PRINCIPLES OF THE TELEPHONE
PART I
SUBSCRIBER'S APPARATUS
McGraw-Hill DookCompatry
Electrical World The Engineering andMining Journal
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Metallurgical and Chemical Engineering Power
Transmitter
Battery
Receiver
Induction Coil
m
Generator
HookSwitch
Contact
Crossing of Wires
not Joined
Jack
Impedance Coil
Conde r
"C^O" ^
1 Ground
Junction of Wires
Symbols used in diagrams.
Electromagnet
Frontispiece
INDUSTRIAL EDUCATION SERIES
PRINCIPLES
OF THE TELEPHONE
PART I
SUBSCRIBER'S APPARATUS
PREPARED IN THE
EXTENSION DIVISION OF
THE UNIVERSITY OF WISCONSIN
BY
CYRIL M. JANSKY, B. S., B. A.
ASSOCIATE PROFESSOR OP ELECTRICAL ENGINEERING
THE UNIVERSITY OP WISCONSIN
AND
DANIEL C. FABER, E. E.
ASSISTANT PROFESSOR OP ELECTRICAL ENGINEERING
THE UNIVERSITY OP WISCONSIN
McGRAW-HILL BOOK COMPANY, INC,
239 WEST 39TH STREET. NEW YORK
LONDON: HILL PUBLISHING CO., LTD.
6 & 8 BOUVERIE ST., E.G.
1916
COPYRIGHT, 1916, BY THE
MCGRAW-HILL BOOK COMPANY, INC.
IHK MAPI.E PRESS YORK PA
PREFACE
In order that this text might appeal to and be of practical use
to men who are actively engaged in the installation, care, and
operation of telephone apparatus, every advantage of contact
with the men engaged in the industry was utilized by the authors,
including conferences especially with the engineers of the Wis-
consin Telephone Company, from whom many valuable sugges-
tions were received.
Although details of construction are not given to any extent
it was the purpose of the authors to clearly set forth the principles
that underlie good construction. The main emphasis, however,
is placed upon the principles of operation of different types or
makes of subscribers' apparatus, together with a discussion of
methods of locating faults and their correction.
This is the first part of the course and is mainly confined to
subscribers' apparatus. The two other parts to follow, which
are in preparation, will treat of central office equipment and out-
side construction.
The authors wish to acknowledge their indebtedness to manu-
facturers of telephone apparatus for their unfailing courtesy in
furnishing illustrative material and data. They are also under
great obligation to Mr. L. Killam, H. L. Miller, and F. J. Mayer
of the Wisconsin Telephone Company for advice and suggestions,
and especially to the latter for reading the manuscript.
C. M. J.
D. C. F.
vu
342914
CONTENTS
PREFACE vii
CHAPTER I
INTRODUCTORY
ART. PAGE
1. Historical 1
2. Telephone Operation 1
3 Telephone Instruments 3
4. The Transmitter 3
5. The Receiver ...'., 4
6. The Generator 4
7. The Ringer 4
8. The Hook Switch ' 4
9. The Induction Coil 4
10. The Battery 4
CHAPTER II
ELEMENTARY ELECTRICAL PRINCIPLES
11. Primary Batteries . 6
12. Electrical Pressure 7
13. Telephone Batteries 7
14. Conductors and Insulators . 8
15. Electrical Resistance 9
16. Resistivity 9
17. Wire Measurement 10
18. Gage Numbers 11
19. Units of Resistance 13
20. Unit of Electrical Pressure 13
21. Electric Current 14
22. The Ampere 15
23. Pressure, Current, and Resistance . 15
24. Electric Circuits . 15
25. Series Circuit 16
26. Parallel Circuit 16
27. Closed Circuit 16
28. Open Circuit . 16
29. Short Circuit . . . . 16
30. Grounded Circuit 17
31. Resistance of a Series Circuit '. 17
ix
x CONTENTS
ART PAGE
32. Resistance of a Parallel Circuit 17
33. Cells in Series 20
34. Cells in Parallel . 20
35. Battery Resistance for Parallel Connections 21
CHAPTER III
MAGNETIC PRINCIPLES
36. Receiver Action 23
37. Magnetism 23
38. Magnetic Substances 24
39. Magnetic Induction 24
40. Experiment 1 24
41. Magnetic Action 25
42. Experiment 2 .....' 25
43. Laws of Magnetic Attraction and Repulsion 25
44. Experiment 3 26
45. Permanent and Temporary Magnets 26
46. Experiment 4 27
47. Magnetic Lines 27
48. Experiment 5 28
49. The Magnetic Circuit 28
50. Electromagnetism 29
51. Experiment 6 29
52. Solenoids 29
53. Experiment 7 30
54. Electromagnets 30
55. Horseshoe Electromagnet 32
56. The Ironclad Electromagnet 32
57. Construction of Electromagnets 32
58. Magnet Wire 33
59. Magnetic Action of Receiver 33
CHAPTER IV
SOUND
60. Sound " 35
61. Velocity of Sound 36
62. Properties of Sound 36
63. Pitch 36
64. Loudness 36
65. Timbre or Quality 36
66. Transmission of Speech 37
67. Experiment 8 , 37
68. Variable Resistance . 38
CONTENTS xi
CHAPTER V
TRANSMITTERS
ART. PAGE
69. The Carbon Transmitter 40
70. White Solid-back Transmitter "... 40
71. New Western Electric Transmitter 43
72. Kellogg Transmitter 44
73. Monarch Transmitter 45
74. Operator's Transmitter 45
75. Carbon Electrodes 45
CHAPTER VI
RECEIVERS AND INDUCTION COILS
76. The Receiver , .V. ........ 48
77. Early Receivers . . . y 48
78. Induced Electric Pressure ". . , . . ,; 48
79. Direct Current : . . .... 49
80. Alternating Currents ; . ... . . .... . . . 49
81. Experiment 9 .;,........ 49
82. The Receiver as a Transmitter . . . , . . , . 50
83. Bipolar Receiver .......;. 50
84. Western Electric Receiver . . . . . . '.'.'.. V 51
85. The Kellogg Receiver . . . .'.....'... . V V . ... 53
86. Operator's Receiver - * * - .' . ... 55
87. Sensitiveness of Receivers . ; .'.-.-. ... 55
88. Direct-current Receiver . . . .;...-. . 56
89. The Automatic Electric Co.'s Direct-current Receiver 56
90. The Monarch Direct-current Receiver 57
91. Self-induction 57
92. Self-inductance 58
93. Mutual Induction ...... ^ 58
94. Impedance ~ 59
95. The Induction Coil . . . , . . . ' 59
CHAPTER VII
SIGNALLING APPARATUS AND CIRCUITS
96. Signalling Circuits . r 63
97. Generators 63
98. The Telephone Generator 66
99. Automatic Switch 68
100. The Ringer 69
CHAPTER VIII
THE SUBSCRIBER'S TELEPHONE SET
101. The Complete Telephone 72
102. The Hook Switch . 72
xii CONTENTS
CHAPTER IX
LOCAL BATTERY SYSTEMS
ABT. p AGE
103. Classification of Local Battery Systems 76
104. Series Telephone System 76
105. Local Battery Circuit 78
106. The Bridging Telephone 78
107. Connections of Bridging Telephone 79
108. Telephone Instruments 80
109. Standard Wall Set 80
110. Hotel Set 82
111. Desk Set 83
CHAPTER X
COMMON BATTERY TELEPHONES
112. General , 87
113. The Condenser 88
114. Manufacture of Telephone Condensers ...:........ 90
115. Analogy for a Condenser 92
116. Action of a Condenser 93
117. Function of Condenser in Telephone Circuit 95
118. Receiver and Transmitter in Series; Condenser and Ringer in
Series 95
119. Induction Coil, No Condenser in Receiver Circuit 96
120. Induction Coil and Condenser in Ringer and Receiver Circuits . 96
121. Retardation Coil in Place of Induction Coil 98
122. Wheatstone's Bridge Connection 99
123. C. B. Wall Sets 100
124. Hotel Sets 100
125. Desk Sets . 101
CHAPTER XI
FAULTS IN SUBSTATION TELEPHONE APPARATUS
126. General 106
127. O. K. or Correct Tests, Local Battery Telephones, Line Discon-
nected 106
128. Side Tone 107
129. Classification of Faults 107
130. Fault Finding, Local Battery Telephones, Substation Apparatus 110
131. Faults in Central Energy Substation Instruments Ill
132. Circuits of C. B. Subscribers' Telephones 112
133. Locating Faults in C. B. Telephones 113
CONTENTS xiii
CHAPTER XII
PROTECTION OF TELEPHONE LINES AND APPARATUS
ART. PAGE
134. Need for Protection 117
135. Sources of Excessive Voltage 117
136. Heating Effect of Current 117
137. Lightning Phenomena * 118
138. Lightning Conductors 120
139. Lightning Arresters 121
140. Carbon Block Arresters . . . . ."-.- 122
141. Self -cleaning Arresters ... . . . . . . 124
142. Location of Lightning Arresters ..,-..... 125
143. Protection against Power Circuits . . . ... .... . .. . 125
144. Fuses ". / . ..... v .. . . . . . . 126
145. Protectors ,. ... V .<...'..'.. 127
146. Protection against Weak Currents 128
147. When Substations Need Protection ' . . , 130
CHAPTER XIII
N,
INSTALLATION
148. Entrance Holes ..... ...,,,. ... ...... . . ... . 132
149. Leading-in Wires ....... . . ; . . . . . . .'. . . . . 132
150. Location of Protector . ..... , . ". . ... . -. v . . . : ; . . . 133
151. The Inside Wiring .... ... . . . . . . 1 ... . . . . 133
152. Ground Wiring ... ........... 134
153. Location of Telephone Set ". . ..... ,.,... . 135
CHAPTER XIV
PARTY LINES
154. Definition . .^v ..... * . /. . .... 137
155. Classification of Party Lines .... . . 137
156. Code Ringing . . . ..... ... . , 138
157. Selective Ringing ....,..., 140
158. Harmonic Ringing . ...... ^. .. 142
159. Extension Bells . . ... . . 144
CHAPTER XV
INTERCOMMUNICATING TELEPHONE SYSTEMS
160. Definition . 146
161. Common Battery Interphone Systems .147
162. Western Electric Intercommunicating System 148
163. The Kellogg Intercommunicating System 153
164. The Monarch Intercommunicating System 154
INDEX * 157
PRINCIPLES OF THE
TELEPHONE
CHAPTER I
INTRODUCTORY
1. Historical. The first mention of the transmission of speech 1
to a considerable distance probably was by Robert Hooke in 1667
who described how he had transmitted sounds through a con-
siderable distance by the aid of a tightly stretched string. Later
developments of Hooke's method of transmitting sound show the
substitution of a wire for the string in the original experiments.
In any case the sounds of speech could be transmitted only a few
hundred feet, and it was not until use was made of electricity
that the telephone became a commercial possibility.
The electric telephone was patented by Alexander Graham
Bell in 1876, and the first public exhibition of it was made that
year at the Centennial Exposition in Philadelphia. Since that
date the number of telephones has grown rapidly; in fact, it is
doubtful if any other invention can show such a rapid commercial
development. At present it is not only possible to carry on a
conversation between New York and San Francisco by wire,
but between New York and Honolulu by wireless telephony.
2. Telephone Operation. The modern telephone system con-
sists of subscribers' instruments, the central office, and the con-
necting lines, so arranged that any subscribers' instrument can
be connected at will to any other instrument of the system. It
is in the central office that connections between subscribers' lines
are made, a switchboard in which the lines terminate being lo-
cated at this place. In the manually operated system, which
will be the only one considered at present, switchboard or central
operators, who make the desired connections by hand, are
provided.
The person making the call first signals the central operator
by turning the crank of the telephone generator in the magneto
1
2 PRINCIPLES OF THE TELEPHONE
system. Turning this crank causes an electric current to flow
through the wires to the central office, where it operates a signal,
showing the operator that a connection is desired. As soon as
the subscriber has signalled the central office he removes the re-
ceiver from the hook and listens until the operator has answered
his signal, when he tells the operator the desired number. The
FIG. 1.
operator then connects the line of the calling subscriber to the
line having the number for which he called.
The next step is to attract the attention of the called sub-
scriber, which is done by ringing his telephone bell. As soon
as the wanted subscriber answers his call by removing the re-
ceiver from the hook, the connections between the two instru-
ments are complete, and the two subscribers can talk with
each other.
In order to understand the manner in which the sound of
speech made at one end of a telephone system is reproduced at
INTRODUCTORY
the other end, it will be necessary first to get rid of the popular
idea that the sounds produced at one end of the wire actually
travel to the other end, for such is not the case. Speech is
transmitted electrically. The sound waves of the voice, at the
transmitting or sending station, set up fluctuating electric
currents which pass over the line and cause a diaphragm at the
opposite end to vibrate as these currents fluctuate, thus repro-
ducing as nearly as possible, by means of the vibrations of the
receiver diaphragm, the original sounds.
FIG. 2.
3. Telephone Instruments. In Figs. 1 and 2 are shown the
main working parts of a common form of subscribers'lnstrument.
4. The Transmitter. The transmitter, as its name indicates,
is used for transmitting or sending the message. The working
parts of the transmitter consist of a thin iron diaphragm about
2% in. in diameter, and two carbon disks about % in. in diame-
ter separated' by a small quantity of granulated carbon. One
of the carbon disks is attached to the iron diaphragm, which
is caused to vibrate by the sound waves of the voice when the
2
4 PRINCIPLES OF THE TELEPHONE
transmitter is in use. The variations in the pressure between
the disks cause the electric current flowing through the trans-
mitter to vary from time to time, thus sending fluctuating cur-
rents over the line. The details of construction and operation
of the transmitter are taken up in a later chapter.
5. The Receiver. The receiver consists of a shell, usually
made of hard rubber, containing an electromagnet and a thin
iron diaphragm. The fluctuating electric currents from the
line flowing through the coil of the electromagnet cause the force
with which the magnet attracts the diaphragm to vary from
time to time, thus causing the diaphragm to vibrate and give
out sounds.
6. The Generator. The generator is a small dynamo and
operates only while the crank is being turned. It is not used
during conversation over the telephone, but is used to signal the
central office when a connection is desired by the subscriber.
7. The Ringer. The ringer is an electric bell which is used
to attract the subscriber's attention when he is wanted at the
telephone.
8. .The Hook Switch. The hook switch is used to connect
or disconnect the receiver and transmitter from the line. When
the instrument is not in use the receiver hangs on the hook, hold-
ing it down, connecting the ringer to the line so that the bell can
be rung when the subscriber is wanted. While the ringer is
connected to the line the talking system, consisting of the trans-
mitter and receiver, is disconnected from the line. When the
subscriber answers his call by removing the receiver, the hook is
raised by a spring and the ringing system is disconnected from
the line, while the talking circuit is connected to the line.
9. The Induction Coil. The induction coil is used to increase
the distance speech can be transmitted. The induction coil
consists of an iron core on which are two separate windings of
insulated wire. The principles of operation are explained later.
10. The Battery. While it is well known that the operation
of the telephone depends upon electricity, the exact nature of
electricity is not known, although a number of laws governing
its action have been determined by observation and experiment.
In order to understand the operation of the telephone, it is
necessary to know something of these laws of electricity. The
electrical supply for telephone work is derived from batteries,
or generators.
INTRODUCTORY 5
In Fig. 2 a local battery telephone is shown; that is, each in-
strument has its individual battery. In large systems the
central battery is used, or, in other words, a single battery in
the central office supplies current for all the instruments in
use.
CHAPTER II
ELEMENTARY ELECTRICAL PRINCIPLES
Electric batteries may be divided into two general classes and
are known as primary and secondary batteries. A primary
battery is a device used to generate an electrical pressure by
means of chemical action; that is, the chemical energy of the
battery is changed into electrical energy. The secondary
battery, or as it is more often called, the storage battery, also
makes use of chemical action in supplying electrical energy. But
before such a battery can give off electricity, it must be charged.
That is, electricity must be passed through
it. Primary batteries are used in local bat-
tery telephone systems, and in the central
battery system storage batteries are used.
11. Primary Batteries. A simple primary
cell or battery may be made by placing a
strip of amalgamated zinc 1 and one of copper
in a glass partly filled with a mixture of
sulphuric acid and water, care being taken
that the metals do not touch each other,
see Fig. 3. As far as can be seen no chemical
action is taking place with the battery as
shown. However, if the two plates be con-
nected by a wire, it may be noticed that
the zinc plate is being consumed and that bubbles of gas are
formed on the surface of the copper plate. If the wire con-
nection be broken, the chemical action ceases. It is evident
that there is some action going on when the plates are con-
nected which does not take place when the connection is broken.
The fact is that an electrical current is flowing in the wire.
The plates of a battery are known as the elements or electrodes,
and the solution in which they are placed is known as the electro-
1 A piece of zinc may be amalgamated by cleaning it with diluted sulphuric
acid and then rubbing the surface with mercury. One part of sulphuric
acid poured into twenty parts of water makes a mixture of the proper
strength.
6
FIG. 3.
GLASS jAff
SAL.
AMMONIAC
SOLUTION
ELEMENTARY ELECTRICAL PRINCIPLES 7
lyte. In the above-mentioned battery the zinc and copper plates
are the elements or electrodes, and the sulphuric acid is the
electrolyte. In order to have an electrical action it is not
necessary to have plates of copper and zinc and an electrolyte
of sulphuric acid, for many other substances may be used in
batteries.
One of the most common types of commercial batteries has
elements of carbon and zinc and an electrolyte of sal ammoniac.
The arrangement of such a battery is shown in Fig. 4. This
type of battery is known as the Le
Clanche cell. Dry cells are modified
Le Clanche cells.
12. Electrical Pressure. The two
plates of a battery are said to be charged
if an electrical current flows from one
to the other when they are connected
by a wire. An electrical pressure, which
causes the electricity to move from one
point to another, exists between the
two charged plates. One of the plates
of a battery is said to be charged posi-
tively (+) and the other is said to be charged negatively ( ).
Electricity behaves in many respects like water, and it is
just as necessary to have a difference in pressure between the
two ends of an electrical conductor if we are to have a current
flow as it is to have a difference in pressure between the two
ends of a water pipe if we are to have the water run through the
pipe. It is easy to see from the above statements that if two
points having equal electrical pressures are connected, no
electricity will move from one point to the other.
As water flows from a point of high pressure to one of lower
pressure, so an electrical current flows from a point of high
pressure or potential, as it is usually called, to one of lower
potential. It is customary, in speaking of two electrical charges,
to speak of the charge having the high pressure as positive (+)
and the one of lower pressure as negative ( ). Accordingly
an electric current flows from a positive to a negative point.
In the wire connecting the electrodes of the Le Clanche cell the
current flows from the carbon (+) to the zinc ( ).
13. Telephone Batteries. In early telephone practice some
form of Le Clanche cell, similar to Fig. 4, was largely used.
8
PRINCIPLES OF THE TELEPHONE
This battery costs little to operate, as the materials used in its
construction are not expensive, and when the battery is idle there
is no waste as the sal ammoniac does not attack the zinc to any
extent except when current is flowing. Such a cell requires little
attention except to replace the water lost by evaporation, and
to replace the zinc element when it has been destroyed.
At present a later development of the Le Clanche battery,
known as a dry cell, is used almost to the exclusion of other forms
of primary cells in telephone operation.
The dry cell has electrodes of carbon and zinc, the zinc being
in the form of a cylindrical cup and, in addition to being one of
the battery elements, it also acts as a container for the electro-
lyte. One form of dry cell is shown in
Fig. 5. The carbon element is in the form
of a rod, and is held in the center of the zinc
cup, without touching it at any point, the
electrolyte occupying the intervening space.
The electrolyte, instead of being a liquid as
in the wet batteries, is in the form of a
paste consisting of sal ammoniac, manga-
nese dioxide, carbon, and water, in varying
proportions depending upon the make of
the cell. Several thicknesses of blotting
paper separate the electrolyte from the zinc
cup so that only the sal ammoniac which is
dissolved in the water can come into contact
with it. The outside of the cell is protected by a pasteboard
covering which also insulates the cell from other cells of the
same battery in case more than one are used. The cell is sealed
with pitch or wax. The standard size of dry cell for telephone
work is 2% in. in diameter and 6 in. high.
Dry cells have replaced wet batteries in telephone work
on account of their smaller size, the fact that they are not
easily broken, can not be spilled, have a low first cost, and
require no attention except renewal when they are worn out.
14. Conductors and Insulators. When the two plates of the
simple battery mentioned above are not connected there is
no current flow from one to the other, as shown by the fact
that there is no chemical action taking place; neither is there
any action when the plates are connected by pieces of wood,
rubber, or glass. If, however, a wire of iron, copper, or other
FIG. 5.
ELEMENTARY ELECTRICAL PRINCIPLES 9
metal join the plates, a current flows from one plate to the
other. In other words, the air, wood, glass, etc., do not conduct
the electricity, while the wires do. All metals or substances
which conduct electricity readily are known as conductors.
Substances which do not conduct electricity readily, such as
rubber, glass, etc., are known as nonconductors or insulators.
Any insulator will conduct some little electricity, however slight
that quantity may be; and every conductor will offer some
resistance to the flow of electricity.
15. Electrical Resistance. Electrical resistance is the name
given to that property of a conductor which resists or opposes
the passage of electricity through it. Electrical resistance may
be compared with the resistance which a stream of water en-
counters in flowing through a pipe, the pressure in one case forc-
ing the water to flow, and in the other case an electrical pressure
causing the electricity to move from one point to another. The
electrical resistance of a conductor depends upon the material
of which the conductor is made, and upon the size and length of
the conductor.
16. Resistivity. As some substances conduct heat more
readily than others, so in the case of electricity some substances
conduct it more readily than others.
TABLE I
Metal
Resistivity in ohms at 0C.
Silver, annealed
8.781
Silver, hard-drawn
Copper, annealed . .
9.538
9 61
Copper, hard-drawn
9 86
Aluminum annealed
15 8
Aluminum, hard-drawn
15.93
Platinum
Iron . .
54.35
58 31
Tin
79 29
Lead
115 1
Mercury
565.9
In order to compare the conductivities of different substances
some unit of conductor must be agreed upon. In scientific
calculations the unit conductor is one whose length is 1 cm. and
whose cross -sectional area is 1 sq. cm. In practical work,
10 PRINCIPLES OF THE TELEPHONE
however, the unit conductor is a piece of circular wire 1 ft.
long and Kooo (0.001) in. in diameter. The resistance of
such a conductor is called the resistivity of the material of which
the wire is made.
Table I gives the resistivities of a number of metals. The
first, silver, has the lowest resistivity, while the last, mercury,
has the highest resistivity of the metals given.
If we wish merely to compare the resistivities of wires of
the same size and lengths but of different materials we can call
the resistivity of silver unity. The relative resistivities are then
as follows:
TABLE II
Metal
Relative resistivity
Silver
1
Copper. .
1.09
Aluminum
Platinum
1.8
6 19
Iron
6.04
Tin
9 03
Lead
13 1
Mercury
64.4
From the table it can be seen readily why so much copper
wire is used in electrical apparatus. Silver, which is a better
conductor and would be more desirable for some work for that
reason, costs several times as much as copper.
The resistance of a conductor depends upon its length. The
longer the conductor, the greater its resistance. Thus a No.
16 wire 200 ft. long will have twice as great a resistance as 100
ft. of the same wire.
A small conductor offers a greater resistance to the flow of
an electric current than does a large one of the same material,
just as a small pipe hinders the flow of water more than a large
one. The resistance of conductors varies inversely as the
areas of their cross-sections, or in other words, inversely as the
quantity of material in a given length.
17. Wire Measurement. In this country the length of wire
is usually given in feet, and the size is specified either by diameter,
cross-sectional area, or gage number. The units used for the
measurement of the diameter and cross-sectional area are not
the inch and square inch, but the mil and circular mil.
ELEMENTARY ELECTRICAL PRINCIPLES 11
The Mil. The unit of length in measuring the diameter is the
Kooo ( = 0.001) in. and is called the mil. A 1-in. cable has a
diameter of 1,000 mils. The diameter of a wire 0.25 in. is equal
to 250 mils, etc.
Circular Mils. A circle whose diameter is 0.001 in. (=1
mil) is said to have an area of 1 cir. mil. Since the areas of two
circles having different diameters are to each other as the squares
of their diameters, to express the cross-section of any wire in
circular mils, when its diameter in mils is given, all that is
necessary is to square the diameter, that is, multiply the diameter
by itself.
