Direct and Alternating Current
DIRECT AND ALTERNATING CURRENT
Prepared to accompany Timbie's Elements of Electricity
CLARENCE E. CLEWELL
Sheffield Scientific School of Yale University
NEW HAVEN, CONN.
COPYRIGHT, 1913, BY
CLARENCE E. CLEWELL
T A- i 'I '
PRESS OF THE TIMES PUBLISHING COMPANY
This book of laboratory directions is designed for the use of
students in courses other than that of Electrical Engineering,
who take a brief amount of work in the fundamental principles
The object of the laboratory work in such cases is to aid in the
understanding of the theoretical and practical items given in the
recitation and lecture room. It is the purpose, therefore, to elimi-
nate in as far as possible all features in the actual work of the
laboratory which would tend to lessen or detract from concen-
tration upon the underlying principles involved in the experi-
ment in hand.
To facilitate the work, each experiment is prefaced where possi-
ble by an assignment of articles in the text book which bear di-
rectly or indirectly on the experiment in question. Further, in a
part of the experiments, diagrams of the electrical connections
will be found, together with simple forms which may be followed
in the recording of observations. These forms are somewhat
more complete in the first than in the advanced experiments in
order to serve at the outset as a guide in the preparation of the
data sheets. In the latter experiments this general scheme of rul-
ing up the data sheet is to be followed by each group of men in
the preparation of data sheets before the laboratory exercise.
After teaching with a text book which was too advanced for the
students and a laboratory manual which was not suited to the
text book, the students nor the equipment, it was thought best to
write a Manual which should be adapted to the specific require-
ments of the coming year. The difficulties encountered by the
student have been observed and the aim of the Manual is to pre-
sent the subject from his standpoint. The emphasis placed on
such practical items as constant potential supply mains in the
wiring diagrams, and the practical points connected with direct
and alternating current generators and motors and power trans-
mission in the text, are intended to familiarize the student with
the principles underlying the operation of standard apparatus
which he may encounter after graduation.
Appreciation for many suggestions and helpful advice in the
make-up of this book is due Professor Chas. F. Scott, Sheffield
Scientific School of Yale University.
CLARENCE E. CLEWELL.
NEW HAVEN, CONN.,
1. Ohm's Law 9
2. Measurement of Armature and of Field Resistance 13
3. A^oltage and Power Losses in Transmission 15
4. Study of Measuring Instruments 17
5. Study of Fuses and Circuit Breakers 21
6. Study of Starting Boxes for Motors 24
7. Study of Electric Lamps 27
8. Building up of the Shunt Generator 30
9. Electrical Features of the Shunt Generator 34
10. Comparison of Shunt and Separate Field Excitation 37
11. Electrical Features of the Compound Generator 40
12. Study of the Storage Battery 44
13. Study of Electric Wiring. 46
14. Shunt Motor Speed Features 48
15. Efficiency of the Shunt Motor by the Brake Method 51
16. Series Motor Speed Features 53
17. Efficiency; Stray Power Test; Brake Test 56
18. Static Torque Test on a Motor 59
19. Shunt Generators in Parallel 61
20. Compound Generators in Parallel 65
21. Resistance and Reactance in Series 71
22. Resistance and Reactance in Parallel 76
23. Study of Three-Phase Circuits 79
24. Study of the Transformer 83
25. Electrical Features of the Transformer and the Transmission
of Power 86
26. Study of the Induction Motor 88
27. Electrical Features of the Induction Motor 90
28. Study of the Synchronous Motor 93
29. Alternators in Parallel 96
30. Study of the Mercury Arc Rectifier 98
INTRODUCTION TO THE LABORATORY WORK.
1. General Hints. While the laboratory work is intended
primarily as an aid to a clear understanding of the text book and
lecture work, to be successful, it must be performed in a system-
atic manner, and with due care in handling electric circuits,
instruments and machinery.
