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TRANSACTIONS 
 
 OF THE 
 
 NEW YORK ELECTRICAL SOCIETY 
 
 MODERN TELEPHONE ENGINEERING 
 
 Lecture delivered before the New York Electrical Society 
 February J4, 1901 
 
 BY 
 
 KEMPSTER B. /MILLER 
 
 NUMBER SIX 
 
 PUBLISHED BY THE 
 
 NEW YORK ELECTRICAL SOCIETY 
 
 J20 Liberty Street, New York 
 
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MODERN TELEPHONE ENGINEERING 
 
 BY 
 
 KEMPSTER, B. MILLER. 
 
 Mr. President and gentlemen of the New York Electrical 
 Society : 
 
 The subject which you have assigned to me, "Modern 
 Telephone Engineering," is one of such breadth that only 
 portions of it can be treated in a single evening, and those 
 portions not with as great an amount of detail as I would 
 desire. 
 
 After careful consideration, I have concluded that the 
 subject matter which is most likely to interest the greatest 
 number of those present is a discussion of a few of the en- 
 gineering problems arising in planning a telephone ex- 
 change, and also a description of some of the methods and 
 means employed in what may be considered modern tele- 
 phone practice. Time will not permit me to deal with the 
 various items in detail, and therefore these remarks should 
 be regarded in many cases rather as suggestions than as 
 complete treatments of the various points. 
 
 The statements which I will make in the course of the 
 evening will relate to what I believe to be good telephone 
 practice, regardless of whether it is the practice of the Bell 
 or of the independent companies. The circuits shown are 
 those of the Bell Company, and the apparatus mostly that of 
 the Kellogg Switchboard & Supply Company, with which 
 company I am connected. I trust that I may be pardoned 
 for appearing partial to the apparatus of this latter com- 
 pany ; but, in the first place, my brethren under the wing 
 of the Bell Company have not exhibited an undue amount 
 of enthusiasm in the matter of furnishing me with cuts and 
 data thereon. In the second place, I have been closely 
 identified with the design and the manufacture of the 
 Kellogg apparatus and am therefore better able to talk 
 of it. In the third place, I think that most of the tele- 
 phone engineers here are perhaps as familiar, if not more 
 so than I am, with the apparatus of the Bell Company, and 
 therefore will be more interested in that of another make. 
 Be this as it may, whatever methods, circuits and apparatus 
 are described will be typical of the telephone art as it exists 
 to-day. 
 
 The first problem arising when a telephone exchange is 
 to be installed in a large city is that of the number, size and 
 location of the central office or offices. The proper solution 
 of this problem depends in all cases upon local conditions 
 and can be attained only after much consideration and cal- 
 culation. 
 
 If the cheapest were the best, the telephone engineer 
 would experience comparatively little difficulty in exactly 
 determining the location of the telephone office or offices, as 
 well as the boundaries of the office districts in any city, after 
 having canvassed the communities and ascertained the 
 topographical location of the present and prospective sub- 
 
 scribers. The most important consideration is that of the 
 quality of service to be rendered. This factor, "quality of 
 service," can not be expressed in dollars and cents ; it must 
 be left to the good judgment of the engineer to so construct 
 the telephone plant that the demands made by the public 
 are satisfied without making the plant so expensive as to be 
 an unprofitable investment. 
 
 Other matters to be considered in the preliminary lay- 
 out of a system are: 
 
 Annual cost of operation, including salaries of operators 
 and the cost of current supply ; fixed charges and rent, 
 interest and depreciation on the office equipment, interest 
 and depreciation on the wire plant, interest and deprecia- 
 tion on the subscribers' apparatus, and annual cost of main- 
 tenance. These factors, unlike the first mentioned, are 
 subject to predetermination with a fair degree of exactness. 
 A brief discussion of each of these various factors as to 
 their bearing on the number and location of offices will 
 probably be of interest. 
 
 I think it is a well established fact that the best telephone 
 service can be given where all the lines of the subscribers in 
 a city run to a single exchange and are there handled by a 
 single multiple switchboard. By this means all calls for 
 connections by any subscriber may be handled by a single 
 operator. Where the number of subscribers, however, is 
 too great to be connected to a single switchboard, or where 
 the distribution is such as to naturally group them about 
 well defined and remote centres, a greater number than one 
 exchange must be installed and lines provided between 
 them for connecting the subscribers in the different ex- 
 changes. Such lines are called trunking lines, in distinc- 
 tion to subscribers' lines, which connect the subscriber with 
 the central office. 
 
 In most cases where trunking is necessary, most calls have 
 to be trunked. I believe the percentage of trunked calls to 
 total calls for Greater New York is in the neighborhood of 
 eighty. A connection made over a trunk line necessarily 
 occupies more time and thereby renders the service a little 
 slower. In fact, the aggregate time required for connection 
 over a trunk line is practically twice as great as where ac- 
 complished by a single operator in a multiple board. The 
 fact that two operators necessarily handle a trunk con- 
 nection necessarily increases the liability to error in mak- 
 ing the connection, and therefore produces less satisfactory 
 service. The trunking apparatus and circuits, in order not 
 only to transmit speech, but to convey the requisite signals 
 between the operators, are necessarily more complex than 
 subscribers' line circuits, and thereby a greater liability 
 to trouble exists than where no trunks are required. 
 So far, therefore, as the question of quality of service is 
 
 ' A Lecture delivered before the New York Electrical Society, February 14, 1901, 
 
MODERN TELEPHONE ENGINEERING. 
 
 concerned, there can be no question but that a single large 
 office serving all of the subscribers in a given city is best. 
 
 Again, if we consider only operating expense, a single 
 office serving the entire city is best. First, because in a 
 single office the total number of operators required is a 
 minimum (it has: been amply .proven by experience that 
 this is so); second; in a truhking ;' system the number of 
 higher paid person-; t'K.iio'jvr.itoi.'s will be greatly increased, 
 as there will be for instance, a manager, a wire chief, a chief 
 operator, etc., for each of the different offices; third, the 
 item of current supply is naturally higher with several 
 offices than with a single large one, as the power system 
 requires more current on account of the greater amount of 
 apparatus and also on account of the machinery being split 
 up into smaller units and working with less efficiency. 
 
 The item of fixed charges and rent includes the salaries 
 paid to the general management, the business and engineer- 
 ing offices, as well as rent. The former items should not 
 differ materially whether one office or a greater number 
 is used. The question of rent, however, is a little more 
 complex and must be accurately determined for each par- 
 ticular city. It will vary from 50 cents to $2 per annum for 
 every square foot of floor space. Where a single office is 
 used this is necessarily located somewhere in the center of 
 the city where rent is highest. Where a number of offices 
 are used, the offices will not be in the center of the city, but 
 will of necessity be at points densely populated. The 
 probabilities are that the rental, which will also include 
 .light, heat, elevator service and janitor service, etc., will be 
 slightly cheaper for a large office located in the heart of the 
 city than for several smaller offices located in the most 
 densely populated portion of their respective districts. 
 
 The interest and depreciation on the office equipment is 
 in most cases smaller for the single office system, for while 
 it is true that a single large multiple board costs more than 
 a number of smaller multiple boards having the same ag- 
 gregate capacity as the large one, yet the addition of the 
 trunking apparatus to the smaller boards, and the fact that 
 the power plants, managers, wire chief's, monitor's and 
 trouble clerk's desks and similar apparatus are duplicated 
 at each exchange, will probably throw the balance in favor 
 of the single large exchange. This matter, however, would 
 need to be determined for each set of conditions, as it can 
 not be said in all cases the office equipment for a single 
 exchange would be cheaper than that of a greater number 
 of exchanges furnishing service to the same number of 
 subscribers. 
 
 All the factors so far considered , that is, the quality of 
 service, the cost of operation, fixed charges and rent, and 
 depreciation on the office equipment, seem to lean in favor 
 of the single office, some of them decidedly so, others not 
 so strongly. Against these must be put, in some cases, the 
 interest and depreciation on the wire plant and the item of 
 annual cost of maintenance. 
 
 By wire plant is understood everything pertaining to the 
 telephone plant, from the subscriber's premises to that point 
 in the telephone office, or offices, where the lines enter the 
 terminal heads. It therefore embraces the underground 
 conduits and cables, the pole lines carrying cables or bare 
 lines, together with all necessary apparatus such as man- 
 holes, drop wires, etc. 
 