EXAMPLES
1. What is the cross-sectional area in circular mils of a wire % in. in
diameter?
Solution
Y in. = 0.25 in.
0.25 in. = 25 Kooo = 250 mils
Area in circular mils equals diameter squared
Diameter = 250 mils
250 2 = 250 X 250 = 62,500 cir. mils.
2. A No. 0000 wire has a cross-sectional area of 211,600 cir. mils. What
is its diameter in mils and in inches?
Solution
Since the cross-sectional area in circular mils is equal to the square of
the diameter in mils, the diameter in mils must be equal to the square root
of the cross-sectional area. In symbols
D 2 = area
and D = \/area
But area = 211,600 cir. mils
Hence D = \/21MH)6
= 460 mils
1 mil = Kooo in.
460
Then 460 mils = = 0.46 in.
18. Gage Numbers. In the United States practically the only
gage now used for copper wire is the American Wire Gage com-
monly called the Brown and Sharpe (B. & S.) Gage. This gage
was devised in 1857 by J. R. Brown, one of the founders of the
Brown & Sharpe Manufacturing Co. In this gage the size of
wire is specified by number. The mathematical law on which
12 PRINCIPLES OF THE TELEPHONE
TABLE III. TABULAR COMPARISON OP WIRE GAGES, DIAMETERS IN MILS
Gage No.
American Wire Gage
(B. & S.)
Steel Wire
Gage
Birmingham Wire
Gage (Stubs')
(British) Standard
Wire Gage
7-0
490.0
500.0
6-0
461.5
464.0
5-0
430.5
432.0
4-0
460.0
393.8
454.0
400.0
3-0
410.0
362.5
425.0
372.0
2-0
365.0
331.0
380.0
348.0
325.0
306.5
340.0
324.0
1
289.0
283.0
300.0
300.0
2
258.0
262.5
284.0
276.0
3
229.0
243.7
259.0
252.0
4
204.0
225.3
238.0
232.0
5
182.0
207.0
220.0
212.0
6
162.0
192.0
203.0
192.0
7
144.0
177.0
180.0
176.0
8
128.0
162.0
165.0
160.0
9
114.0
148.3
148.0
144.0
10
102.0
135.0
134.0
128.0
11
91.0
120.5
120.0
116.0
12
81.0
105.5
109.0
' 104.0
13
72.0
91.5
95.0
92.0
14
64.0
80.0
83.0
80.0
15
57.0
72.0
72.0
72.0
16
51.0
62.5
65.0
64.0
17
45.0
54.0
58.0
56.0
18
40.0
47.5
49.0
48.0
19
36.0
41.0
42.0
. 40.0
20
32.0
34.8
35.0
36.0
21
28.5
31.7
32.0
32.0
22
25.3
28.6
28.0
28.0
23
22.6
25.8
25.0
24.0
24
20.1
23.0
22.0
22.0
25
17.9
20.4
20.0
20.0
26
15.9
18.1
18.0
18.0
27
14.2
17.3
16.0
16.4
28
12.6
16.2
14.0
14.8
29
11.3
15.0
13.0
13.6
30
10.0
14.0
12.0
12.4
31
8.9
13.2
10.0
11.6
32
8.0
12.8
9.0
10.8
33
7.1
11.8
8.0
10.0
34
6.3
10.4
7.0
9.2
35
5.6
9.5
5.0
8.4
36
5.0
9.0
4.0
7.6
37
4.5
8.5
6.8
38
4.0
8.0
6.0
39
3.5
7.5
5.2
40
3.1
7.0
4.8
ELEMENTARY ELECTRICAL PRINCIPLES 13
this gage is based is, the ratio of any diameter to the next smaller
is a constant number.
For practical purposes tables are prepared giving the gage
number, diameter in mils or inches, cross-sectional area in cir-
cular mils, and other data that may be useful, depending upon
the completeness of the table. The numbers usually range
from 0000 to 40. The diameter of No. 0000 is 460 mils and of
No. 40, 3.145 mils. The student will thus see that the larger the
gage number the smaller the diameter. A wire table for ordinary
practical calculations is given on page 14. This table was
prepared by the Bureau of Standards and is published in circular
No. 31 together with others of greater accuracy and detail.
The telephone companies still use the " Standard Wire Gage,"
otherwise known as the New British Standard (N.B.S.) Gage,
for copper wire and the Birmingham Gage for steel wire. The
other steel wire gage used in this country is now known as the
Steel Wire Gage (Stl.W.G.). Manufacturers of wire as a rule
prefer that the size of wire be specified in decimal fractions of an
inch, without the use of gage numbers. A comparative table of
the four gages used by telephone companies is given in Table III.
19. Unit of Resistance. In order that the resistance of dif-
ferent wires may be compared, a unit of resistance has been
adopted. This unit of resistance is known as the ohm. The
resistance of 1,000 ft. of No. 10 copper wire at a temperature of
20C. or 68F. is about 1 ohm.
The standard ohm is defined as the resistance offered to the
flow of an unvarying electric current by a column of mercury
106.3 cm. long, 14.4521 grams mass, and of a constant cross-
sectional area, at a temperature of melting ice.
20. Unit of Electrical Pressure. Just as it has been neces-
sary to choose a unit of resistance, so it has been necessary to
choose a unit of electrical pressure or electromotive force, as
this pressure is usually called. The unit of pressure is known
as the volt. Electromotive force or electrical pressure is not
electricity; it is merely the force that causes electricity to move,
so that the voltage of a battery is no measure of the quantity
of electricity that can be obtained from that battery, but is
merely a measure of the electrical pressure existing between the
two plates. The pressure in a standard dry cell is about 1J^
volts. A voltmeter is an instrument for measuring electrical
pressure. Since the electrical pressure of a battery exists be-
14
PRINCIPLES OF THE TELEPHONE
tween 'its two electrodes, in order to secure a voltmeter read-
ing of this pressure the instrument must be connected to the
electrodes.
TABLE IV. NUMBER, DIMENSIONS, AND RESISTANCE OF STANDARD AN-
NEALED COPPER WIRE (SOLID)
No.
Diameter
Area
Weight
Resistance in ohms at 68F.
A.W. Gage
In mils
Circular mils
Lb. per 1,000 ft.
Ohms per 1,000 ft.i
6
162.0
26,250.0
79.46
0.3951
7
144.3
20,820.0
63.02
0.4982
8
128.5
16,510.0
49.98
0.6282
9
114.4
13,090.0
39.63
0.7921
10
101.9
10,380.0
31.43
0.9989
11
90.74
8,234.0
24.92
1.260
12
80.81
6,530.0
19.77
1.588
13
71.96
5,178.0
15.68
2.003
14
64.08
4,107.0
12.43
2.525
15
57.07
3,257.0
9.858
3.184
16
50.82
2,583.0
7.818
4.015
17
45.26
2,048 .
6.200
5.064
18
40.30
1,624.0
4.917
6.385
19
35.89
1,288.0
3.899
8.051
20
31.96
1,022.0
3.092
10.15
21
28.46
810.1
2.452
12.80
22
25.35
642.4
1.945
16.14
23
22.57
509.5
1.542
20.36
24
20.10
404.0
1.223
25.67
25
17.90
320.4
0.9699
32.37
26
15.94
254.1
0.7692
40.82
27
14.20
201.5
0.6100
51.46
28
12.64
159.8
0.4837
64.90
29
11.26
126.7
0.3836
81.84
30
10.03
100.5
0.3042
103.2
31
8.928
79.70
0.2413
130.1
32
7.950
63.21
0.1913
164.1
33
7.080
50.13
0.1517
206.9
34
6.305
39.75
0.1203
260.9 .
35
5.615
31.52
0.09542
329.0
36
5.000
25.00
0.07568
414.8
37
4.453
19.83
0.06001
523.1
38
3.965
15.72
0.04759
659.6
39
3.531
12.47
0.03774
831.8
40
3.145
9.888
0.02993
1049.0
1 For hard-drawn copper wire increase these values by 2.7 per cent.
21. Electric Current. In order to have a flow of electricity
from one point to another it is necessary that a difference in
ELEMENTARY ELECTRICAL PRINCIPLES 15
electrical pressure exist between the two points, and that these
points be connected by a conductor.
The rate of flow in any circuit depends upon the pressure or
voltage, and the resistance of the path through which the cur-
rent flows. It is evident that a greater rate of flow will take
place under a high pressure than under a low one. Thus a
pressure of 2 volts would cause twice as much electricity to flow
through a circuit in a given time as a pressure of 1 volt. The
resistance which has to be overcome in any conductor determines
the current, since the greater the resistance, the more the current
will be held, back or hindered in flowing through the conductor.
22. The Ampere. The ampere is the unit of current, and is
the current that will flow in a circuit having a resistance of 1
ohm under an electrical pressure of 1 volt. An ammeter is an
instrument for measuring the rate of flow of electricity in a
circuit.
23. Pressure, Current, and Resistance. It has been shown
above that electric current depends upon the pressure and re-
sistance of a circuit, or that for different pressures and resistances
different currents will flow. Thus if a certain current is flowing
in a circuit and it is desired to change that current, it is neces-
sary to change either the pressure or resistance of the circuit.
The units of pressure, resistance, and current have been so
chosen that if two of these quantities are known, the third can
be easily calculated. Since 1 ampere represents the current in a
circuit having a resistance of 1 ohm under a pressure of 1 volt, in
order to have 2 amperes flow in the same circuit it would be neces-
sary to have a pressure of 2 volts, etc. A simple way of express-
ing this relationship is: Current equals pressure divided by
resistance, or
Volts
Amperes = Ohms
In telephone operation the pressure remains constant, so that
the fluctuating current which is necessary to send a message is
obtained by the rapid variations in the resistance of the trans-
mitter when it is being used.
24. Electric Circuits. An electric circuit is a system of con-
ductors and apparatus connected so that a current, under certain
conditions, may flow from one point to another point on the con-
ductors. With reference to the manner in which the conductors
are connected, we have two general classes of circuits; namely,
16
PRINCIPLES OF THE TELEPHONE
series and parallel. With reference to the possibility of current
flowing, circuits are classed as closed and open.
25. Series Circuit. A series circuit is one in which the current
must flow through each part of the circuit in succession. The
current has the same strength at whatever point in the circuit
it is measured. Fig. 6a is a series circuit consisting of a magneto
generator, two wires and a telephone ringer.
26. Parallel Circuit. Parallel circuits are shown in Fig. 6&.
The current from the generator divides and goes through the
bell coils in parallel. Thus a parallel circuit is one consisting
FIG. 6a.
FIG. 6&.
FIG. 6c.
FIG. 6d
of two or more individual circuits connected in parallel, or, in
short, a parallel circuit is a divided circuit.
27. Closed Circuit. When the conductors are connected so
that a current can flow, the circuit is said to be closed. Fig. 6a
is also a closed series circuit.
28. Open Circuit. An open electrical circuit is one in which
some part is disconnected so that a current can not flow. Fig.
6c shows an open circuit, opened at the jack, and at the magneto.
29. Short Circuit. A short circuit is said to exist when a
shunt or parallel circuit of comparatively low resistance has been
connected to the main circuit. A shunt is one of the branches
of a parallel circuit. Fig. 6d shows a connection from the point
a to the point b short-circuiting the generator.
ELEMENTARY ELECTRICAL PRINCIPLES
17
30. Grounded Circuit. A circuit is said to be grounded when
any part of it is connected to the ground. Such a circuit is
shown in Fig. 6e. In many telephone systems the ground is
made a part of the circuit, but one wire being used. This
is made possible by the fact that wet earth is a good conductor
of electricity.
FIG. 6e.
31. Resistance of a Series Circuit. When conductors are
connected end to end so that the total current must flow through
each conductor in succession, the joint resistance of the con-
ductors or the resistance of the circuit is the sum of the resist-
ances of the individual conductors so connected. Thus in Fig.
7 the resistance of the receiver circuit is 70 + 10 + 87.5 + 175
+ 10 + 32 = 384.5 ohms.
IO OHMS
10 OHMS
FIG. 7.
32. Resistance of a Parallel Circuit. When several conductors
are connected in parallel, as the ringers in Fig. 66, the joint con-
ductivity is the sum of the conductivities of the several branches.
This will be more evident if we consider a hydraulic analogy.
Suppose two large water mains are connected by several small
18 PRINCIPLES OF THE TELEPHONE
pipes. It is very evident that the current of water flowing from
one main to the other main is equal to the sum of the currents
in the small pipes. That is, the joint conductivity is equal to
the sum of the conductivities of the small pipes. In exactly an
analogous manner, when the circuit consists of several conductors
in parallel, the total current is the sum of the currents in the
several parallel conductors. This fact, together with Ohm's
law, enables us to calculate the joint resistance in the following
manner :
Suppose the resistances of the ringers in Fig. 66 are R^ R 2 ,
and Rz respectively, and that a difference of electrical pressure
of E volts exists between their terminals; then by Ohm's law the
current in ringer 1 is
E
l ^R,
E
current in 2 is iz = -5-
Kz
and
E
current in 3 is i s = -5-
riz
If R is the joint resistance, the total current is
7-^
" R
But ii + i* + *s = I ]
Therefore
E] = E_ E_ E_
R R\ RZ Rs
or
1=1+1+1
R RI RZ RS
Solving this for R, we get
RiXRzX Rs
Ri X R* + Ri X Rz + #2 X fla
If the resistances of the bells are equal then
R\ = RZ Rs, and
That is, the joint resistance is equal to one-third the resistance
of one bell.
ELEMENTARY ELECTRICAL PRINCIPLES 19
EXAMPLE
Three ringers whose resistances are 1,000 ohms, 1,600 ohms, and 2,000
ohms are bridged across a line to which is connected a storage battery of
24 volts.
(a) What is the current in each bell?
(6) What is the total current given out by the battery?
(c) What is the joint resistance of the ringers?
Solution
(a) By Ohm's law the several currents are
* l = iloo = a 24 am P-
24
= 0.015 amp.
-,
and
24
>* = Poo = - 012 am P-
(6) The total current is equal to the sum of ii + i 2 + i 3 or
7 = 0.024 + 0.015 + 0.012 = 0.051 amp.
(c) The joint resistance may be calculated in two ways. First by Ohm's
law
R E
= -j
24
=
or by the formula
D
it =
R 1 XR 2 XR S = 1,000 X 1,600 X 2,000
= 3,200,000,000
RiXR 2 = 1,600,000
R! X Ra = 2,000,000
R 2 x Rz = 3,200,000
#1 X R* + Ri X R s + #2 X # 3 = 6,800,000
and
_ 32,000
~68~
_ 32,000 8,000
68 17
= 470.6 ohms, nearly.
To calculate the joint resistance of any number of conductors connected
in parallel, we proceed in exactly the same way. It is not necessary to show
how any more formulas are calculated. A general rule will suffice.
Rule. To find the joint resistance of any number of parallel conductors,
3
20
PRINCIPLES OF THE TELEPHONE
divide the product of the resistances of all of the conductors by the sum of the
products obtained by multiplying together all of the resistances less one. The
same resistance must not appear in any partial product more than once.
33. Cells in Series. In the application of Ohm's law the
electromotive force E must be the total electrical pressure in
the circuit. It is, therefore, neces-
sary to be able to calculate the
pressure when cells are connected
in series or in parallel. Cells are
said to be connected in series when
G the carbon electrode of one is con-
^J^^=
=^r=/3>=
^==^=--
n a
r
L
i.
C
a
6
/
/?
FIG. 8.
-M/txz' c/Che
FIG. 9.
nected to the zinc electrode of the next and so on as shown in
Fig. 8.
The analogous diagram of tanks in series may help to show
how the total pressure is calculated. The hydrostatic pressure
at A is evidently the sum of the pressures due to the elevations
of the water AB + BC + CD; that is, the sum of the pressures
in the individual tanks. Similarly, the electrical pressure be-
tween the terminals 1 and 2 is the sum of the pressures across the
cells a, 6, and c. In general, if E is the pressure of one cell
and n cells are connected in series, the total pressure is nE.
34. Cells in Parallel. Fig. 9 is a diagram of tanks and cells
connected in parallel. It is evident that the hydrostatic pres-
ELEMENTARY ELECTRICAL PRINCIPLES 21
sure exerted by the water in tank A is the same as that in B
and C, since the height of the water is the same in each. The
total pressure is equal to that of one tank. The three tanks
could be replaced by one large tank, and as long as the water
was maintained at the same height, the pressure at the orifice
would be exactly the same in the two cases.
When cells are connected in parallel the total pressure is
equal to the pressure of one cell, and the three cells a, 6, and
c, can be replaced by one large cell having the same cross-section
of zinc and carbon as the three cells taken together.
When tanks are connected in parallel it is evident that each
supplies only a part of the current. The same principle holds
with reference to cells connected in parallel each cell supplies
only a part of the total current. The student can readily verify
the law of pressures by connecting three cells in parallel and
then connecting a voltmeter to terminals 1 and 2, Fig. 9,
and comparing the voltmeter reading with the reading given when
the voltmeter is connected to each cell separately.
35. Battery Resistance for Parallel Connections. The effect
of connecting cells in parallel is to increase the current capacity
and decrease the internal resistance. In so far as the internal
resistance of one cell is concerned, it may be considered as a
conductor whose resistance is r. Three cells in parallel will thus
be the equivalent of three resistances in parallel. It has been
shown that when three equal resistances are in parallel, the
joint resistance is equal to one-third of the resistance of one
wire. Accordingly, the joint internal resistance of a battery
of m parallel cells is
ra
EXAMPLE
Five cells each having an internal resistance of 1 ohm are connected in
parallel. What is the joint resistance?
Solution
Since the resistances of the cells are the same, the joint resistance is
K of 1 ohm = 0.2 ohm.
QUESTIONS
1. Explain the steps necessary in order to send a telephone message from
one point to another.
2. Name the main parts of a telephone instrument and give briefly the
uses of each.
22 PRINCIPLES OF THE TELEPHONE
3. Of what use is a battery in telephone work?
4. Explain how a simple battery is made. .
5. What is a conductor? An insulator? Name the five best conductors
of which you know. The five best insulators.
6. What are the elements of a battery? What is the electrolyte?
7. What is meant when an object is said to be "charged?"
8. What do you understand by electrical pressure? Compare it with
water pressure.
9. What are the elements and electrolyte of the Le Clanche cell? Explain
briefly how a dry cell is made.
10. What is electrical resistance? Upon what does the resistance of a
conductor depend?
11. Which has the greater resistance, copper or silver? Iron or copper?
Iron or lead?
12. Why is a unit of resistance necessary? What is this unit?
13. What is the unit of electrical pressure? By what other name is
electrical pressure usually known? What is the voltage of a dry cell?
14. What causes an electrical current to flow? What conditions are
necessary in order to have a current of electricity?
15. What is the unit of current? Define it in terms of volts and ohms.
16. What is the relation between volts, amperes, and ohms?
17. What is the rate of current flow through a coil of telephone wire
having a resistance of 1 ohm, if the ends are connected to the terminals of a
dry cell? If another coil of half the length is used, what will be the current
in amperes?
18. Define the following: Circuit, open circuit, closed circuit, short cir-
cuit, series circuit, parallel circuit, shunt.
19. Four resistances of 80, 95, 40, and 50 ohms are connected in series.
What is the joint resistance?
20. Four series ringers of 80 ohms resistance each were connected in
parallel. What was the joint resistance?
21. Four dry cells, each having an electromotive force of 1.4 volts and
an internal resistance of 1 ohm, were connected in series to a circuit whose
resistance was 10 ohms. What was the current in the circuit?
22. Suppose the cells mentioned in question 21 were connected in parallel
to the same circuit, what current would flow?
CHAPTER III
BINOING POSTS ,
MAGNETIC PRINCIPLES
36. Receiver Action. An examination of a telephone receiver
shows the working parts to consist of a permanent magnet on
which coils of fine insulated wire are wound, and a thin iron
disk mounted close to, but not touching, the poles of the magnet.
This arrangement is shown in Fig. 10 for a
single-pole receiver, which was the earliest
type in use.
This examination also shows the iron disk
or diaphragm to be attracted by the per-
manent magnet. Since the outer edge of
the disk can not move, the disk will become
slightly "dished," as the center is drawn in
toward the magnet. When the receiver is
not in use this pull will be steady, and there
will be no movement of the disk. If the
strength of the magnet be increased, how-
ever, it will exert greater attraction for the
disk, and the latter will be pulled closer to
the magnet pole. On the other hand, if the
force of the magnet's attraction be decreased
the diaphragm will spring away from the
pole, and return more nearly to its original shape.
When the receiver is in use, the strength of the magnet is
changed from time to time by the fluctuating line current which
flows through the coil of the receiver. If these changes in the
force of the magnet take place rapidly enough, the disk will
vibrate at such a rate that sounds will be produced by it. In
order to understand how an electric current flowing in the coil
can change the strength of the magnet, it will be necessary to
investigate a few of the relations existing between electricity
and magnetism.
37. Magnetism.- The magnet, as first known, existed in the
form of a certain iron ore known as magnetite (so named in
23
DIAPHRAGM'
FIG. 10.
24 PRINCIPLES OF THE TELEPHONE
honor of the city of Magnesia, where the ore having this pecu-
liarity was discovered) which has the property of attracting
pieces of iron. The strange force by which the particles of iron
were attracted was likewise known as magnetism. These first
magnets were natural magnets.
It was found, somewhat later, that artificial magnets could
be formed by subjecting pieces of iron to the influence of a
magnetizing force. One of the early methods of producing such
artificial magnets, was by stroking or rubbing a piece of iron with
a piece of magnetic ore or natural magnet. There are at present
other methods of producing artificial magnets. The first arti-
ficial magnets were in the form of a bar as shown in Fig. lla.
FIG. lla. FIG. lib.
38. Magnetic Substances. Iron in its various commercial
forms, such as wrought iron, cast iron, steel, etc., is strongly
magnetic and is known as a magnetic substance. Substances
such as wood, glass, copper, etc., which can not be made to
act as magnets, are known as nonmagnetic substances.
39. Magnetic Induction. When a magnetic substance is
magnetized by coming into contact with a magnet, the substance
is said to have been magnetized by induction, or the magnetism
is said to be induced in the substance.
40. Experiment 1. Apparatus:
Bar Magnet.
Iron Filings.
Wire Nails.
(a) Dip the ends of the bar magnet into the iron filings,
and note that the filings cling to the ends of the magnet. The
parts to which filings cling are called the poles of the magnet.
Those points to which no filings cling are known as neutral
points.
(6) Rub a knife blade or other piece of steel with one end
of the magnet; always move the magnet in the same direction
along the knife. Dip the end of the blade in the filings. Has
the blade become magnetized?
(c) Hold one end of the bar magnet against the head of a
MAGNETIC PRINCIPLES 25
nail and dip the point of the nail into the iron filings. Note
that a magnet pole has been developed on the point of the nail.
(d) Try the last experiment with a piece of wood or short
piece of copper wire in place of a nail, and note that there is no
evidence of these substances becoming magnetized.
41. Magnetic Action. If a bar magnet be suspended by a
fine thread attached to its center, the magnet will turn so that
one end will point in a northerly direction and the other in a
southerly direction, no matter what the original position of the
magnet may be. The end of the magnet that will point toward
the north is called the North pole (marked N.), and the other end
is called the South pole (marked S.). A compass needle is merely
a very light bar magnet.
42. Experiment 2. Apparatus:
Horseshoe Magnet.
Two Bar Magnets.
(a) Suspend a bar magnet by a fine thread attached to its
center. When the magnet has come to rest, it will point north
and south. Mark the end that points north, to indicate the
N. pole. Suspend the second bar magnet in the same way
and mark its N. pole. Bring the N. pole of the first bar magnet
near the N. pole of the suspended one. Observe that the two
poles repel each other. Now bring a N. and S. pole near each
other and observe the strong attraction exerted between the
two.
(b) Using the suspended bar magnet as in the first part of
the experiment, test the poles of the horseshoe magnet, and
mark the N. pole.
43. Laws of Magnetic Attraction and Repulsion. If the N.
pole of one bar magnet is brought near the S. pole of the other,
a strong attraction is exerted between the two; but if the two N.
or two S. poles are brought together, they repel each other;
hence we can write two laws governing the action of one magnetic
pole on another, as follows: (1) Like magnetic poles repel each
other, and (2) unlike magnetic poles attract each other.