The electric current is somewhat intangible and when making
connections and carrying out tests, the only way to conduct the
work intelligently is to form at the outset definite ideas of the
laws which govern the flow of current in the circuit involved.
A most important item to observe is the distinction between
electric pressure (electromotive force) expressed in volts, and
electric current expressed in amperes, and quantitative ideas
should be gained early in the work regarding the usual values
of these units. The most common value of pressure in com-
mercial lighting is approximately 110 volts. The normal voltage
is different in different plants ; it may be 108, or 110, or 115, etc.
In each case the aim is to maintain the normal value at all times.
The general term "110 volts" is used to mean a constant po-
tential circuit of the 110-volt class. An ordinary carbon filament
incandescent lamp consumes one-half an ampere with a pressure
of 110 volts across its terminals. The principal circuits used in
the laboratory tests are 110 and 220 volts. Electric railway cir-
cuits are ordinarily 550 volts, and five 110-volt lamps are con-
nected in series in the cars for lighting.
In connecting up a set of apparatus and the instruments for a
test, some idea should be had of what each component part of the
circuit stands for in its relation to the whole, as well as a fairly
definite idea of what will happen when the main switch is closed.
As a safeguard, the Instructor should always be asked to check
2 LABORATORY MANUAL
over the connections before power is applied through the closing
of the main switch.
As to the connections themselves, a switch, together with a fuse
in each side of the line, should be installed. The switch should be
closed and opened quickly at the start to see if all is correct, and
after the switch has been closed and the work started, each piece
of apparatus should be watched closely for the first few minutes
to note if any part of the circuit is being overheated. Should any
trouble exist, open the switch at once and trace the connections
to find the error. A heavy current will flow through a low re-
sistance at 110 or 220 volts, and hence, in using apparatus having
low resistance an adequate additional resistance must always be
placed in the circuit before closing the switch.
2. Material Required for the Work. Each man is to have
the following items :
(a) Paper for use in recording data with carbon paper for
duplicating the data sheets.
(b) A pad of ruled loose leaf note paper on which the report
is to be written, together with suitable report covers.
(c) A pad of cross section paper the same size as the loose leaf
paper for curve plotting.
It is suggested that students provide themselves with a pair of
w r ireman 's pliers and a screw driver.
3. Instruments. Electrical measuring instruments are deli-
cate and expensive and they should be handled with the same
care that one would use in the handling of a high grade watch
or equivalent piece of jewelled apparatus. Damage to instru-
ments through carelessness is chargeable to the students involved.
Ammeters. An ammeter is a low resistance instrument and is
intended for the measurement of the electric current which flows
through its coil. The important item is to avoid sending through
the ammeter a current which is larger than it is intended to
carry. A safeguard is a short circuiting switch as shown in Fig.
1. When this switch is closed the current flows through the
switch and the ammeter is thus protected. When the observa-
tions are to be made, the switch is opened and the current flows
through the instrument. This precaution, while not always
necessary, is sufficiently important to make it desirable in the
Millivoltmeters and Shunts. A resistance of known value R
may be connected in the circuit (the shunt in Fig. 1), and a low
reading voltmeter known as the millivoltmeter ("thousandths of
a volt" meter) may be connected across its terminals to deter-
mine the current flowing through the resistance R. If e is the
reading on the millivoltmeter in millivolts (thousandths of a
volt), the current is then equal to
-i- R or
These instruments may be calibrated to read amperes directly,
or what is more common in the laboratory, the shunt is stamped
Supply Mains (110 Volts D C.)