 It is easy to see that the average length of subscribers' 
 lines will be greatest when a single office is used, and will 
 steadily decrease as the number of offices is increased. It 
 
 has been pointed out by Mr. F. J. Dommerque that in a 
 given community the average length of subscribers' lines 
 varies inversely as the square root of the number of offices. 
 In other words, if the average length of subscribers' lines 
 with one office is L, then the average length of subscribers' 
 
 lines with N offices is approximately L' = 
 
 I N 
 
 This 
 
 law is not strictly true, but gives a close approximation. 
 Mr. Dommerque, in planning the re-equipment of the sys- 
 tem of the Chicago Telephone Company on an ultimate 
 basis of fifty thousand (50,000) subscribers, found that 
 the average length of line for the city of Chicago, if 
 equipped with one exchange, would be twenty thousand 
 (20,000) feet. From this figure and from the law of varia- 
 tion just stated, the curve shown in Fig. I has been 
 plotted, giving the average length of subscribers' line for 
 any greater number of offices up to sixty. It was found 
 that the actual measurements of subscribers' lines for the 
 various proposed layouts for the city of Chicago closely 
 coincided with the figures of this curve, so closely in fact 
 that such a curve was found to be a practically reliable 
 source of information for any number of offices. A simi- 
 lar curve might be plotted for trunk lines and order wires 
 connecting such offices, which curve would, of course, 
 
 t 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 56 60 
 NUMBER OP OFFICES IS THE EXCHANGE 
 
 FIG. i. SUBSCRIBERS' LINE CURVE. 
 
 start with a zero ordinate for one office and rapidly rise 
 with the number of offices. 
 
 It is interesting to conceive these curves, when carried to 
 their limits ; that is, when there were 50,000 central offices 
 for 50,000 subscribers. The curve of subscribers' lines 
 would then have a zero ordinate, while the curve for the 
 trunk lines would have a maximum ordinate which would 
 be equal in feet to the distance from one subscriber's sta- 
 tion, through all of the other subscribers' stations. This 
 means that there would be no subscribers' lines whatever, 
 but that from each subscriber's station there would be a 
 trunk line corresponding to every one of the 49,999 other 
 stations, as these trunk lines would extend through all sta- 
 tions. We have an actual example of such a system as 
 this in the ordinary house telephone system, where every 
 subscriber has his own central office and where every 
 subscriber's line extends to every other subscriber. 
 
 If there is but one exchange in the city, the proper loca- 
 tion for it may be found by marking upon the map all of 
 the subscribers present and prospective and then determin- 
 ing the center of gravity of the system, so to speak. This 
 may readily be done by drawing a straight line through 
 the map in such a manner as to have one-half of all the sub- 
 scribers on one side of it and the other half on the other. 
 
MODERN TELEPHONE ENGINEERING. 
 
 In a similar manner another line may be drawn at right 
 angles to the first, bisecting the total number of subscribers, 
 and the intersection of these two lines will probably be 
 fairly close to the proper place for the central office. 
 
 A remote group of subscribers, not cut by these two lines, 
 tvould tend to draw the true center of distribution from the 
 point so located toward it. In order to avoid errors from 
 this source, a number of other pairs of bisecting lines should 
 be drawn, the lines of each pair being at right angles. 
 Three such pairs of co-ordinates, 30 degs. apart, would 
 locate three centers, probably close together, and the center 
 of the triangles formed by these centers is a very close ap- 
 proximation to the center of distribution of all the sub- 
 scribers. 
 
 With this location determined, it is an easy matter to de- 
 termine the total average distance of the subscribers from 
 it, and therefore the total wire-mileage of the subscribers' 
 lines. With the average subscribers' distances as the base, 
 a curve such as just shown, may be plotted, which will 
 show in a close enough degree of approximation what the 
 average length of subscribers' line would be for anv other 
 number of central offices. 
 
 Of course, as the number of offices increases, the wire- 
 mileage of the trunk lines and order wires increases and 
 figures concerning this must be worked out in consideration 
 of the locations of the various offices and the amount of 
 traffic between them. This latter factor, the traffic between 
 offices, varies greatly according to the kind of districts con- 
 nected, and it is therefore almost impossible to give general 
 figures as to the mileage of trunk lines and order wires. It 
 may be said, however, that the statistics of the large cities 
 of this country and Europe show that the trunk and order 
 circuit wire-mileage is approximately one-fifth of that of 
 the subscribers' wire-mileage. It is evident that this figure 
 can be used as a basis for laying out new exchanges, 
 for, by the very nature of things, there can be no definite 
 ratio between the two. 
 
 If it is found necessary for any reason to divide the sys- 
 tem up into office districts, a great amount of care must be 
 taken as to the proper boundaries for these districts. Of 
 course, rivers, water fronts and railroads will frequently 
 aid in the determination of these boundaries. There is fre- 
 quently a tendency toward the use of large business streets 
 and thoroughfares as boundary lines between districts, but 
 investigation will show that in many cases this is not good 
 practice, as such streets or thoroughfares should be the 
 centers of distribution rather than the boundaries of them. 
 Diagonal thoroughfares between offices should be used as 
 trunk line routes, rather than boundaries of districts, owing 
 to their directness and to the fact that a large number of 
 subscribers are usually located in their vicinities. 
 
 Having decided on the boundaries of the districts, their 
 respective centers may be determined, and between these 
 the most available routes for trunk lines determined. 
 
 In figuring the cost of the wire plant for various layouts 
 of a city, it will not do to assume the cost to be in direct pro- 
 portion to the wire-mileage required by the systems using 
 one office, or a greater number, for there is a factor enter- 
 ing the problem which greatly favors the single office or, I 
 might say, greatly favors the fewer number of offices. This 
 factor is due to the fact that with the single office system, or 
 with the system nsing few offices, a greater amount, in 
 proportion, if large conduit or cable may be used, and 
 therefore the cost per conduit foot and per mile will be con- 
 
 siderably smaller than where a greater number of offices 
 are used. 
 
 In general, it may be said that the interest and deprecia- 
 tion on the wire plant decreases as the number of offices 
 increases, and this is the main, and, in fact, practically the 
 only item to offset the advantages of the fewer number 
 of offices or the single office. 
 
 The interest and depreciation on the subscribers' ap- 
 paratus would be the same, whether one or many offices 
 were used. 
 
 A limitation to the size of switchboards has heretofore 
 necessitated a greater sub-division of offices in very large 
 cities, than any of the considerations so far pointed out 
 would warrant. 
 
 Until recently, it has been the policy of the Bell Tele- 
 phone Company to establish no central offices having 
 switchboard capacity for over 6000 lines. This limit was 
 maintained because of the fact that it has not been practi- 
 cable to build multiple switchboards having a larger capa- 
 city than that. This limit in the capacity of the board has 
 been due entirely to mechanical, rather than electrical con- 
 ditions, and on account of it, it has been impracticable to 
 build such exchanges as those of New York and Chicago 
 with a single office, if other conditions had warranted it. 
 
 There has recently developed, however, a strong tend- 
 ency on the part of the Bell and the Independent com- 
 panies to build central offices with a much larger equipment 
 than 6000. The Cuyahoga Telephone Company, of Cleve- 
 land, is now equipped with a multiple board having 
 a capacity of nine thousand six hundred (9600) lines in- 
 stalled, and with an ultimate capacity of about twenty 
 thousand (20,000). The board at Paris, France, equipped 
 by the Western Electric Company, has a present capacity 
 of something like nine thousand three hundred (9300) 
 lines, and the Kinloch Telephone Company, of St. Louis, 
 Mo., has a present equipment of eight thousand eight hun- 
 dred (8800) lines. I am informed that a number of 
 exchanges operated by the Bell Company are now 
 being planned or built, having a capacity of nine thousand 
 (9000) lines and upward. 
 
 I believe fully that the future systems in large cities will 
 be operated with fewer and fewer central offices and con- 
 sequently with larger switchboards. This belief is rendered 
 tenable by the present tendency of operating companies 
 and also recent improvements in switchboards by which 
 the mechanical limitation of six thousand (6000) lines is 
 removed and by which as many as twenty-five thousand 
 (25,000) lines can be handled in a single straight, multiple 
 switchboard and even as great a number as one hundred 
 thousand ( 100,000) lines by the use of systems now shown 
 to be practicable for a smaller number. 
 