The action of the compass in taking a north and south position
can be understood, since investigation has proved that the
earth is a gigantic magnet, having one magnetic pole near the
earth's north pole, and the other magnetic pole near the earth's
south pole.
A horseshoe magnet is another common form of artificial
26
PRINCIPLES OF THE TELEPHONE
magnet, one .of the ends being a N. and the other a S. pole, as
shown in Fig. 116.
44. Experiment 3. Apparatus:
Bar Magnet.
Nail.
Piece of Watch or Clock Spring about 3 in. long/ 1
(a) Repeat the third part of Experiment 1, and note that as
long as the magnet and nail are in contact the point of the nail
will hold a considerable quantity of the filings, but as soon
as this contact is broken the nail loses the greater part of its
magnetism and most of the filings drop. The nail, being of soft
iron, is only a temporary magnet.
FIG. 12a.
FIG. 126.
(b) Repeat the experiment using the piece of watch spring
in place of the nails. Even after the contact between the
spring and the magnet has been broken, the spring retains
the greater part of its power to pick up the filings. The spring,
being of hard steel, has become a permanent magnet. Try the
bar magnet used in this experiment with a file to see whether
or not it is of hard steel.
45. Permanent and Temporary Magnets. Artificial magnets
which retain their magnetism a long time are known as perma-
nent magnets. Wrought iron may be strongly magnetized, but
as soon as the magnetizing force is removed it loses the greater
part of its magnetism. Hard steel, when once magnetized,
will retain its magnetic properties indefinitely. Both these forms
of iron are made use of in telephone work; in some cases we use
hard steel because we want the part to retain its magnetism, as
in the magnets of a ringer, and in other cases we want to magnet-
ize the part temporarily and then want the same part to lose its
MAGNETIC PRINCIPLES 27
magnetism a moment later, as in the receiver diaphragm. The
action of these parts will be explained later.
46. Experiment 4. Apparatus:
Bar Magnet.
Piece of Watch Spring.
Be sure that the spring used in Experiment 3 is magnetized.
After testing the spring by dipping in the filings, cut it into several
short pieces, and test each piece for magnetic properties. A
magnetic pole will exist at each end of each of the small pieces,
and each will have the same power of attraction as the original
magnet.
47. Magnetic Lines. The property by which a magnet will
FIG. 13. FIG. 14.
attract pieces of iron or other magnetic material has given
rise to the conception of magnetic lines. Magnetic lines are
the imaginary lines along which the forces of attraction and
repulsion are exerted. The space surrounding a magnet in
which these forces are exerted is known as the magnetic field.
Each individual line forms a complete loop or circuit passing
through the poles of a magnet, and the number of these lines
which pass through the poles determine the strength of the mag-
netic field.
A bar magnet is surrounded by these lines which enter at
one pole and leave through the other, as shown in Fig. 12a. Mag-
netic lines are considered as passing out of the magnet at the
N. and into the magnet at the S. pole.
The distribution of magnetic lines under different conditions
is shown in Figs. 12a and 126, 13, and 14.
A magnet can not be produced with but one pole. If a bar
magnet is broken into a number of small pieces, each piece
will have a N. and a S. pole.
28 PRINCIPLES OF THE TELEPHONE
48. Experiment 5. Apparatus:
Horseshoe Magnet.
Bar Magnet.
Iron Filings.
Sheet of Smooth, Stiff Paper.
(a) Lay the bar magnet on the table, and over it place a sheet
of paper. Sprinkle iron filings over the paper. Tap the paper
gently while sprinkling the filings and note that the arrangement
of the filings is similar to the diagram of the magnetic field shown
in Fig. 12a.
(6) Repeat the above experiment using the horseshoe magnet
instead of the bar magnet.
49. The Magnetic Circuit. The path of the magnetic lines
is known as the magnetic circuit. Thus in Fig. 12a the magnetic
circuit is made up of two parts, the steel of the magnet and the
air through which the lines pass. A magnetic circuit is said
to be closed when the circuit is composed entirely of magnetic
substances, such as iron or steel. Whenever pieces of iron or
steel are brought into a magnetic field, the lines pass through
them very readily and they become magnetized. If the material
is soft iron, when taken out of the field it will lose most of its
magnetism, while if it is hard steel it will retain its magnetism,
both substances behaving the same as if they had been in actual
contact with a magnet.
Magnetic lines tend to pass along the path offering the least
resistance, the same as electric currents. Iron and steel offer
the least resistance to the passage of magnetic lines, so are used
for magnetic circuits whenever possible. Air offers from 1
to 10,000 times as much resistance to magnetic lines as iron,
depending upon the degree of magnetization. Copper, glass,
paper, and other nonmagnetic substances offer the same resist-
ance to magnetic lines as air. Magnetic circuits through sub-
stances other than iron are usually made short, so that the number
of lines in the magnetic field will be as great as possible. The
horseshoe magnet is stronger than the bar magnet of the same
size because the magnetic circuit through air is shorter. That
air does offer considerable resistance to magnetic lines can be
seen from the fact that it is necessary to bring a magnet quite
near a piece of iron before any attraction is noticed. The re-
sistance which any substance offers to the passage of magnetic
lines is known as reluctance.
MAGNETIC PRINCIPLES
29
50. Electromagnetism. Any conductor carrying an electric
current is surrounded by a magnetic field, as shown in Fig. 15.
The dark spot in the center of the figure represents a cross-
section of the wire.
51. Experiment 6. Apparatus:
Two Feet of Bare Copper Wire.
Dry Cells.
Iron Filings.
Compass.
(a) Dip the wire in the iron filings and note that the filings
do not stick to the wire. Now connect the ends of the wire to
the terminals of the dry cells; place the wire in the filings and
FIG. 15.
observe that the filings stick to the wire, although it 'is not a
magnetic substance. Disconnect one end of the wire and repeat
the experiment. It is evident that .the magnetic field exists only
as long as the current is flowing.
(6) Place the wire over the compass so that it is parallel to
the needle and close the circuit. Observe that the needle is
deflected, showing the magnetic action of the current. Reverse
the current through the wire by reversing the connections to the
battery, and note that the needle is deflected in the opposite
direction, showing the magnetic field has been reversed.
52. Solenoids. If a wire carrying a current be wound into
a coil, as shown in Fig. 16, the magnetic lines surrounding each
turn of the coil will be in the same direction as those of the
other turns, and the result will be a magnetic field similar to
that of a cylindrical bar magnet. A coil so arranged and carry-
ing a current is called a solenoid. A solenoid behaves exactly
like a bar magnet. At one end of the solenoid a N. pole exists,
30
PRINCIPLES OF THE TELEPHONE
while at the other end a S. pole exists, depending upon the direc-
tion of the current flowing in the wire. A reversal of the current
will cause a reversal of the magnetic field.
The strength of the magnetic field of any coil depends upon the
number of turns in the coil and upon the current flowing in the
coil, since the magnetic field of a solenoid is due to the added
effect of all the turns in the coil.
53. Experiment 7. Apparatus:
Solenoid.
Dry Cells.
Compass.
(a) Connect the solenoid to the dry cell and by placing one
end near the N. pole of the compass observe whether it attracts
or repels the compass.
FIG. 16.
If it attracts the N. pole of the compass, what kind of a pole
is it? If it repels? Mark the end with an N. or S.
Now test the other end the same way and observe that it is
of opposite polarity. Mark this end according to its polarity.
(6) By reversing the connections at the battery, cause the
current to flow in the opposite direction through the coils.
Test as before with the compass and note that the pole which
was marked N. in the first case has now become a S. pole, and the
one which was marked S. has become a N. pole.
54. Electromagnets. If an iron core be placed in a solenoid,
it becomes what is known as an electromagnet. Since magnetic
lines pass through iron much more readily than through air,
the same magnetizing force can produce a stronger field through
iron than through air or some other nonmagnetic substance.
MAGNETIC PRINCIPLES
31
Hence the purpose of the iron core is to increase the strength of
the magnetic field of the coil without increasing the current or
the number of turns in the coil.
FIG. 17.
The electromagnets used in telephone work are of three general
forms, classified according to the form of the iron core.
One form, the bar electromagnet, consists of a solenoid wound
on a straight iron core, as shown in Fig. 17. An examination
FIG. 18.
shows that the magnetic circuit contains a long air gap. If
this air gap be shortened, the number of lines and therefore the
strength of the magnetic field will be increased without changing
the current or coil in any way.
32
PRINCIPLES OF THE TELEPHONE
55. Horseshoe Electromagnet. One of the easiest ways of short-
ening the air gap is to bend a bar electromagnet in the form of a
horseshoe, as shown in Fig. 18. To facilitate manufacture,
however, the core of the horseshoe electromagnet is usually made
in three parts instead of being bent as shown. Fig. 19 shows the
general form of commercial horseshoe magnets, consisting of two
spools and the yoke joining their cores. Since such a magnet
usually is arranged to attract an armature, the latter further
decreases the air gap, as shown, to the short spaces between the
armature and poles.
56. The Ironclad Electromagnet. The ironclad or tubular
electromagnet is shown in Fig. 20. In this type, the coil is wound
FIBER
PAPER
CORE.
COIL
ARMATURE
FlG. 19.
on an iron core and is surrounded by a tubular shell. Such a
magnet has the advantages of occupying small space, and of
having its magnetic field confined strictly within the shell so
that there are no stray lines to affect other apparatus which may
be near.
57. Construction of Electromagnets. The coils for electro-
magnets are usually wound in the form of spools. Such a spool
may be entirely of fiber so that it can be removed from the core
if desirable, or the fiber ends may be forced on the iron core as
shown in Fig. 18. In the latter form several layers of paper are
wrapped around the core to insulate it thoroughly from the coil.
On this spool the insulated wire of the coil is wound in layers.
Sometimes the layers are further insulated from each other by
a thickness of paper.
MAGNETIC PRINCIPLES
33
CORE
58. Magnet Wire. The insulated copper wire used in wind-
ing coils for electromagnets is known as magnet wire.
Most magnet wire is covered with either silk or cotton. Of
the two, silk has the higher insulating properties and is used
largely on very fine wire, as a cover-
ing of silk is thinner than one of
cotton. Cotton is used almost ex-
clusively on the larger sizes. Silk
or cotton insulated wire has either
one or two layers of the insulating
materials, and is known as single
silk (or cotton) covered, and double
silk (or cotton) covered. When
two layers are used they are wound
in opposite directions. As both
silk and cotton absorb moisture
readily, wire insulated with these
materials is sometimes saturated
with melted paraffine, shellac, var-
nish, or some other insulating com-
pound to make it waterproof.
More often the coil is so treated
after being wound.
Enameled wire is a later develop-
ment in the insulation of magnet
wire. Enameled wire is made by-
coating the wire with liquid enamel
which is then baked on. The ad-
vantages of this wire are that it is FIG. 20.
waterproof, will stand high tem-
peratures, and the covering of insulating material is very thin.
59. Magnetic Action of Receiver. If an electric current be
sent through the coil of the receiver in such a direction that the
magnetic lines set up by it are in the same direction as those of
the permanent magnet, the strength of the magnet will be in-
creased and the disk will be drawn closer to the pole. If a cur-
rent be sent through the coil in the opposite direction, however,
so that the magnetic lines due to the current oppose those of the
magnet, the strength of the magnet will be decreased and the
diaphragm will spring away from the pole.
When a fluctuating current flows through the coil, the magnetic
34 PRINCIPLES OF THE TELEPHONE
field of the coil will increase with increasing current and decrease
as the current decreases, and these changes will cause changes
in the strength of the field of the permanent magnet. Thus,
whether or not the lines induced by the coil are in the same
direction as those of the permanent magnet, there will be changes
in the strength of the magnetic field whenever there are varia-
tions of the current flowing in the coil. Hence the diaphragm
will, vibrate in harmony with the changes of current.
QUESTIONS
1. What is a magnet? What is magnetism?
2. What is an artificial magnet? How made?
3. What is the pole of a magnet? How many poles do magnets have?
4. What is a compass needle? Which is the N. pole of a magnet?
5. What is a magnetic substance? Name the most common magnetic
substance.
6. What is meant by magnetic induction? Give an illustration of mag-
netic induction. Can magnetism be induced in a bar of iron without having
the bar come into contact with a magnet?
7. Give the law of attraction and repulsion between magnet poles.
8. What is a permanent magnet? What kind of material is usually used
in permanent magnets?
9. What is a magnetic line? What is a magnetic circuit?
10. What is a magnetic field? Upon what does the strength of a magnet
depend?
11. Which offers the greater resistance to magnetic lines: Air or steel?
Steel or copper? Wrought iron or air?
12. Why is a horseshoe magnet stronger than a bar magnet of the same
size?
13. If a telephone receiver is examined it will be noticed that there is a
steady pull between the disk and the magnet. How can this be explained?
14. What would be the effect if the disk and magnet poles should be
moved closer together?
15. What relation exists between an electric current and magnetism?
16. What is a solenoid? Explain how a solenoid is similar to a bar
magnet.
17. What happens when the current stops flowing in a solenoid? When
the current is reversed what happens? Upon what does the strength of a
solenoid depend?
18. What is an electromagnet? Why is an iron core used? What are
the common forms of electromagnets? Name the advantages of each.
19. How is magnet wire insulated? Name advantages of each kind of
insulation.
20. Explain how a fluctuating electric current flowing in the receiver coil
will affect the diaphragm. Explain fully.
CHAPTER IV
SOUND
60. Sound. Sound is produced by the vibration of some
body, and is transmitted through space in the form of waves in
the air; hence sound may be defined as wave motion in the air,
capable of affecting the sense of hearing.
If a stone be dropped into a pond of water, a disturbance
is set up which spreads in the form of waves in ever-widening
circles.
If a tuning fork be started vibrating, sound is produced.
The sound travels from the source of disturbance in the form
of air waves. Investigation and experiment have shown that
the air moves forward and backward in the direction in which
the sound travels. At one instant the air in front of the fork
is condensed, while that behind it is rarefied, and the next in-
stant the air in front of the fork is rarefied while that behind it
is compressed. The waves thus travel as a series of compressions
and expansions. The sounds which issue from a telephone
receiver are caused by the rapid vibration of the iron diaphragm.
Air is not the only substance that will transmit sound waves,
water, wood, iron, etc., being useful in this respect. The early
telephone experiments mentioned in the first chapter depended
upon the transmission of sound waves through a tightly stretched
wire. That some material medium is necessary for the trans-
mission of sound waves can be shown by placing an electric bell
under the receiver of an air pump and exhausting the air. As
the air is exhausted from the receiver, the sounds from the bell
grow weaker and weaker until they cease entirely when the air
has been all exhausted, although the bell may be seen in full
operation all the time.
Since vibrating bodies produce sound waves, it is to be ex-
pected that sound waves are capable of causing certain bodies to
vibrate when the waves come into contact with such bodies.
This is shown by the fact that a person talking in a room where
4 35
36 PRINCIPLES OF THE TELEPHONE
there is a piano will cause certain wires of the instrument to
vibrate and thus give out sounds. Another proof is that a
heavy clap of thunder will often cause the windows of a house
to shake violently. In the telephone the sound waves of the
voice are directed against the diaphragm of the transmitter,
causing it to vibrate.
61. Velocity of Sound. It is well known that sound waves
take a considerable amount of time to travel from one point
to another. Experiments have shown that sound travels at
the rate of about 1,090 ft. per second. In connection with this
statement it is interesting to compare the speeds of electricity
and light with that of sound. Electric waves and light travel
with the' same speed, which is in round numbers 186,000 miles
a second, or about 930,000 times as fast as sound, since sound
travels about 1 mile in 5 sec. That light travels at a much
higher speed than sound can be verified easily by watching a
locomotive at a distance and observing how long a time is re-
quired for the sound to reach the ear after one sees the steam
issuing from the whistle.
62. Properties of Sound. The properties of sound depend
upon three different quantities: pitch, loudness, and timbre
or quality.
63. Pitch. Pitch is determined by the rate of vibration of
the sounding body; that is, the number of vibrations per second
determine whether the sounds given off will be "high" or
"low," a high rate of vibration giving a higher pitched sound
than a low rate of vibration. The short wires of a piano give
off high-pitched sounds because their rate of vibration is rapid,
and the longer bass strings which vibrate at a slower rate give
off lower tones.
64. Loudness. Loudness of sound depends upon the distance
through which the sounding body vibrates. The distance
through which the vibrating body moves is called the ampli-
tude. Thus when a piano key is struck a sharp blow, the
amplitude of the string will be greater than when a light blow
is given the key. In the former case, the sound is louder or
stronger than in the second case, though the pitch is the same.
Loudness depends upon the energy of the vibration.
65. Timbre or Quality. Quality is that property of sound
not due to pitch or loudness, that enables us to tell one sound
from another. For an example, a violin and piano may be sound-
SOUND 37
ing the same note, yet a difference in quality can be detected.
This difference is not due to pitch or loudness. The char-
acteristics of the waves given out by the two strings are dif-
ferent. This perhaps can be made clearer by considering a
water wave. When such a wave is examined it is seen that
many small waves surmount it. Similarly a string or other
sounding body can start waves which consist of a fundamental
wave and also small waves. These small waves are called
overtones, and so change the wave form, and thus the quality
of the sound, that we are able to tell one person's voice from
another's, or to distinguish between the sounds of different
musical instruments.
66. Transmission of Speech. If the sounds of speech were
simply in the form of waves of a given pitch, they could be
transmitted over the telephone lines by merely opening and
closing the circuit at the transmitter the required number
of times per second. For every time the circuit was opened
or closed there would be change of current through the receiver
and a corresponding magnetic action which would cause the
diaphragm to move. The loudness of the sound, which depends
upon the amount of movement of the receiver diaphragm,
could be controlled by variations in the strength of the current.
However, the vibrations due to the sound of the human
voice are very complex, due to overtones and the variations
of both pitch and loudness which take place hundreds of times
a second. Hence to transmit such sounds is much more difficult
than we might at first imagine, since the current flowing in the
telephone circuit must vary with the slightest variation in the
sounds to be transmitted, whether these variations be in timbre,
pitch, or loudness.
67. Experiment 8. Apparatus:
Telephone Receiver.
Two Dry Cells.
Copper Wire.
Coarse File.
(a) Connect one dry cell in series with the receiver as shown
in Fig. 21. Attach one of the wires to the tang of the file. Draw
the end of the other wire along the file so as to open and close the
circuit repeatedly, in the meantime observing that the sound
given off by the receiver is merely a series of clicks, which occur
whenever the circuit is opened or closed. The pitch of the sound
38
PRINCIPLES OF THE TELEPHONE
depends upon the rapidity with which the wire is drawn along
the file.
(6) Repeat the above with two cells in series, and note that
the only change is that the sound produced by the receiver is
louder than when one cell is used. This shows that the loudness
of sounds depends upon the energy of the vibrations of the
diaphragm.
68. Variable Resistance. With the telephone parts connected
as shown in Fig. 22, a change in resistance in any of the parts
causes a change in the current flowing in the circuit. Hence,
instead of opening and closing the circuit to send variable cur-
FIG. 21.
rents over the line, the resistance of the transmitter is changed
from time to time by the sound waves of the voice. This
variable resistance is obtained by the use of. carbon.
Carbon is found in a number of well-known forms, such as
charcoal, graphite, lampblack, etc. Hard carbon, similar to
arc-lamp carbon, is used in telephone transmitters. The
property of carbon which makes it suitable for this work is that
the electrical resistance of a contact made of this substance can
be regulated by the pressure applied. This resistance, which
depends in a large measure upon the closeness of contact of the
carbon parts, is decreased when the pressure is increased, and
increased when the pressure is reduced. Such a contact is very
sensitive, the slightest variation in pressure causing a change in
SOUND
39
its resistance. In the transmitter the changes in pressure neces-
sary to cause variations in the electrical resistance of the carbon
parts are produced by the vibrations of the diaphragm, and since
the diaphragm is very sensitive and responds to the slightest
FIG. 22.
variations of pitch, loudness, and quality of the sound waves
of the voice, the pressure on the carbon parts varies according
to the characteristics of these sound waves.
CHAPTER V
TRANSMITTERS
69. The Carbon Transmitter. In the earlier forms of trans-
mitters such as the Edison and Blake, the variations in resist-
ance were obtained through the action of the diaphragm on a
single disk of carbon. The use of such instruments was limited
by the fact that currents heavy enough to give the required
transmission burned the surfaces of the carbon electrodes at the
points of contact, soon destroying them. In order to provide a
large number of points of contact, between which the current is
divided, the granulated carbon type of transmitter was developed.
This type is used at present to the exclusion of all others, and
consists of two carbon disks, one stationary, the other movable
and arranged to vibrate with the diaphragm, separated by a
small quantity of granulated carbon. The greatest drawback
to the early adoption of this type was the tendency of the
granulated carbon to "pack" into a compact mass, which
rendered the transmitter useless. In order to overcome this
tendency the solid-back type was developed.
70. White Solid-back Transmitter. The White solid-back
transmitter which has been the standard of the Bell companies for
many years, is shown in section in Fig. 23. The case, A, is made
of brass, having a heavy cover, B, to which is attached the hard-
rubber mouthpiece M. The mouthpiece serves to collect the
sound waves and concentrate them upon the diaphragm, D, which
is a thin iron or aluminum disk having its edge covered with
rubber, R. As shown in Fig. 24, two springs, S, and $', bear on
the diaphragm. The short one holds the edge of the diaphragm
firmly against the cover, B, and the long one rests on the dia-
phragm to dampen its vibrations and render it less sensitive
to outside noises.
The electrodes, which are two polished carbon disks, E and
E', are contained in a brass chamber consisting of two parts.
The rear electrode, E, which is the larger of the two, is firmly
secured within the brass cup, F. The cup, F, is attached to the
40
TRANSMITTERS
41
FIG. 23.
FIG. 24.
42 PRINCIPLES OF THE TELEPHONE
bridge, G, by means of the pin and set-screw. The front carbon
is fastened to the stud, 0, the shank of which passes through the
diaphragm and is held in place by two check nuts. A thin
mica washer, M, is clamped between the head of the stud and the
threaded ring, N, the outer edge of this washer being held be-
tween the cap, H, and the cup, F. The center of the mica washer
is therefore rigidly attached to the front electrode and partakes
of its movements, while the outer edge is fastened to the rear
electrode which is fixed. Any changes in relative position of
the electrodes can take place only through the bending of the
mica washer. In addition to holding the front electrode in its
normal position, the mica washer closes the chamber containing
the electrodes and keeps the granulated carbon with which this
space is filled from falling out. The front electrode is insulated
from the frame by the mica washer, and by the fiber lining, L,
which keeps the granulated carbon away from the sides of the
cup. Since one terminal is connected to the front electrode by
the flexible connection, C (see Fig. 24) and the other to the frame
of the transmitter, any current which passes through the instru-
ment must flow through the granulated carbon.
The operation of the solid-back transmitter is as follows:
The sound waves of the voice of the person speaking cause
vibration of the diaphragm, which, being rigidly connected to
the front electrode, causes that to vibrate also, as the mica washer
which holds it in place is very flexible. Since the back electrode
is held stationary, the granulated carbon is subjected to varia-
tions in pressure. As a result the current flowing through the
transmitter is varied.
71. New Western Electric Transmitter. The new Western
Electric transmitter, shown in Fig. 25, is a modified form of the
White instrument. As in the White transmitter, . the front
electrode is carried on a mica washer and is connected by a
stud to the center of the diaphragm, and the rear electrode is
fixed in the bottom of the electrode chamber. This chamber
is attached to the back of a metal cup, S (which takes the place
of the bridge in the White transmitter) by the threaded part, C.
This not only holds the chamber in place, but also holds the
outer edge of the mica washer firmly between the two parts.
The metal cup and diaphragm are insulated from the shell
of the transmitter at R, so that neither of the electrodes is
TRANSMITTERS
43
FIG. 25.
FIG. 26.
44
PRINCIPLES OF THE TELEPHONE
connected to the exposed metal parts. Of the terminals, shown
in the figure, TI is connected to the cup, and the other, T z ,
which is insulated from the cup, is connected to the front electrode
by a flexible connection.
