T K 
 
GIFT OF 
 
 '12 'NVP IVd 
 
GIFT 
 FER 11 1914 
 
 ELEMENTARY 
 ELECTRICAL TESTING 
 
 MONOGRAPH 2 
 
 JOINT COMMITTEE SERIES 
 
 NATIONAL EDUCATION' ASSOCIATION EDITION 
 
 X 
 
 CONTRIBUTED BY THE TECHNICAL STAFF OF THE 
 
 WESTON ELECTRICAL INSTRUMENT CO. 
 NEWARK, N.J. 
 
THE JOINT COMMITTEE MONOGRAPH SERIES 
 
 The following is a list of Monographs written or being 
 written by the technical staff of the manufacturers mentioned, 
 and issued in co-operation with the Joint Committee on Physics. 
 These Monographs are intended to convey to teachers the 
 point of view of men of affairs as to the principles and facts 
 worth teaching to high school students in each specialty. 
 
 1. Announcement. 
 
 2. Elementary Electrical Testing. 
 
 Weston Electrical Instrument Company, Newark, N. J. 
 
 3. Edison Storage Batteries. 
 
 The Edison Storage Battery Company, Orange, N. J. 
 
 4. Hydraulic Machinery. 
 
 Gould Pump Company, Seneca Falls, New York. 
 
 5. Mechanics of the Sewing Machine. 
 
 Singer Sewing Machine Co., Singer Building, New York City. 
 Others are projected. 
 
ELEMENTARY 
 ELECTEICAL TESTING 
 
 MONOGRAPH 2 
 JOINT COMMITTEE SERIES 
 
 NATIONAL EDUCATION ASSOCIATION EDITION 
 
 CONTRIBUTED BY THE TECHNICAL STAFF OF THE 
 
 WESTON ELECTRICAL INSTRUMENT CO, 
 * 
 
 NEWARK, N. J. 
 
COPYRIGHT, 1914 
 
 BY 
 WESTON ELECTRICAL INSTRUMENT CO. 
 
ARGUMENT 
 
 THIS monograph is prepared as evidence of our belief in 
 the new movement to make education more practical. 
 The Weston Electrical Instrument Company is 
 convinced that the public high school instructors desire to use 
 pedagogic material drawn from real life and prefer to perform 
 laboratory experiments which teach the fundamentals of science 
 as far as possible with commercial equipment, such as the student 
 will later use or see used, and with which he will later be expected 
 to be familiar. 
 
 Assured that teachers want to know what technical men 
 in the industries believe to be fundamental, we have prepared 
 brief suggestions relating to the preparation of a course in 
 electrical measurements. Representatives of our house have 
 been observing the work of schools for several years. In issuing 
 this monograph, we have consulted authorities on pedagogics. 
 We have also received the advice of school men and have then 
 printed such matter as appealed to our judgment. 
 
 Some of the suggestions offered may be beyond the present 
 scope of high school physics, although extremely elementary 
 from our standpoint. 
 
 Since this is neither presented as a text book, nor offered 
 as a complete outline of a course of study, but merely contains 
 material offered for consideration, it is probable that, as this 
 movement progresses, exercises which are at present too advanced 
 may later be acceptable and even necessary. 
 
 Exercises credited to authors of text books have in some 
 instances been abbreviated but not altered in detail, even though 
 in our judgment and in the judgment of members of the Joint 
 Committee changes could well be made. We have, however, 
 
 280152 
 
4- ARGUMENT 
 
 appended our opinions when we differed considerably with 
 the author, or believed suggestions were appropriate. 
 
 We hereby express our thanks to the authors and publishers 
 who permitted us to quote from their works. 
 
 We wish to acknowledge our indebtedness to the members 
 of the Joint Committee on Physics who have advised us in 
 both the general plan of the monograph and in the selection 
 and arrangement of the material. All of our suggestions which 
 were adjudged by the committee members to be impracticable 
 under existing conditions pertaining to schools have been 
 eliminated. However, every effort has been made to make 
 this monograph represent the judgment of our Staff, rather 
 than that of the educators consulted. 
 
 WESTON ELECTRICAL INSTRUMENT COMPANY. 
 
CONTENTS 
 
 SUBJECTS 
 
 PACK 
 
 Argument '. 3 
 
 The Extensive Use of Electrical Energy 7 
 
 Current 8 
 
 Connections for Voltmeters and Ammeters 13 
 
 Power 21 
 
 Resistance 23 
 
 Insulation 25 
 
 The Weston Direct-current Ammeter 33 
 
 The Weston Direct-current Voltmeter . . 37 
 
 EXPERIMENTS 
 
 I. Resistance and Current in a Divided Circuit 8 
 
 II. The Measurement of the Current, Voltage, and Power of a Model 
 
 Lighting Circuit. Two-wire System 10 
 
 III. The Fall of Potential along a Conductor 13 
 
 IV. Fall of Potential in a Lamp Bank 15 
 
 V. The Number of Watts for one B.T.U. per Second 17 
 
 VI. Heating Effect of an Electric Current 18 
 
 VII. Efficiency Test of an Electric Motor 21 
 
 VIII. The Determination of Low Resistances by the Ammeter and 
 
 Voltmeter Method 23 
 
 IX. The Determination of High Resistances by the Voltmeter Deflec- 
 tion Method 25 
 
 X. The Wheatstone Bridge 27 
 
 XL The Slide Wire Bridge 30 
 
 XII. The Effect of Temperature on the Resistance of a Lamp Filament 31 
 
 XIII. The Study of an Ammeter 35 
 
 XIV. The Study of a Voltmeter 39 
 
 XV. The Action of a Simple Cell 40 
 
 5 
 
ELEMENTARY ELECTRICAL TESTING 
 
 THE EXTENSIVE USE OF ELECTRICAL ENERGY IN 
 NUMEROUS INDUSTRIES 
 
 In re-outlining the course of study in high schools, due con- 
 sideration should be given to the fact that changes in the relative 
 importance of certain industries have recently taken place, and 
 that these changes affect both the home and the civic life of our 
 people in various communities. 
 
 The new census gives figures to support the statement that 
 electrical industries are economically more important than the 
 chemical industries in point of the number of wage-earners 
 employed and in the value of the product manufactured each 
 year. 
 
 It is estimated that ten billion dollars are now invested in 
 electrical industries. About one-fourth of this represents the 
 capital invested in power developments of a semi-public nature. 
 
 The educational significance of these facts is two-fold. : 
 
 First. In emphasizing that more attention must be given by 
 instructors to the industrial applications of electricity, and to 
 physics generally. 
 
 Second. In the direct answer which they give to the question 
 as to what constitutes practical ideas from the business man's 
 point of view. We will elaborate this point. The greater por- 
 tion of this tremendous investment is dependent upon the main- 
 tenance of circuits having a pressure of 110 volts or over. Accord- 
 ingly the first things that students should learn are the facts con- 
 cerning these circuits. For developing means of teaching these 
 and other facts the country is dependent upon its educators. 
 It is from teachers only that the student may obtain comprehen- 
 
 7 
 
8 ELEMENTARY ELECTRICAL TESTING 
 
 sive ideas relating to the materials, arrangement and methods 
 of distribution of electrical circuits. Just as the water and gas 
 mains, with their innumerable outlets and ramifications, supply 
 a city with two necessities, the electrical conductors give us 
 access to an agent a source of energy which has become 
 more necessary to us than gas, and in frequent instances is 
 almost as indispensable as water. And this agent, this source 
 of energy, is an electric current. 
 
 It seems logical and proper, therefore, that a course in ele- 
 mentary electrical measurements should begin with the con- 
 sideration of the subject from the standpoint of current. 
 
 CURRENT 
 
 The following exercise, published in the Manual of Fuller 
 and Brovvnlee,* Experiment 77, is here given in part as an illus- 
 tration of a method of teaching current distribution. 
 
