Aber: 5 pea a oe A 6) bes} Regulator, Automatic Voltage. . . .. . 57 Resistance . . . a | opmitatare ho Method of pede, Reruiacan bees. ver .03 Standardi@iasiiy 5. casusm ebro) eh terial isis Resonance . . pm oet fo aPlZS Return-loop System ‘of Distribubon eo ee OO Ran oe Mains aes ee Cee Rey = abe tst oo Rotary Converters: favee soa ole Reel oe tO Rotary Field! \ au. © see ssn ay LAD Rotary Transformers and Revulaion of ROO Mya oui se diy, 5, mame kode ee reaarevie. cy LSU, Safety Device for Transformers ... . 378 Sag and Stress in Overhead Wires . . . . 241 Search) Lamps) 0 J : ile (hte oe OOD. Self Inductance, Minar és Reding Ag) geley Otjlnesyand Gircultsuens wou, aes Palo Series Incandescent Lamps . 24, 25, 422 Series-Parallel Incandescent Systems . . . 26 Semmes oystem sArG Lishtnow em. wee ce se od Danger from . . Se aE ae, OU Distribution of Potential ODM aaa. alo Regulation Of 75% cablsbese 6 wene wt sag, FOL AltematmeosCurrentm game amet y tem eones) eG Ol DISEEIDU TON Nan ie aet aemure ined omer crs ALS Service Connections or Loops. . .. . . 255 Seven-wire Systems .. sts Single-phase Parallel "en tovner Sens eo Skin ect 5 acon eae cio ctw Seal el oO Slips sae ys) ee emew a ie beak cc eer ALS. Sockets ees ht ele em ars, 6 OOD Split -Insulationysc.ursn: so eee eee saa eee StoravesBatteriesgs swyse ‘ Ps . » 7 y. o} ~ % ‘ yur f . ' i a . ne By ye 2 ‘ 5 aA ie taal ‘. ry 5a wr af wo, : , 4 7 7 ’ «' . . * ‘ > “ a? , F * } ‘ : ed ” S . 1 » | ris ri we 3 J - ri. & r -

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