72. Kellogg Transmitter. The Kellogg Switchboard and
Supply Co.'s transmitter is shown in section in Fig. 26. It will
be immediately noticed that the chief difference between this
instrument and those previously discussed is that the electrode
cup is made a part of the diaphragm, D, and therefore partakes
FIG. 27.
of its movements. In order that the moving parts may not be
too heavy to respond readily to the sound waves of the voice,
the diaphragm is made of hard-drawn aluminum with the elec-
trode chamber stamped in its center. The diaphragm, D, the
front carbon disk, E, which is attached to the bottom of the
chamber, the granulated carbon, C, and the mica washer, M,
are the movable parts. The disk E' is stationary, as it is rigidly
attached to the bridge, G. This bridge is a straight piece of
hard-drawn brass.
To prevent the transmitter's taking up outside noises' and
being affected by mechanical vibration which might inter-
TRANSMITTERS
45
fere with talking, the diaphragm rests on a soft pad, P. Two
damping springs having cushioned tips have been provided as
in the White instrument. The working parts of this trans-
mitter are all insulated from the case.
73. Monarch Transmitter. The Monarch transmitter shown
in Fig. 27 differs from those already studied in having both
its electrodes mounted on flexible mica washers which support
the carbon chamber. The rear electrode, which is attached
to the bridge, is the only fixed part. The diaphragm is of
aluminum and is separated from the case by an insulating ring.
The flexible connection between one terminal and the front
electrode is shown in the figure. The ^_____________,
stud of the rear electrode, which is
insulated from the bridge, is connected
to the other terminal.
74. Operator's Transmitter. In
Fig. 28 is shown a special form of
transmitter for switchboard operators'
use. As this instrument is provided
with a plate which rests on the oper-
ator's breast, the long curved mouth-
piece is always in the proper position
for use. The breast transmitter and
watch-case receiver described in the
next chapter make up the operator's set.
In cases where the operator is compelled to leave the switch-
board frequently to attend to other duties, as in small exchanges,
many of the advantages of the breast transmitter are lost.
In such cases a transmitter of the same form as the subscribers'
instrument is suspended by adjustable cords in front of the
operator.
75. Carbon Electrodes. The disks for use in transmitters
are made of specially prepared hard carbon. The faces in
contact with the granulated carbon are made as nearly true
as possible, and are highly polished. The reverse sides are
copper plated and then soldered to the backing plates of brass.
When assembled, the electrodes must be parallel to each other
if good results in operation are to be obtained.
The granular carbon is very hard, uniform in size, and free
from dust. As mentioned above, a great deal of trouble was
caused by the packing of the granulated carbon in the earlier
FIG. 28.
46 PRINCIPLES OF THE TELEPHONE
transmitters, due to moisture, unevenness in size of carbon
grains, and by wedging apart of the carbon disks. These
difficulties were overcome by making the chamber containing
the carbon grains waterproof; by making the grains of uniform
size and hard enough not to crush in service; and by improve-
ments in manufacture, so that the electrodes are always parallel
to each other.
Any transmitter can be packed by pulling the diaphragm
forward so as to widely separate the electrodes. This allows
the carbon granules to settle and wedge the electrodes apart.
In the earlier types this could be done by placing the lips against
the mouthpiece and drawing in the breath. In order to prevent
this, modern mouthpieces are slotted at the base.
According to a recent report of the American Telephone
and Telegraph Co. there were designed, constructed, and in-
stalled, during the 37 years from 1877 to 1914, 53 improved types
and styles of telephone receivers and 73 types and styles of
transmitters. These figures do not include hundreds of minor
improvements made in both transmitters and receivers.
QUESTIONS
1. How are sounds transmitted by the telephone? Does sound actually
travel from one instrument to the other?
2. What are the parts of the telephone used in transmitting the sounds of
speech?
3. Will a telephone work if the battery be removed? Why not?
4. What do you understand sound to be? How is sound produced?
How transmitted from place to place?
5. How fast does sound travel through air? Compare the speed of sound
with that of light. With electricity.
6. Upon what three things does the quality of sound depend?
7. What is pitch? Loudness? Timbre?
8. Explain how a telephone receiver produces sound.
9. Explain how sound waves can cause the transmitter diaphragm to move.
10. What are the characteristics of the waves set up by the sounds of
speech?
11. Why can not the sounds of speech be transmitted by repeatedly open-
ing and closing the telephone circuit as in the experiment?
12. Examine carefully as many different makes of transmitters as pos-
sible. What differences do you find?
13. Explain how changes in transmitter resistance can cause the receiver
to operate. How are these changes in resistance caused?
14. Why is carbon used in transmitters?
TRANSMITTERS 47
15. Explain briefly the construction and action of the solid-back trans-
mitter.
16. What is meant by packing of a transmitter? How is packing caused ?
17. Does the carbon transmitter ever open the battery circuit? Answer
this question by studying the transmitters shown in Figs. 23 to 27.
CHAPTER VI
RECEIVERS AND INDUCTION COILS
76. The Receiver. The telephone receiver makes use of the
fluctuating electric currents to reproduce the sound waves which
caused these current variations at the transmitting end of the
line. Receivers are electromagnetic in their action, as has been
briefly explained in an earlier chapter.
77. Early Receivers. Early receivers were of the single-pole
type; that is, the diaphragm was influenced by only one pole of
the magnet. An early form of receiver is shown in Fig. 10, the
parts being named. The operation of such a receiver is due to
the magnetic action of the current flowing through the coil,
which either weakens or strengthens the magnetic field of the
permanent magnet, and thus causes the diaphragm to vibrate
in unison with the changes of current strength.
The magnetic circuit of this type of receiver contains a very
long air path ; hence a considerable current is required to produce
the required changes in magnetic force. Another serious objec-
tion to this type of receiver is the ease with which the adjustment
is disturbed, owing to the magnet being attached to the shell
at the end farthest from the diaphragm.
78. Induced Electric Pressure. A further investigation of the
relations existing between magnetism and electricity shows that
when a wire is moved in a magnetic field so as to cut the magnetic
lines, an electrical pressure is set up in the wire. The value of
this pressure depends upon the rate of cutting the magnetic
lines, or, in other words, the number of lines cut per second.
A pressure generated by the relative movement of a conductor
and a magnetic field, is called an induced pressure.
The direction of the induced pressure depends upon the direc-
tion of the cutting of magnetic lines. Hence a movement of a
conductor in one direction through a magnetic field will cause
a pressure in one direction, and a movement in the opposite
direction will generate a pressure in the opposite direction. A
pressure can be induced in a coil by changing the strength of the
RECEIVERS AND INDUCTION COILS 49
magnetic field inside the coil. Since a magnetic line makes a
complete loop or path, it is evident that if the number of lines
inside a coil are changed, some lines must be cut by the coil
during the change. Increasing the strength of the field inside
a coil sets up a pressure in one direction, while decreasing the
number of lines sets up an opposite pressure, because the lines
are cut in opposite directions during these changes.
The induced pressure will be maintained only so long as the
relative motion of conductor and field is kept up, or while
magnetic lines are being cut.
In general we may say that whenever the magnetic field sur-
rounding a conductor varies in intensity an electrical pressure
will be set up in the conductor, and if the circuit be closed a
current will flow.
Induced pressure may be either direct or alternating, depend-
ing upon whether the magnetic lines are cut continuously in one
direction or the direction of cutting is reversed from time to
time.
79. Direct Current. A direct current flows continuously in
one direction, although its strength may vary from time to time.
The flow of a current of electricity caused by the pressure of a
battery is in one direction.
Direct currents may be divided into two classes, continuous
and pulsating. A continuous current is one the strength of
which does not change materially from instant to instant. A
pulsating current, however, is a direct current the strength of
which may vary from time to time without change in the direc-
tion of flow. Continuous currents are used for lighting and power
purposes. Pulsating currents are made use of in telephone
practice.
80. Alternating Currents. An alternating current is one
which varies continuously in strength and changes direction
periodically.
81. Experiment 9. Apparatus:
Two Telephone Receivers.
About 50 ft. of Annunciator Wire.
Connect two telephone receivers by about 25 ft. of copper wire.
Have a person in another room to assist you and see if sounds
can be transmitted without using any batteries in the circuit.
It will be seen from the above, since no battery or other source
of power is used, that the only energy used in operating this
50 PRINCIPLES OF THE TELEPHONE
telephone is that of the sound waves themselves. This energy
is very small; hence the resultant current sent from one station
to another is likewise small, and sounds can be transmitted
only a short distance. It was early realized by those interested
in the development of the telephone that if the telephone was to
become of any commercial value, one capable of transmitting
speech to a greater distance was necessary.
82. The Receiver as a Transmitter. Two receivers connected
as shown in Fig. 29 formed the first practical telephone for the
transmission of speech, and constituted Bell's invention. The
operation of such a telephone is as follows:
Suppose that A is the sending or transmitting station, and B
the receiving station. The sound waves due to the sounds of
speech strike the diaphragm at A and cause it to vibrate in unison
FIG. 29.
with the waves of sound. That is, every variation in the pitch,
loudness, or timbre of the sounds affects the diaphragm. The
vibrations of the diaphragm cause variations in the strength of
the magnetic field, since every vibration causes a change in the
length of the air gap between the disk and the pole of the magnet,
and thus increases or decreases the number of magnetic lines
which pass through the coil.
Every time the magnetic field is disturbed, induced currents
are set up in the- coil. These electrical currents flowing through
the coil at B cause the diaphragm at B to vibrate in unison with
that at A, and thus produce sound waves like those which
cause the diaphragm at A to vibrate.
The receiver seemed to be quite satisfactory, for it would work
when large enough currents could be made to flow through it.
Hence efforts were made to improve the transmitter, which re-
sulted in the development of the carbon transmitter. We have
observed that the carbon transmitter does not generate its own
current, but merely controls the current from some outside
source, such as batteries.
83. Bipolar Receiver. In the bipolar type of receiver the air
gap is very much shorter than in the single-pole receiver, since
RECEIVERS AND INDUCTION COILS 51
both poles are near the diaphragm. The working parts of the
receiver are attached to the case near the diaphragm, or are
arranged in an inner metallic case so that the adjustment is
independent of the outer case.
84. Western Electric Receiver. Fig. 30 shows the con-
struction of the Western Electric bipolar receiver. The shell is
of hard rubber and is made in three parts. Two permanent bar
magnets are employed, being fastened together so as to form a
single horseshoe magnet. Two soft-iron pole pieces P and P'
are attached to the ends of the magnet near the diaphragm.
Each one of the soft-iron poles is surrounded by a coil of very
fine insulated copper wire, marked M and M' in the figure. Im-
mediately in front of the poles is placed the sheet-iron dia-
phragm D which must not touch the pole pieces even when
vibrating through its widest range. One of the magnet poles is
N. and the other is S. The diaphragm forms a part of the
magnetic circuit, and where the lines enter the diaphragm a
S. pole is formed, and where the lines leave the diaphragm a
N. pole is formed. Thus the diaphragm acts as an armature
and by the attraction of the magnet is constantly bent or dished
toward the pole pieces.
The coils on the pole pieces are connected so that the mag-
netic lines set up by a current passing through them will make
one a N. pole and the other a S. pole. The currents flowing
through the coils in one direction tend to strengthen the field of
the permanent magnet, and currents flowing in the opposite
direction tend to weaken the field of the permanent magnet.
The diaphragm will spring away from the pole pieces when they
are weakened, and when the current ceases the diaphragm will
be drawn back toward the pole pieces. When the magnetic
field set up by the coils assists the field of the magnet, the
diaphragm will be drawn nearer to the pole pieces, and when the
current stops the diaphragm will again spring back to its normal
position.
From this it will be seen that if a current flows first in one
direction and then in another, or an alternating current flows in
the receiver coil, the diaphragm will answer every impulse of
current, no matter from which direction it comes. Alternating
currents flow in circuits where an induction coil is used.
If the receiver were not equipped with a permanent magnet,
its magnetic field would be strengthened by a current flowing
52
PRINCIPLES OF THE TELEPHONE
through the coil in either direction, and the diaphragm would be
attracted or drawn in toward the pole whenever a current
flowed. However, an alternating current flowing in the receiver
FIG. 30.
FIG. 30a.
coil will alternately strengthen and weaken the field of a perma-
nent magnet, in the first case drawing the diaphragm out of its
FIG. 306.
normal position, closer to the pole, and when the field is weak-
ened, allowing it to spring farther away. Hence, when a perma-
nent magnet is used, an alternating current is capable of pro-
RECEIVERS AND INDUCTION COILS 53
ducing a greater vibration of the diaphragm than would be the
case if a soft-iron core were used. The pitch is also an octave
lower.
The resistance of the coils M and M ' is usually about 60 ohms
for the pair, or 30 ohms for each coil.
The magnet is attached to the case by means of a threaded
block which screws into the internal thread B. This arrangement
allows of close adjustment of the distance between the pole
faces and the disk, and as a result of the close coupling, changes
in temperature do not readily affect this adjustment. - The
latest type of this receiver no longer has exposed binding posts.
This is shown in Fig. 30a and b. This receiver is the standard
in use by the Bell Co.
85. The Kellogg Receiver. The Kellogg receiver with in-
ternal binding posts for the wires, or cords, as they are com-
monly called, is shown in section in Fig. 31. The shell, S, and
FIG. 32.
the cap, E, are of composition rubber, the shell consisting of a
single piece. In order to make the cap stronger and less liable
to split under hard usage, a perforated copper disk is molded into
it. The diaphragm, D, is firmly clamped between the cap and
the brass cup, C, to which the permanent magnets are at-
tached. Therefore, the adjustment which is made between the
diaphragm and the poles of the magnet at the time of manu-
facture is permanent.
The receiver can be completely taken apart, without breaking
any connections, by removing the cap. Fig. 32 shows the re-
ceiver with shell removed. The permanent magnets, P and
P f , Fig. 31, are placed side by side with corresponding poles at
opposite ends and bolted together at the rear end, holding the
block of iron, H, firmly between them, in effect forming a U
54
PRINCIPLES OF THE TELEPHONE
I
magnet. At the diaphragm end the soft-iron pole pieces are
attached. The two pole pieces are separated by a part of the
brass cup, and are firmly clamped between the permanent mag-
nets by the brass bolt, B. Brass being a
nonmagnetic substance, as has been
shown above, has no effect on the mag-
netic field of the magnet. In order that
no strain may be placed upon the bind-
ing posts when the receiver is in use, the
cord is firmly tied to the block, H.
As receiver cords are a considerable
source of annoyan.ce, it is interesting in
connection with this receiver to note the
F 33 method of fastening the metal tips to
the flexible strands of the cords. Fig.
33 shows the details of making this connection. The cord tip
is first wrapped tightly with wire, .and the strands are brought
back over this and firmly held in place by a metal clamp, as
FIG. 34.
shown. The tip, which is turned and bored from a solid brass
rod, is then soldered over this special clamp.
The particular advantages of a receiver with internal binding
posts are that the binding posts, being inside the case, are not
RECEIVERS AND INDUCTION COILS 55
subject to injury; the cord at point of contact is not subject to
damage; the user of the receiver cannot receive shocks from the
same, since he can not touch the posts; and the receiver has a
very neat appearance.
86. Operator's Receiver. The operator's receiver (or watch-
case receiver, as it is often called on account of its shape) is
shown in Figs. 34 and 35.
This instrument is a double-
pole receiver; hence the opera-
tion is the same as that of the
hand receivers described above.
The permanent magnet consists
of steel rings, P, which are cross-
magnetized so that a N. pole ex-
ists on one side and a S. pole on
the other. The soft-iron pole pieces are clamped between the
bottom ring and the case, as shown in the sectioned view.
87. Sensitiveness of Receivers. The sensitiveness of a re-
ceiver depends upon the strength of the permanent magnet and
upon the diameter and thickness of the diaphragm, and its
distance from the magnet poles. A thin diaphragm responds
very readily to currents of high frequency (or rapid vibration)
and gives clear and sharp tones, while a thicker one is more
rigid and responds readily only to those currents having low
frequency. About )-{ oo in- is the average thickness of sheet iron
used in receiver diaphragms, the diameter being about 2J in.
For successful operation a thick diaphragm must be larger than
a thin one. The chief objection to a very sensitive receiver is
that it reproduces any disturbances which the line may have
taken up as faithfully as it does the sound from the transmitting
end. A very sensitive receiver, therefore, would be unsuitable
for use on a grounded line or on a metallic circuit of poor con-
struction. On a long metallic circuit of good construction, the
simple weakening of the transmitter current may be compensated
to some extent by using a sensitive and delicate receiver.
To secure loudness, the receiver must be arranged so that the
alternating currents from the line produce the largest possible
movement of the diaphragm in both directions from its normal
position. The strength of the permanent magnet must be de-
signed with reference to the properties of the diaphragm and <the
strength of the line currents. If the permanent magnet be too
56 PRINCIPLES OF THE TELEPHONE
strong, the diaphragm will be dished excessively, and no con-
siderable movement is possible when the magnet is made still
stronger, although a large movement takes place in the opposite
direction when the magnet is weakened. If the permanent
magnet be too weak, the movement of the diaphragm will be
large when the magnet is strengthened, and small when it is
weakened. Hence, if the permanent magnet be either too strong
or too weak, the receiving will be imperfect.
88. Direct-current Receiver. A later development in receiver
construction is the direct-current receiver.
In common battery telephone practice when the line is in use
there is a steady current flowing from the central office to the
subscriber's instrument to energize the transmitter of the
latter. In the direct-current receiver this current is made to flow
through the windings of the receiver, thus producing a magnetic
field which takes the place of that due to the permanent magnets
in the common receiver. Hence no permanent magnets are re-
quired. This action will be more fully understood when the
common battery system is studied in a later chapter.
89. The Automatic Electric Co.'s Direct-current Receiver.
The Automatic Electric Co.'s direct-current receiver is shown in
FIG. 36.
Fig. 36. The working parts of the receiver are contained within
the brass cup, S. The winding consists of a single coil mounted
on the core, C, which in turn is attached to the center of a U-
shaped iron stamping having its ends, P and P', bent up and
partially surrounding the coil.
The direct current flowing in the line magnetizes the core and
steel stamping. From the shape of the magnetic circuit, it is
evident that at any given time the end of the core near the dia-
RECEIVERS AND INDUCTION COILS
57
phragm will be a N. pole, and the ends, P and P f , will-be S. poles,
or vice versa. This receiver, then, has all the advantages of the
bipolar receiver in having both its poles near the diaphragm.
When the talking circuit is open there is no current flowing in the
coil, and the diaphragm will be perfectly flat at a short distance
from the poles. However, as soon as the talking circuit is closed,
direct current flows through the coil and draws the diaphragm
toward the poles of the magnet, exactly as is done in the case of
the polarized receivers. When the fluctuating voice currents
flow through the coil they will either strengthen
or weaken the direct current flowing in the
coil, and will cause the magnetic field to fluc-
tuate, thus causing a movement of the dia-
phragm. This receiver is designed to operate
when current values range from 0.45 to 0.80
amp.
90. The Monarch Direct-current Receiver.
The Monarch direct-current receiver shown
in Fig. 37 operates on quite a different plan
than the one just discussed. This receiver
has, instead of permanent magnets, two soft-
iron cores, S and /S', on which are mounted
two long coils, C and C'. Soft-iron pole pieces
are attached to the ends of these cores, as
shown, and carry the two coils, M and M 1 '.
The two sets of coils are connected in parallel to the cords of the
receiver. The coils C and C' have a somewhat lower resistance
than the coils M and M' '; and, owing to the fact that they have
a larger number of turns and have iron cores of larger size, they
have a greater impedance and are known as impedance coils.
The direct current of the line, therefore, will flow very readily
through the two long coils and will magnetize the cores, giving
the effect of a permanent magnet, but on account of the high
impedance of these coils, the high-frequency voice currents will
flow more readily through the coils M and M', and will affect
the magnetic strength of the cores exactly as do the coils of the
polarized receiver.
91. Self-induction. A most important property of electric
circuits is their action and reaction upon each other. Fig. 15
shows that every current carrying wire is surrounded by a mag-
netic field. The current in the wire builds up this field. It has
FIG. 37.
58 PRINCIPLES OF THE TELEPHONE
also been shown that whenever the intensity of a magnetic field
around a conductor changes either by being built up, by decay,
or by relative motion in such a way that the magnetic lines cut
across the conductor, an electromotive force is developed in the
conductor. This electromotive force is in such a direction as to
oppose any change in the current flowing or in the magnetic
field surrounding the conductor. This principle of electro-
magnetic induction evidently does not depend upon the source
of the magnetic field. Hence an e.m.f. is induced in a conductor
when the current in the conductor changes, for every change in
current is accompanied by a change in the density of the mag-
netic field which is due to the current. This principle of in-
ducing an e.m.f. by variations in the current flowing is known as
self-induction.
When an e.m.f. is impressed upon a circuit, the current can
not rise to a maximum value at once on account of the counter-
pressure of self-induction.
It is very evident that the value of the counter e.m.f. of self-
induction depends upon the rate at which the flux surrounding
the conductor changes; hence it will depend upon the shape of the
circuit. If the conductor is wound into a coil of such shape that
all of the magnetic lines thread through it, the counter-pressure
of self-induction for a corresponding change in the current will
be greater than when the conductor is straight.
92. Self-inductance. If a conductor is wound into a coil so
that if the current varies at the rate of 1 amp. per second the
pressure of self-induction is 1 volt, the coil is said to have unit
self -inductance. Self-inductance is thus the numerical value
of the property which causes a counter-pressure of self-induction
to be developed. It may also be defined as the ratio of the flux
threading through a coil to the current producing it. For any
given coil with an air core the self-inductance is a constant
quantity. If the coil has an iron core the self-inductance is not
constant for the reason that the flux does not vary uniformly
with the current. In other words, the permeability of iron is
not constant, but varies with the current.
93. Mutual Induction. If the magnetic flux produced by a
current in one coil threads through a neighboring coil, an electro-
motive force is also induced in the second coil. This e.m.f. is
said to be due to mutual induction. In so far as the physical
principles are concerned, self and mutual induction are alike.
RECEIVERS AND INDUCTION COILS 59
The only difference is that in one case the e.m.f. is induced in the
circuit in which the current is flowing, and in the other case the
e.m.f. is induced in a neighboring but separate circuit.
94. Impedance. In any circuit that has self-induction any
change in the current will be opposed. When the current is
increasing the induced e.m.f. opposes the increase and when the
current is decreasing the e.m.f. induced is in such a direction as
to oppose the decrease. The value of this opposing e.m.f. de-
pends not only upon the self-inductance of the circuit or coil,
but also the rate at which the current is changing.
If an alternating e.m.f. of a given value is impressed upon a
coil which has some self-inductance, the resulting current will be
smaller than if a constant direct e.m.f. were impressed upon the
same coil. The ratio of the e.m.f. to the current flowing is
called the impedance. In other words, the alternating e.m.f.
divided by the impedance gives the current in the circuit. The
higher the impedance, the smaller the current. As the im-
pedance increases with the frequency, the higher the frequency
the smaller the current in an inductive circuit. The currents
in a telephone line which are produced by the voice are of high
frequency, hence only a small current will pass through a coil
which has considerable inductance.
95. The Induction Coil. In order to transmit telephone
messages to any considerable distance it is necessary to use
FIG. 38a.
induction coils in the circuit to increase the voltage, so that
the resistance of the line may be overcome.
The induction coil is designed to raise the voltage of the
line current, which is controlled by the transmitter. The current
through the transmitter and battery is a direct current but is
pulsating, due to the variable resistance of the transmitter.
Another function of the induction coil is to change this pulsating
direct current to an alternating one. An induction coils are
shown in Figs. 38a and 386.
The induction coil consists of an iron core and two windings
60
PRINCIPLES OF THE TELEPHONE
of insulated wire, known as the primary and secondary, as shown
in the diagram of Fig. 39. The core is made of a bundle of iron
wires which have been softened by annealing. The primary
winding consists of from 250 to 600 turns of insulated copper
wire, and the secondary of from 1,500 to 3,500 turns. The
primary is made of larger wire than the secondary, and the two
windings are not connected inside the coil. The primary is
connected in the circuit with the transmitter and batteries, as
TIG. 386.
shown in Fig. 40, and the secondaries are connected to the line.
Hence the only currents which flow in the line are the alternat-
ing currents induced in the secondaries of the coils.