 EXPERIMENT I 
 RESISTANCE AND CURRENT IN A DIVIDED CIRCUIT 
 
 Object. To compare (a) the currents in the branches of 
 a divided circuit with the resistance of those branches; (6) 
 the total resistance with the resistance of the branches. 
 
 Apparatus. Lamp board like that shown in Fig. 1; 32- 
 candle-power lamps to fill board; 3 ammeters; voltmeter, with 
 connecting wires; connections to 110 volts B.C. circuit. 
 
 Experimental. Proper connections for a circuit of two 
 branches, like that shown in Fig. 1, are to be made. The resist- 
 ance in each branch of the circuit consists of an equal number 
 of similar incandescent lamps, connected in parallel. The 
 ammeters are so connected that the total current through both 
 branches can be read and also the individual current in each 
 branch. The terminals of a voltmeter, which is not shown, 
 are to be connected to the terminals of any portion of the cir- 
 cuit whose resistance is desired. 
 
 * Published by Allyn & Bacon, Boston. 
 
RESISTANCE AND CURRENT IN A DIVIDED CIRCUIT 9 
 
 All the lamps on both sides are to be turned on and reading 
 of each ammeter recorded. The voltmeter is then connected 
 in succession to the terminals of each branch circuit and to the 
 terminals of the combined circuit and the readings obtained 
 recorded in tabular form near the top of the left-hand page. 
 All the lamps but one on one branch are then turned out, leav- 
 ing all the lamps in the other branch of the circuit burning. 
 Readings of the voltmeter and ammeters are taken and recorded 
 
 QOQ 
 
 i 
 
 FIG. 1. Lamp Board, Ammeters and Connections. 
 
 as before. Make the following additional combinations in the 
 two branches and record the results: 2 lamps and 3 lamps; 2 
 lamps and 4 lamps; 2 lamps and 5 lamps. 
 
 A simple diagram of connections should be made, and a brief 
 description of the method of making the tests should be given. 
 From the readings of the instruments the resistance of each 
 branch and the resistance of the entire circuit should be calculated 
 for each case by the application of Ohm's Law. 
 
 Discussion. Does increasing the number of lamps in parallel 
 in a circuit increase or decrease the resistance of the circuit? 
 
10 ELEMENTARY ELECTRICAL TESTING 
 
 When the number of equal known resistances are connected in 
 parallel, give a rule for finding the combined resistance. 
 
 EXPERIMENT II 
 
 THE MEASUREMENT OF THE CURRENT, VOLTAGE, AND 
 POWER OF A MODEL LIGHTING CIRCUIT. TWO-WIRE 
 SYSTEM 
 
 This experiment is from Wiley's Loose Leaf Manual.* It 
 is more elaborate but attains the same end as the preceding one. 
 
 Apparatus in the Laboratory: Model lighting circuit board; 
 110-volt direct current. 
 
 Apparatus from the Stock Room: Ammeter; voltmeter; 
 yard stick; 15-ampere fuses. 
 
 The line wires are No. 14 German silver, 180 ohms per mil- 
 foot, to furnish in the limited space of the laboratory conditions 
 similar to those of an actual incandescent lighting circuit in a 
 three-story building. (See Fig. 2.) 
 
 All readings of current, voltage, etc., should be recorded on a 
 single outline diagram of the line. 
 
 The binding posts at the end of the line are to be regarded 
 as the source of power. Measure the voltage at these terminals 
 and the current in each section of the line at this voltage. 
 
 Measure the distance from the terminals to the first group of 
 lamps, also the distance from each group to the next succeeding 
 group. (Measure in each case to the point on the mains at which 
 the short leads from the lamps are attached. The resistance 
 of the short copper leads to the lamps may be neglected.) 
 
 From its length and mil-foot resistance, compute the resist- 
 ance of each section of the line. Compute also the " drop " 
 over each section and the voltage at each group of lamps. Check 
 your computed voltage by comparison with the voltmeter read- 
 ings across the lamps. 
 
 Since the voltage may fluctuate, one voltmeter should be kept 
 connected across the supply terminals, and the voltage at the 
 lamps should be read from a second voltmeter when the voltage 
 at the terminals is at the proper value. 
 
 * Published by John Wiley & Sons, Inc., New York. 
 
THE MEASUREMENT OF THE CURRENT 
 
 11 
 
 I -h 
 
 g 
 
 
 
 
 
 
 I - 
 
 ) 
 
 /^^c\ A 
 
 
 J 
 
 3 
 
 1 ' 
 
 ,__/ 
 
 i 
 
 "-IP) 
 
 
 
 o 
 
 
 
 ^^s 
 
 B" 
 
 
 ^] + 
 
 
 
 o 
 
 
 
 
 s 
 
 
 P 
 
 
 
 
 
 
 
 
 
 
 
 
 H 
 
 1 
 
 
 T 
 
 
 
 
 
 
 o 
 
 
 
 
 o 
 
 
 
 
 HH 
 
 
 
 o 
 
 
 
 
 
 
 
 
 
 o 
 
 
 PH 
 
 
 
 o 
 
 
 
 
 q 
 
 
 
 
 o 
 
 
 fc 
 
 i t 
 
 
 
 o 
 
 
 
 
 
 
 
 o 
 
 
 r 
 
 P - 1 
 
 ) 
 
 f 
 
 
 o 
 
 
 ( 
 ( 
 
 XttU-c^ 
 
 
 
 o 
 
 
 M 
 
 O 
 
 
 1 
 
 
 __/ 
 
 1 
 
 1 
 
 * 
 
 J_ 
 
 -^ff\ 
 
 ((( [( < )] 
 
 
 
 
 
 1 
 
 
 
 o 
 
 
 
 
 1 
 
 1 
 
 
 
 o 
 
 
 o 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 o 
 
 
 
 
 o 
 
 
 
 
 o 
 
 
 
 
 
 
 
 
 
 
 
 o 
 
 (pj 
 
 !> 
 
 
 o 
 
 
 1 
 
 
 
 o 
 
 
 
 
 o 
 
 wf 
 
 j) 
 
 
 
 
 
 H 
 
 
 
 o 
 
 
 
 
 o 
 
 
 
 
 o 
 
 
 E 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 n 
 
 
 
 
 /iS^J 
 
 o 
 
 T 
 
 
 
 
 %^ I 
 
 
 1 
 
 
 
 
 4 ti 
 
 . 
 
 
 
 
12 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 From your data determine also: 
 
 1. The watts delivered at the binding post terminals. 
 
 2. The watts expended in each section of the feed wire. 
 
 3. The watts expended in each group of lamps. 
 
 4. The total watts supplied to the lamps and the total line 
 
 loss. 
 
 5. The " efficiency " of the line, i.e., the percentage of the 
 
 total power supplied which is delivered to the lamps. 
 
 6. The resistance of each group of lamps, and the average 
 
 hot resistance of each lamp. 
 
 FIG. 3. Connection for Ammeter with Self-contained Shunt. 
 
 FIG. 4. Connection for Ammeter with Detachable Shunt. 
 
 If conditions permit, we would suggest that a teacher would 
 better have small groups of students use a part of the actual 
 lighting circuit in the school, in place of a model, and follow the 
 directions given in the experiments as far as possible. 
 
 This plan is suggested by the letter on page 22 of our Mono- 
 graph B-l, written by Mr. C. W. Parmenter, Head Master of 
 the Mechanics Arts High School, Boston. The plan is also used 
 by Prof. H. H. Higbie of the Wentworth Institute, Boston, Mass. 
 
THE FALL OF POTENTIAL ALONG A CONDUCTOR 13 
 
 CONNECTIONS FOR VOLTMETERS AND AMMETERS 
 
 The student's attention should be called to the fact that 
 an ammeter should always be connected in series with the line, 
 whereas a voltmeter should be connected across the line. See 
 Figures 3, 4 and 5. 
 