 The fundamental object of the multiple switchboard is 
 to place within reach of every operator a line terminal or 
 spring jack, as it is called, for every subscriber's line enter- 
 ing the exchange. As is well known, the switchboard is 
 divided into sections, each section being of such size as to 
 allow an operator, without undue exertion, to reach over 
 its entire surface. On each section are placed a certain 
 number of line signals, by which the subscriber is enabled 
 to attract the attention of the operator and a corresponding 
 number of line jacks, by means of which the operator may 
 make the initial connection with that line in response to 
 such signals. These jacks, which are associated with the 
 line signals, are termed answering jacks. In addition to the 
 
MODERN TELEPHONE ENGINEERING. 
 
 line signals and answering jacks are provided what are 
 termed multiple jacks, there being one of these on each 
 section for every line in the exchange. To express this in 
 a little different way ; the various lines entering the central 
 office are divided up into suitable groups, these groups 
 terminating in line signals and answering jacks at the vari- 
 ous sections of the multiple switchboard. In addition to 
 this, all the lines entering an exchange are carried to every 
 section of an exchange, each line terminating in every sec- 
 tion in a multiple jack. An operator seated at any section, 
 in response to a display of one of her line signals, inserts 
 one plug of a pair into the corresponding jack,, and, having 
 ascertained the number of the subscriber desired, inserts 
 a corresponding plug of the pair into the multiple jack of 
 the subscriber called for. As every subscriber's line has a 
 multiple jack at every section of the switchboard, any 
 operator is able to complete by herself any connection 
 called for at her board. 
 
 The size of the section is limited by the reach of the 
 operator, and it is this fact that places a mechanical limit 
 to the size of an ordinary multiple switchboard. It has 
 been found impracticable in most cases to place the jacks 
 closer than on half-inch centers, and under these conditions 
 no more than six thousand (6000) of the multiple jacks 
 could be placed, in addition to the other apparatus, within 
 the space afforded by one section. 
 
 Two distinct methods have been followed with the intent 
 of removing this limit. Each has been productive of excel- 
 lent results ; each has been used separately, but up to the 
 present time they have not both been incorporated into the 
 same installation. The first of these methods is an obvious 
 one and contemplates the reduction in the size of the spring 
 jacks with a view to getting more of them within the reach 
 of the operator. The mechanical difficulties in this line 
 have been great, owing to the fact that the circuits of the 
 switchboard were so complicated as to require three, four, 
 and sometimes even five contacts to each spring jack. The 
 Western Electric Company in the Paris board succeeded in 
 reducing the dimensions of the jack to three-eighths of an 
 inch (-in.) centers, both horizontally and vertically, and 
 thus rendered practicable the installation of a switchboard 
 having a capacity for considerably over nine thousand 
 (9000) lines. I believe that there are other boards which 
 have been installed with success having this size of jack, 
 and that others are at present in course of construction for 
 large cities in this country. This jack had three contacts, 
 and I believe is the smallest that has ever been put into ex- 
 tensive use. 
 
 Quite recently the company with which I am connected 
 has perfected a system of circuits for a multiple switch- 
 board so simplified that only two contact points are required 
 for each jack ; that is, merely a tip spring and sleeve, these 
 two contacts always standing open and never engaging 
 each other. By the use of this two-wire system, it is per- 
 fectly practicable to build the jacks on -in. centers, hori- 
 zontally and vertically, and thus place within the reach of 
 a single operator no fewer than 25,000 lines. 
 
 The second method of increasing the possible size of 
 multiple switchboards was devised by Mr. M. G. Kellogg, 
 and is now working in two large exchanges in this country. 
 In this system, which is known as the divided multiple sys- 
 tem, the switchboard and lines are divided each into four 
 divisions. The switchboard consists of four multiple 
 boards, all in the same office, and to these is connected one 
 
 of the groups of lines, in the same manner as if they were 
 in separate exchanges. With this arrangement, if no further 
 provisions were made, it would be possible for any sub- 
 scriber in one division to obtain a connection with any 
 other subscriber in his own division, but not with any one 
 in any of the other three divisions. In order to enable any 
 subscriber in, say, the A division to obtain a connection 
 with a subscriber in the B, C or D division, means are pro- 
 vided, not for trunking between the divisions, but for en- 
 abling the calling subscriber to signal directly to one of 
 the operators in the division in which the line of the called- 
 for subscriber belongs. This necessitates the use of four 
 line signals and four answering jacks for each line, cne of 
 each in each division, and of means whereby the subscriber 
 may display any one of the signals to the exclusion of the 
 others. Instead of trunking between exchanges, therefore, 
 the subscriber sends his call directly into the division of 
 switchboards to which the desired subscriber belongs. 
 
 By the use of this divided multiple switchboard it is seen 
 that the ultimate capacity of any exchange is increased ap- 
 proximately fourfold; not quite this, however, because 
 the extra line signals and answering jacks take up a portion 
 of the room which would otherwise be available for the 
 multiple jacks. 
 
 The significance of what I have said concerning the two 
 methods of increasing the size of multiple switchboard will 
 now be apparent. If the four division exchange were in- 
 stalled on -in. centers, the capacity of such an exchange 
 would be something over 90,000 lines operating without 
 giving the operator as great a reach as they have at present 
 in some multiple switchboards, and between the subscribers 
 of this mammoth exchange" there would be no trunking 
 whatever. 
 
 You may say that the divided multiple system introduces 
 complications in the circuits, which is true ! You may also 
 say that operators will find difficulty in handling the jacks 
 on ^-in. centers, which may or may not be true ! But with 
 the demand for telephone service growing with an ever 
 increasing rate as at present, who will say that the 100,000- 
 l:ne switchboard will not be required in the near future, 
 and who at present can conceive of any other way of hand- 
 ling it? 
 
 During the past five years a complete revolution has been 
 worked in the circuits and apparatus of telephone ex- 
 changes, due to the introduction of the systems known un- 
 der such names as "common battery," "central battery'' and 
 ''central energy." These systems, when properly equipped, 
 have proven vastly superior from both an economic and 
 operating standpoint to the old magneto systems, wherein 
 local batteries were used for talking and magneto gen- 
 erators for making the calls. The local primary battery at 
 the subscribers' station has been superseded by a large 
 storage battery at the central office, which also takes the 
 place of the old magneto generator. The mechanical signal 
 or drop has given place to the lamp signal. By these inno- 
 vations the apparatus at the subscribers' station nas been 
 rendered simpler and more compact, that at the central 
 office more reliable, and the operation of the entire system 
 has been rendered automatic as far as the subscriber is 
 concerned, and as much so as desirable as far as the op- 
 erator is concerned. Such a system employs batteries at 
 the central office for furnishing all current, for signaling by 
 the subscriber and for the operation of the talking ap- 
 paratus. These batteries are invariably secondary, rather 
 
MODERN TELEPHONE ENGINEERING. 
 
 than primary, for large exchanges, and operate almost uni- 
 versally at the voltage given by ten cells in series, i. e., 20 
 to 24 volts, according to state of charge. 
 
 For charging these batteries and for furnishing current 
 for ringing the subscribers' bells complete power plants 
 are required, these consisting of suitable generating units, 
 driven cither electrically or mechanically, together with 
 suitable controlling apparatus. Most companies depend 
 
 
 KIG. 2. ClkCU 
 
 upon electrical power, the charging and ringing machines 
 being motor generators running directly from city mains. 
 
 It may be imagined that the failure of any source of 
 energy in a large telephone system is a matter of most seri- 
 ous moment. The cutting off of one branch of the current 
 supply would cause consternation in the entire business 
 organization of a community. In order to prevent any such 
 breakdown, all portions of the power plant are made in 
 duplicate, and where it is possible duplicate primary sources 
 of power are also made available. Frequently the large 
 telephone companies employ their own steam generating 
 plant, which they use constantly, with the current from 
 city mains as a reserve supply. 
 
 The determination of the size of the storage batteries is 
 the primary problem in the engineering work in connection 
 with the telephone power plant, as, having once determined 
 it, the size of the charging machines and the details of the 
 other apparatus are readily ascertained. We know in a 
 given city approximately the number of subscribers to be 
 provided for in the present installation, and we make a 
 guess at what the future number will be. Past experiences 
 have proven that these guesses are almost always too small, 
 for the growth of telephone service in the United States 
 has far exceeded expectations. We also know by statistics 
 from other cities approximately the number of calls each 
 subscriber will make per day. This figure ranges from 
 about eight calls per day in smaller and less active cities to 
 something like thirty calls per day in our largest business 
 centers. Experience has also taught us that the average 
 length of time from the time a subscriber takes his tele- 
 phone off his hook in making the call to the time the connec- 
 tion is completed with the subscriber called for will vary 
 not far from six seconds, if the service is prompt ; and that 
 the average length of conversation is somewhat less than 
 two minutes. Knowing these figures and the average 
 length, and therefore the average resistance of the various 
 subscribers' lines, and of the connecting apparatus, the 
 number of ampere-hours required for the subscribers' cir- 
 cuits is readily determined. To this must be added the 
 number of ampere-hours required for the operators' trans- 
 mitter circuits, the supervisory lamp circuits, and any other 
 circuits that may exist. The whole may be reduced to a 
 
 certain number of hundredths of ampere-hours per con- 
 nection, a most convenient unit. 
 