The operation of the induction coil is as follows: The pul-
sating current flowing through the primary induces a pulsating
magnetic field in the iron core, increasing as the current increases
and decreasing as the current decreases. The secondary, being
wound on the same core as the primary, must cut the magnetic
FIG. 39.
lines every time the field changes. Thus a voltage will be in-
duced in the secondary winding every time there is a change in
the value of the core magnetism. When this field increases
there will be a voltage induced in one direction, and when it
decreases a voltage will be induced in the opposite direction.
An alternating current thus flows in the line. Since the mag-
netic field is of the same value through each of the turns of
both the primary and secondary, at any instant the voltage of
one turn of the primary which causes the change of field must
RECEIVERS AND INDUCTION COILS 61
equal the voltage caused by that field in one turn of the second-
ary. Since the turns are all in series, the primary and secondary
voltages will be in the same ratio as the number of turns in
the primary and secondary windings. Thus for a coil having
500 primary 'turns and 2,000 secondary turns, the voltage ratio
would be 1 to 4; and if the primary pressure were 3 volts, the
secondary would be 12 volts.
The induction coil helps transmission by permitting the use
of a low-resistance battery circuit; by producing alternating
currents ; and by producing currents of higher voltage than could
be successfully handled by a transmitter.
0^3=
*
t
FIG. 40.
QUESTIONS
1. Explain the action of a telephone receiver.
2. Why is a permanent magnet used instead of a soft-iron core? Give
two reasons.
3. Explain how a pressure can be generated by induction.
4. What is a direct current? How is it produced?
6. What is an alternating current? How is it produced?
6. Are most induced currents direct or alternating? Why?
7. Can a receiver be used as a transmitter? If so, explain its action
as such.
8. What are the objections to the use of a single-pole receiver?
9. Explain the construction of a double-pole receiver.
10. Upon what does the sensitiveness of a receiver depend?
11. Why are receivers not made as sensitive as possible?
12. What are the advantages of inside binding posts?
13. Why should the magnet be attached to the case near the diaphragm?
14. For what purpose is an induction coil placed in a telephone circuit?
15. What are the principal parts of an induction coil?
16. Explain how an induction coil increases the voltage in a telephone
system.
17. What determines the ratio of the primary voltage to secondary
voltage?
62 PRINCIPLES OF THE TELEPHONE
18. In what ways does an induction-coil help transmission?
19. Why is an alternating current better than a pulsating direct current
in the operation of a receiver?
20. Explain the operation of the Monarch direct-current receiver.
21. Explain self-induction; mutual induction. Cross talk is usually due
to mutual induction.
22. What is the self -inductance of a circuit if a pressure of 2 volts is
induced when the current in the circuit varies at the rate of 50 amp. per
second? The unit of self-inductance is called a henry.
23. What quantities determine the current strength when an alternating
pressure is connected to a circuit?
CHAPTER VII
SIGNALLING APPARATUS AND CIRCUITS
96. Signalling Circuits. So far, only the transmitting and
receiving circuits (or, as they are generally called, the talking
and listening circuits) have been considered. In addition to
these circuits, some means must be provided for signalling the
subscriber when he is wanted, and likewise for him to signal the
central operator when he wants some other party.
In the local battery system, that is, the system where each
telephone instrument has its own batteries, a hand generator
or small dynamo and a bell or ringer form the signalling circuit.
97. Generators. In the study of magnetism it was said that
if a conductor be moved in a magnetic field, so as to cut the
magnetic flux, an electric pressure is generated, and if the
circuit be closed a current will flow. The operation of the
electric generator is dependent upon this principle.
Faraday, a noted scientist, discovered this principle of magneto-
electric induction in 1831. Following out his experiments along
this line, he made the first dynamo known. This model consisted
of a disk of copper 12 in. in diameter, which was mounted on a
shaft so that it was free to rotate with the shaft. A permanent
magnet was so placed that its poles embraced the disk and the
magnetic lines of the permanent magnet passed through the disk.
When this disk was caused to rotate between the poles of the
magnet, the magnetic lines were cut and an electrical pressure
was induced.
Since Faraday's time the dynamo has been developed through
numerous types and with many modifications of designs, but
all have been developed on the principle which he laid down and
which has been stated as follows:
When a conductor is moved in a magnetic field so as to cut
the magnetic lines, there is an electromotive force induced in
the conductor, in a direction at right angles to the direction of
motion and also at right angles to' the direction of the magnetic
lines.
While the Faraday disk dynamo did not generate anyj3on-
7 63
64 PRINCIPLES OF THE TELEPHONE
siderable pressure, it led the way to the development of other
forms of the dynamo, in which the arrangement of the conductors
with respect to the field was better suited to the development of
large pressures and currents.
The necessary elements of a dynamo for the generation of
current are a magnetic field and a conductor or conductors which
can be caused to move across the field. The simplest form of
such a machine is shown in Fig. 41. A loop of wire is mounted
on a shaft so that it can be rotated on its axis and cut through
the magnetic lines passing from N. to S. At the instant at which
the loop is in the vertical position, as shown by the full lines, the
^_^ conductors are moving paral-
lel to the magnetic lines, and
since there are no lines being
cut, no pressure is induced.
When the loop is in the hori-
zontal position, as shown by
the dotted lines, it is moving
FIG 41 o ne mag-
netic lines; and as the rate of
cutting magnetic lines is a maximum in this position, the pres-
sure induced will be a maximum.
The direction of the pressure induced, when the conductor
is passing through the field in this direction of rotation, is from
front to back on the right side of the loop, and from back to front
on the left side of the loop. The pressures in the two sides of
the loop will tend to cause current to flow in the same direction
around the loop and the pressure in the loop will be the sum of
the pressures induced in the two conductors.
When the loop is moved 90 degrees further it will again be in
a vertical position, but the conductor which was at the top in
the first case will now be at the bottom, and the other will be at
the top. Advancing the loop past this point, a pressure will
again be induced in the conductor, but the position of each con-
ductor with respect to the N. and S. poles will be reversed. Since
the positions of the conductors have been interchanged and the
current has the same direction with respect to the magnetic field
and the direction of motion, it will flow through the loop in a
direction opposite to that of the first half revolution. That is,
the direction of the flow of current through the loop will reverse
every 180 degrees.
SIGNALLING APPARATUS AND CIRCUITS 65
If two rings be attached to the loop, one being connected
to each end, and mounted so they will rotate with the shaft
and loop, the pressure at these rings will reverse each time
the loop has passed through 180 degrees. Current can be taken
from these rings by placing a brush on each of the rings, as
shown in Fig. 42 and connecting them to a conducting circuit.
FIG. 42.
Since the direction of the induced pressure changes each
180 degrees, the flow of current in the circuit will reverse cor j
respondingly. That is, an alternating current will flow in the
circuit. This changing of current from zero to a maximum
value, then decreasing to zero, reversing, building up to a
maximum value in the opposite direction, and then decreasing to
FIG. 43.
zero again, is shown by Fig. 43 and is known as a cycle. The
number of cycles per second is called the frequency. When the
loop is in the vertical position as shown by full lines in Fig. 41,
the pressure is zero corresponding to point A, or zero degrees,
Fig. 43. The curve above the line ABC shows positive voltages,
and that below shows negative voltages.
66
PRINCIPLES OF THE TELEPHONE
If this generator be supplied with a commutator as shown
in Fig. 44, the output will be a pulsating direct current. It
has been shown above that the current in the armature coil
reverses every time the conductors pass from one pole to the
next, and that an alternating current flows in the coil. HOW-
FIG. 44.
ever, the coil is so connected to the commutator that every
time a conductor passes through a neutral point, the brush
contact is changed from one segment to the next; thus the
current through the brushes is always in the same direction.
The curve in Fig. 45 shows the effect the commutator has on
the alternating voltage represented in Fig. 43.
90
360'
FIG. 45.
98. The Telephone Generator. The telephone generator or
magneto is usually an alternating-current generator, one form
of which is shown in Fig. 46.
The magnetic field of such a generator is produced by U--
shaped permanent magnets. In the figure, five permanent
magnets are shown, but the number used varies from two to
six in different classes of service, depending upon the ringing
power required. Several small magnets instead of one large
one are used, because it is easier to properly shape the smaller
ones, and also because small pieces of steel can be magnetized
more readily than large ones. . It is necessary that the permanent
magnets retain their strength indefinitely, if the generator is
SIGNALLING APPARATUS AND CIRCUITS 67
to remain in service. Cast-iron pole pieces are usually attached
to the permanent magnets in order to have the pole face conform
to the shape of the armature, and thus have a small air gap for
the magnetic lines to cross. The pole pieces are also made of
use in holding the magnets together, and are usually connected
with each other by brass rods which hold them the proper dis-
tance apart. It is very necessary that this adjustment be main-
tained, as there is little clearance between the pole faces and the
armature. If it ever becomes necessary to take a magneto-
generator apart, care should be taken in assembling the magnets
so as to place all like poles together. In case of the reversal of
FIG. 46.
one magnet in a five-bar generator, only three bars will be
effective.
The armature consists of a large number of turns of fine
insulated copper wire wound on an iron core. A common form
of core shown in Fig. 47 is of cast iron or is built up of thin,
soft-iron sheets having the shape shown in the end view of the
armature. If built up, enough sheets to form a core of the
desired length are riveted together. Inside the hollow shaft, S,
is a brass rod, P, which is insulated from the shaft by a fiber
sleeve, F. One end of the coil is connected to the insulated
brass rod and the other end is connected to the armature core,
and thus through the armature shaft and bearings is connected
68
PRINCIPLES OF THE TELEPHONE
to the frame of the machine. The bearings for the armature
shaft are of brass and are attached to the pole pieces. On one
end of the armature shaft is a pinion driven by a larger gear
when the crank is turned.
Another form of armature known as the H-type is shown
in Fig. 48. The core is held between two end plates, B and
B', having projecting studs, S and S', which take the place of
FIG. 47.
the shaft. The main advantage of this type of armature is
that the winding space is large, as it is not obstructed by
the shaft, and will accommodate a large number of turns. One
end of the coil is connected to the core, and the other to the
insulated pin, P, which passes through the stud, S. This
form of armature is used in the Kellogg generator shown in
Fig. 46.
FIG. 48.
99. Automatic Switch. Every telephone generator is provided
with an automatic switch which disconnects the generator from
the line when it is not in use. Such a switch is operated by a
lengthwise movement of the main generator shaft. In early
types of telephones the required lengthwise movement of the
shaft was produced by the subscriber's pushing in on the crank
while ringing. In modern telephones the switch is operated auto-
matically when the subscriber first turns the crank by the use of
some device similar to that shown in Fig. 49. In the device
shown the shaft can be moved within the hub of the large
gear. The spring, S, between the hub of the gear and the collar,
C, on the shaft holds the crank to the right when it is not in use.
SIGNALLING APPARATUS AND CIRCUITS 69
However, as soon as the crank is turned, the notch, F, in the
collar, B, forces the shaft to the left before the large gear com-
mences to turn, and operates the switch; since the lengthwise
movement of the shaft takes place under less force than is
required to cause the armature to turn. The distance which
the[shaft moves to the left is determined by the distance between
the hub of the gear and the collar, C.
Local battery telephones may be divided into two general
classes; series and bridging. The automatic generator switch
used must be different in the two cases. These differences
are taken up in a later chapter devoted to these two classes
of instruments.
FIG. 49.
100. The Ringer. The bell used in telephone signalling is
ordinarily known as a polarized bell or ringer because the moving
part or armature is permanently magnetized or polarized.
In Fig. 50 is shown a common form of telephone ringer.
M and M ' are the coils composed of a large number of turns of
small wire wound upon soft-iron cores, both cores being at-
tached to the iron yoke, Y. The iron armature, A, is pivoted
at F, and has the clapper rod, C, attached to its center; hence
any movement of the armature results in a movement of the
clapper. The armature is supported by the brass bar, B, which
can be adjusted to vary the distance between the poles of the
electromagnet and the armature. The permanent magnet is
not in contact with the armature; hence the armature is in-
fluenced by the permanent magnet by induction, and since
the N. pole of the permanent magnet is opposite the center
of the armature, a S. pole will be induced at that point and
the two ends of the armature will become N. poles, exactly as if
the armature were a part of the permanent magnet.
70
PRINCIPLES OF THE TELEPHONE
When no current is flowing in the coils, either end of the
armature will attract either magnet core, as the armature is
polarized by the permanent magnet.
When current flows through the winding of an electromagnet,
one core becomes a S. pole, and the other becomes a N. pole.
Hence in the ringer shown in the figure current flowing in one
direction through the coils will cause the core M to be a N. pole
and the core M' to be a S. pole. Since the armature has a N. pole
at each end, one end of the armature will be attracted by the S.
pole of the electromagnet, and the other will be repelled by the
N. pole of the electromagnet, and the armature will be tilted,
causing the clapper to move to one side. If the current in the
coils be reversed, the polarity of the magnet cores will be reversed,
causing the armature to be tilted in the opposite direction.
When alternating current is used in ringing, the polarity of the
magnet cores reverses just as often as the current in the coils
reverses in direction. Hence every time the current reverses
the armature is tilted from one side to the other, causing the
ball of the clapper to strike one of the gongs. The rapidity of
ringing depends upon the frequency or rate of reversal of the
alternating current. As the quality of the ring depends largely
SIGNALLING APPARATUS AND CIRCUITS 71
upon the position of the gongs, they are made adjustable with
reference to the ball of the clapper.
The Kellogg ringer has no provision for adjustment of the
distance between the armature and magnet poles after leaving the
factory. As this adjustment is very close, a strip of German
silver is placed between the armature and magnet poles to keep
them from coming into direct contact and thus freezing or stick-
ing together. The gongs are adjustable as in the ringer described
above.
CHAPTER VIII
THE SUBSCRIBER'S TELEPHONE SET
101. The Complete Telephone. The following separate pieces
of telephone apparatus have been discussed : The battery, trans-
mitter, induction coil, receiver, generator, and ringer. These
parts make up the talking and signalling circuits, and in order to
have successful commercial operation must be connected in a
certain manner.
It is necessary to save battery current when the transmitter
is not in use; hence some means must be provided for opening
the battery circuit when the instrument is idle, since the trans-
mitter itself never opens this circuit, but merely changes or
controls the resistance of the same.
The receiver must also be disconnected from the line when
not in use, so as to leave the line free for signalling currents.
The signalling circuit must be disconnected when the talking
circuit is being used.
102. The Hook Switch. The above connections are estab-
lished and broken by the hook switch. In order to make the
action of the hook switch as nearly automatic as possible, the
hook is made the most convenient point on which to hang
the receiver when it is not in use, as it is not rigidly attached to
the telephone. When the receiver is on the hook, its weight pulls
the hook down and holds the battery or transmitting circuit
open, thus saving battery current, and also holds the receiver
circuit open, leaving the line free for signalling purposes. When
the receiver is off the hook, however, a spring raises the latter,
and closes the circuits through the receiver and transmitter so
that the line can be used for communication.
The Western Electric hook switch for wall telephones, shown
in Fig. 51, is commonly known as the short-lever type. The
hook is pivoted to the bracket, and when the receiver is re-
moved the lever is raised by the hook spring which at the same
time closes the switch contacts. When the receiver is hung on
the hook, its weight pulls the lever down and compresses the
72
THE SUBSCRIBER'S TELEPHONE SET
73
hook spring, allowing the switch springs to fall back against the
two short springs which act only as supports. These springs
are so spaced that when the lever is clear down, all the contacts
shown will be open. The spring arrangement shown is varied
to suit the conditions under which the hook is used.
The Stromberg-Carlson hook switch, shown in Fig. 52, oper-
ates on a somewhat different principle. The bracket, F } on
FIG. 51.
FIG. 52.
which the hook and switches are mounted, is a one-piece steel
punching. The spring, S, raises the hook when the receiver is
in use. When the receiver is placed on the hook its weight
pulls it down, and the rubber roller, P, forces apart the springs
A and D, thus opening the contacts. The hook can be re-
moved from the telephone cabinet for shipping or transportation
FIG. 53.
purposes if desired without disturbing the adjustment of the
switch springs.
The Kellogg hook switch, shown in Fig. 53, is of the long-
lever type, having all parts mounted on the casting, F, which
is firmly secured to the backboard of the telephone. The switch
springs are operated by means of the fiber roller, P. In the
switch shown, when the hook is up the two upper contacts be-
tween A, B, and C are open. When the hook is down this con.
74
PRINCIPLES OF THE TELEPHONE
dition is reversed and the lower contacts are closed. These
switch springs are of German silver, having platinum contact
points. Platinum points are used in hook switches because this
metal does not corrode readily and the contacts are therefore
easy to maintain.
Hook switches for use in desk stands are of somewhat different
design. The Western Electric switch is contained in the barrel
of the stand, and is operated by the lever very much as in the
wall telephone of the same make.
The Kellogg desk stand hook switch is arranged somewhat
differently, as is shown in Fig. 54, the switch springs being placed
FIG. 54.
in the base of the instrument. The hook lever is in two parts.
One part is pivoted at P, and the other at'D to the spring B;
and the two points are pinned together at C. When the hook is
raised the spring contact is closed. When the lever is depressed
the point C is moved to the right and D is forced downward,
opening the contact between A and B. Other spring arrange-
ments are used for different classes of service, as mentioned above.
QUESTIONS
1. Of what does the signalling circuit of a telephone consist? Show
by diagram.
2. For what is the generator used? The ringer?
THE SUBSCRIBER'S TELEPHONE SET 75
3. Upon what principles does the operation of a dynamo depend?
4. In the simple dynamo shown in Fig. 41 why do not the voltages
generated in each side of the loop oppose each other?
5. What kind of a current will be generated by the simple dynamo
mentioned above? Explain.
6. Of what use is a commutator? Are magneto generators ever provided
with a commutator?
7. How is the magnetic field of a telephone generator constructed?
8. Describe two types of magneto armatures.
9. For what is the automatic switch used? How does it operate?
10. Why is the ordinary telephone ringer said to be polarized ?
11; Explain the action of the ringer, including the effect of the permanent
magnet on the armature.
12. For what is the hook switch used? Explain in a general way how the
hook switch works.
13. Name the parts of the talking circuit.
14. Show how the Kellogg desk stand hook switch operates.
CHAPTER IX
LOCAL BATTERY SYSTEMS
103. Classification of Local Battery Systems. It has been
mentioned previously that local battery telephones are of two
classes : series and bridging. The series instruments are so named
because the generator and ringer are placed in series with each
other. The bridging instruments are so called because the bell
and generator are separately bridged or connected in parallel
across the line.
104. Series Telephone System. Fig. 55 is a diagram showing
the connections of the various parts of the series telephone.
FIG. 55.
When the receiver is on the hook the signalling circuit is closed
at the point C. If the central operator wishes to call the left
station of Fig. 55, ringing current is sent over the line, let us say,
entering at terminal LI, and leaving at L 2 as follows: Following
the line from LI, we reach the spring, R, of the generator. For
the operation of the series telephone ringer, a generator with an
automatic switch, as shown in Fig. 49, is required. This switch
76
LOCAL BATTERY SYSTEMS 77
contains two springs, R and Q, insulated from each other except
when the switch is closed. The spring R is in contact with the
insulated pin, P, to which one end of the armature coil is con-
nected; and the spring Q is attached to the frame of the generator,
to which the other end of the armature coil is connected. Hence
these two springs form the terminals of the armature coil. One
of the terminals, either TI or T 2 , is connected to the line, and
the other terminal is connected to one end of the bell or ringer
coil. When the generator is not in use the circuit is closed,
through the two springs, R and Q, as shown in Fig. 55, thus
forming a low-resistance shunt for the generator and allowing
the ringing currents to flow readily through the bell. If these
currents had to flow through the high resistance of the armature
coil, in addition to the bell coil, they would be much weakened.
As soon as the subscriber has heard his bell ring, he removes
the receiver from the hook which is raised by a spring, opening
the signalling circuit, at the same time closing the talking and
listening circuits through the contact points, a and 6. The
diagram at the right in Fig. 55 shows the connections after the
receiver has been removed from the hook, the contacts a and b
being closed and contact c opened.
Referring to the diagram at the right of Fig. 55, the talking
current can be traced through the receiver, reproducing the
sounds of the operator's voice, and through the secondary of
the induction coil to the point j. From j the current flows
through the wire to contact a, through the receiver hook and to
L 2 . From./ to the hook there are two paths for the current, one
through the wire connecting j and a, and the other through the
primary of the induction coil, the transmitter, and battery; but
the resistance of this latter circuit is so much higher than that
of the first that practically all of the current flows through the
wire directly from j to a.
From the diagram of connections it is evident that battery
current is flowing in the transmitter circuit when the receiver
hook is up, whether the transmitter is in use or not, since the
latter never opens the circuit. It is the variation of current in
the primary of the induction coil, however, which causes current
to flow in the secondary, and a continuous direct current (no
matter what its value may be) can not produce current in the
secondary. Hence there is no interference with the talking
current from the central operator's instrument.
78 PRINCIPLES OF THE TELEPHONE
105. Local Battery Circuit. The circuit made up of the
battery, transmitter, primary of the induction coil, and the con-
necting wires form what is known as the local battery circuit.
When the subscriber talks, a variable current is set up in the
transmitter circuit on account of the variations in resistance of
the transmitter. This current flows from the battery, through
the transmitter, through the primary of the induction coil, and
through the contact a, through a part of the hook, and through
contact 6, back to the battery. The variations in the current
flowing in the primary of the induction coil cause an alternating
current to be induced in the secondary of the coil. This current
flows through the circuit as outlined above, out over the line,
and is finally converted by means of the receiver at the receiving
station into mechanical energy of sound.
As soon as the subscriber is through talking he hangs the re-
ceiver on the hook. The hook, being pulled down, opens the
talking circuit and closes the signalling circuit, thus leaving the
instrument ready to receive future signals.
If the subscriber wishes to signal the central operator, he
turns the generator crank while the receiver is on the hook.
Whenever the generator crank is turned, the shaft is moved
horizontally and the pressure of the hard-rubber tip, L, Fig. 49,
opens the circuit by pressure on the spring R, allowing the cur-
rent to flow through the line. If the contact between R and Q
were not broken, the current from the generator would flow
through the shunt instead of the line.
106. The Bridging Telephone. Fig. 56 is a diagram of the
connections of the bridging telephone. It will be seen that the
ringer and generator are connected in parallel across the line.
The generator is provided with an automatic switch which opens
the circuit when the generator is not in use, so that no current
can be shunted from the ringer or talking circuit. The switch
has three springs: R, Q, and /. The ends of the armature coil
are connected to spring R and to the frame of the generator.
When the subscriber is called, the ringing current passes from
LI to spring Q, then to spring J, through the bell coils and to the
other side of the line.
When the subscriber wishes to call central or some other sub-
scriber, the motion of the ringer shaft to the left opens the bell
circuit between Q and J and closes the generator circuit between
R and /. The ringing current leaves through R to Q, then to
LOCAL BATTERY SYSTEMS
79
line at LI and returns by L 2 and the connecting wires to the
generator frame to which the other generator terminal is con-
nected. The pin, P, is insulated from the generator frame.
Bridging bell coils are wound to a resistance of 1,000 to 2,500
ohms, varying with different manufacturers.
The number of turns on the bell coils is much greater than with
the series bells; therefore the current necessary for ringing
is much less than with the other type. Again, the impedance
of the bell coils is so high, compared with that of the circuit
through the telephone receivers, that the amount of high-fre-
FIG. 56.
quency talking current shunted is of no consequence in the
operation of the system.
107. Connections of Bridging Telephone. The connections
of the bridging telephone, Fig. 56, show that there is no bottom
contact for the hook switch. When the receiver is on the hook
the only path for current from the line is from LI through the
ringer to L 2 . When the receiver is removed from the hook, the
contacts a and b are closed, establishing the listening and talking
circuits, which are the same as in the series instrument.
When the subscriber wishes to signal the central operator he
turns the generator crank, which closes the contact between R
and Q and connects the generator to the line, and at the same
80
PRINCIPLES OF THE TELEPHONE
time opens the contact between Q and J and thus disconnects
the ringer so that none of the calling current is shunted from the
line.
108. Telephone Instruments. Telephone instruments com-
monly used are of the types known as wall and desk sets; the
former so called because they are usually attached to the wall,
and the latter because they are intended for use on a desk or
table.
109. Standard Wall Set. A common form of magneto wall
set is shown in Figs. 1 and 2; and a view of another instrument
FIG. 57a.
FIG. 576.
of the same type but of different make is shown in Figs. 57a
and, 576.