 FIG. 5. Connection for Voltmeter. 
 
 EXPERIMENT III 
 THE FALL OF POTENTIAL ALONG A CONDUCTOR 
 
 The nature of a divided circuit and the current in it may have 
 been called the primary idea in the preceding experiments, but 
 they served as well as means of discussing the theory that*it is 
 pressure which actually forces the current through. 
 
 Schools have for a long time given an exercise showing the 
 " Fall of Potential along a Conductor," and it merely remains 
 for the teacher to give this exercise a more practical atmosphere 
 to have it continue to be one of the strongest on the list.* 
 
 Apparatus Required: Weston ammeter, range 3 amperes; 
 Weston voltmeter, range 3 volts; 1 meter slide wire bridge; 
 storage cell or 2 constant primary cells in series; No. 18 (B. & S. 
 gauge) 40-mil Weston alloy wire; No. 24 (B. &. S gauge) 20-mil 
 Weston alloy wire. 
 
 A length of 40-mil alloy wire is stretched over the meter rod 
 and securely clamped. The resistance of 40-mil alloy wire is 
 approximately 0.50 ohm per meter. 
 
 One storage cell, with the ammeter in series, is connected with 
 the stretched wire, an extra piece about % meter long is included 
 
 *See Experiment No. 29, Laboratory Manual in Physics. Wauchope, 
 Scott, Foresman & Co., New York and Chicago. 
 
14 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 in the circuit and its length is regulated until the current flowing 
 is 3.00 amperes. See Fig. 6. 
 
 The 3-volt range of the voltmeter is also connected as shown 
 and primarily the drop over the entire length of wire is found. 
 
 The resistance of 1 meter of wire is then determined by the 
 pi 
 
 formula R = . 
 
 It must be borne in mind, however, that this is riot a " zero " 
 method, and theoretically, at least, is not correct. That is to 
 
 V^ 
 
 ^: 
 o 
 
 y 10 o oi 
 
 r*i 
 
 
 
 
 
 
 I. 
 
 n 
 
 
 
 o 
 
 
 II 
 
 FIG. 6. 
 
 say, since current is necessarily flowing through the voltmeter 
 when in use, the instrument therefore forms part of a divided 
 circuit by shunting the resistance wire. 
 
 If an improperly constructed voltmeter of low resistance is 
 employed, its introduction will affect the indications of the ammeter 
 by materially reducing the total resistance of the circuit, and its 
 own indications will be erratic and disproportionate when differ- 
 ent lengths of the resistance wire are being tested. 
 
 Weston voltmeters have sufficiently high resistances to pre- 
 vent the irregularities referred to, but nevertheless the best 
 method for determining the total resistance of the wire is to ascer- 
 tain the drop over definite lengths of the wire, such for instance 
 
THE FALL OF POTENTIAL IN A LAMP BANK 15 
 
 as 10 centimeters, tabulate the results, obtain a mean value and 
 determine the total resistance by direct proportion. 
 
 When sufficient data have been secured relating to the 40- 
 mil wire, a piece of 20-mil is substituted, and the test repeated. 
 A current of about 1.00 ampere may be used. The resistance 
 of 20-mil alloy is about 2 ohms per meter. 
 
 A length of copper wire may also be tested, preferably one 
 having a diameter of 0.020 inch, and the relative resistance 
 of copper and Weston alloy wire determined. 
 
 Caution: Weston alloy wire has about 23.5 times the resist- 
 ance of copper wire, and therefore the experimenter must be 
 cautioned to add enough alloy wire to the circuit to compensate 
 for this difference, before the current is completed through the 
 copper wire. 
 
 The fall of potential along a conductor may also be beauti- 
 fully shown with this apparatus by substituting a wire the 
 resistance of which is not uniform. Such a wire may easily be 
 prepared by scraping or filing a resistance wire at irregular dis- 
 tances while stretched, and by coating it with a little solder here 
 and there, afterwards smoothing with emery paper. 
 
 The two voltmeter leads are then held, one in each hand, 
 so as to span a section of the wire, and the deflection is noted and 
 compared with the results obtained with other sections of the 
 same length. 
 
 EXPERIMENT IV 
 THE FALL OF POTENTIAL IN A LAMP BANK* 
 
 Question. What is meant by " Fall of Potential "? 
 
 Apparatus. 110-volt current; three electric lamps; volt- 
 meter. 
 
 Directions. Pass the current through the three lamps con- 
 nected in series as shown in the diagram. It requires electrical 
 pressure to force the current through the lamps. This pres- 
 sure is measured by the voltmeter. Connect the terminals 
 of the voltmeter to a and d to obtain the volts pressure required 
 to drive the current through all three lamps. In like manner 
 find the difference of electrical pressure between a and c for two 
 lamps, and between a and b for one lamp. 
 
 * From "Manual in Physics," by Joseph A. Wauchope. 
 
16 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 Results. Volts pressure required for three lamps = 
 Volts pressure required for two lamps = 
 Volts pressure required for one lamp = 
 
 Discussion. Potential is only another expression for electrical 
 pressure. Difference of potential means difference of electrical 
 pressure, and fall of potential means fall of electrical pressure. 
 A voltmeter is used to measure the potential difference between 
 any two points in an electrical circuit, just as pressure gauges are 
 used between any two points of a water pipe. The further away 
 from the pumping station the greater the fall of pressure of the 
 water in pounds per square inch, due to the resistance of the pipes. 
 
 FIG. 7. 
 
 D.C. AMMETER 
 
 So the further the electric current goes in a conductor the 
 greater the fall of potential in volts, due to the electrical resistance 
 of the conductor. The greater the resistance the greater the 
 fall of potential. Do the lamps used in this experiment all have 
 the same amount of resistance? Why do not the lamps give 
 light in this experiment?* 
 
 * In the "Physical Laboratory Manual," by Ball, Hauptman and Bateman, 
 issued by the Cooper Union Supply Store, New York, a different method is 
 given, which consists in employing a lamp bank, a gold-leaf electroscope, 
 and a Weston voltmeter. 
 
 They call attention to the fact that the leaves of the electroscope will 
 diverge less and less as the knob of the electroscope is connected at the 
 points of decreasing potential, and then add : 
 
 "Now use a Weston voltmeter in place of the electroscope and note 
 the readings of the voltmeter for the same points. Note how much more 
 accurately the potential difference can be estimated by means of the volt- 
 meter than by means of the electroscope. 
 
WATTS FOR ONE BRITISH THERMAL UNIT 
 
 17 
 
 FIG. 8. 
 
 EXPERIMENT V 
 
 NUMBER OF WATTS FOR ONE BRITISH THERMAL UNIT 
 
 PER SECOND 
 
 In " Physics," by Mann and Twiss,* the following exercise 
 
 is suggested, which has long 
 
 been successfully used in 
 
 schools to teach that the 
 
 electrical energy delivered to 
 
 an incandescent light is trans- 
 
 formed into heat energy. 
 
 An ordinary 16-candle- 
 
 power, 110-volt, electric lamp 
 
 is placed in a jar containing a 
 
 measured amount of water 
 
 and a thermometer (Fig. 8). 
 
 The heat from the lamp 
 
 warms the water. If the jar 
 
 contains 2 pounds (1 quart) 
 
 of water and if a Fahrenheit thermometer is used, the tem- 
 
 perature of the water rises about 1J F. per minute i.e., 
 the water is heated at the rate of 3 B.T.U. 
 a minute. 
 
 A voltmeter V and an ammeter A measure 
 the electric power supplied to the lamp. If 
 the voltmeter reads 110 volts, and the am- 
 meter reads J ampere, the power supplied is 
 110 (volts) Xj (ampere) =55 watts. Hence, 
 roughly, 55 watts = 3 B.T.U. per minute; or 
 1100 watts = 1 B.T.U. per second. 
 