 Knowing the total number of ampere-hours required of 
 the storage battery in a day, a size of battery should be se- 
 lected having that capacity in ampere-hours, and also one 
 having a discharge rate in amperes equal to one-sixth of the 
 total ampere-hours required. Experience has shown that 
 the maximum discharge rate is equal to about one-sixth of 
 the ampere-hours required per day. 
 
 The circuits of a telephone exchange are of such a com- 
 plex nature as to defy adequate explanation in the short 
 time available. 
 
 In the modern common battery exchange the operation 
 is such that when a subscriber removes his receiver from 
 the hook, a lamp will be automatically lighted in front of 
 the operator at the central office, who handles the calls of 
 the group of subscribers to which this particular subscriber 
 belongs. The operator in response to this signal makes 
 connection with the subscriber's line by inserting a plug 
 into the spring jack of that line, and after having ascer- 
 tained his wants, completes the connection with the sub- 
 scriber called for by means of a second plug attached to 
 the first. As soon as a connection is thus made with a sub- 
 scriber's line, the line lamp normally in control of the sub- 
 scriber is disassociated from the line, but another lamp asso- 
 ciated with the plug is placed under the control of the sub- 
 scriber, this lamp being known as a supervisory lamp. 
 
 In Fig. 2 is shown a complete line circuit in diagram, in- 
 cluding the apparatus at the subscriber's station, which is 
 shown at the left, and the apparatus at the central office, 
 which is shown at the right. The apparatus at the sub- 
 
 FIG. 3. CIRCUIT. 
 
 scriber's station consists of an induction coil, a receiver, 
 a granular carbon transmitter, a two micro-farad con- 
 denser, a looo-ohm bell and an automatic hook-switch 
 mounted in a suitable manner. It will be noticed from 
 the diagram that the circuit between the two sides of 
 the line is held open to direct currents by the condenser, 
 and to all currents by the hook-switch contact, while the 
 receiver is on the hook. This leaves the bell in operative 
 relation to the line, for the purpose of receiving calls ; the 
 path of the ringing current, which current is, of course, 
 alternating, being through one side of the line, through the 
 bell and condenser to the other side of the line. When the 
 receiver is removed from its hook, the circuit between the 
 two sides of the line is closed, allowing the passage of 
 direct current through the secondary of the induction coil 
 and the transmitter in series. At the same time another cir- 
 cuit containing the transmitter, the receiver, the primary of 
 the induction coil and the condenser is closed. The ob- 
 
MODERN TELEPHONE ENGINEERING. 
 
 ject of this latter circuit, which is local to the subscriber's 
 station, will be pointed out later. 
 
 At the central office, in addition to the answering jack 
 and the multiple jacks, there are two relays, one termed 
 the line relay, which is normally in circuit across the line 
 in series with the 24-volt storage battery common to all 
 
 tip and sleeve strands of the calling plug are connected by 
 the two halves of the other winding of the repeating coil 
 between which is connected to the same battery. 
 
 This repeating coil, and there is one of them for each pair 
 of cords and plugs, has four windings, each having ap- 
 proximately 3300 turns, all of these windings being on the 
 
 FIG. 4. CIRCUIT. 
 
 lines, and the other termed the cut-off relay, which is in an 
 auxiliary circuit, and is adapted to open the circuit through 
 the line relay and the battery as soon as a plug is inserted 
 into any jack belonging to its line. Under normal condi- 
 tions, both relays remain unoperated and the conditions are 
 as shown. As soon, however, as the subscriber removes 
 his receiver from its hook, the circuit between the two 
 sides of the line is made complete at the subscriber's station 
 and the current at the central office energizes the relay and 
 doses the circuit containing the line lamp and the battery. 
 This lamp is located in immediate proximity to the answer- 
 ing jack, and the operator, seeing it illuminated, inserts a 
 plug into the answering jack in response to the call. This 
 plug is one of many pairs within the reach of the operator 
 and has three contacts, two of which, the tip and sleeve, are 
 adapted to engage the two springs of the answering jack, 
 and thus complete the talking circuit. The third is adapted 
 to engage the ring contact of the jack, and being in connec- 
 tion with the live side of the battery, causes a current to 
 pass from this side of the battery through the ring contact 
 of the jack and the coil of the cut-off relay back to the 
 other side of the battery. The energization of the cut-off 
 relay serves to open the circuit of the line relay, as already 
 described, and this, when operated, extinguishes the lamp. 
 One of the cord circuits is now shown in Fig. 3. In this 
 the answering and calling plugs are shown at the extreme 
 left and right-hand portions, respectively, each having, as 
 beforesaid, three contacts, two of which, the tip and sleeve, 
 are connected by wires shown in heavy lines, which form 
 part of the talking circuit, and the third of which is con- 
 nected through the supervisory signaling apparatus to the 
 live side of the battery, as shown. The tip and sleeve sides 
 of the answering plug are connected together through the 
 two halves of one side of a repeating coil, between which 
 is connected the common battery. In the same manner the 
 
 same magnetic core. It is evident that these connections 
 allow direct current from the storage battery to pass out 
 over the metallic circuits of the two connected lines to ener- 
 gize the transmitters. A fluctuation in the current flowing 
 in either line will, however, by induction between the wind- 
 ings of the repeating coil, cause a corresponding fluctuation 
 ir, the current of the other line. In this way conversation 
 between two connected lines is made possible, it being by 
 induction rather than conduction. 
 
 t INC * CORD 
 
 FIG. 5. CIRCUIT. 
 
 In the sleeve strand of the cord of each plug is connected 
 a supervisory relay, which is energized, as is readily seen, 
 only while current is flowing over the line to which the 
 corresponding plug is connected, i. c., while the receiver 
 of that line is off its hook. These relays control the circuit 
 of supervisory lamps, by the illumination or darkness of 
 which the operator is enabled to ascertain the condition of 
 the connected lines. On the right of this figure is shown 
 
MODERN TELEPHONE ENGINEERING. 
 
 the ringing and listening keys. The former is a device for 
 switching the alternating current generator into the circuit 
 of the calling plug and its connected line ; the latter con- 
 nects the operator's head telephone, which is shown in the 
 center of the diagram, across the two sides of the cord, in 
 order to enable the operator to communicate with either 
 subscriber. 
 
 The operation of this circuit and apparatus can best 
 be understood by referring to Fig. 4, which shows two 
 subscribers' lines equipped as already described, connected 
 by a cord circuit at the central office. Consider the line 
 at the left to be that of the calling subscriber and that at 
 the right the called. In the first place, the removing of the 
 calling subscriber's receiver from its hook illuminates the 
 line lamp, which remains lighted until the operator answers 
 by the insertion of the answering plug into the answering 
 jack. The operator, before inserting the calling plug into 
 the jack of the called subscriber, must ascertain whether 
 or not the line of that subscriber is already busy. This is 
 automatically accomplished, for upon the touching of the 
 
 lighted when one or both of the subscribers hang up their 
 receivers, which act will, by opening the circuit of the line, 
 cause the corresponding supervisory relay to let go of its 
 armature and thus remove the shunt about its lamp. When 
 both lamps are thus lighted, the operator knows that the 
 connection is no longer desired, and without further in- 
 quiry pulls down the plugs. 
 
 It is evident that any fluctuation in the resistance of the 
 current of a transmitter at one subscriber's station will 
 cause a corresponding fluctuation of the current flowing in 
 the line, and this fluctuation will pass through the two 
 windings forming one side of the repeating coil, which will 
 act inductively on the two windings forming the other side 
 of the repeating coil and produce a corresponding fluctua- 
 tion in the other line. This latter fluctuation will pass 
 through the secondary winding of the induction coil and 
 the transmitter at the subscriber's station, and induce a cor- 
 responding fluctuation in the local circuit containing the re- 
 ceiver and the condenser. 
 