M [ 1
rt Circuiting /
To the Load
Fig. 1. Two types of instrument for measuring current: (a) am-
meter, carrying the full current; (b) shunt with millivoltmeter for
measuring the voltage drop in the shunt. Note the short circuiting
switch for the protection of the instrument in each case.
with the current which flows through the circuit when the milli-
voltmeter to which it is connected indicates its full scale deflec-
tion. Thus a millivoltmeter whose full scale deflection is 100,
when used with a shunt stamped 10, indicates 10 amperes when
the millivoltmeter registers its full deflection, that is, each scale
division indicates one-tenth of an ampere. All shunts in the
laboratory are numbered according to the instrument with which
they are to be used, and in all cases the number on the shunt and
on the instrument must correspond,
4 LABORATORY MANUAL
Voltmeters. A voltmeter is a high resistance instrument and,
hence, is actuated by a relatively small current flowing through
its coil. The voltmeter is intended for use across a circuit and
measures the pressure E effective between its terminals. If the
resistance of the voltmeter is R, and it is connected across 110-
volt mains, the current flowing through the instrument is 110/R.
Thus the voltmeter is, in reality, a current-measuring instrument
on a small scale, but its divisions are calibrated in volts and the
current is usually a very small fraction of an ampere or a negli-
gible quantity in most of the practical experiments. The range
of a voltmeter may be increased by the use of an external supple-
mental resistance termed a multiplying coil, although in many
of the simpler tests this is unnecessary, since instruments of a
suitable range are available.
4. Use of Instruments. Always keep an instrument away
from stray magnetic fields like those in the neighborhood of some
electromagnets and some generators and motors. Outside mag-
netism may affect the accuracy of an instrument very appre-
Where the wires from an instrument cross the floor and there
is danger of it being pulled off the table, the wires should be an-
chored to a table leg or should pass through holes in the table
provided for this purpose.
No instrument should be used where the reading to be taken
is less than one-third of its full scale deflection, since the accuracy
of an instrument is greater for higher than for lower readings.
Observe and record the zero deflection of each instrument be-
fore connecting it into the circuit, also record the correction con-
stant of each instrument on the data sheet. If this constant is
appreciable, each observation should be duly corrected. Note on
the data sheet the number of each instrument used somewhat as
"Weston Voltmeter No. 65.
Zero deflection, 0.5 volt; Correction constant, 0.97.
5. Rheostats. A suitable rheostat or adjustable resistance
should be employed in most cases, both as a protection for circuits
of low resistance and to afford the required adjustments of the
current throughout the experiment. Always select a rheostat
that will carry the required current without overheating and that
will give the necessary variations of resistance.
So-called field rheostats designed for use in series with the
shunt field windings of generators, are adapted to low current
values, and must not be used where larger currents are in-
volved. Look on the rheostat for its current rating.
A bank of incandescent lamps connected in parallel may be
used as a rheostat. A lamp bank is a safe device to use for this
purpose because the resistance cannot be reduced to zero. Note
that when one lamp is turned on the resistance is twice as great
as when two are on. In other words, the more the lamps turned
on the less the resistance. Each 16 candle-power carbon lamp
when burning has a resistance of about 220 ohms and carries one-
half an ampere at 110 volts.
Other types of rheostats for special purposes are usually avail-
able in the instrument rooms.
6. Circuit Breakers and Fuses. When a circuit breaker
opens, always open the main switch before closing the breaker,
then close the switch. If trouble is still on the line, the breaker
is apt to open when the switch is closed.
If a fuse blows, open the main switch before replacing the fuse,
and after the new fuse has been installed close the main switch.
See that the fuse is of the proper size for the circuit it is to pro-
7. Constant and Variable Factors in the Experimental Work.
In many of the experiments, observations are made of the
changes which take place in one or more factors when certain
conditions are maintained constant and certain other factors are
varied according to some prescribed method. This gives rise to
three classes of observation items, namely, constants, independent
variables and dependent variables.
For example, in Experiment 1, under item 1, Order of Work,
with a given size and kind of wire and a constant current flowing,
the lengths of the wire are to be varied and the changes in the
volts drop for these different lengths are to be observed. Here
the constants are the size and kind of wire and the current ; the
independent variable, that which is changed directly by the ope-
rator, is the length of the wire ; and the dependent variable is the
6 LABORATORY MANUAL
In each experiment, the student is to indicate on the data
sheet, preferably at the top of the various columns, to which of
these classes of observation items the data belongs.