 The first determination of this relation 
 was made more accurately by the same Joule 
 who made the first determination of the rela- 
 tion between the B.T.U. and the foot-pound. 
 The apparatus (Fig. 9) does not differ in 
 principle from that used by Joule. A coil 
 of platinum wire is placed in a jar con- 
 
 taining a measured quantity of water and a thermometer. A 
 
 FIG. 9. 
 
 Scott, Foresman & Co., Publishers, New York and Chicago. 
 
18 ELEMENTARY ELECTRICAL TESTING 
 
 voltmeter V and an ammeter .A, arranged as shown in Fig. 7, 
 measure the number of watts of electric power used in heating 
 the coil. The number of B.T.U. given up by the coil to the 
 water in a certain time is obtained by multiplying the number of 
 pounds of water in the jar by the number of degrees F. rise in 
 temperature. 
 
 Joule's experiments have been repeated many times by many 
 other scientists, using different current strengths and different 
 kinds of coils. The results of all of these experiments show that 
 there is a constant ratio between the heat unit and the unit of 
 electric power; and they give the accurate value of this ratio as 
 
 1055 watts = 1 B.T.U. per second. 
 4.2 watts = 1 gram-calorie per second. 
 
 The above exercise, as described, deals merely with the rela- 
 tion that the power units (watts and B.T.U. per second) bear 
 to each other. 
 
 EXPERIMENT VI 
 HEATING OF AN ELECTRIC CURRENT 
 
 The following directions, reprinted from " Laboratory Exer- 
 cises in Physics," by Fuller and Brownlee, seem to be more 
 practical than the preceding one.* 
 
 Object. To measure the number of calories of heat fur- 
 nished by an incandescent lamp and to calculate the cost. 
 
 Apparatus: Calorimeter; thermometer; 16-candle-power 
 incandescent lamp; porcelain keyless socket; voltmeter; amme- 
 ter; source of 110-volt current; graduate, or balance and weights; 
 flexible insulated wire for connections; watch or clock with 
 second hand. 
 
 By allowing a lamp to heat a known weight of water for a 
 measured time, we may find the calories per second furnished 
 by the lamp. If we know the current and voltage of the lamp, 
 we may estimate the heat liberated per kilowatt-hour. Although 
 all the heat liberated by the lamp will not be measured in this 
 
 * See also Experiment 92 in "Experimental Physics," by Smith, Tower 
 and Furton. Ginn & Co. 
 
HEATING OF AN ELECTRIC CURRENT 
 
 19 
 
 experiment, yet the efficiency of the lamp as a heater, as used 
 here, compares favorably with regular heating appratus. 
 
 Experimental. A porcelain keyless socket is connected to a 
 110-volt line with an ammeter between the socket and the 110- 
 volt terminals (Fig. 10). A voltmeter is connected across the 
 terminals of the socket. A lamp is then screwed into the socket 
 and the switch closed in the circuit to make sure that the con- 
 nections are correct and that the instruments read in the proper 
 direction. The lamp is then turned off till needed. 
 
 Into a nickel-plated brass calorimeter is placed 250 grams of 
 water at a temperature six or seven degrees below room tem- 
 perature. 
 
 FIG. 10. 
 
 This is stirred thoroughly with a thermometer and the tem- 
 perature noted; immediately the current is turned on through 
 the lamp, which is inserted in the calorimeter, the exact time in 
 minutes and seconds being noted. 
 
 The time and the temperature of the water are recorded 
 in the tabular form near the top of the left-hand page, the volt- 
 meter and ammeter also being noted and recorded. The lamp 
 should be immersed until the tip rests on the bottom of the 
 calorimeter, and the thermometer should stand in the calorim- 
 eter beside the lamp. For the next five minutes the lamp 
 
20 ELEMENTARY ELECTRICAL TESTING 
 
 burns inverted in the water. By moving the lamp up and down 
 in the water, never raising it more than a quarter of an inch 
 from the bottom, the water can be kept stirred and so of equal 
 temperature throughout. 
 
 The calorimeter should not be handled during the experiment. 
 The voltmeter and ammeter should be frequently observed, and 
 if there is any variation, the average reading for the whole time 
 should be the one recorded and used. 
 
 When the lamp has been in the water exactly five minutes, 
 take it out promptly, stir the water vigorously with the thermom- 
 eter, and read and record the temperature. 
 
 Using fresh quantities of water, repeat the test twice. The 
 water equivalent of the calorimeter should be obtained from 
 the instructor. 
 
 Make a sectional drawing of the calorimeter with lamp and 
 thermometer in place and with the connections of the instrument 
 shown. 
 
 A brief description of the method of the experiment should 
 accompany the drawing. 
 
 From the weight of the water, with the water equivalent of the 
 calorimeter added, and the change of temperature, the number of 
 calories furnished in five minutes can be calculated. The number of 
 watt-seconds is found by multiplying volts, amperes, and seconds 
 together. From these two results calculate the calories per 
 watt-second and per kilowatt-hour. As the time and the weight 
 of water are the same in all three tests, the averages of tempera- 
 ture changes, volts, and amperes w T ill be used in the calculation. 
 The problem called for in the conclusion should be worked out 
 in the note-book, using the local rate for electricity. 
 
 CALCULATED RESULTS 
 
 Corrected weight of water (water + water equivalent of calorimeter). ... g. 
 
 Average temperature change in five minutes c. 
 
 Calories furnished in five minutes cal. 
 
 Calories furnished per second cal. 
 
 Watt-seconds of energy used in five minutes w. s. 
 
 Calories per watt-second 
 
 Calories per kilowatt-hour 
 
 Cost of current per kilowatt hour c. 
 
EFFICIENCY TEST OF AN ELECTRIC MOTOR 21 
 
 Discussion. Explain any way in which heat generated by 
 the lamp may escape without being measured in this experiment. 
 
 Conclusion. At the price of . . . ff per kilowatt-hour, the cost 
 of raising 4 liters of water from 15 to 100 C. will be . . . 
 if an electric heater of the same efficiency as the lamp is employed. 
 
 POWER 
 
 A series of exercises designed to emphasize the difference 
 between work and power as measured in electrical units (e. g., 
 watt-hour and kilowatt) is commercially of primary importance. 
 
 In " A High School Course in Physics/ 7 by Gorton,* will be 
 found the following terse and lucid definition: 
 
 " POWER OF AN ELECTRIC CURRENT. Since power refers to 
 the rate at which work is done or energy expended, it may be 
 found by simply dividing the total energy expended by the time. 
 In an electric circuit, therefore, the power is measured by the 
 product of the potential difference and the current strength; or, 
 Power (watts) = volts X amperes. ' ' 
 
 EXPERIMENT VII 
 EFFICIENCY TEST OF AN ELECTRIC MOTOR 
 
 There is a rapid increase in the number of school laboratories 
 which have a small electric motor. These motors are usually 
 of recent design, and of \ horse-power or over in rating. 
 
 The student who uses motors of this type is usually impressed 
 by the fact that he is working with real commercial quantities of 
 energy and with actual life-size machines. 
 
 A determination of the brake horse-power and efficiency of 
 such a motor is considered an essential part of the laboratory 
 course in many of our high schools. Such a test is very well 
 described in " Physics," by Mann and Twiss, as follows: 
 
 " As with simple machines, water motors, and steam engines, 
 the most important thing about electric machines is the efficiency. 
 
 " Suppose that we have bought a 110-volt motor that is built 
 to develop two horse-power, and we wish to test its power and 
 efficiency in order to see whether it does what is claimed for it. 
 
 * D. Appleton & Company, Publishers. New York and Chicago. 
 
22 
 
 ELEMENTARY, ELECTRICAL TESTING 
 
 The voltmeter F, whose terminals are attached to those of the 
 motor (Fig. 11), measures the D P at the motor. It should read 
 110 volts. The ammeter A, placed in the power circuit in series 
 with the machine, measures the number of amperes flowing 
 through the motor. Suppose it reads 16 amperes. Then the 
 power supplied to the motor (power in) is 
 
 110 (volts) X 16 (amperes) = 1760 watts. 
 