 One of the functions of this local circuit, which has 
 
 tip of the calling plug to the ring contact of the multiple 
 jack, she will get a click in her head telephone if the line 
 is busy, and silence if it is free. If the line is free, she in- 
 serts the plug to its full extent, and presses her ringing 
 key, which rings the subscriber's bell. As soon as the call- 
 ing plug is inserted the supervisory lamp, shown at the 
 right-hand lower portion of the figure, will be illuminated, 
 because its circuit will be completed from ground through 
 the battery, through the 9O-ohm resistance coil and the 
 lamp ; thence through the third strand of the cord, the test 
 ring contact of the jack, and to ground through the cut-off 
 relay. This current will not only illuminate the lamp, but 
 will operate the cut-off relay of the line called for as well. 
 As soon as the subscriber of this line responds, he will 
 close the circuit between the two sides of his line and thus 
 allow current to pass over the line to operate the trans- 
 mitter. This current will pass through the supervisory re- 
 lay, which will then be operated to close the low-resistance 
 shunt about the lamp and thus extinguish it. Both lamps 
 are now out, and the operator pays no further attention to 
 the connection until she sees one or both of the supervisory 
 lamps again lighted. The corresponding lamp will be 
 
 FIG. 7. 
 
 already been referred to, is that of putting the receiver in 
 inductive relation to the line without subjecting it to the 
 passage of direct current through it, which circumstance 
 might tend, if the receiver were connected up the wrong 
 way, to demagnetize its magnets. This local circuit is, 
 however, designed to accomplish still another result, this 
 being the formation of a comparatively short path, through 
 which the fluctuations set up by the transmitter may more 
 readily pass ; in other words, it is supposed to act as the 
 local circuit of a sort of a booster to the outgoing telephone 
 currents. The fluctuation in current caused by the trans- 
 mitter pass through the condenser and primary winding of 
 the induction coil, and thus induce on the secondary, which 
 is in the line, fluctuations which tend to increase the fluctua- 
 tions directly produced in the line by the transmitter. 
 
 So far, only such means have been described as would 
 admit of the connecting of two subscribers whose lines 
 terminate in the same exchange. Where several offices are 
 used in the same system, the connections between sub- 
 scribers in different offices are, as has already been pointed 
 cut, made by trunk lines extending between the offices. 
 These trunk lines are snoken of as outgoing or incoming, 
 
s 
 
 MODERN TELEPHONE ENGINEERING. 
 
 according to which end of them is referred to. Thus an 
 outgoing trunk line at one office would be an incoming 
 trunk line at another. The distinction is obvious. The 
 outgoing trunk lines usually terminate in jacks, in the 
 regular sections of the multiple switchboard, while the 
 incoming trunk lines usually terminate in plugs and cords 
 and special apparatus, arranged at special incoming trunk 
 sections, which sections are provided with multiple jacks in 
 the same manner as the regular sections. 
 
 In Fig. 5 I have shown the circuits of a trunk line. 
 The outgoing end of it at one office being shown at the 
 left of the dotted line, while the incoming end, with its 
 
 FIG. 8. LOAD CURVE. 
 
 accompanying apparatus, is shown at the right. This cir- 
 cuit is somewhat complicated and will be better understood 
 in connection with Figs. 6 and 7, which shows a subscriber 
 in one exchange connected through the local cord circuit at 
 that exchange, and a trunk line with a subscriber at another 
 exchange. Assuming that the subscriber at the left has sent 
 in a call, the operator at one of the regular sections of the 
 switchboard will answer it in the ordinary way by inserting 
 a plug into the answering jack, and having learned that 
 the connection desired is for a subscriber in another ex- 
 change, she presses her order wire key and communicates 
 over the order wire with the incoming trunk operator at one 
 
 FIG. Q. LOAD CURVE. 
 
 of the incoming trunk sections of the branch exchange. 
 The incoming trunk operator repeats the order and gives 
 the number of an unused trunk line to the operator at the 
 first exchange. The incoming trunk operator at the second 
 exchange then tests the line in the ordinary way with the 
 trunk line plug, and if it is found free, she completes the 
 connection by inserting the plug in the multiple jack of that 
 line. As soon as this operator completes the connection, 
 she sets the automatic ringing key, shown just to the left 
 of her plug, which locks in the ringing position, because 
 its controlling relay is not operated. The ringing generator 
 
 is provided with an interrupter, each revolution 'of which 
 switches the ringing current on to the line for one second 
 and the 24-volt battery on to the line for five seconds. Thus 
 the subscriber's bell rings for one second of every six until 
 lie removes his receiver from its hook. When he does this, 
 the current from the 24-volt battery or from the ringing 
 generator, whichever happens to be in circuit, passes 
 through the primary magnet of the ringing key, and trips 
 the latter into its normal position. By this means the bell 
 of the subscriber is rung at intervals until he responds 
 without any further attention on the part of the operator. 
 The insertion of the trunk plug into the jack of the called 
 subscriber's line allows current to flow from the battery 
 through the test relay and disconnect lamp, shown at the 
 extreme lower right-hand portion of the figure, and thence 
 through the cut-off relay to ground. This does not light 
 the disconnect lamp, because that lamp is already shunted 
 by the armature of another relay contained directly in the 
 trunk line circuit, this latter relay having been operated by 
 the insertion of the plug into the trunk line jack at the 
 
 
 FIG. 12. FRONT OF PARIS EXCHANGE. 
 
 other exchange. As soon as the ringing key is tripped, the 
 current from the battery at the incoming end of the trunk 
 line flows through the metallic circuit of the subscribers' 
 line to furnish current for talking. This operates the sup- 
 ervisory relay in the incoming cord circuit, and puts a 
 ground on the sleeve side of the cord circuit at the first 
 office, thereby operating the calling supervisory relay at 
 that office, and this shunts the corresponding supervisory 
 lamp and extinguishes it, thus showing to the regular opera - 
 tcr at the first office that the subscriber has responded. As 
 soon as the subscriber called for hangs up his receiver, the 
 supervisory relay at the incoming trunk section lets go of 
 its armature and removes the ground from the sleeve side 
 of the trunk line. This causes the calling supervisor]' relay 
 at the first office to lef go of its armature and remove the 
 shunt from around the corresponding lamp. When both 
 lamps of the regular pair of cords at the first office are 
 lighted, the operator at that office pulls down the connec- 
 tion. By thus removing the calling plug from the trunk 
 jack, current ceases to flow in the trunk line relay at the 
 
MODERN TELEPHONE ENGINEERING. 
 
 FIG. 10. PARIS EXCHANGE. 
 
 second office, which removes the shunt from the disconnect 
 lamp at that office and allows it to be lighted for the first 
 time. This is a signal for the incoming trunk operator 
 to disconnect. 
 
 It will be seen from the above circuits that when two 
 subscribers are connected over a trunk line, they are under 
 the control of the originating operator. The disconnect 
 signals from both subscribers go direct to this operator, and 
 it is not until she has pulled down this connection that the 
 disconnect signal is given to the operator at the other ex- 
 
 change. The latter operator has no means of listening 
 in on the circuit. 
 
 An interesting feature about such trunking systems is 
 the means by which the incoming trunk operator at the 
 second office informs the calling subscriber and the regu- 
 lar operator that line called for is busy. When she finds 
 on testing that a line is busy, she inserts the trunk line plug 
 into a "busy back" jack, which jack has connected in its 
 circuit the machine giving an interrupted current which 
 gives a peculiar, familiar, and, to most subscribers, an ex- 
 
 FIG. M-riTTSBURO EXCHANGE. 
 
IO 
 
 MODERN TELEPHONE ENGINEERING. 
 
 tremely disagreeable sound. This sound is heard by botli 
 operator and subscriber at the first office, and the discon- 
 nection is made. 
 
 * * * * 
 
 There is probably no method of determining the relative 
 activity of any city at different times of the day or on dif- 
 ferent days, so well as by watching the telephone exchange 
 in that city in operation. For keeping track of their busi- 
 ness and being able to handle it to the greatest advantage, 
 telephone companies usually make at frequent intervals 
 what they term " peg counts." These show the total num- 
 ber of calls received and connections made during each of 
 the twenty-four hours of the day. These results, if plotted 
 in the form of a curve, in which the abscissae represent time 
 and the ordinates represent total number of calls per hour, 
 
 the use of telephone service. From the hour of 6 A. M. 
 each of the curves rise rapidly, corresponding to the 
 awakening of the city, reaching a maximum for the day in 
 the neighborhood of ten o'clock, A. M. They then fall off 
 
 FIG. 13. FRONT OF PITTSBURO SECTIONS. 
 
 show graphically and in a striking way the amount of tele- 
 phone business being transacted at any given hour. The 
 curves (Fig. 8) , show what I believe has never been shown 
 before that is, peg counts, taken the same day, of two 
 opposition exchanges in the same city. These exchanges 
 have about the same number of subscribers, not far from 
 6000. The dotted curve represents the telephone activity 
 in the Bell exchange, while the solid curve represents that 
 of the Independent exchange. The rates of the Bell ex- 
 change are considerably higher than those of the Inde- 
 pendent, and from this fact we are enabled to draw several 
 conclusions from the form of the two curves. 
 