8. Suggestions for the Work in the Laboratory. The fore-
man of each group of men is to prepare a ruled and labelled data
sheet on the paper for that purpose before the work of taking
observations is begun. There is to be one data sheet for each
man in the group and one extra copy is to be made for placing on
file with the Instructor.
To secure instruments before beginning the work, fill out a re-
quisition card with complete designation of each instrument de-
Before disconnecting the apparatus, have the data sheets ap-
proved by the Instructor so that if any observations are missing,
they may be taken without the trouble of re-connecting the appa-
At the end of the experiment, return all instruments to the
supply room, disconnect all wires and arrange the apparatus in
the same condition in which it was found.
The usual order of the work in a given laboratory period may
be summarized as follows :
1. Recitation on the assigned articles in the text book and on
the Theory in the Manual.
2. General inspection of the apparatus to be used.
3. Data sheets ruled up and the columns of same labelled by
the foreman of the group.
4. Requisition card filled out and instruments secured from
the supply room.
5. Connections made, and inspected by the Instructor before
power is applied to the apparatus.
6. Observations taken for securing the required data.
7. Data sheets approved and stamped by the Instructor be-
fore the connections are taken apart.
8. Connections taken down, instruments returned to supply
room and apparatus put in order.
9. Carbon copy of data placed on file with the Instructor, and
assignment of experiment for the following exercise ascertained.
9. Written Report. The written Report is to consist (a) of
the original data sheet; (b) a diagram of the connections em-
ployed in each item of the given experiment showing the instru-
ments on the drawing labelled with their respective laboratory
numbers; (c) answers and calculations based on the heading
Written Report at the end of each experiment in the following
directions. These report pages are to be bound by a brass clip
into the laboratory cover, and on this cover the blanks are to be
duly filled in with the name of the student, number of the ex-
periment, and so on.
The following rules will be observed in connection with the
written reports :
(a) The written report on a given experiment is to be handed
in one week after the performance of that experiment, that is,
where there is one laboratory exercise each week, the written re-
port is to be handed in at the laboratory exercise next following
that on which the experiment was performed.
(b) An extension of time will be granted, provided a written
valid excuse is presented on the form provided for this purpose,
on the due date of the report, approved by the Instructor, and
attached to the data sheet in the report when the latter is handed
(c) Reports handed in after the expiration of two weeks from
the date on which the experiment was performed (except in case
of illness) will be given reduced credit.
NOTE: The actual work on the written reports of the various experi-
ments has been reduced to a minimum consistent with an understand-
ing of what was done in making the observations. The advantageous
time to make up such a brief summary of the work performed in a
laboratory exercise is within a week after the actual performance of the
work. Hence the preceding requirement (a) stating that the report is
due one week from the laboratory exercise involved.
Experiments 1 to 20, inclusive, constitute
the Direct Current portion of the Manual.
See articles 48, 49, 50, 52, 53, 73 and 81 in the text book (Tim-
bie's Elements of Electricity).
The purpose of this experiment is to verify the relations of
pressure or electromotive force (E) in volts, current (7) in am-
peres, and resistance (R) in ohms in a circuit as expressed by
Theory. Nearly all electrical problems involve a knowledge
of Ohm's law. This law may be expressed by the following equa-
Pressure Volts E
Current= ^ r . or Amperes = ~ - r ; or /=
Resistance , Ohms R
obviously this expression may be written in the following forms :
E = RI and R= -j.
In the use of this law, it is very important to consider 7, E and
72 in the particular portion of the circuit involved, and further,
the law applies only to that portion of E which is used in over-
coming resistance. The voltage E measured between the ends of
a simple wire or across the terminals of a motor armature when
stationary may be substituted in the formula for Ohm's law with
the corresponding current, for the calculation of the resistance
R, because all of E is used in overcoming resistance in these cases.