 " A brake is applied to the axle of the motor and the readings 
 made. 
 
 LIN 
 
 FIG. 11. 
 
 " Let us suppose these readings to be as follows: 
 
 " Circumference of brake wheel, 2 feet; revolutions per minute, 
 
 360; pull on brake, 82.5 pounds. Then the power obtained from 
 
 the motor is: 
 
 82.5(lbs.) X2(ft.) X360(r .p.m.) =59,400 ft.-lbs. per min. 
 
 " Dividing this by 33,000 ft.-lbs. per min. to reduce it to 
 horse-power, we get 1.8 horse-power. 
 
 " So 1760 watts of electric power was supplied to the motor 
 to make it do work at the rate of 1.8 horse-power. This result 
 enables us to compare the efficiency of this motor with that of 
 
DETERMINATION OF LOW RESISTANCES 23 
 
 others; but it does not state what the real efficiency of this motor 
 is, because the power in is expressed in watts, and the power out 
 is expressed in horse-power."* 
 
 RESISTANCE 
 
 The most accessible means for measuring quantities with 
 commercial accuracy are the ones most frequently ignored. In 
 the measurement of resistance it is perhaps true that instructors 
 neglect the voltmeter-ammeter method because it seems so ob- 
 vious. Certainly no quicker method could be selected than that 
 of placing an ammeter in series, and then connecting a voltmeter 
 across the terminals of the conductor to be tested. Nothing 
 else is necessary except perhaps a rheostat to regulate the amount 
 of current. 
 
 Every student knows enough of algebra and of Ohm's law 
 to solve for the resistance when he has taken such measurements. 
 The student who is thus early asked to test the resistance of each 
 phase of an induction motor, the fields of a direct-current motor, 
 the ballast coils in an arc lamp, or any other electrical device, 
 starts with a fair appreciation of what ordinary commercial 
 measurements of resistance are like. 
 
 The following experiment fully describes the advantages and 
 limitations of this method: 
 
 EXPERIMENT VIII 
 
 THE DETERMINATION OF LOW RESISTANCES BY THE 
 AMMETER AND VOLTMETER METHOD 
 
 Apparatus Required: Weston ammeter; Weston millivolt- 
 meter, 500 millivolt range; Weston voltmeter, 3 and 15 volts; 
 unknown low resistance; storage battery, 3 cells; Weston alloy 
 or carbon rheostat. 
 
 This method is identical in principle with the one by which 
 the fall of potential along a conductor is determined. Com- 
 mercially, it is constantly used for testing the resistances of bus- 
 bars, dynamo armatures, shunts, etc. 
 
 * See also Experiment 37, Wauchope's Manual in Physics. Scott, Foresman 
 & Co. New York and Chicago. 
 
24 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 A Weston ammeter, preferably with a detachable shunt 
 having a standard drop, is connected in series with the resistance 
 material to be measured, a rheostat, and a source of steady 
 current. 
 
 Current is regulated until a suitable ammeter deflection is 
 obtained. 
 
 If the resistance of the material to be tested cannot be deter- 
 mined approximately by calculation, a voltmeter should first be 
 connected across its extremes a and b (see Fig. 12) and a pre- 
 
 FIG. 12. 
 
 liminary test made to learn whether the potential between these 
 points is too great to permit the use of the millivoltmeter. If 
 this is not the case, the millivoltmeter is substituted, its deflection 
 noted, and the resistance between a and b is found by the formula 
 
 / being the current and E the e.m.f. indicated by the millivolt- 
 meter. 
 
 Strictly, the value obtained is not that of the resistance 
 material only, but of the combined resistance of the material 
 and the millivoltmeter in parallel. 
 
 To determine the resistance accurately, a more refined method 
 must be used. 
 
 A variation of the above method, which is useful for measur- 
 ing armature resistance, etc., results from the use of a single 
 millivoltmeter, and a known low resistor approximately equal 
 
DETERMINATION OF HIGH RESISTANCES 25 
 
 to the unknown resistance, in place of the shunt. Then by first 
 connecting the millivoltmeter across the resistor, the current 
 through the circuit may be computed by Ohm's law. Next, 
 by placing the instrument terminals across the unknown resist- 
 ance, the fall of potential through it and also its resistance may 
 be then determined. With steady current, this method is as 
 accurate as the preceding one, but since it is assumed that the 
 conditions do not change between tests, it cannot be relied upon 
 with fluctuating current. 
 
 INSULATION 
 
 An ideal condition of affairs would be to have a perfect con- 
 ductor of electricity protected by a perfect insulator. We would 
 then always secure 100 per cent efficiency, irrespective of the 
 length of cross-section of our conductor. But unfortunately 
 we have neither, since all conductors have resistance and all 
 insulators are imperfect conductors. The result is that there 
 is always a current discharge on a line. This discharge or 
 " leakage " is increased by the use of poor or worn insulating 
 material and by dampness. 
 
 In all electric light and power stations, leakage is regarded as 
 waste, involving a direct financial loss. Therefore, it is of great 
 importance to be able to " keep up the line insulation " and to 
 understand how to locate leaks or " grounds." 
 
 EXPERIMENT IX 
 
 THE DETERMINATION OF HIGH RESISTANCES BY THE 
 VOLTMETER DEFLECTION METHOD 
 
 Apparatus Required: Weston direct-current voltmeter, 150- 
 volt range; high unknown resistance; direct-current line voltage, 
 100 to 120 volts. 
 
 On a direct-current line, in order to successfully determine 
 insulation resistance, it is necessary to use a Weston permanent 
 magnet movable-coil type of high resistance voltmeter, with a 
 uniformly divided scale. The resistance of the instrument 
 should be known. 
 
26 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 The normal voltage of the line is then found by connecting 
 the voltmeter across the line, as shown in Fig. 13, and the deflec- 
 tion noted. 
 
 FIG. 13. 
 
 One binding post of the instrument is then grounded by con- 
 necting it with any convenient water, gas or steam pipe, and 
 the other binding post connected directly with one of the gener- 
 ator terminals. The other generator terminal is connected with 
 the line to be tested and the resultant deflection also noted. 
 See Fig. 14. 
 
 = 6 
 
 FIG. 14. 
 
 The resistance of the line (in ohms) is then found by the for- 
 mula : 
 
 Y SXR 7? 
 
 Xgr-B. 
 
 S = First deflection in scale divisions; 
 S' = Second deflection in scale divisions; 
 R = Instrument resistance. 
 
THE WHEATSTONE BRIDGE 
 
 27 
 
 Example : 
 
 =120; 
 
 ' = 20; 
 
 = 15000; 
 
 X= 
 
 X = 75000. 
 
 The insulation resistance of a line or cable may readily be 
 determined in this manner if it is not too high. 
 
 Sometimes, when the insulation is very poor, a fine class- 
 room illustration can be made by grounding the line through an 
 ammeter in series with an incandescent lamp, as shown on 
 Fig. 15. 
 
 =" G 
 
 FIG. 15. 
 
 EXPERIMENT X 
 THE WHEATSTONE BRIDGE 
 
 The Wheatstone bridge remains the time-honored instru- 
 ment for the highly accurate determination of resistance. In 
 commercial testing laboratories, it is regularly used for routine 
 and special work, demanding greater precision than can readily 
 be obtained by more rapid methods. Its use is also common 
 for measurements in which the large currents or voltages required 
 by other methods are either unobtainable or objectionable. 
 As an illustration, reference might be made to its use in telegraph 
 and telephone tests for faults and grounds. The Wheatstone 
 bridge is operated by specially trained workers, and for educa- 
 tional purposes it should not be classed with voltmeters, ammeters, 
 and wattmeters as an instrument with which every high-school 
 graduate should be familiar. 
 