It is seen that the essential parts are all mounted within the
cabinet or on the outside where they are most convenient for
the user. The transmitter is carried on a hollow adjustable
arm, through which the wires pass to the interior of the cabinet.
The ringer is mounted on the inside of the door, and as the
gongs are on the outside it is necessary for the clapper rod to pass
through a hole in the door. The induction coil is mounted on
the inside of the door, as is the condenser, the use of which in
magneto telephones will be explained in a later section on party
lines. In some other makes of telephones the induction coil is
not mounted on the door, but is placed in the cabinet. The hook
LOCAL BATTERY SYSTEMS
81
switch is of the short-lever type, the switch being inside the
cabinet. The magneto is mounted on a shelf, and has its shaft
FIG. 58.
FIG. 59.
extending through the right side of the box. The receiver, as
in all telephones of this type, is connected with the instrument
by means of a flexible cord.
FIG. 60a.
FIG. 606.
In order that connections between the parts mounted on the
door and those within the box will not be broken when the door is
opened for inspection or other purposes, the hinges are made a
82 PRINCIPLES OF THE TELEPHONE
part of the conducting circuit. A special form of spring joins
the two leaves of the hinges in order that the circuits may not
be opened through corrosion and wearing of the parts.
A wiring diagram of the series instrument of this type is
shown in Fig. 58, and a diagram of the bridging set is shown in
Fig. 59. A comparison of these diagrams with the simplified
ones preceding will show the connection of the parts to be
practically the same.
Another wall set of standard make is shown in Figs. 60a and 606.
The set shown in the figure does not contain a condenser, but a
place is provided for one immediately under the induction coil.
FIG. 61.
The wiring of this set differs in one respect from that shown in
Fig. 57. The wires are carried in a conduit from the apparatus
inside of the box to that on the door and accordingly the hinges
do not form a part of the electrical circuit. The variation of the
resistance in the hinge contacts is thus obviated.
110. Hotel Set. Figs. 61 and 62 show a local battery telephone
of small size, commonly known as the residence or hotel type. It
is, in fact, a simple magneto box cabinet containing all the talk-
ing and signalling apparatus except the batteries, which are
placed in any convenient location away from the instrument,
This telephone has all the operating advantages of other types,
LOCAL BATTERY SYSTEMS
83
and is installed where a larger cabinet would be objectionable,
and where the writing shelf is not necessary. All of the working
FIG. 62.
parts are of standard size, and the wiring is practically the same
as that of the standard instrument previously shown.
FIG. 63.
111. Desk Set. A desk set consists of the desk stand, com-
prising the transmitter, receiver, hook switch, and induction
84
PRINCIPLES OF THE TELEPHONE
coil; the desk box, containing the magneto and ringer; and the
battery box, although the latter is often omitted, as the batteries
may be set in an out-of-the-way place where a box is not re-
quired. A desk stand and desk box are shown in Figs. 63 and
64. The wiring diagram of a desk set does not differ materially
from a wall set except that as the apparatus is not all mounted
FIG. 64.
in one cabinet, flexible conductors are used to connect the vari-
ous parts. A wiring diagram for the Stromberg and Carlson
desk set is shown in Figs. 65 and 66.
An apparatus consisting of a combination of receiver and
transmitter, and known as a hand telephone is shown in Fig.
67. Although these are not in common use", They are made in
LOCAL BATTERY SYSTEMS
85
two styles. The one shown in the figure opens and closes the
circuits in the usual way, that is, through the medium of the hook
BATTERIES
FIG. 65. Desk telephone wired for series operation.
DRY BATTERIES
FIG. 66. Desk telephone wired for bridging operation.
switch. Another make has no hook switch, the ringing circuit
being open and the talking circuit closed by the pressure of a
lever in the barrel of the instrument.
86
PRINCIPLES OF THE TELEPHONE
QUESTIONS
1. What is meant by a local battery telephone?
2. Explain the difference between the two types of local battery telephones.
3. Trace the path of current in series telephone: when the operator is
signalling the subscriber; when the subscriber is answering his call; and when
the subscriber is signalling the central office. Use diagrams.
4. What objections are there to the use of series instruments on party
lines?
6. What is the difference between a series and bridging ringer?
FIG. 67.
6. Trace the circuits of the bridging telephone as you did for the series
instrument in question 3.
7. Why does not the current from the line flow through the battery when
the hook is up, since the circuit is closed?
8. Explain why the talking current does not flow through the ringer,
instead of the receiver, in the bridging telephone.
9. Explain the difference between the generator switches used in series
and bridging instruments, and give the reasons for the difference.
10. Why does not the battery current flowing through the primary of the
induction coil interfere with the talking current from the line which passes
through the secondary?
CHAPTER X
COMMON BATTERY TELEPHONES
112. General. The common battery or central energy tele-
phone system is so named on account of the fact that the current
for the operation of the system is supplied from a central or
common source instead of from batteries at each subscriber's
station. The common source is invariably a storage battery 1
located at the central office.
The electrical characteristics of common battery substation
apparatus are the same as those of the local battery equipment,
but on account of the central source of energy some of the
apparatus found in local battery installation is not used, and
some other equipment is added. The design and connections
are also modified and changed. The operation of the induction
coil in connection with the condenser in the common battery
system is somewhat the more complicated.
A common battery transmitter has a higher resistance than the
local battery transmitter. This is due to the fact that the
voltage employed is much higher in the former than in the latter
systems. In general exchange practice the higher voltage is not
needed for transmission, but for signalling. The resistance of
the line is the same in the two cases. Therefore, to reduce
the current in the talking circuit, the resistance of the various
parts is increased. The resistance of standard common battery
transmitters of various makes is in the neighborhood of 100
ohms.
The common battery induction coil differs also somewhat
from the induction coil of the local battery system. In the
latter system the electrical pressure employed is comparatively
low, approximately 4 volts. This is too low for efficient trans-
mission, so an induction coil, is used which transforms the low
primary pressure to a secondary pressure which is sufficiently
high to force the current to the other instrument.
1 The storage battery will be explained in connection with central office
equipment.
9 87
88 PRINCIPLES OF THE TELEPHONE
In common battery systems the primary voltage is high enough
so that, instead of using a step-up coil, the coil may be entirely
omitted, or one that slightly lowers the pressure may be em-
ployed. Induction coils, however, vary in this respect con-
siderably, being designed for the particular instrument circuit in
which they are used. As a general rule it is advisable never to
replace any part of an instrument equipment with that of an-
other manufacture unless investigation shows that the appara-
tus which it is proposed to substitute is designed for that type
of circuit.
The added feature of the common battery system is the con-
denser, the construction and use of which will be explained
in the succeeding paragraphs. Instead of each instrument
having a hand generator for signalling the central office, the
current for calling the operator, or central, is supplied by the
common battery. The ringing generator is thus omitted from
the common battery system.
113. The Condenser. When an insulated electrical conductor
is connected to a battery or some other source of electrical
FIG. 68.
pressure, the conductor becomes charged; that is, a sufficient
quantity of electricity flows into the conductor to raise its
potential or pressure to that of the battery. If a conductor
be connected to the positive terminal of a battery, it becomes
positively charged, and if it be connected to the negative ter-
minal it becomes negatively charged. A condenser is an ar-
rangement of conducting plates which are insulated from each
other, and therefore can be charged by connecting them to a
source of electrical pressure.
The most simple form of a condenser consists of two or more
conducting plates close together, but separated by some in-
sulating material called the dielectric. Fig. 68 is a cross-
section of a simple condenser of this type. The heavy horizontal
lines represent a cross-section of the conducting material, and
the fine dots represent the insulating material.
COMMON BATTERY TELEPHONES 89
Capacity of a Condenser. The capacity of a condenser
is measured by the quantity of electricity required to charge it
to a difference of pressure of 1 volt. The unit of capacity is
called the farad, a word derived from the ^name Faraday. A
condenser is said to have a capacity of 1 farad when a charge of
1 coulomb raises its potential by 1 volt. The farad is entirely
too large for practical purposes, and so the microfarad, which is
one-millionth of a farad, is used. Condensers are thus rated in
microfarads.
The capacity of a condenser depends upon several factors;
the number and dimensions of the sheets of conducting material,
the material and thickness of the dielectric. The greater the
area of the sheets of conducting material and the thinner the
layers of insulation, the higher the capacity.
Furthermore, if the dielectric is paraffined paper, the capacity
is 1.9 to 2.4 times as large as it would be if air of the same thick-
ness were used. This property of the insulating material upon
which the capacity of the condenser depends is called the di-
electric constant or specific inductive capacity.
The principal materials used in the manufacture of telephone
condensers are tin-foil, paper, and paraffine.
The tin-foil is made from an alloy of about 90 per cent, lead
and 10 per cent. tin. This is rolled out until it is very thin.
In the preparation of the foil, great care is taken to insure purity
of the product and freedom from grit, which would puncture
the condenser when assembled and pressed.
The paper employed is a special grade of what is commercially
known as rice paper. It is white in color, very flexible, of
high tensile strength, and quite tough. Condenser paper is
purchased in several thicknesses varying from 0.0005 to 0.001
in. It is put up in rolls of different widths, depending upon the
finished dimensions of the condenser for which it is intended.
There are two main reasons for the use of paraffine: first,
its insulating properties are quite high, and thus it reinforces the
dielectric strength of the paper; and second, its dielectric constant
is somewhat higher than that of paper alone. When a dielectric
with a large constant is used, the dimensions of a condenser of a
given capacity are less than when the dielectric constant is small.
This constant for paraffine ranges from 1.9 to 2.4, depending
somewhat upon the temperature. The capacity of a condenser
may be calculated by the following formula:
90
where
PRINCIPLES OF THE TELEPHONE
C = 884 X 10- 10 X -?- microfarads
6
k = dielectric constant
S = area of dielectric between conducting plates in
square centimeters
t = thickness of dielectric in centimeters
in- 10 =
10,000,000,000
EXAMPLE
A condenser is made of 501 sheets of tin-foil separated by sheets of par-
affined paper 0.007 in. thick. The overlapping portions of the sheets of
FIG. 69.
tin-foil are 10 in. by 10 in. as shown in Fig. 69. Calculate the capacity of
the condenser. \
Solution
If there are 501 sheets of tin-foil, there are 500 sheets of paraffined paper
between the sheets of tin-foil. The total area of these sheets of paper will be
Therefore
Then
600 X 10 X 10 X 6.45 = 322,500 sq. cm.
S = 322,500 sq. cm.
t = 0.001 X 2.54 = 0.00254 cm.
K = 2.3 about
C = 884 X 10-io x 2 - 3X500 microfarads
26 microfarads nearly.
The capacity of paper condensers varies greatly with the rate of charge
and discharge.
114. Manufacture of Telephone Condensers. The process of
manufacturing telephone condensers is well shown in Fig. 70.
COMMON BATTERY TELEPHONES
91
The machine used for the winding of the condensers is usually
provided with six spindles arranged to carry the rolls of the paper
and foil in the manner indicated. A collapsible mandrel upon
WINDING
MANDREL.
FIG. 70.
which the tin-foil and paper are wound is shown to the right.
The first step in the assembly of the condenser is the winding of
a few turns of paper, only, on the mandrel to form a core. This
is done to avoid sharp bends
in the inner layers of the foil.
Very thin strips of brass
about Y in. wide and about
an inch longer than the
width of the foil are at-
tached to each strip of foil.
These brass strips are used
to connect the tin-foil to the
terminals on the condenser
case. In some makes of
condensers the connecting
strips are placed midway in
the foil strip to reduce the
plate resistance and thus de-
crease the loss of energy, and
heating of the condenser.
The required number of
turns of paper and foil are F
then wound on the mandrel,
the foil is cut off and a few extra layers of paper wound on for
protection.
The condensers, after being assembled in pressing "jigs,"
are next placed in perforated baskets and immersed in a large
tank of molten paraffine, a cover is placed on the tank and the
air exhausted until a desired degree of vacuum is obtained.
92 PRINCIPLES OF THE TELEPHONE
After the condensers have remained in the tank for about an
hour the air is again admitted. This forces the paraffine into
the remote recesses of the condensers. By hydraulic presses the
condensers are next subjected to heavy pressure which removes
all excess paraffine and forces the plates together as closely
as possible. This process increases the capacity of the con-
denser as is evident from the formula given, which shows
that the capacity increases as the thickness, t, of the dielectric
decreases.
The partially completed condensers are now tested for capacity
and insulation resistance at a voltage of at least double the work-
ing value. Those passing this test are next placed in moisture-
proof containers. The containers are lined with pasteboard,
the condenser placed in position and the case is filled with
paraffine. The terminals are next placed in position and the
cover is soldered on, after which a further test is made to check
the capacity and insulation. A finished condenser is shown in
Fig. 71.
115. Analogy for a Condenser. A better understanding of
the action of a condenser may be had by considering an analogy.
Suppose we have an air tank that under 1 atmospheric pressure
holds a certain definite quantity of air, say 5 Ib. We can define
the capacity of the vessel in terms of the number of pounds of
air it holds, and call it a 5-lb. tank.
If the pressure is doubled, the tank will hold 10 Ib. of air. Since
we have defined the capacity of the tank in terms of unit (1
atmosphere) pressure, we can not call it a 10-lb. tank. A 10-lb.
tank under the same conditions will hold 20 Ib. of air.
Furthermore, if the tank be exhausted, evidently no back
pressure will be exerted when air is first admitted to the tank.
As soon as some air is admitted to the tank, back pressure begins
to manifest itself, and when the back pressure equals the maxi-
mum applied pressure, no more air enters the tank. We thus
see that the amount of air entering per unit of time depends upon
the back pressure, and this back pressure will depend upon the
capacity of the tank. For instance, if we put 5 Ib. of air in a
10-lb. tank, the back pressure will be one-half as great as when 5
Ib. of air are put into a 5-lb. tank. We can then say that unit
capacity of a tank is such that when 1 Ib. of air is forced into
it the pressure will be equal to 1 atmosphere. Evidently a cer-
tain amount of work will be done in forcing the air into the
COMMON BATTERY TELEPHONES 93
tank, and we could define unit capacity in terms of the work
expended.
The capacity of electrical conductors is analogous to the
capacity of the air tank discussed above. The capacity of a
condenser or system of conductors is usually defined in terms of
the quantity of electricity required to raise the difference of
pressure between the terminals by 1 volt. In accordance with
this definition the quantity of electricity that a condenser will
contain is equal to the product of the capacity and pressure.
116. Action of a Condenser. If a condenser has one of its
plates connected to each side of a battery circuit, it will become
charged; that is, a quantity of electricity will flow into the con-
denser due to the battery pressure, and one plate will become
positively and the other negatively charged. After a condenser
has been connected to a direct-current circuit for a short time,
there will be no flow of current to or from the condenser, since
the condenser becomes fully charged almost instantaneously;
and when charged, the difference in pressure between its plates
is^the same as that of the battery or other source of charging
current. If the pressure in the circuit be decreased or reversed,
the charge will flow out of the condenser and back through the
circuit.
When an alternating pressure is impressed upon a condenser
the action is somewhat different. As the pressure increases from
zero to a maximum a current flows into the condenser, one side
becoming charged positively and the other side negatively.
The current flows as long as the pressure is changing, and the
back pressure of the condenser is always just equal to the applied
pressure. When the applied pressure begins to decrease, the
current begins to flow out of the condenser. When the applied
pressure is reversed the current flows into the condenser in the
opposite direction. This continues until the applied pressure
.again attains a maximum value, when the current again is re-
versed. These fluctuations of current continue so long as the
applied pressure fluctuates or changes. An alternating current
may thus flow in a circuit containing a condenser. The exact
value of such a current will depend upon the applied e.m.f., the
frequency, the capacity, and the resistance of the circuit. The
algebraic expression for a current in a circuit having capacity and
resistance is
94 PRINCIPLES OF THE TELEPHONE
E
I =
' (27T/C) 2
where
E = applied e.m.f.
R = resistance in ohms
/ = frequency of the applied e.m.f.
TT = 3.1416
and C = capacity in farads.
EXAMPLES
1. A pressure of 110 volts at 60 cycles is impressed upon a circuit whose
resistance is 5 ohms and capacity % microfarad, what is the current?
Solution
Given
E = 110 volts
R = 5 ohms
/ = 60
C = % X 10~ 6 farads
To find I
110
7 =
5 2 +
(27r X 60 X H X 10- 6 ) 2
110
V 25 + (2* x 20)
110
, A/25+ (7,955) 2
110 110
" V (7,955) 2 ~~ 7,955
as 25 is negligible in comparison with (7,955) 2 = 0.013 amp.
2. Suppose that in problem 1 the frequency were increased to 600, what
would the current be then?
Solution
The solution is exactly the same as the foregoing, except for/ we substitute
600. The equation for current becomes
/-- =y$=
3 X 10 6 \ 2
ITT X 600/
110
A/25 + (795.5) 2
=0.13 amp., nearly.
COMMON BATTERY TELEPHONES 95
This shows that when the resistance is small, the current increases or varies
directly as the frequency so long as the pressure remains constant. Both the
voice currents and ringing currents are of high enough frequency to give an
appreciable current through a condenser. The frequencies of voice currents
range between 100 and 2,500 cycles per second in ordinary telephonic
communication.
117. Function of Condenser in Telephone Circuit. The func-
tions of a condenser in a telephone circuit are determined some-
what by the system of connections employed. The physical
basis for its use is the action of a condenser with reference to
direct and alternating currents.
The subscriber's apparatus in the common battery system
comprises a transmitter, receiver, and ringer. Direct current is
used to operate the transmitter, while alternating current is
preferable for the operation of the receiver and ringer. As a
condenser will not permit the passage of direct current, but will
permit the flow of an alternating current, a condenser is con-
nected into that part of the circuit through which only alternating
current is to flow. The points of connection will depend upon
the system of connections used. There are several different
connections used in practice of which the following are the most
common :
118. Receiver and Transmitter in Series; Condenser and
Ringer in Series. What is perhaps the simplest connection is
Receiver
FIG. 72.
indicated in Fig. 72, the bell being bridged across the line in
series with the condenser. Since the condenser will allow the
ringing current to flow in the circuit, the instrument is ready to
receive signals from central at any time when the receiver is on
the hook. As direct current from the central battery can not
flow through the condenser, there is no battery current flowing
as long as the talking circuit is open.
96 PRINCIPLES OF THE TELEPHONE
When the subscriber desires to signal central, he merely re-
moves the receiver from the hook, which closes contact a, thus
completing the talking circuit and allowing battery current to
flow in his circuit. As soon as the talking circuit is closed, the
battery current flowing lights a small electric lamp in the central
office, thus attracting the attention of the operator.
When a circuit like that shown in Fig. 72 is used, the talking
current flows directly through the receiver. If the receiver
should happen to be connected in the line the wrong way, the
effect of its permanent magnet would be largely destroyed, for
usually there is enough battery current employed on a line to
overcome entirely the permanent magnets of a receiver, if it
flows through the receiver in such a way as to oppose them. A
receiver with line battery flowing through it in this manner will
have only about half the efficiency that it should have.
119. Induction Coil, No Condenser in Receiver Circuit.
Fig. 73 shows a more common arrangement of the circuits of a
C.B. telephone. The receiver as shown is connected to the
Receiver-
FIG. 73.
talking circuit, through the induction coil only. In this case the
circuits are so arranged that only the currents induced in the
secondary winding, due to the variations in battery current,
flow through the receiver.
120. Induction Coil and Condenser in Ringer and Receiver
Circuits. Another system of connections very extensively used
is that shown in Fig. 74. This diagram shows that when the
hook switch is open no direct current can flow. Alternating
current can, however, be sent over the line to operate the ringer.
When the receiver is removed from the hook the receiver cir-
cuit is connected in series with the condenser. In such a connec-
tion the condenser minimizes the inductive effect of the second-
ary winding of the induction coil and the receiver windings,
COMMON BATTERY TELEPHONES
97
increasing the efficiency of transmission. A brief consideration of
the principles involved will make clear how the condenser in-
creases the sensitiveness of the receiver. Let us consider the
receiving circuit closed and the subscriber listening. Under
this condition the direct line current flows through the primary
of the induction coil and transmitter. The transmitter offers a
fixed resistance to the flow of current. The line current fluctuates
in volume according to the sound waves causing the distant trans-
mitter to vibrate. With the substation circuit closed and the
transmitter at rest the condenser is charged to a difference of
FIG. 74.
potential equal to that across the transmitter. Now when a
pulsating current flows in the primary, an alternating current is
induced in the secondary. At one instant it flows through the
receiver into the transmitter circuit, and as a reversal occurs it
flows into the condenser but does not pass through; it is retained
there only during the interval required for the current to reverse
in the coil when the condenser discharges into the circuit through
the receiver. This oscillating action of the condenser increases
the sensitiveness of the receiver in reproducing the vibrations
of the distant transmitter. This is the main reason for the use
of a condenser in a receiver circuit. In this connection it per-
forms two functions; to prevent direct current from flowing
98
PRINCIPLES OF THE TELEPHONE
through the ringer, and to reinforce the action -of the induction
coil and thus increase the sensitiveness of the receiver. This
system of connections is standard with the American Telephone
and Telegraph Co. and is widely used both in this country and in
England.
A modification of the American Telephone and Telegraph
Company's system of connections is shown in Fig. 75. An
examination of this diagram will show that the transmitter,
the receiver, and the primary of the induction coil are in the
line circuit, while the secondary, the transmitter, and the
condenser form a local circuit. When the transmitter is sta-
tionary that is, when the subscriber is listening the pulsating
I
^wvw^--j
FIG. 75.
current causes a variation in the potential at the terminals of
the condenser. These variations in pressure cause the con-
denser to be charged and discharged, thus reinforcing the
fluctuations in the receiver. A similar action takes place when
the transmitter is used.
121. Retardation Coil in Place of Induction Coil. The self-
inductance of a coil prevents the current in the coil from reaching
a maximum value at the .instant of maximum pressure. The
growth, or increase or decrease, of current through such a coil is
retarded in time, and hence a coil with large self-inductance is
called a retardation coil. A retardation coil differs from an
induction coil in that it contains only one winding. The use of
such a coil in the subscriber's circuit is shown in Fig. 76. The
function of the retardation coil will be readily understood from
the following:
COMMON BATTERY TELEPHONES
99
Assuming the receiver on the hook, the ringing circuit may be
traced from line conductor 1 to branch 3, ringer 4, conductor 5,
switch-hook points 6 and 7, around the receiver by way of the
shunt 16, thence through condenser 15 to the other side of the
line 2. With the receiver off the hook, as shown in the diagram,
it will be observed that two parallel paths are provided, one con-
taining the condenser and receiver and the other the retardation
coil 13 and the transmitter. When the subscriber is listening, the
passage of the high-frequency voice currents is opposed by the
retardation coil, but they have practically free passage through
the condenser and receiver. The direct current for the trans-
FIG. 76.
mitter on the other hand can not pass through the condenser
but passes quite freely through the retardation coil. Such a,
combination of retardation or impedance coil and condenser pro-
vides an automatic means of separating the high-frequency voice
currents from the direct current. No direct current ever flows
through the receiver. The system of connections shown in Fig.
76 is that employed by the Kellogg Switchboard and Supply Co.
122. Wheatstone's Bridge Connection. A very interesting
substation circuit is that shown in Fig. 77. The principle of the
retardation coil is again employed to keep the direct current out
of the receiver. As shown, the circuit consists of four coils, two
retardation coils, and two noninductive resistance coils. These
100
PRINCIPLES OF THE TELEPHONE
four coils are connected so as to form the four arms of the WJieat-
stone bridge. The two parallel paths from A to B have the
same resistance, hence the steady direct current entering at A
divides, one half passing by way of ACB and the other by way
of ADB. The potentials of the points C and D are equal, and
hence no direct current flows through the receiver. When,
however, high-frequency voice currents enter at A, their passage
is opposed much more by the retardation coil between A and D
than by the noninductive coil between A and (7; hence they pass
FIG. 77.
to C through the receiver to D and then to B. Such a combina-
tion of noninductive and inductive coils, when properly balanced,
successfully keeps the steady direct current out of the receiver.
The condenser is placed in the ringing circuit only. This system
of connections has been largely used by the Dean Electric Co.
in connection with their common battery telephones.
123. C.B. Wall Sets. A common form of wall set is shown in
Fig. 78, this particular one being of Kellogg manufacture. The
wiring diagram for this instrument is shown in Fig. 76.