In delivering current to an electric motor in motion, however, a
portion only of the pressure (or voltage E) at the terminals of
the motor armature is used in overcoming resistance in the arma-
ture windings, the remainder of the pressure going to overcome
the voltage generated in the armature and thus being available as
useful pressure in the rotation of the machine. In this latter
case, the voltage recorded by a voltmeter at the terminals of the
armature cannot be used in the formula for Ohm 7 s law because a
portion only of this voltage is used in overcoming resistance.
When the resistance of a simple wire is determined by measur-
ing E and I and substituting inphm's law, the method is known
as the voltmeter-ammeter method of measuring resistance.
Supply Mains (110 Volts D C.)
r i r i
Short Circuiting /
Wires to be Measured
Protective Resistance (Lamp Bank)
Fig. 2. Apparatus and connections for measuring the resistance of
a wire by the voltmeter-ammeter method.
Current Supply. 110 volte Direct Current.
Apparatus Required. (1) Lengths of various sizes and kinds
of wire; (2) protective resistance; (3) yard stick; (4) incan-
descent lamps arranged for series or parallel connection; (5)
fuses; (6) suitable voltmeter with flexible leads; and (7) an am-
Order of Work. Make a diagram of the exact connections em-
ployed in each of the following items, labelling each instrument
on the drawing with its laboratory number.
1. Place a suitable protective resistance in series with the am-
meter and the wire to be tested as shown in Fig. 2. With a
given constant current flowing through the wire as indicated by
the ammeter, observe the volts drop across 1/3, 2/3 and the en-
tire length respectively of the wire and observe the supply volt-
age. Record these observations as in Form 1.
Length of Wire
2. With the voltmeter across the entire length of the wire used
in item 1, observe the volts drop for three different values of cur-
rent and record as in Form 1.
Supply Mains (110 Volts D C.)
-4i r i
^ ^ Voltmeter
Fig. 3. Wiring diagram for a study of the voltage and current re-
lations in a series circuit, made up, in this case, of two incandescent
lamps connected in series.
3. Repeat items 1 and 2 for the second and third samples of
4. With two lamps in series with the ammeter as in Fig. 3, ob-
serve the current and volts drop across each lamp and the supply
voltage, using Form 2.
Supply Mains (110 Volts D C.)
11 f T
^~ ^7 Ammeter
r - ,
Lamp Bank (Lamps in Parallel)
Fig. 4. Diagram for a study of the voltage and current relations
in a parallel circuit, made up, in this case, of several incandescent
lamps in parallel.
5. With the lamps connected in parallel, all the lamps turned
off except one, and an ammeter in series with this one lamp as
shown in Fig. 4, observe the current in and the volts drop across
this lamp. With the same lamp turned on, repeat for a second
lamp, observing the current in the second lamp by the difference
in the readings of the two ammeters. Use Form 2.
Written Report. The written report is to consist of the origi-
nal data, and a diagram of connections as specified in Article 9
of the Introduction, together with answers to the following ques-
1. On what does the resistance of a wire depend ?
DIRECT CURRENT 13
2. On what does the voltage-drop in a wire depend ?
3. Explain briefly the results under item 1, Order of Work,
and calculate the resistance in Ohms for each of the three obser-
4. Explain briefly the results under item 2, Order of Work,
and calculate the resistance in Ohms for each of the three obser-
5. Same for items 3 and 4, Order of Work.
6. Prom the observations taken, what effect has the size and
kind of wire on the resistance ?
7. What distinguishes the relations of voltage and current in
the component parts of series and parallel circuits to the supply
voltage and current ?
Measurement of Armature and of Field Resistance.
See Articles 75, 81 and 115 in the text book (Timbie's Ele-
ments of Electricity).
The purpose of this experiment is to measure the resistance of
the armature and of the shunt and series field windings of a gen-
erator by the voltmeter-ammeter method.
That portion of a circuit carrying relatively high current
usually has a low resistance in order to reduce the losses in the
circuit. Hence, an armature and a series field of a generator,
through which the main part of the generated current flows, have