28 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 Many teachers question the wisdom of attempting to teach 
 beginners the principle and operation of this instrument, as much 
 because of the indifferent success which so frequently results as 
 because of its special character. This discussion is intended for 
 those teachers who feel that they must attempt its use. The 
 slide-wire form is here suggested, not only because it is already 
 in common use, but also because it lends itself to a close corre- 
 lation between the " Fall of Potential " exercises which should 
 precede it. 
 
 Apparatus Required: Slide wire bridge; Weston student 
 galvanometer; Weston ammeter; resistance box; resistance 
 
 FIG. 16. 
 
 wire; coil of unknown resistance; one or two cells; incandescent 
 lamps. 
 
 If four incandescent lamps are arranged in multiple series, 
 as shown in Fig. 16, and connected with the line, they will serve 
 excellently to demonstrate the principle of the parallelogram 
 of forces, as exemplified by the Wheatstone bridge. Even if 
 the rest of the exercise is omitted, this part should be given as a 
 class-room demonstration. The instrument should preferably 
 be a zero center ammeter. 
 
 Suitable questions would be as follows: 
 
 If an ammeter is connected (as shown), what will happen? 
 
 Will it indicate, and if not, why not? 
 
 If one of the lamps is removed, what will be the result? 
 
THE WHEATSTONE BRIDGE 
 
 29 
 
 If a lamp of different size (resistance) is substituted, will it 
 make any difference? 
 
 If, instead of the lamps, a parallelogram of resistance wire 
 is constructed so that the four sides have the same resistance, 
 and a battery and galvanometer are connected, as shown in Fig. 
 17, the current will split where A and B unite, pass through 
 A and X and also through B and R, reuniting where R and X 
 join. The four " arms " of the bridge being alike, no current 
 will flow through the galvanometer when connected as shown. 
 In other words, the potential at the junction of AX and BR 
 
 FIG. 17. 
 
 will be the same. The battery and galvanometer may then 
 be interchanged without affecting the balance. 
 
 To construct a practical bridge, it is only necessary to sub- 
 stitute for A , B } X , and R conductors of large area and negligible 
 resistance with suitable gaps for the insertion of resistance coils 
 and the material to be measured. See Fig. 18. If then, A and 
 B are each 100 ohms and R is also 100 ohms, a balance is secured 
 by adjusting X, and when no current flows through the gal- 
 vanometer the resistance of X will be 100 ohms. If the resistance 
 of X is to be determined, R is made adjustable and is varied 
 until it equals X. The resistance of R and X, if equal to each 
 other, may differ greatly from A and /?, and an equilibrium 
 be established, provided A and B equal each other. Finally, 
 A, B, R, and X may all differ in resistance and a balance be 
 
30 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 secured, provided a proportionality exists in their respective 
 resistances for instance, when 
 
 A = 10, = 100, 
 
 and 
 
 350. 
 
 These conditions obtain when an uneven bridge or ratio is used 
 in making measurements. 
 
 FIG. 18. 
 
 EXPERIMENT XI 
 THE SLIDE WIRE BRIDGE 
 
 In the slide-wire form of bridge A and B consist of a single 
 wire of uniform resistance. Contact may be made at any point 
 on this wire, and a scale is provided to determine the position 
 of contact. See Fig. 19. The galvanometer is preferably con- 
 nected as shown, an unknown resistance is inserted at X and a 
 box of coils at R. The slide- wire contact is at first placed cen- 
 trally, and an approximate balance secured by varying R. A 
 place is then found on the slide wire where perfect balance is 
 secured. 
 
 Since the scale is divided into a thousand parts, the resistance 
 of X can be closely determined by simple proportion. 
 
EFFECT OF TEMPERATURE 
 
 31 
 
 EXPERIMENT XII 
 
 THE EFFECT OF TEMPERATURE ON THE RESISTANCE OF 
 A LAMP FILAMENT* 
 
 Apparatus: 110- volt direct current; series lamp resistance or 
 any suitable adjustable resistance; switch with 6-ampere fuses; 
 ammeter; voltmeter; lamps to be tested. (These should be 
 of several types.) 
 
 The difference between the resistance of an incandescent lamp 
 filament when the switch to power is first closed and the resistance 
 
 FIG. 19. 
 
 of the same filament an instant later when the filament has been 
 raised to its normal operating temperature, is a matter of con- 
 siderable commercial importance. With the old type carbon 
 filaments, where the temperature effect is to give a smaller 
 initial current, the phenomenon could be safely allowed to adjust 
 itself. With the newer metallic filament lamps in which the first 
 rush of current may be much greater than the normal operating 
 current, some protection to the line, when the power switch is 
 first closed, is frequently necessary where large groups of lamps 
 are being supplied. The character and extent of this temperature 
 
 *From "The Loose Leaf Laboratory Manual," published by John 
 Wiley & Sons, Inc. 
 
32 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 effect varies with various types of lamp filaments will be apparent 
 from the following study. 
 
 Method. Connect your apparatus with the 110-volt circuit 
 as shown in the diagram, using first a carbon filament test lamp. 
 (The voltmeter is here connected around the ammeter so that 
 the latter may read the true current through the lamp.) Put 
 a fuse in the line to protect the ammeter. See Fig. 20. 
 
 Data and Results. Take readings of both the ammeter and 
 voltmeter with all the lamps in the lamp board in series with the 
 test lamp. Then cut out one lamp at a time, and take readings 
 of the voltmeter and the ammeter for each step, until the last 
 
 EFFECT OF TEMPERATURE ON THE 
 RESISTANCE OF AN INCANDESCENT LAMP 
 
 FIG. 20. 
 
 reading is for the test lamp alone. The ammeter and the voltmeter 
 should be read simultaneously. 
 
 Compute the resistance of the test lamp for each current. 
 
 You have no actual measurement of temperature here, but 
 as the current increases the temperature of course rises very 
 perceptibly for the last two or three readings. 
 
 Plot a curve from your data, using the resistances of the 
 lamp as ordinates, the current as abscissae. 
 
 What conclusion may be drawn regarding the effect of increase 
 of temperature on the resistance of the filament? 
 
 Test in the same way lamps having a treated carbon filament 
 and lamps having a metal filament as in the tantalum and tungsten 
 lamps. Plot curves for each and compare the filaments as to 
 their change in resistance with rise in temperature. 
 
EFFECT OF TEMPERATURE 33 
 
 Addenda. Connect the voltmeter across the test lamp only, 
 and note the ammeter reading. Compare this with the ammeter 
 reading taken with the voltmeter connected across both the 
 lamp and ammeter. Which ammeter reading gives the true 
 current through the lamp, and why? Which voltmeter reading 
 gives the true voltage across the lamp, and why? When would 
 it be best to connect the voltmeter across both the ammeter 
 and test piece? When across the test piece only? 
 
 Give detailed reasons for your answer. (See " Elements of 
 Electricity," p. 413.) 
 
 PROBLEMS: 1. Measure on a Wheatstone bridge the resistance .of the 
 tungsten filament lamp at room temperature (20 C.). Using the tempera- 
 ture coefficient of tungsten as .0051, and the resistance found for the lamp 
 in the above experiment when on its rated voltage, compute the temperature 
 of the filament when used on its rated voltage. 
 
 2. In an experiment like the above the voltmeter was connected across 
 the test lamp only. If the resistance of the voltmeter is 16,500 ohms, the 
 voltmeter reading is 110 volts, and the ammeter reading is .285 amp., what 
 is the true current through the lamp filament? 
 
 THE WESTON DIRECT-CURRENT AMMETER 
 
 For pedagogic purposes the Weston ammeter with detachable 
 shunts has marked advantages over the instrument with a self- 
 contained shunt. The instructor may call attention to the 
 fact that a " shunt " is really a part of the main conductor carry- 
 ing the current and that only a small fraction of the total cur- 
 rent passes through the movable system of the instrument. 
 