 It will be noticed that from twelve at midnight until six 
 o'clock both curves are very low, and gradually fall until 
 three o'clock, after which they slowly rise until six. This' 
 shows conclusively that the hour of 3 A. M. is that of least 
 business activity, except, perhaps, in those lines, such as 
 burglary and other allied industries which seldom lequire 
 
 FIG. 14. FRONT OF IMTTSHURG SINGLE SECTION. 
 
 to a minimum, corresponding to the lunch hour, between 
 twelve and one, and again rise to a second maximum be- 
 tween the hours of four and five in the afternoon, after 
 which they rapidly fall during the evening hours. The 
 fact that the Independent exchange has cheaper telephones 
 
 FIG. 17. REAR OF TITTSBURG SINGLE SF.CTION. 
 
 than the other will, perhaps, account for the fact that its 
 subscribers begin work a little earlier in the morning, reach 
 their maximum activity at a little earlier hour, take a little 
 shorter time for lunch, go home a little later in the evening 
 
MODERN TELEPHONE ENGINEERING. 
 
 
 FIG. 15. REAR OF ST. LOUIS SWITCHBOARD. 
 
 FIG. l6. REAR OF PITTSBURG MULTIPLE. 
 
12 
 
 MODERN TELEPHONE ENGINEERING. 
 
 and work a little more all night, all of which facts are in- 
 dicated by these two curves. 
 
 In Fig. 9 I show two curves representing telephonic 
 activity of the same city on two different dates. It will 
 be noticed that the dotted curve, which represents a 
 Monday, shows a greater number of calls than the solid 
 curve, which was taken on a Wednesday. It is usually true 
 that Monday is about the busiest day in the week in tele- 
 
 FIG. 18. LINE CONSTRUCTION OLD ST. LOUIS EXCHANGE. 
 
 FIG. 24. CABLE HEADS. 
 
 phonic exchanges. It would be interesting to trace the 
 significance of the different humps or characteristics of 
 these curves, but it will be noticed that whatever hump is on 
 one is present, or at least suggested, on the other. The 
 pronounced hump on the dotted curve at the extreme right 
 
 is undoubtedly due to the theater business, between seven 
 and eight in the evening. Probably the reason why this 
 was more pronounced on one curve than on the other was 
 that there was a special attraction or special set of attrac- 
 tions, in the entertainment line, on the particular day on 
 which the curve was taken. Such curves as the^e have 
 been said by someone to represent the pulse of a city, and I 
 think that the longer one thinks about it, the more apt 
 
 Fill. IQ. PITTSBURG POWER PLANT. 
 
 FIG. 2O. PITTSEURG STORAGE BATTERIES. 
 
 this term appears. Every city has a heart in its telephone 
 exchange, and this heart is throbbing with as great a regu- 
 larity as that of any living creature. 
 
 I will now, in conclusion, show a number of pictures of 
 telephone exchanges and apparatus, which I think will be 
 
MODERN TELEPHONE ENGINEERING. 
 
 FIG. 21. P1TTSBURG REPEATING COII. RACK. 
 
 of general interest, and which, perhaps, will make some of 
 the statements which I have made more clear to those not 
 well posted in telephony. My first view in this series (Fig. 
 10) is that of the interior of the exchange installed by the 
 Western Electric Company, at Paris, France. This board 
 is next to the largest in the world, it having a capacity of 
 something like 9300 lines. 
 
 The next view (Fig. n) is that of the multiple board 
 installed for the Pittsburg & Allegheny Telephone Com- 
 
 and Allegheny, all operating in conjunction with the ex- 
 change here shown. The total present equipment of these 
 offices is about 7000 lines. 
 
 Fig. 1 2 shows the front view of one of the sections of the 
 great switchboard at Paris, shown in Fig. 9. In this board 
 mechanical annunciators of the electrically self-restoring 
 type were used, which can be seen at the extreme top of the 
 section. Those signals are automatically restored when the 
 operator inserts a plug into the answering jack, but this ar- 
 
 
 
 FIG. 22. LINE CONSTRUCTION, OLD ST. LOUIS EXCHANGE. 
 
 pany, of Pittsburg, Pa., by the Kellogg Switchboard & 
 vSupply Company. This board is now wired for 4080 lines 
 and has an ultimate capacity of 6000. It forms the main 
 office of the Pittsburg & Allegheny exchange, there being 
 six branch offices located in the various parts of Pittsburg 
 
 FIG. 23. CABLE VAULT, ST. LOUIS EXCHANGE. 
 
 rangement has a marked disadvantage over the more 
 modern form of boards wherein the line signals are located 
 in the immediate vicinity of the answering jacks. With the 
 drops, as shown in this section, the operator had first to 
 look to the top of the section, fix the number of the signal 
 
MODERN TELEPHONE ENGINEERING. 
 
 FIG. 26. NEW ST. LOUIS DISTRIBUTING BOARD, NO. I. 
 
 
 FIG. 27. NEW ST. LOUIS DISTRIBUTING BOARD, NO. 2. 
 
MODERN TELEPHONE ENGINEERING. 
 
 displayed in her mind, and then look down and pick out the 
 corresponding answering jack in which to insert the plug. 
 In the later boards, where the line signal is next to the cor- 
 responding answering jack, she is saved this change of 
 vision, and also this mental operation. She simply plugs 
 into the jack next to the signal displayed without any re- 
 gard to its number. 
 
 In Fig. 13 is shown a front view of several of the sec- 
 tions of the Pittsburg exchange, a complete view of which 
 
 FIG. 25. OLD ST. LOUIS DISTRIBUTING BOARD. 
 
 was just seen, and in Fig. 14 a closer-view of one of the 
 sections. Here the multiple jacks are plainly seen in the 
 upper portion of the board, the answering jacks and line 
 signals being shown below. One of the key shelves is 
 raised, showing the wiring of the ringing and listening 
 keys, and also the springs of these keys. Below this one 
 of the front panels is removed, showing some details of 
 the wiring of the connecting rack of the section. 
 
 The next view (Fig. 15) is that of the rear of the 
 St. Louis switchboard. The cables leading to the mul- 
 tiple jacks are shown in the upper portion of this picture, 
 and below them the cord racks, the supervisory relays 
 and the running box containing the vaiious line cables are 
 clearly shown. 
 
 Fig. 16 is a similar view of the rear sections of the Pitts- 
 burg exchange, a good idea of the multiple cables being 
 afforded. 
 
 Fig. 17 shows with greater detail the rear of one of the 
 Pittsburg sections. The multiple cables are seen at the top 
 and below these are the relay boxes, one of which is open, 
 disclosing the supervisory relays at the top and the line re- 
 lays of that particular position. In the lower portion of 
 the picture is shown the connecting rack, to which all of the 
 wiring of this particular section is led. 
 
 In Fig. 1 8 is shown the turning section of the multiple 
 cables. This is at the end of the switchboard and shows 
 where the multiple cables are turned downward into the 
 running box, where they run along back of the sections, 
 and are joined to their proper answering jacks and signal- 
 ing circuits on the connecting rack of each section. 
 
 In Fig. 1 9 is shown what might be called the heart and 
 lungs of a telephone exchange. This is the power plant of 
 
 the Pittsburg & Allegheny exchange. On the right is seen 
 the power switchboard, which is of marble, and is provided 
 with voltmeters and ammeters, circuit breakers, and all 
 switches necessary for handling the machines and batteries. 
 In the center of the picture are shown the two ringing ma- 
 chines and on the right the charging machines for charging 
 the storage batteries. 
 
 I will now show (Fig. 20) the storage batteries of 
 this same exchange, these comprising ten cells each of 
 chloride accumulator, each cell being of 800 ampere-hours 
 capacity. The tanks are of wood, lined with lead, and the 
 floor and walls are of tile laid in cement. At the upper 
 left-hand portion of the picture may be seen the heavy 
 copper bus-bars forming the discharge leads of these bat- 
 teries. These are of solid copper, 3 ins. x in. in thickness. 
 
 The next view (Fig. 21) shows the repeating coils of the 
 Pittsburg & Allegheny system, there being one of these coils 
 for each pair of cords in the entire exchange. These coils 
 are iron clad and are enclosed, as shown, in a practically dust- 
 proof cabinet. It is through these coils that the current from 
 the storage battery is led to the various cord circuits, whence 
 it passes to the lines of connected subscribers to furnish 
 talking current. 
 