124. Hotel Sets. The common battery hotel or residence set
is designed to be used in places where it is desirable to economize
space, as are the local battery sets of the same type. These
COMMON BATTERY TELEPHONES
101
cabinets are made either of wood or pressed steel; the latter have
been in growing favor of recent years. A Stromberg-Carlson
steel hotel set is shown in Fig. 79. An open view of this set,
with a part of the cover cut away to show the connections of the
FIG. 78.
transmitter, is given in Fig. 80, while Fig. 81 shows a Western
Electric hotel " phone."
125. Desk Sets. The common battery desk set consists of
the desk stand, and the desk box containing the ringer; the latter
102 PRINCIPLES OF THE TELEPHONE
FIG. 79.
FIG. 80.
COMMON BATTERY TELEPHONES
103
may be of either wood or steel. In Fig. 82 are shown the parts
of the Kellogg desk set. The desk stand itself contains all the
working parts of this telephone, except the ringer and its con-
FIG. 81.
FIG.
denser. A study of the circuit diagram of this set in Fig. 83
shows that two condensers are employed one in the base of the
desk stand, and the other, as stated above, in the ringer box.
10
104
PRINCIPLES OF THE TELEPHONE
The reason for the employment of two condensers is that with
such an arrangement only two conductors are needed between
the desk box and desk stand. With the addition of extra con-
ductors between the desk box and desk stand, a single condenser
can be made to serve in this place as readily as it does in the
wall type previously discussed, as in all other respects the wiring
and arrangement of parts are practically the same.
The general practice of some other companies is to mount only
the transmitter, hook switch, and receiver in the desk stand, all
other parts being mounted in the desk box. There are many
other forms of common battery telephones, including special
FIG. 83.
forms of wall telephones, adjustable desk stands, hand telephones,
etc., which we will not discuss in this course, although they have
a considerable field of usefulness.
QUESTIONS
1. In what ways does a central battery telephone system differ from a
local battery system?
2. Do you see any advantage in a C.B. system? In an L.B. system?
Explain.
3. What is a condenser? How does it work?
4. Why is a condenser necessary in a C.B. telephone system? Explain
its action.
6. How are telephone condensers made?
COMMON BATTERY TELEPHONES 105
6. What is the method of signalling the central operator in a C.B. system?
7. What is the objection to having the receiver directly in the talking
circuit?
8. Show by diagrams how the circuits of a C.B. telephone are arranged
so that the receiver is not directly connected to the line.
9. Diagram and explain the Wheatstone bridge method of connections.
10. A pressure of 100 volts alternating at a frequency of 60 cycles is
connected to a circuit having a resistance of 1 ohm and a capacity of 3 micro-
farads. What is the current?
11. If the resistance of a circuit is small in comparison with the capacity
reactance of a circuit, how does the current in the circuit vary with the
frequency?
12. What is a farad? What is a microfarad?
13. What is meant by dielectric constant? If we use a dielectric whose
constant is high, how will the capacity of a condenser compare with the
capacity of one of same size but with air as the dielectric?
14. Explain fully the action of the condenser when connected as shown in
Fig. 74.
15. Explain the retardation coil. If an alternating e.m.f. be connected
to a retardation coil, will the current reach its maximum value at the same
time as the applied e.m.f.?
CHAPTER XI
FAULTS IN SUBSTATION TELEPHONE APPARATUS
126. General. In order that a telephone may give efficient
service any trouble or difficulty in operation must be promptly
located and removed. Troubles are called faults, and the
process of locating the trouble is called " trouble shooting" or
fault finding.
As the telephone is an apparatus which makes use of me-
chanical, electrical, and magnetic principles, trouble may develop
in any one of these classes.
Mechanical troubles are usually disclosed by the faulty
operation, or nonoperation of the electrical devices, hence in
their localization, electrical principles are used.
The most common fault in an electrical apparatus is due either
to short circuits or open circuits. In one way or another these
cause most of the telephone troubles. Where batteries and a
magneto ringer are used, these may fail by exhaustion, that is,
the batteries may be used up; and the magnets on the generator
may be too weak or reversed.
If a telephone is in good operating condition, the different parts
will behave under test in a certain positive manner. The first
step in localizing trouble is to perform what are known as O. K.
or correct tests. These are five in number, and are as follows:
127. O. K. or Correct Tests, Local Battery Telephones, Line
Disconnected.
1. When the magneto is turned, the bells should ring.
2. No generator current should flow through the coils of the
receiver when the hook is down.
3. A spark should be seen at the hook-switch contact when it is
moved up and down.
4. Under normal conditions no battery current should flow
through the receiver.
To test for battery current in the receiver hold the receiver to
the ear and short-circuit the line terminals. If you hear clicks,
the battery current is flowing through the receiver.
106
SUBSTATION TELEPHONE APPARATUS
107
L,
5. If in Fig. 84 points LI and L 2 are bridged or short-circuited,
a very strong side tone should be heard in the receiver when one
blows or speaks into the transmitter.
128. Side Tone. An examination of Fig. 84 will make clear
what is meant by side tone. If LI and L 2 are short-circuited and
the receiver raised from the hook, it is evident that the currents
induced in the secondary of the induction coil must pass through
the receiver; that is, any sound causing a vibration in the trans-
mitter can be heard at the receiver.
This is called side tone. When this
circuit has low resistance, as when
the line terminals are short-circuited,
the sound given out by the receiver
will be comparatively loud, or a strong
side tone will be heard.
129. Classification of Faults. If
the above-mentioned conditions are
not fulfilled, there is a fault in the
apparatus which must be found.
Trouble manifests itself by the in-
activity or faulty operation of some
part of the telephone set. This in-
activity is the symptom of trouble,
and it is more convenient to classify
the faults with reference to the symp-
toms disclosed; hence the following
classification.
Bell Does Not Ring. In the series
type of instrument the , subscriber's
bell should ring whenever the generator crank is turned. If
the bell does not ring under such conditions, it may be due to
an open line. To test for open line, connect the two line ter-
minals of the telephone; if the bell rings when the generator
is operated, the line is open. If the bell does not ring when
the line terminals are short-circuited, the fault may be in the
generator, ringer, switch contacts, or inside wiring. Examine
inside wiring and be sure that all connections are firm and that
there are no broken wires. Examine the generator switch and
see that the switch contact is open when the crank is turned;
otherwise the generator is short-circuited through this switch.
Examine the hook-switch contacts and see that the contacts are
FIG. 84.
108 PRINCIPLES OF THE TELEPHONE
all clean, and that the upper ones are closed when the hook is
up. If no faulty contacts or connections are found, the trouble
is probably an open circuit in the generator or ringer coil. Place
one finger on the frame of the generator and another on the
spring at the end of the armature, turn the crank, and see if a
shock can be felt. If no shock can be felt, an open circuit in the
armature coil exists. If the generator proves to be all right,
the ringer coils must be defective.
If a bridging instrument is being tested, failure to ring might
be due to a short-circuited line. To test for this, disconnect
the line wires, and if the bell rings the trouble is on the line.
Some bridging sets are so arranged that when the generator is
cut in, the ringer is automatically cut out, in order that no
ringing current may be shunted from the line by the local bell.
Be sure that the instrument is not of such a type before making
further tests. If such be the operation of the set, of course the
bell will not ring when the crank is turned, and in order to con-
tinue the test the ringer must be connected across the line.
If disconnecting the line wire does not locate the trouble, pro-
ceed with the test as for the series instrument, examining the
switches, wiring, etc., remembering that in the case of a bridging
set the generator switch must be closed instead of opened when
the crank is turned.
If the bell does not give a strong, clear ring, the trouble is
perhaps mechanical rather than electrical, and the ringer should
be adjusted as directed under ringer adjustments below.
Can Not Call the Central Operator. This condition may be due
to weak or defective generator. To test, disconnect the line
wires, place the fingers across the line terminals of the telephone,
and feel for current when the crank is turned. If no current be
felt, test the generator directly with the fingers by placing one
on the frame and another on the spring at the end of the
armature, as above. If no current be felt, the armature winding
is open. If current be felt, the circuit is open in the switch or
wiring. If the generator turns hard, there is a short-circuit in
the generator or some other part of the telephone.
Can Not Hear nor be Heard. This condition is probably due
to an open listening circuit. Examine hook contacts and see
that the contacts are closed when the hook is up. With the
hook up, turn the generator crank. If the generator can not be
heard in the receiver, the circuit is open. Short-circuit the
SUBSTATION TELEPHONE APPARATUS 109
secondary of the induction coil and ring as before. If the gen-
erator is now heard, it proves that the secondary or the induction
coil is open. To test the receiver and cord, place the fingers
across the receiver terminals. If the receiver be open, current
can be felt when the crank is turned.
Can Hear but Can Not be Heard. This condition is evidently
due to some fault in the transmitter circuit. The defect may be
due to weak or worn-out cells, poor contact at the hook switch,
an open or short circuit in some other part of the primary circuit
or in the secondary of the induction coil.
First examine and if possible test the cells with an ammeter.
If they are found to be in good condition, examine all connections
and hook-switch contacts. To test the transmitter, short-circuit
the line at LI and L 2 , Fig. 84, and move the hook up and down.
If a spark appears at the hook-switch contact a when contact
is broken, test for side tone. If the side tone is found to be
medium, the trouble is a weak transmitter. If there is no side
tone, or if it is very weak, either the transmitter is short-circuited
or the primary of the induction coil is short-circuited.
If, when the hook is moved up or down, no spark appears at
the hook-switch contacts, then short-circuit the transmitter. If
this gives a strong click in the receiver, the transmitter is open;
if no click is heard in the receiver, then short-circuit the primary
of the induction coil and move the hook switch up and down.
If a spark appears, the primary of the induction coil is open; if
no spark appears, the wiring is open.
Can be Heard but Can Not Hear. In such cases the trouble is
usually in the receiver circuit, and is probably due to a defective
receiver or to a short-circuit in the receiver cords.
Intermittent Faults. Whenever there is a complete break or
short circuit, the fault is complete and lasting unless repaired.
There is another class of faults which are more difficult to localize,
namely, occasional faults, or those that last for only a short time,
while in the interval the apparatus works satisfactorily. Such
faults are due, as a rule, to loose contacts or open circuits which
may become closed under vibration, change in temperature,
movement of some part, etc.
In locating faults of this nature careful inquiries must be
made concerning the circumstances and conditions under which
the fault appears or is manifest, and how it affects the operation
of the telephone. One of the most common sources of inter-
110 PRINCIPLES OF THE TELEPHONE
mittent trouble is the local cord circuit. The conductors may
become broken, and in a certain position maintain close enough
contact to make transmission possible, while in other positions
the conductors may be separated so as to form a complete break.
Then, again, some of the strands in one conductor may become
broken, pierce the covering, and form a short circuit with the
other conductor while externally the cord may show no defect
whatever.
Test for Faulty Cord. Whether or not the cord is at fault may
usually be determined by putting the receiver to the ear and then
continuously blowing in the transmitter, while the cord is
pulled, twisted, and wound in different ways. If the fault is in
the cord, this movement will cause interruptions in the noise
due to blowing into the . transmitter. Another method is to
connect the cord and receiver to a dry cell directly, and then to
listen while the cord is pulled, bent, and twisted. If there are
any faults in the cord, they will be disclosed by clicks or splutter-
ing sounds in the receiver.
To facilitate the work in localizing faults the following tabular
arrangement has been prepared.
130. Fault Finding, Local Battery Telephones, Substation
Apparatus. Disconnect the line wires before beginning the tests.
Begin with the five O.K. tests mentioned at the beginning of this
chapter. If possible test the cells with an ammeter.
I. Bells ring weakly.
A. Adjust ringer and turn magneto.
1. If bells ring O.K. the bells were out of adjustment.
2. If bells still ring weakly.
a. Hold hook down and turn magneto.
(a) If there is magneto current in receiver, hook is
crossed with spring.
(6) If there is no current in the receiver, the
magnets of the magneto are weak. This may be
due to a reversal of one or more magnets.
II. Bells do not ring.
A. Bridge fingers across line and turn magneto.
1. If current is felt, the ringer coils are open, or the
connecting wires are open, j
2. If no current is felt.
a. Short-circuit magneto terminals and turn the
magneto.
SUBSTATION TELEPHONE APPARATUS 111
(a) If it turns hard, magneto coils are short-
circuited.
(6) If it turns easy, magneto coils are open-
circuited.
777. Can hear but can not be heard.
A. Short-circuit line at LI and L 2 .
1. If there is a spark at hook switch and
a. Medium side tone, then
(a) Poor transmitter, or
(6) Weak dry cells,
b. Weak or no side tone
(a) Transmitter short-circuited or
(b) Primary of induction coil is short-circuited.
2. If there is no spark at hook-switch contacts, short-
circuit transmitter and
a. If, when hook switch is closed, a strong click is
heard in receiver, then transmitter is open.
3. If no click results, then short-circuit primary coil,
and
a. If a spark appears at hook, primary coil is open.
b. If no spark results, wiring is open.
IV. Can not hear nor be heard.
A. Lift hook switch and turn magneto.
1. If no current flows in receiver, bridge fingers across
receiver terminals and turn magneto again.
a. If you feel current effect, receiver or cord is open,
2. If you feel no current, short-circuit secondary of
induction coil and turn magneto.
a. If there is current in the receiver, secondary of
induction coil is open.
b. If no current flows in receiver, wiring is open.
131. Faults in Central Energy Substation Instruments. The
foregoing remarks apply mainly to faults commonly found in
connection with local battery apparatus and circuits. In many
respects they also apply to common battery substation instru-
ments. As the batteries in a common battery system are not
located at the substation, any faults in connection with them are
quickly located and remedied. This fact also makes the localiza-
tion of faults in a common battery telephone somewhat easier,
as a steady source of current is always assured. A common
source of trouble with common battery circuits is in connection
11
112
PRINCIPLES OF THE TELEPHONE
with the leading-in wires where they are fastened to damp walls.
The dampness will in time cause deterioration of the insulation
and grounds will result. Good practice requires that where the
leading-in wires pass through a window or door frame they should
be protected by a porcelain tube. The wires should enter through
a hole sloping downward from within. Where the walls are
damp, the leading-in wires should be run on porcelain knobs so
as to avoid all contact with the walls.
132. Circuits of C.B. Subscribers* Telephones. As indicated
in Figs. 72 and 85, there are two common methods of connecting
the receiver and transmitter in a C.B. subscriber's set. In Fig.
FIG. 85.
72 the receiver and transmitter are in series; that is, they are
connected in such a manner that the transmitter current also
passes through the receiver. In this diagram no induction coil
is used. A similar arrangement may be employed with an induc-
tion coil, as indicated in Fig. 85. This arrangement in practice
is known as side-tone wiring.
The other method is that shown in Fig. 86. This shows the
transmitter removed from the receiver circuit, and placed between
the hook and primary of the induction coil. This arrangement is
known as side-tone reduction wiring. The reason for these two
designations will presently appear. When the arrangement
shown in Fig. 85 is employed, the variations of current in the
SUBSTATION TELEPHONE APPARATUS
113
primary of the induction coil induce currents in the secondary.
These secondary currents have a high frequency, hence can flow
quite readily in the receiver circuit. Then, again, as the current
in the transmitter varies, a variation in potential or pressure
will result across the terminals of the receiver circuit. As the
potential increases, the condenser will be charged, and as the
potential decreases, the condenser will discharge. There are
thus two sets of currents flowing in the receiver circuit. When
the arrangement of Fig. 86 is employed, only the induced cur-
rents flow in the receiver circuit. There will thus be side tone
FIG. 86.
in either case, but it will be weaker when the side-tone reduction
connection is used. Referring to Fig. 85 we see that there are
three circuits in the common battery telephone set :
1. The talking or battery circuit is from LI through the primary
of the induction coil to the hook switch at a, and then through
transmitter and to L 2 .
2. The listening circuit is from b to the receiver, to the second-
ary of the induction coil and condenser back to b.
3. The ringing circuit is from LI through ringer and condenser
to L 2 .
133. Locating Faults in C.B. Telephones. In testing a sub-
scriber's set for faults some difficulties may be experienced which
are not evident from the simplicity of the connections. An ex-
114 PRINCIPLES OF THE TELEPHONE
amination of Figs. 85 and 86 will show that current may flow
through the receiver from three directions: namely, from the
line through the primary of the induction coil, from the line
through the ringer, and through the condenser. The trans-
mitter may receive current from the line through the ringer, or
through the secondary of the induction coil. These different
possible sources of current may cause difficulty in locating
trouble. The first step in locating trouble is to examine the
hook switch to see if it makes good contact, if there is current
on the line, and also if the trouble is in the listening or talking
circuits.
The most common faults in C.B. subscriber's apparatus with
bridged ringer may be located in the following manner :
Can Not Call Central Operator. This is due generally to line
trouble or, on party lines, to another receiver off the hook. If
neither of the above is the cause, examine hook contacts. Hold
the hook down and short-circuit the receiver momentarily. Clicks
in the receiver 'mean receiver spring b, Fig. 85, is crossed with hook.
Next move hook up and down and look carefully for sparks.
If no spark appears, the other spring a is crossed with the hook.
Next short-circuit the condenser. If no spark be seen when the
short-circuiting wire is removed, it is a sign that an internal
short circuit exists in the condenser. On desk sets a damp
cord will also prevent the subscriber's signalling the operator.
Bell Does Not Ring. This may be due to the bell being out
of adjustment, switch-hook contacts crossed, or an open circuit
in the ringer.
To adjust the ringer proceed as follows:
First, loosen lock nut and adjust front bearing screw, so that
the armature will move freely but not be loose. After the ad-
justment has been made, hold the screw and tighten the lock
nut.
Second, adjust the stroke of clapper ball, if ringer be adjustable
in this respect. Move bell gongs outward as far as possible, and
adjust the armature so that the clapper ball has a stroke of about
Min.
Third, adjust the gongs. Move left gong toward the clapper
ball so that when the left end of the armature is lifted and
quickly released, the ball will strike the gong once only and will
not remain in contact with it. Make the same adjustment for
the right gong.
SUBSTATION TELEPHONE APPARATUS 115
If the bell be a biased one, the biasing springs should next be
adjusted. The biasing spring should give sufficient tension to
produce a clear and even ring on each gong, and should be
tightened or loosened to give the desired effect.
If the bell does not ring after adjustment, ask the operator for
a ring, hold the hook down, and see if the generator is heard in
the receiver. If such be the case, the hook contacts are crossed.
If no sound be heard in the receiver, raise the hook and see if
clicks are heard in the receiver when the binding posts are short-
circuited. If such be the case, the primary and secondary of
the induction coil are crossed. If none of these. tests locate
the trouble, it is probably due to an open-circuited ringer coil.
Can Call Central but Can Not be Heard. This means that
when the receiver is raised from the hook the circuit through
the primary of the induction coil and transmitter is complete
and that current is on the line. If under these conditions the
subscriber can not be heard, the trouble must be in the trans-
mitter, and as the circuit is closed the transmitter must be
short-circuited or packed.
Can be Heard but Can Not Hear. This condition indicates a
fault in the receiver circuit. It may be in the secondary of the
induction coil, the receiver, or the wiring. Short-circuit the
secondary of the induction coil and listen for a click in the
receiver. If no click is heard in the receiver, either the re-
ceiver or connections are open; if a click is heard, the secondary
of the induction coil is open.
These simple methods apply mainly to a C.B. telephone set
whose connections are shown in Figs. 85 and 86. If the con-
nections differ radically, the general principles may still apply,
although the procedure for localizing the fault may differ
somewhat.
QUESTIONS
Explain how you would locate the causes of the following local battery
faults :
1. Series bell does not ring.
2. Bridging bell does not ring.
3. Can not call operator.
4. Can not hear or talk.
5. Can hear but can not be heard.
6. Can talk but can not hear.
Explain how you would locate the causes of the following central battery
faults:
12
116 PRINCIPLES OF THE TELEPHONE
7. Can not call operator.
8. Bell does not ring.
9. Can get central but can not talk.
10. Can talk but can not hear.
Give complete adjustments for:
11. Ordinary polarized bell.
12. Biased bell.
CHAPTER XII
PROTECTION OF TELEPHONE LINES AND APPARATUS
134. Need for Protection. Whenever a telephone circuit
receives a voltage higher than that for which it is designed,
excessive currents may flow, overheating and possibly destroying
the apparatus connected to the circuit. The excessive currents
also increase the fire hazard and may cause injuries to persons
coming into contact with the circuit even at some distance from
the point of application of the excessive voltage.
135. Sources of Excessive Voltage. The sources of excessive
voltage against which telephone apparatus must be protected
may be classified under the following heads :
1. Lightning.
2. High- volt age power circuits.
3. Low-voltage power circuits.
The low-voltage currents may cause damage in two ways :
1. By heating.
2. By electrolytic action.
To prevent, or at least reduce to a minimum, the danger from
these sources, protection devices of various kinds are used.
136. Heating Effect of Current The heating effect of an
electric current is proportional to the square of the current
flowing and to the resistance of the circuit within which it
flows. If we wish to calculate the heat developed by a given
current within a given resistance in a given time, we use the
formula
Heat = 0.24PRI calories
/ is the current in amperes.
R is the resistance of the circuit in ohms.
t is the time in seconds during which the current has been flowing.
A calorie is the amount of heat required to raise the temperature
of 1 gram of water 1C.
EXAMPLES
1. A current of 30 amp. flows through a resistance of 5 ohms for J^ hr.
How many calories of heat are developed?
13 117
118 PRINCIPLES OF THE TELEPHONE
Solution
Formula H = 0.24/ 2 7ft calories
7 = 30 amp., R = 5 ohms, t = 1,800 sec.
Then
H = 0.24 X 30 2 X 5 X 1,800
= 0.24 X 900 X 5 X 1,800
= 1,944,000 calories.
2. Assuming that all of the heat developed is utilized in heating water,
to what temperature would the heat developed in the circuit mentioned
in example 1 raise 10 gal. of water?
Solution
I gal. of water weighs 8.33 Ib.
10 gal. of water weigh 83.3 Ib.
1 Ib. = 453.6 grams
83.3 Ib. = 83.3 X 453.6 = about 37,785 grams.
As 1 calorie will raise the temperature of 1 gram of water 1C., the tem-
perature to which 1,944,000 calories will raise 37,785 grams is 1,944,000 -f-
37,785 or 51.4C. This is the equivalent of 92.5F.
These problems seem to indicate that the current alone is re-
sponsible for the heating. This view is correct, but it must not
be forgotten that in a given resistance the current is directly
proportional to the pressure and, hence, it is just as correct to
consider the heating effect to be proportional to the square of
the pressure; for by Ohms' law
1-^
R
Then
*-
which, when substituted in the formula for heat, gives:
E 2 E 2
Heat = 0.24 ^ X Rt = 0.24 ^ t.
The danger from excessive voltage or pressure is thus of two
kinds : It may cause excessive currents, and it may puncture the
insulation, causing short circuits. Two kinds of protective
devices are thus necessary, one to prevent excessive currents and
the other to prevent the entrance of high pressures.
137. Lightning Phenomena. Although lightning is the oldest
manifestation of the dissipation of large quantities of electrical
energy, it is the least understood today. We shall not go into the
TELEPHONE LINES AND APPARATUS 119
theories of lightning, but it may be of interest to point out certain
characteristics of lightning and their modern explanation. The
old idea, one still commonly held, is that lightning is a simple
discharge of electricity between clouds, or between clouds and
the earth. This conception is in some respects inadequate, al-
though it does seem to state what one actually sees. Perhaps
the more correct explanation of lightning phenomena is that
due to Dr. Steinmetz. His explanation is that instead of being a
rupture of the air under an excessive high voltage, lightning is
in reality an equalization of stresses within the ether. Lightning
may be compared to the breaking of a piece of glass which has
been rapidly chilled and thereby filled with internal tension and
compression strains. If such a piece of glass is scratched it
will suddenly break all over. So, with our present knowledge,
we must consider as the most probable explanation although
not certain by any means that lightning discharge is the
phenomena of the equalization of internal electric stresses in the
cloud, and is analogous to the splintering or breaking of an
unevenly stressed brittle material like glass.