 A clear conception of the operation of a Weston ammeter 
 may be obtained by means of a water-main analogy. The water 
 main or trunk A through which water is flowing under pressure 
 as in city water mains (Fig. 21), may be fitly compared with the 
 conductor or bus-bar F (Fig. 22). The section B of reduced 
 diameter corresponds with the shunt E, and the small pipes C 
 may be compared with the leads (7. It is obvious that, since the 
 pipes C are of smaller dimensions than B, less water will flow 
 through them; and, provided that the proportion between 
 B and C is fixed and constant, the quantity of water flowing 
 through the meter D will depend upon the total quantity flowing 
 through B, and hence D may be calibrated to indicate the total 
 
34 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 flow of water instead of merely indicating the quantity passing 
 through C. 
 
 On the same principle the scale of the instrument H may be 
 figured to indicate any required amperage; but, although the 
 
 FIG. 21. 
 
 total current flowing through F may be 20,000 amperes or more, 
 the sensitivity of the movement of H to current is so great that 
 only about .3/100 ampere is required to produce a full scale 
 deflection, and under proper conditions no current in excess of 
 
 Fio. 22. 
 
 this amount will ever pass through H. The conductors G, 
 together with the movable coil and resistors inside of H, are so 
 proportioned that the flow of current through H is limited to the 
 proper amount. But, nevertheless, instead of merely indicating 
 
THE STUDY OF AN AMMETER 35 
 
 the amount of current passing through its movement, H may 
 be calibrated to correctly indicate the total current flowing 
 through FEF. 
 
 Finally, the quantity of current which will flow through G 
 will depend upon the resistance of the shunt E; therefore, since 
 shunts of different resistance and current capacity may be used 
 in place of E, the instrument H may be used for an unlimited 
 number of ampere ranges.* 
 
 EXPERIMENT XIII 
 THE STUDY OF AN AMMETER f 
 
 Apparatus: Ammeters of several types. 
 
 Ammeters may be divided into two classes: (1) Thermal 
 in which the movement of the index is secured through the change 
 in length of a wire when heated by the current passed through 
 it. The heat generated, and therefore the change in length, 
 is here proportional to the square of the current; and 
 
 (2) Electromagnetic in which the motion is due to the 
 magnetic field produced when current is sent through the coils 
 or coil of the instrument. Electromagnetic ammeters may be 
 of three types: (a) Solenoidal; (6) permanent magnet; (c) 
 elect rody namometer . 
 
 Data and Results. Examine instruments of several of the 
 above types and report upon the following features for each : 
 
 1 . The type of instrument. 
 
 2. The scale: range and graduation. Divisions uniform or 
 
 varying width. Why? 
 
 * A shunt in electrical parlance was formerly defined as a resistor connected 
 in multiple with an indicating instrument carrying the main current to 
 reduce the current flowing through the latter; but, since the introduction 
 of Weston instruments, "shunt" has become the trade name for a constant 
 resistor of special form, having dimensions which will permit its practical 
 use in series with a conductor carrying a current, independent of the current 
 capacity of its indicating instrument. 
 
 t From " The Loose Leaf Laboratory Manual." John Wiley & Sons, 
 Inc. 
 
36 ELEMENTARY ELECTRICAL TESTING 
 
 3. The construction: 
 
 (a) Fixed parts; nature, material, construction and 
 
 arrangement. 
 (6) Moving parts; construction, bearings; control by 
 
 which they are returned to zero; damping. 
 (c) Shunt. 
 
 4. The connections by which the current enters and leaves 
 
 the instrument. The circuit from bind post to bind post. 
 
 5. The resistance of each coil and of each shunt separately. 
 
 The resistance of the instrument from terminal to 
 terminal. 
 
 Explain the construction and arrangement of each essential 
 part by simple diagrams, and show by a simple diagram how 
 the parts are assembled in the complete instrument. Show the 
 direction of current through each instrument and explain the 
 reason for the deflection of the moving parts by the current. 
 State in each case whether the instrument is suited for the mea- 
 surement of direct or alternating current and why. 
 
 If the instrument is suited for both direct and alternating 
 current, will the same calibration do for both kinds of power? 
 Explain the reason for your answer. 
 
 By means of a millivoltmeter measure the millivolt drop 
 across each instrument on full scale reading and compare this 
 with the computed value. Account for any difference. 
 
 Show by diagrams the proper directions of the instrument 
 to a line to measure the current taken by a motor. 
 
 PROBLEMS. The full scale reading of a millivoltmeter is 100 millivolts. 
 The resistance of the moving coil is 5 ohms. 
 
 (a) What resistance must a shunt be in order to be used with this milli- 
 voltmeter to have the scale read amperes? 
 
 (6) What must the resistance of a shunt be in order that the full scale 
 reading may indicate 10 amperes? 
 
 SUGGESTIONS FOR FURTHER STUDY 
 
 If the "drop" of a West on station shunt of specified ampere 
 capacity is 50 millivolts, what will its resistance be between its 
 potential terminals? 
 
THE STUDY OF AN AMMETER 37 
 
 Detach the shunt of a Weston ammeter with removable 
 shunts, and measure the resistance of the instrument by means 
 of a bridge. 
 
 Note: Minimum current should be used, and the battery 
 key kept depressed to prevent the disturbing effect of currents 
 induced by the motion of the movable coil. 
 
 If the drop of a shunt is 50 millivolts, the instrument for 
 which it is intended must necessarily give a full scale deflection 
 with 50 millivolts. What then will be the amount of current 
 required to give a full scale deflection when the instrument is 
 used without a shunt? 
 
 Explain what the effect would be if the leads from the shunt 
 to the instrument were altered in length or resistance. 
 
 Determine the power (in watts) consumed by the instrument 
 with shunt when in circuit with full scale deflection. 
 
 THE WESTON DIRECT-CURRENT VOLTMETER 
 
 The Weston direct-current movable coil voltmeter consists 
 essentially of a light rectangular coil of copper wire usually 
 wound upon an aluminum frame, pivoted in jeweled bearings 
 and mounted to rotate in an annular space between the soft iron 
 core and the specially formed pole pieces of a permanent magnet. 
 A light tubular pointer is rigidly attached to the coil and moves 
 over a calibrated scale. 
 
 The current is led into and from the coil by means of two spiral 
 springs, which serve also to control its movement. This move- 
 ment is due to the dynamic action between the current flowing 
 through the coil and the magnetic field of the permanent magnet. 
 (See Fig. 23.) The pointer becomes stationary and the coil attains 
 a position of equilibrium when the opposing forces of the springs 
 equal the force caused by the rotary tendency of the coil. Since 
 the magnetic field is uniform and the torsion of the springs 
 proportional to the deflection, the scale divisions are practically 
 uniform. 
 
 The well-known aperiodic or " dead-beat " quality of Weston 
 instruments is produced in this type by foucault currents gen- 
 erated in the aluminum frame when rotating through the mag- 
 netic field. These Foucault currents have a sufficient influence 
 
38 ELEMENTARY ELECTRICAL TESTING 
 
 on the movement of the coil to cause it to come to rest almost 
 instantly and without friction. 
 
 Ball, Hauptman and Bateman, when dealing with the subject 
 of Potential Differences,* make the following statement, which 
 we quote not only because it is well expressed, but also since it 
 serves excellently to describe the proper conditions under which 
 voltmeters may be uesd : 
 
 " It has already been stated and shown in the previous experi- 
 ments that, when certain electrical generators are employed, the 
 P.D.'s between bodies charged from such generators may remain 
 unchanged even when these bodies are connected by a conductor. 
 This effect may, under proper conditions, be made the basis of 
 measurement of P.D. The instruments generally employed for 
 the commercial measurement of P.D. are based upon this principle. 
 It must be noted, however, that the use of such instruments 
 is admissible only when the current which passes through them 
 does not alter the P.D. between the points to which they are 
 connected." 
 