 No single feature of telephone work has shown greater 
 advancement than that of outside line construction. In 
 place of the overhead wire, so common a few yeais ago, 
 and still too common, the lines in systems are being placed 
 underground to a greater and greater extent. My next pic- 
 ture (Fig. 22) illustrates a piece of overhead construction, 
 showing lines as they were about to enter the exchange of the 
 old system at St. Louis, Mo. In marked contrast to this, the 
 
 s 
 
 FIG. 28. KELLOGG DISTRIBUTING BOARD. 
 
 next (Fig. 23) is a picture which shows how the subscribers' 
 lines now enter the exchange building in that same St. Louis 
 system. This is a picture of an underground cable vault 
 and shows with what care and system the cables coming 
 in from the street are led into the cable shaft leading to the 
 exchange room. 
 
 The next view (Fig. 24) shows a common method of term- 
 inating the lines of telephone exchanges, or rather termin- 
 ating the outside construction work. The line cables may be 
 seen leading up from the lower part of the picture These 
 enter the cable heads, which are, in this case, cast-iron 
 boxes hermetically sealed, and the various wires of the 
 
i6 
 
 MODERN TELEPHONE ENGINEERING. 
 
 
 M 
 
 o 
 o 
 
 a 
 
 i 
 
 6 
 
 
 s 
 
 r 
 
 II 
 
 *fc- X 
 
MODERN TELEPHONE ENGINEERING. 
 
 cables are distributed to insulated contact points on the in- 
 side of the boxes. These contact points extend to the 
 outside of the boxes through insulated bushings, and from 
 thence the circuit is led through protecting devices, after 
 which they are again arranged into cables which proceed 
 from the top of the cable heads and are then led to the 
 switchboard terminals. 
 
 This brings up the subject of distributing boards. A 
 little thought will show that some means must be provided 
 for distributing the various line wires which enter the ex- 
 change to their proper terminals on the switchboard, and to 
 
 FIG. 32. SINGLE CUT-OFF RELAY. 
 
 enable such changes to be made in this distribution as re- 
 quired. If such provision were not made and the line cables 
 were run directly to the switchboard, the wires in one 100- 
 pair cable being led, for instance, to the first position, and 
 the wires in the second loo-pair cable to the second posi- 
 tion, etc., it would be necessary at any time when a change 
 in a subscriber's number was desired, to open the cable, 
 take out the proper wire and 
 fasten it to one of the other 
 cables leading to the proper sec- 
 tion of the board. The changing 
 about of the various line wires 
 from one part of the board to 
 another is a very common occur- 
 rence, and to do it in the manner 
 j ust suggested would be wholly 
 impracticable. In order to pro- 
 vide means for systematically 
 making such changes, recourse 
 is had to what are called dis- 
 tributing boards or frames. These 
 assume a great variety of forms, 
 but the principle upon which they 
 
 are designed is as follows: In one portion of the distributing 
 board are placed clips suitably arranged, in which the wires 
 of the line cables may terminate. On another portion of 
 the distributing board is arranged another group of clips 
 or connectors in which the separate wires of the switch- 
 board cables may terminate. We thus have all connections 
 from the lines, and also all connections from the switch- 
 board wired in a permanent manner to the various con- 
 nectors on the respective portions of the distributing 
 board. The gap between the terminals of any pair of wires 
 on the line side of the distributing board, and those of the 
 corresponding pair leading from the switchboard is filled 
 by means of bridle or jumper wires. The distributing 
 board is so arranged that these jumper wires may be run 
 from any pair of connectors on the line side to any pair 
 on the switchboard side, and if the distributing board is a 
 good one, these jumper wires are laid with perfect system 
 and may be changed as often as desired. 
 
 In Fig. 25 is shown a distributing board which is 
 happily a relic of the past, but which in its day was dis- 
 tinctly superior to others and may be said to have served 
 its purpose well. This picture was taken from the inside 
 of the distributing board which was in the form of a hollow 
 square, one corner of which is shown. This accommodated 
 about 4000 lines entering the old Bell exchange at the 
 corner of Fourth and Pine Streets, in St. Louis. The line 
 wires were practically all aerial and were brought to a large 
 tower at the top of the exchange building ; they were then 
 led down in suitable cables, as shown in this picture, which 
 cables were fanned out for attachment to the clips on the 
 upper portion of the distributing board. In the same man- 
 ner the switchboard cables were led up to the lower set of 
 clips on the distributing board. Connection between any 
 line wire and any switchboard wire was then completed by 
 means of the jumper wires, as readily seen. 
 
 It is the object of modern distributing boards to do 
 away with the confusion in these jumper wires and to lead 
 them as far as possible in parallel directions in their courses 
 from the line to the switchboard sides of the frames. My 
 next view (Fig. 26) shows the switchboard side of the dis- 
 tributing board of the present main exchange of St. Louis, 
 which exchange takes the place of the old exchange, the dis- 
 tributing board of which was just shown. The switch- 
 board cables may be seen leading up from beneath. They 
 are fanned out to the lightning arrester terminals shown on 
 the right-hand portion of the board, these terminals being 
 omitted from the left-hand portion, which latter portion is 
 reserved for future growth. These arresters contain a 
 
 FIG. 37. STRIP OF JACKS. 
 
 device which will open the line and ground it if an ex- 
 traneous current of sufficient intensity to be considered 
 dangerous comes in over the line. Such currents are 
 termed sneak currents, and would do great damage but 
 for these little protectors. The protectors also include 
 _static arresters, by means of which a high potential current 
 is caused to jump to ground rather than go on through the 
 intricacies of the switchboard. The jumper wires are led 
 from the terminals on this side of the distributing board 
 through iron rings, some of which may be seen at the ex- 
 treme left of the picture'and to the'line side of the distribut- 
 ing board, a view of which (Fig. 27) is now shown. The 
 terminals on this .side are arranged in horizontal rows, and 
 the jumper wires coming from the line side, after passing 
 through their respective rings, are led up or down as the 
 case may be, to the particular horizontal row on which the 
 desired terminal exists, and are then led along the cor- 
 responding horizontal shelf to the proper terminal. The 
 
i8 
 
 MODERN TELEPHONE ENGINEERING. 
 
 cables leading in from the street are permanently con- 
 nected to these horizontal rows of terminals and may be 
 seen coming in at the bottom portion of the board through 
 the trap door in the floor. 
 
 The next view (Fig. 28) shows a Kellogg distributing 
 board, this particular one being that at the Pittsburg ex- 
 change already referred to. The peculiar feature of this 
 board is that the line and switchboard terminals, instead of 
 being arranged on opposite sides of the frame work, as on the 
 board just described, are arranged on alternate vertical strips, 
 there being a strip of switchboard terminals and then a strip 
 of line terminals, and so on. The arresters in this case are 
 put on the cable heads, which may be seen in the rear of the 
 view. Jumper wires are led up or down from the respect- 
 ive line terminals through the large iron rings seen at the 
 bottom and top of the board, and then horizontally to the 
 
 FIG. 36. STRIP OF LAMPS. 
 
 proper strip of line terminals, and then up or down to the 
 proper terminal on this. The cables from the cable heads 
 in the rear are led up to the line strips from beneath, as 
 clearly shown, while the cables leading to the switchboard 
 may be seen in the iron running frame at the top of the 
 picture. 
 
 I have had a few slides prepared representing in detail 
 parts of the apparatus used in some of the latest Kellogg 
 exchanges. I show here (Fig. 29) a subscriber's automatic 
 switch-hook with the receiver upon it. The next picture 
 (Fig. 30) shows the same hook with the receiver removed. 
 It will be seen that the long lever serves to bend the central 
 spring into engagement with the lower or upper set of springs 
 according to whether the telephone is in use or not. This 
 accomplishes the changes of circuit by which the operator 
 is automatically called and by which she is later given the 
 signal to disconnect, which changes are made, of course, 
 entirely without the volition or knowledge of the sub- 
 scriber. 
 
 Passing on to the central office, I will show (Fig. 31) 
 a strip of line relays similar to the ones used in the Pitts- 
 burg exchange. These relays are mounted twenty per 
 strip, as shown. 
 
 The next cut (Fig. 32) shows a single line cut-off relay. 
 The magnet of this is enclosed in an iron shell which forms a 
 portion of the magnetic circuit. The armature of this relay, 
 when attracted, causes the two long springs to break contact 
 with the lower springs, and to make contact with the two 
 upper springs. I may say, in passing, that all contacts in 
 the best relays of this description are made of platinum.' 
 These relays are about i ^ ins. in diameter, and the shells 
 are of soft iron about y% in. thick. 
 