 If, therefore, an instrument is used to measure this P.D., it 
 should, to perform its functions properly, be so constructed that 
 it is extremely sensitive to current. 
 
 Weston voltmeters fulfill these conditions. It is true that 
 they are conductors and require a small current to render them 
 operative, and that there are conditions under which they would 
 not indicate correctly. For instance, they could not be used 
 with marked success to determine the static charge of a Holtz 
 machine or a Ley den jar. Neither would they correctly indicate 
 the potential of a Zamboni dry pile,f nor should they be used to 
 directly test the e.m.f. of a Weston standard cell. 
 
 But since Weston voltmeters require only about 1/100 ampere 
 with full scale deflection, there will be no appreciable fall in 
 potential when they are connected across any commercial source 
 of current, ranging from a light and power plant to a dry cell. 
 
 An exercise similar to the following one,- on the " Study of 
 a Voltmeter," should in our opinion form part of -every labora- 
 tory course in physics. 
 
 * Experiment No. 107, Laboratory Experiments, 1913. 
 t Deschanel's Natural Philosophy, 1870. 
 
THE STUDY OF A VOLTMETER 39 
 
 EXPERIMENT XIV 
 THE STUDY OF A VOLTMETER* 
 
 Apparatus Required: Voltmeters of several types. 
 
 Voltmeters may be of the types suggested in preceding 
 experiment, or may be electrostatic. 
 
 Examine instruments of several types and report upon the 
 following for each : 
 
 1. The type of instrument. 
 
 2. The scale: range and graduation. Divisions uniform or 
 
 varying width. Why? 
 
 3. The construction: 
 
 (a) Fixed parts: nature, material, construction and 
 
 arrangement. 
 (6) Moving parts: construction; bearings; control by 
 
 which returned to zero; damping, 
 (c) Series coils. Double-scale instruments. 
 
 4. The connections by which current enters and leaves the 
 
 instrument. The circuit from binding post to binding 
 post. 
 
 5. The resistance of the instrument. Resistance per volt. 
 
 Explain the construction and arrangement of each essential 
 part by a separate diagram, and show by a simple diagram how 
 the parts are assembled in the complete instrument. Show the 
 direction of current through the instrument in each and give the 
 reason for the deflection of the movable parts. Why does the 
 reading indicate the difference of potential at the terminals? 
 State in each case whether the instrument is suited to the meas- 
 urement of direct or alternating voltages and why.f 
 
 * From " Physical Laboratory Notes," by J. M. Jameson.* 
 t See also "Electrical Instruments and Testing," (Chapter IV), by Schnei- 
 der and Hargrave, published by Spon & Chamberlain, N. Y., also "Lessons 
 in Practical Electricity," Lesson XVIII, Swoope, published by D. Van 
 Nostrand Co., N. Y. 
 
40 
 
 ELEMENTARY ELECTRICAL TESTING 
 
 Suggestions for Further Study. Note or measure the resist- 
 ance of one or more ranges of a Weston voltmeter of the movable 
 coil type. 
 
 Determine the current actually flowing through the instru- 
 ment with full scale deflection. 
 
 Determine the power (in watts) required to produce a full 
 scale deflection with different ranges. 
 
 FIG. 23. 
 
 Explain why a voltmeter the resistance of which is high (in 
 ohms per volt) is preferable to one of low resistance. 
 
 Explain the construction of a 
 ment and field. 
 
 Weston " soft-iron " move- 
 
 EXPERIMENT XV 
 THE ACTION OF A SIMPLE CELL 
 
 Since generators and storage cells have taken the place of the 
 primary batteries once largely used for the commercial produc- 
 tion of current, the exercise which follows may be rated by many 
 instructors as too abstract to deserve a place in a practical physics 
 
THE ACTION OF A SIMME CELL' 
 
 41 
 
 course. However, nearly every modern text book still retains 
 it, and it remains for the instructor to decide whether it suffices 
 to merely refer to Volta's discoveries, or emphasize their signifi- 
 cance by means of an exercise which enables the student to pro- 
 duce current by chemical action. 
 
 Apparatus Required: Low range, direct-current, 5-ampere 
 Weston ammeter; low range, direct-current, 1.5- or 3-volt Weston 
 voltmeter; strips of sheet zinc, 2X6 inches; strips of sheet 
 copper, 2X6 inches; glass jar, about 5 inches in diameter and 4 
 inches deep; ten per cent sulphuric acid solution; mercury; 
 copper wire. 
 
 FIG. 24. 
 
 FIG. 25. 
 
 Copper leads are to be soldered to the plates, the latter being 
 bent so that they may hang in the solution from the edge of the 
 jar. 
 
 The voltmeter is first connected and deflection noted, which 
 will be about 1 volt. See Fig. 24. 
 
 The ammeter is then substituted and deflection also noted. 
 Current when circuit is first completed will be about 1 ampere; 
 but since polarization rapidly sets in, the current at once decreases. 
 Why? 
 
 Note the large number of hydrogen bubbles passing to the 
 surface of the liquid from the zinc plate. Why? 
 
 Substitute an amalgamated zinc plate. Note that no hydro- 
 
42 ELEMENTARY ELECTRICAL TESTING 
 
 gen forms except when the circuit is closed, and that the current 
 is comparatively constant. Why? 
 
 With the ammeter in circuit, decrease the distance between 
 the zinc and copper poles, and note the increase in current. Why? 
 See Fig. 25. 
 
 Disconnect the ammeter and try the above experiment with 
 the voltmeter. Note that there is no appreciable difference 
 in the e.m.f. Why? 
 
 With the plates hanging from the edge, connect both instru- 
 ments successively, and note their deflections. Determine the 
 resistance of the circuit (cell, ammeter and leads) by the formula 
 
 R = resistance of circuit; E = e.m.f.; / = current. 
 
 When both instruments are connected at the same time, the 
 indicated e.m.f. of the voltmeter is much less than it is when the 
 ammeter is not in parallel. Why? 
 
 NOTE. In making the above experiments the student may be 
 permitted to connect the ammeter as stated, because the 
 resistance of the cell is sufficiently high to limit the flow of current 
 to a safe amount. He should, however, be cautioned about 
 using an ammeter in this manner when the resistance of a cell 
 is likely to be low, and consequently its current is large. For 
 instance, an ordinary dry cell, when new, will have so low a 
 resistance that it will sometimes give as much as 30 amperes 
 for a short time when short-circuited through a Weston ammeter. 
 Another point of importance is that the resistance of a Weston 
 voltmeter is so high that a small decrease in the resistance of 
 the circuit, of which it forms a part, caused by bringing the plates 
 closer together, will have too slight an effect to materially change 
 the total current flowing under the conditions.* 
 
 * See also Gorton's "High School Physics," Chapter XVIII; Fuller and 
 Brownlee's "Laboratory Exercises," Experiment 68; "Physics," Mann and 
 Twiss, page 58; and "Physics Laboratory Manual," by Cavanagh, West- 
 cott and Twining. Ginn Company, Publishers, New York. 
 
THIS BOOK IS DUE ON THE LAST DATE 
 STAMPED BELOW 
 
 AN INITIAL FINE OF 25 CENTS 
 
 WILL BE ASSESSED FOR FAILURE TO RETURN 
 THIS BOOK ON THE DATE DUE. THE PENALTY 
 WILL INCREASE TO SO CENTS ON THE FOURTH 
 DAY AND TO $1.OO ON THE SEVENTH DAY 
 OVERDUE. 
 
 NOV 21 i 936 
 
 
 
 
 
 
 APR 8 1*40 
 
 
 
 
 JUN 41 19 
 
 
 - 9hn'52JPB 
 
 
 liggl!^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 LD 21-100m-8,'34 
 

 ssoisa, 
 
 UNIVERSITY OF CAUFORNIA LIBRARY