 In the latest Kellogg exchanges the line and cut-off re- 
 lays are mounted side by side on the same strip. This 
 view (Fig. 33) shows such a strip containing twenty of each 
 
 relays, the line relays being the smaller of the two. The 
 strip represents a unit of twenty lines, which unit is car- 
 ried throughout the entire scheme of wiring in the central 
 office. The next illustration (Fig. 34) shows the opposite 
 side of this relay strip, showing the connecting wires from 
 the various terminals. These terminals pass through walls 
 in the iron mounting strip and are readily accessible for 
 soldering in the wires. In mounting the line and the cut-off 
 relay in such close proximity, many of the wires which 
 would otherwise have to be formed up in cables are reduced 
 to the short, stiff bare wires clearly shown in this picture. 
 Coming now to the line signals themselves, in Fig. 35, I 
 show a strip of drops, these being mounted on J^-in. centers 
 so as to correspond to the spacing of the jacks. These were 
 used before the adoption of the line lamp signals by the Kel- 
 logg Company, a strip of which signals is shown in Fig. 36. 
 
 This strip is formed from a single 
 piece of hard rubber about 10 ins. 
 long and }4 in. thick. The lamps, 
 one of which is shown in the front 
 of the view, are so arranged as to 
 slide into their sockets between the 
 jack springs in such manner as to 
 make the proper circuit connections. 
 In front of each lamp is a little lens, 
 usually of opalescent glass, this 
 lens being carried in a spun brass 
 cap. The advantages of the line 
 lamp signal over the old form of me- 
 chanical signal is obvious, the principal ones being the 
 absolute freedom from complexity, cheapness, and the fact 
 that they may be removed instantly if found defective and 
 replaced by good ones. A defective line lamp may be 
 taken out and another one substituted in less than one-half 
 minute. Another beauty of the line lamp is that when it 
 is out of order it may be thrown away, which is much easier 
 
 FIG. 38. CORD AND PLUG IN SECTION. 
 
 than repairing a mechanical drop. These lamps usually 
 operate at 20 or 24 volts, take about ampere and give ap- 
 proximately ^ candle power. The calculated life of these 
 lamps, with their present degree of perfection and with the 
 average rate of use, is about thirty years, but, of course, 
 this figure remains to be verified by actual service, for 
 unfortunately the telephone business is not yet old enough 
 to prove the statement. 
 
 I now show (Fig. 37) what is by all odds the most import- 
 ant piece of apparatus in a telephone exchange the spring 
 jack. This is a strip of the two-point jack already referred 
 to, and it will be seen that they are mounted twenty to a strip 
 on spaces corresponding to those of the lamp jack. The 
 
MODERN TELEPHONE ENGINEERING. 
 
 same jack is used for both answering and multiple. This 
 strip is built up of hard rubber, brass and German silver, 
 the front being a solid piece of hard rubber, slightly over 
 10 ins. long, and drilled for receiving the sleeve contacts. 
 The rear of the strip is a piece of hard rubber milled to re- 
 ceive the line springs and the rearwardly projecting lugs 
 from the sleeve contacts. This rear strip is reinforced by 
 a heavy brass strip, into which the screws holding the line 
 springs are threaded. In this particular strip the jacks 
 are on |-in. centers, the strip being 7/16 in. high. The 
 spacing with this same construction is readily reduced to 
 |4 in. Where a smaller spacing than this is required, as in 
 a board of extreme size, a somewhat different construction 
 would be used. 
 
 I have spoken throughout the evening of cords and plugs, 
 with the details of which some present may not be familiar. 
 
 FIG. 40. CABLE FANNED OUT. 
 
 The next picture (Fig. 38) shows one Of each, the plug 
 being cut away to expose the method of attaching the cord, 
 the cord having its various layers removed to better show 
 its construction. This plug has two contact points instead 
 of three, as described in connection with Western Electric 
 or Bell system. It will be noticed that the two strands of 
 the cord are composed of twisted tinsel, over which z wrap- 
 ping of floss silk and a braid of cotton are placed. Thus 
 insulated, the two conductors are bound together by a 
 wrapping of cotton, after which they are enclosed in a 
 spiral of brass wire for protection. Over the spiral of wire 
 are placed two layers of linen braiding throughout the en- 
 tire length of the cord, and for about 10 ins. back of the 
 plug a third reinforcing layer is put on. These cords vary 
 in length, according to the size of the multiple board, this 
 length being anywhere from 36 ins. up to nearly 100 ins. 
 
 The next view (Fig. 39) shows a strip of ten supervisory 
 relays, these relays being the ones used in connection with 
 the cord circuit for controlling the supervisory lamps. These 
 are iron clad, being made in the same shell as is used for the 
 cut-off relay. The armature, a disk of iron about i ins. in 
 diameter, is suspended in front of the poles of the electro- 
 magnet by a thin leaf spring. The working parts of the 
 relay, including the armature and contact points, are en- 
 closed in a brass cap, which cap locks on the shell with a 
 bayonet joint. 
 
 The next and last view ( Fig. 40) shows a few of the cables 
 used in switchboard work, as they are formed up in the fac- 
 tory. Some of these are made from standard 21 and 41- 
 pair cables in long lengths with their ends formed 
 and laced to fit the particular portion of the switchboard 
 for which they are required. Some of the other cables are 
 formed by hand, exactly as shown, directly from the 
 insulated wire, which is usually twisted in pairs. These 
 cables, as you will see, remind one more of diagrams of 
 nervous systems, sometimes seen in medical books, than 
 anything else I can think of. 
 
 The design of telephone switchboards and apparatus is a 
 matter requiring attention to almost infinite detail. Besides 
 having a knowledge of the requirements of telephone serv- 
 ice and a thorough appreciation of the engineering prob- 
 lems involved, the designer must have a peculiar faculty, 
 which has been termed "telephone sense." Ability to dis- 
 tinguish between apparatus and circuits that will work suc- 
 cessfully in practice, from those that will work only in the 
 laboratory, is not acquired by all ; and the problems are so 
 remote from those of other branches of electrical engineer- 
 ing, that it is seldom that precedent taken from other 
 branches can be found. 
 
 Those who have the best interest of the telephone busi- 
 ness at heart are now designing their apparatus and cir- 
 cuits on the basis that telephone systems in small villages 
 must have as good transmission as those in the largest 
 cities, with the idea in view that in the future the whole 
 continent will be one vast telephone exchange, the various 
 large cities being the main offices, and the long-distance 
 lines, the trunk lines between them, and the small villages 
 the branch exchanges. 
 
 I regret that time, together with a lack of definite 
 information, prevents me from treating of Dr. Pupin's 
 recent invention, but this subject is one of such apparent 
 importance, that to devote less than a whole evening to it 
 would be to do both the inventor and the audience an 
 injustice. 
 

PUBLISHED TRANSACTIONS 
 
 OF THE 
 
 NEW YORK ELECTRICAL SOCIETY 
 
 NO. I. ELECTRICAL PROGRESS OF THE YEAR, 1887 
 
 By JOSEPH WETZLER, December 28, 1887. 
 
 NO. 2. A PRACTICAL METHOD OF CALCULATING AND DESIGNING 
 
 DYNAMOS AND MOTORS 
 
 By FRANCIS B. CROCKER, March 28, 1888. 
 
 NO. 3. THE SOCIAL SIDE OF THE ELECTRIC RAILWAY 
 
 By T. C. MARTIN, March 12, 1890. 
 
 NO. 4. ELECTRICITY AT HIGH PRESSURES 
 
 By ELIHU THOMSON, March 29, 1899. 
 
 NO. 5. SYSTEMS OF ELECTRIC TRANSMISSION AND DISTRIBUTION 
 
 By CHARLES PROTEUS STEINMETZ, November 16, 1900. 
 
 NO. 6. MODERN TELEPHONE ENGINEERING 
 
 By KEMPSTER B. MILLER, February 14, 1901. 
 
 NO. 7. ALTERNATING CURRENT MOTORS 
 
 By CHAS. F. SCOTT, April 25, 1901. (/ press.) 
 
UNIVERSITY OF CALIFORNIA LIBRARY 
 
 UNIVERSITY OF CALIFORNIA LIBRARY 
 BERKELEY 
 
 Return to desk from which borrowed. 
 This book is DUE on the last date stamped below. 
 
 APR 
 
 21 1948 
 
 LD 21-100m-9,'47(A5702sl6)476