If such a discharge, or even a small portion of it, happens to
pass through a telephone instrument, those parts connected with
the line at that time will probably be destroyed either by having
their windings fused by the heavy current, or by having the
insulation of the windings punctured by the high voltage. Of
course, the person using the telephone when such a stroke occurs
would be very fortunate to escape without serious injury. There
are two somewhat different lightning effects encountered in
telephone work. The first of these is the direct stroke, which
has been discussed above; the second is that due to induction.
Whenever a discharge takes place, electromagnetic waves move
out in the ether somewhat like water waves in a lake when a
disturbance takes place at some point. These electromagnetic
waves when they cross telephone lines induce, currents in the
wires. The effect of electromagnetic induction due to a lightning
discharge is usually neglected. There is, however, another kind
of induction which must be considered, namely, electrostatic
induction. To understand this, suppose a heavily charged cloud
moves up to the region over the line. If the cloud is negatively
charged, a positive charge will be induced on the telephone line,
and an equal negative charge will have a tendency to pass to
earth. If the approach of the cloud is slow enough, this free
120 PRINCIPLES OF THE TELEPHONE
negative charge will pass to earth by gradual leakage over the
insulators. If the approach of the cloud is rapid, and if the
potential difference between its charge and the earth is great,
the free charge on the line may puncture the insulation and
pass to earth. When the telephone line is metallic and well
insulated upon poles or other fixtures and not connected with
the earth by conducting material, the inductive effect of the
lightning discharge will not be so great as upon a line whose
ends are grounded.
The telephone line, as a rule, does not offer an easy and direct
path for the lightning discharge between the cloud and the earth,
owing to its horizontal position, and usually receives only a por-
tion of the discharge which finds its way to or from the earth
over several poles nearest the main path of the discharge. The
total quantity of electricity in a lightning discharge is not
great, but as the voltage is high the energy is comparatively
great, and as the duration of the discharge is short, the power is
very high.
138. Lightning Conductors. Before the laws governing the
dissipation of electrical energy were well understood, it was
supposed that lightning would obey Ohm's law, and that when-
ever a discharge took place it would follow the easiest path to
earth, and that if an easy path were provided it would protect all
others. It is not merely a charge of electricity that must be
conducted to earth, but a large quantity of energy must be
dissipated as quickly as possible and in such a way as not to be
destructive.
Lightning is an oscillatory discharge; that is, in effect it can
be compared to the action of a compressed or extended spring
which is suddenly released. The spring does not dissipate its
energy in one swing from the extended position to its position of
rest, but it overshoots the neutral position and oscillates back
and forth for some time. If, however, the spring is immersed
in some viscous material, it will not overshoot its position of
equilibrium, but will dissipate its energy in moving slowly
from the extended position to its position of rest. The material
will not be violently disturbed and no harm will be done.
In a lightning discharge there is a certain amount of energy
to be dissipated, and it may be that a single rush of electricity
in one direction does not suffice to dissipate all of the energy.
If the path has a moderately high resistance, a single rush may be
TELEPHONE LINES AND APPARATUS 121
sufficient, but if the resistance of the path is low a single rush is
not sufficient, and the discharge will oscillate until all the energy
is turned into heat. The rush in either case, however, is likely
to be violent and the discharge will not always take the easiest
path but will make its own paths, which are sometimes quite
unexpected.
The relatively high resistance of iron as compared with that of
copper makes the use of iron wire for lightning rods on telephone
poles beneficial in damping the oscillations of the flash, and thus
permitting the discharge to leak away slowly and without
side flashes. Its high melting point and cheapness are also ad-
vantages. The wire must not be too small, however, or there
will be risk of its fusing.
An electric current, like matter, seems to possess inertia. That
is, it takes some time to start a current, and likewise when its
flow has once been established some time is required to reduce it
to zero. Thus, when a source of e.m.f. is connected to a circuit,
the resulting current will not at once, or immediately, reach a
maximum value. This is due to the fact that the establishment
of a current in a circuit is accompanied by the storage of energy
in the space surrounding the circuit. To store energy in the mag-
netic field requires time. When the circuit is broken the energy
is returned to the circuit and thus does not permit the current
to drop instantly to zero. This property of a circuit is called
inductance. Whenever a wire is bent, its inductance is in-
creased, and a greater opposition is presented to the establish-
ment of a current. It is thus evident that wire used for lightning
rods should have no sharp bends, and in fact, should be as free
from bends as possible. As already stated, a lightning discharge
is of an oscillatory character with an exceedingly high frequency.
This high frequency increases the reactance of a bend or kink
to such an extent that the discharge is liable to jump across to
some other conductor and not pass around the bend
139. Lightning Arresters. There are two ways, then, in which
lightning can affect a telephone circuit : by electrostatic induction
and by direct stroke. The object of lightning arresters is to
protect the subscribers' station apparatus, cables, and central
office equipment against damage from both these causes. The
operation of lightning arresters depends upon the fact that
current due to a lightning discharge will jump across a short air
gap more readily than it will pass through a coil or other piece of
122
PRINCIPLES OF THE TELEPHONE
FIG. 87.
apparatus having considerable impedance. It has been stated
above that when high-frequency current flows in a coil having
impedance, a high counter-pressure is set up, which tends to
hold back the current. As lightning has a frequency many times
higher than that of currents used in telephone practice, lightning
will not pass through the coils and windings of telephone in-
struments readily if some other con-
venient path to ground be provided.
In Fig. 87 is shown a diagram of a
lightning arrester and connections.
The two short plates are connected to
the line; and the long plate is connected
to ground. The gaps between the
plates are made very small so that any
current due to lightning finds an easier path to ground through
the air gap than through the coils of the instrument. Arresters
were formerly made with metal blocks of the general form shown,
but proved to be quite unsatisfactory on account of the fact
that heavy discharges were likely to fuse the plates and fill
the gaps with molten metal,
thus destroying the arrester
as well as putting the line
out of commission.
140. Carbon Block Ar-
resters. The carbon block
arrester is in common use on
account of the fact that dis-
charges between the carbon CARB
blocks do not melt or fuse
the blocks readily; hence
the arrester is not difficult to
maintain. One form of car-
bon arrester is shown in Fig.
88. This arrester consists
CARBON
GROUND
PLATE:
FIG. 88.
of carbon blocks, having the two inside ones connected to the
ground, and the outside blocks connected one to each side of the
line. Between the outside and inside blocks are placed separa-
tors of mica, which are perforated with a number of circular holes
through which the discharge takes place when the arrester oper-
ates. Forms of micas are shown in Fig. 89.
The American Telephone and Telegraph Co. uses two types of
TELEPHONE LINES AND APPARATUS
123
open space "cut-outs," as the carbon block arresters are some-
times called.
The cut-out employed at substations and at the central office
consists of small carbon blocks, one of which is connected to the
telephone circuit and the other to earth, separated by thin sheets
of mica 0.0055 in. thick. A small cavity in one of the opposite
faces of the carbon is filled with a button of fusible metal which
melts at about 160F. A carbon with
the fusible button is shown in Fig. 90.
" The distance between these carbons is
such that electricity at over 350 volts will
pass from the carbon connected with the
telephone circuit across the space to the
opposite carbon and thence to earth.
When this escape of current to earth
takes place a tiny arc in the space be-
tween the carbons may be sufficient to warm the carbons and
cause the fusible metal to flow from its recess and fill the space
between the carbons and thus establish a permanent connection
to ground. If the escape of current to earth does not sufficiently
heat the carbon to cause the fusible metal globule to flow, a
permanent connection to ground may not be established. In
FIG. 89.
FIG. 90.
many instances, however, even if the fusible metal globule does
not fuse, small particles of carbon from the carbon blocks are
broken loose by the sudden current discharge and these may
partially ground the line.
"For the protection of aerial and underground cable conductors
extended by open wires over J^ mile in length, the open space
cut-out is not as sensitive as the one above described. In this
124
PRINCIPLES OF THE TELEPHONE
case the discharge surfaces consist of two metal blocks and these
are separated by means of a mica 0.011 in. in thickness."
The Kellogg Co. equips its magneto telephones with carbon ar-
resters of the type shown in Figs.
91a and 916. The metal back
plates are semicircular, and are
connected to the line. The front
disk of carbon is connected to
ground and is separated from
the line plates by a thin sheet
of perforated mica.
FIG. 9l. 141. Self-cleaning Arresters.
Continual electrical discharges
between blocks are likely to cause deposits of fine carbon dust
which interfere with the operation of the arrester. This can be
FIG. 916.
removed readily, however, by taking the arrester apart and
cleaning the parts. To obviate the necessity for frequent clean-
ing a cut-out has been devised in which
the discharge gap is wedge-shaped, be-
ing narrower at the top than at the
bottom. It is claimed that the carbon
particles will not lodge between the
carbons when such a gap is used. The
construction of the Roberts " self-clean-
ing arrester" is shown in Fig. 92.
A self-cleaning arrester for outdoor
installation is shown in Fig. 93. This arrester has three carbon
blocks, one of which is connected to ground and one to each side
of the line.
FIG. 92.
TELEPHONE LINES AND APPARATUS
125
There are numerous arresters on the market, but they are
practically all of the carbon type, and work on the same principles
as those outlined above. Another make is shown in Fig. 94.
142. Location of Lightning Arresters. Arresters may be made
a part of the instrument, or connected to the line at the point of
entrance to the building. Good practice requires 'that an
arrester be placed in the latter position if thft line inside the build-
ing is of any considerable length.
143. Protection against Power Circuits. The voltage of light-
ning and power circuits is always higher than that of the tele-
phone line; hence it is neces-
sary to protect the latter
against possible contact, either
partial or complete, with the
former. To accomplish this
several types of protective
devices are used. These de-
FIG. 93.
FIG. 94.
vices must protect against an almost infinite number of possible
conditions of voltage and current strength.
The protection against damage from crosses with high-pressure
power circuits is accomplished by means of the same devices as
used for guarding against damage by lightning or static discharges,
namely, the open space cut-out. The open space cut-out operates,
as previously explained, by grounding the line. If the disturbing
voltage is of short duration such as a lightning discharge no
other protection is necessary. When, however, the disorder
is due to a cross with a high-tension or other power line, the
126
PRINCIPLES OF THE TELEPHONE
resulting current will probably continue to flow even if the tele-
phone circuit is grounded. In fact, the grounding may even
increase the current by reducing the resistance. In order to
guard against such accidents fuses are used.
144. Fuses. A fuse is a piece of conductor made of an alloy
having a low melting point and forming part of the circuit.
The action of fuses depends upon the heating effect of the
electric current flowing through them. They are, therefore,
FIG. 95.
used not to protect against high voltages, but to guard against
the flow of currents which might damage an instrument by
overheating the windings of its various parts. Fuse wires are
designed to melt when a current above a predetermined strength
flows, thus opening the circuit and breaking the current before
the parts of the instrument are overheated.
A fuse does not offer much protection against lightning, as a
lightning discharge may destroy an instrument before the tern-
Frank B. Cook SAnips!-- 600. Volts'.
FIG. 96.
perature of the fuse is high enough to melt it, or it may even
jump across the gap made by a blown fuse. Fuses used in tele-
phone circuits are invariably of the enclosed type. The mica
fuse having a small copper wire between two sheets of mica, as
shown in Fig. 95, is quite commonly used. The fuse wire is
attached to metal terminals at each end, by means of which the
fuse is held in the block.
Another type of enclosed fuse consists of a fusible wire con-
tained within a tube of fiber or porcelain. Usually the wire is
TELEPHONE LINES AND APPARATUS
127
surrounded by some nonconducting powder to assist in destroy-
ing the arc when the fuse wire is vaporized. An enclosed fuse is
FIG. 97.
shown in Fig. 96, and a porcelain fuse block, with fuses, is
shown in Fig. 97.
145. Protectors. Arresters and fuses are used singly or in
combination, and are known as protectors.
FIG. 98.
The Western Electric 58A protector, shown in Fig. 98, affords
protection against lightning, high voltage, and heavy power
currents. It consists of a carbon block arrester, designed to
128 PRINCIPLES OF THE TELEPHONE
operate at 400 volts, and two enclosed fuses designed to carry 5
amp. continuously and to operate at 7 amp. As protection
against a continued arc after a lightning or high-voltage discharge,
a plug of lead is placed in the outside arrester blocks. The
operation of this arrester is as follows : Assume that one side of
the line has come into contact with a 600-volt trolley wire. The
potential of this line is immediately raised to that of the trolley
wire, and the arrester operates. At the time of the operation
of the arrester, an electric arc is set up between the two plates.
This arc has a very much lower resistance than that of the original
air gap; hence the current flow through the arrester tends to
increase. If this rises to a value of above 7 amp., the fuse will
blow in a short time, disconnecting the arrester and telephone
instrument from the line. However, should this current only
reach a value of 5 or 6 amp., the fuse might not blow for some
m
FIG. 99.
time, in which case the fusible plug of the arrester would be
melted by the heat of the arc, and the metal would run down
between the two arrester plates, short-circuiting them and
allowing a large current to flow. This large current would at
once cause the fuse to operate and disconnect the arrester from
the line. After the fusible plug has been melted the arrester
must be taken apart and the metal removed before the arrester
can be put into service again.
Fig. 99 shows another type of protector which, in addition to
the arrester and fuse, consists of a switch by which the telephone
can be entirely disconnected from the line during the time of a
storm. The protector as shown is supposed to take care of one
side of the line only, and is sufficient for grounded lines. On
full metallic lines, however, a double-pole arrester of the same
type is used.
146. Protection against Weak Currents. Very weak currents,
usually called "sneak" currents, may flow if the telephone line
is crossed with low-voltage power or lighting lines, or with
comparatively high-voltage lines through a high resistance.
TELEPHONE LINES AND APPARATUS
129
In central-energy systems these sneak currents may be caused
by a ground on one of the line wires, or the crossing of two wires
without being subjected to a foreign potential. The danger
from such currents lies in the fact that the heat generated by
them accumulates and thus raises the temperature of the coil
through which they flow to an excessively high value. The
accumulated heat causes deterioration of the insulation and may
cause open circuits.
In order to protect against currents that are too small to
operate the fuse, and which are harmful only when permitted to
flow for a considerable time, a circuit grounding device called a
heat coil is used. The heat coil consists of a coil of fine German
silver wire which forms a part of the telephone circuit, Fig. 100.
The general principles of the operation of heat coils will be
readily understood by reference to Fig. 101, which shows a
FIG. 100.
FIG. 101.
section of one type of this protective device. The coil A is
wound on a metal bobbin D which is soldered to a metal stud Q.
The sneak current, on passing through the coil, heats the bobbin,
and if it flows for a sufficient length of time the accumulated
heat will melt the solder when the stud Q is pushed away by the
glass rod C breaking the circuit. The glass rod is actuated by
the spring S. To ground the current the lug Q is pushed against
a grounding contact.
In some makes of heat coils the soldered connection is under
tension. When the solder melts, a spring breaks the circuit
through the apparatus and grounds the line by making contact
with a grounded lug. This type of heat coil will be more fully
explained when the protection of central office equipment is
taken up.
The action of the heat coil is to protect the apparatus against
prolonged currents of just sufficient strength to overheat its
windings, such currents being below 1 amp. and consequently
130 PRINCIPLES OF THE TELEPHONE
below the range of practical fuse operation. For local battery
systems the heat coils are made very sensitive, since they have to
protect apparatus which, on account of its high resistance and
low heat conductivity, may be injured by comparatively weak
currents. In this system the telephone and ringing currents,
which under ordinary conditions flow over the line between a
substation and the central office, will not operate the heat coil.
The telephone current is of minute strength and the ringing cur-
rent, though greater, is of comparatively short duration. The
quantity of heat developed by these currents in the heat coil is
so small that there is no danger of its operation.
In the common battery systems the line from the central office
to the substation carries, in addition to the ringing and telephone
currents, the direct current for the transmitter. If the line is of
very low resistance, this current may attain a considerable
strength. For this reason it has been necessary to provide a
heat coil of sufficiently low resistance to carry this current with-
out operating, but in addition all the pieces of the telephone
apparatus in the line circuit have been designed so as to withstand
the heating effect of this current for an indefinite time.
The resistance of the sneak current arrester for local battery
systems is about 46 ohms. The effect on telephone transmission
of the resistance of these arresters in the line has been made the
subject of carefully conducted experiments, and it has been found
that their effect is quite imperceptible on local service transmission.
On the other hand, in order to secure high-efficiency telephone
transmission in common battery systems, it is necessary that the
resistance be kept as low as possible. The sneak current arrester
for this system is designed with a resistance of about 3.6 ohms.
This is found sufficiently high to develop the necessary amount of
heat to cause the arrester to operate promptly on dangerous
currents.
147. When Substations Need Protection. As regards the
necessity for protection, subscribers' stations are of two types;
exposed and unexposed. An exposed station is one which is
liable to be affected by lightning discharges, or by the line coming
into contact with high-voltage transmission and other electric
light and power wires. Ordinarily, therefore, the line of any
subscriber's station which is connected with the central office
through aerial wiring or cable is considered exposed. A station is
likewise considered exposed where a building is fed by a tap from
TELEPHONE LINES AND APPARATUS 131
an aerial cable, even though this tap may be carried underground.
Wherever a station is so located that electric light wires or other
circuits carrying a pressure of over 250 volts are liable in case of
failure to come into contact with the wall wiring of the telephone,
the station should be considered as exposed, and the substation
should be protected.
Subscribers' stations connected directly to the central office
through an underground cable are considered unexposed, and
therefore need no protection.
CHAPTER XIII
INSTALLATION .
148. Entrance Holes. Before making any holes for the en-
trance of the leading-in wires, the location of the protector must
be decided upon. Having decided upon the location of the
protector, one entrance hole sloping downward from within
should be made, care being taken that the distance between
the protector and the entrance hole is as short as possible. In
locating entrance holes and protectors, it is desirable to locate
LINE.
NOT LESS
THAN I
nq i
NOTE:-
THE: SPACING OF PORCELAIN
SUPPORTS SHALL NOT EJCCE.E.O 18"
SUPPORTS SHALL, BE PLACED
APPROXIMATELY 2' FROl^ CORNERS
JOIST
\
V/o
Th
T
'A
FIG 2
LESS
N 1"
FIG. 102.
both so as to give the shortest and most direct connection for
the ground wire.
149. Leading-in Wires. The wires extending from the pole
or fixture to the building should be attached to a support on the
outside of the latter. The distance from the last outside support
to the entrance hole should not be over 1 ft., Fig. 102. From
132
INSTALLATION
133
the last support a twisted pair of wires should be used to enter
the building. To prevent water following the wire into the
building, a drip-loop should be made at the leading-in wires at
a point immediately below the entrance hole, Fig. 103. All
entrance holes should have porcelain tubes for the protection of
the leading-in wires, and these tubes should project a short
distance from the hole. The leading-in wires should not come
into contact with any part of the building; and if these wires
must be extended through walls, floors, or partitions, they should
be enclosed in porcelain tubes in the same manner as in passing
through the outside wall. Porcelain tubes should always be
firmly secured so that they will not slip out of place. If it is
necessary to carry the leading-in wires any distance inside the
FIG. 103.
building before they reach the protector, they should be supported
by porcelain knobs or cleats.
150. Location of Protector. The protector should be mounted
upon the wall in such a manner that the fuses are vertical, and
should be placed as near as possible to the point where the lead-
ing-in wires enter. It is essential that the protector be not ex-
posed to water or dampness; if such is the case, a protector de-
signed for outside service should be used. The protector should
be mounted away from all combustible materials.
151. The Inside Wiring. The wires used on the inside of the
building after the protector has been passed should be what is
known as inside wire, and may be either single, double, or triple
conductor wire, depending upon the requirements. As neatness
is a desirable characteristic of all inside wiring, it is essential that
wires should be run only horizontally and vertically, and in
14
134 PRINCIPLES OF THE TELEPHONE
as workmanlike manner as possible. As far as possible, such
wires should be concealed. Wherever picture molding is pro-
vided, wires may be conveniently carried along this molding;
or if the latter is not available, they may be carried along the
mopboard, in corners, etc., but should never cross open walls
or ceilings.
All wire must be fastened in such a manner as not to injure
its insulation. For this purpose insulated staples, cleats, or
insulated tacks may be used.
Telephone wires should never be run through hollow partitions,
under floors, or other places where there is any liability of coming
into contact with electric light wiring. When it is necessary to
cross any open electric light or power wire, pipes, or other con-
ducting material, the telephone wires should not come within
2 in. of these wires or pipes, and should be protected by
porcelain tubes, or several wrappings of friction tape. Whenever
practicable, wires should be run above pipes and conducting
materials which it is necessary for them to cross. There should
be no coils or knots made in any of the wires at the protector or
telephone set terminals, or any other part of the inside wiring.
Where necessary to splice wires of the system within the build-
ing, all joints should be soldered and carefully wrapped with
rubber and friction tape. If tracer wires or wires of different
colors are used, corresponding wires should always be spliced
together.
152. Ground Wiring. In order to secure the best service from
protectors, it is necessary that the ground wire be properly in-
stalled. The ground wire should run as directly as possible from
the protector to ground, and should have no kinks, coils, knots,
or sharp bends. Where necessary to carry a ground wire through
an outside wall, a separate hole should be provided at least 3 in.
distant from the entrance hole. If necessary to protect this
wire from injury, it should be protected by a wooden molding or
enclosed in a nonmetallic conduit, and never run in an iron
pipe.
The ground connection may be made through a water or gas
pipe, or to a ground rod driven in permanently damp earth.
Whenever connections are made to pipes, preference should be
given to water pipes; and when made to gas pipes, should be at
a point between the meter and the street so that the removal of
the meter will not break the ground connection. When ground
INSTALLATION 135
connections are made to pipes, some form of clamps is used. Fig.
104 shows a good form. Steam or hot-water pipes or other parts
of heating systems are not desirable as ground connections. If
it be necessary to use a ground rod, the latter should be located
within the building if possible. In connecting the ground wire
to the ground pipe or rod, the pipe or rod should be thoroughly
clean, the ground wire wrapped around it a number of times, and
the connection soldered.
153. Location of Telephone Set. A wall set should be so
located that the mouthpiece of the transmitter will be at the
most convenient height for the average person using the same.
The height from the floor to the center of the transmitter should
be about 4 ft. 10 in. A telephone set
should not be located where it will
be injured by doors or movable furni-
ture, or where it will interfere with
persons passing through the room.
The set should not be mounted on a
damp wall, if the same can be avoided,
nor near a window that is liable to
be opened during a storm. However,
if such be the only space available, a FlG 104
waterproof board should be mounted
on the wall, and the telephone in turn mounted on this. Vibrat-
ing partitions and noisy locations should be avoided.
The wall sets should be fastened firmly and securely to the
wall. In attaching a set to a wooden or plastered wall, round-
head wood screws should be used. To fasten a set to brick,
cement, or stone walls, holes should be drilled in the wall in
proper position for the screws. The holes should then be
plugged, and the set fastened with round-head wood screws.
Care should be taken that thoroughly dry woocl is used, and that
the plugs are large enough to hold securely. Expansion bolts
may be used in place of the above. If a set is to be attached to
a hollow tile wall, holes should be drilled at the proper places and
toggle bolts used to attach the same.
A desk stand should be placed where it is most accessible and
convenient for the subscriber. If possible, the bell box should
be so located that the cord can be connected directly to the
terminals in the box, so as to prevent the cord from lying on the
floor or where it might be exposed to dampness or damage.
136 PRINCIPLES OF THE TELEPHONE
QUESTIONS
1. Why is it necessary to protect telephone instruments from lightning
and high-voltage light wires?
2. What is the principle of operation of lightning arresters? Why will
lightning jump across a small air gap rather than pass through a telephone
instrument?
3. Explain the construction of the carbon block arrester. What are
its advantages?
4. Explain the construction of fuses and give their uses.
6. Describe and explain the operation of the Western Electric 58A
protector.
6. What is meant by exposed and unexposed subscribers' stations?
7. When a telephone is being installed, where should the protector be
located?
8. How are the leading-in wires carried through outside walls, partitions,
etc.?
9. How should inside wiring be done?
10. How should the ground wire be run, and the ground connections
made?
11. What precautions should be observed in locating the telephone set?
12. Why are the leading-in wires not allowed to touch any part of the
building, while the inside wiring may be run along molding, etc?
13. Explain the function and action of sneak-current arresters.
14. What is the difference between sneak arresters for common battery
systems and for local battery systems?
CHAPTER XIV
PARTY LINES
154. Definition. The simplest form of a telephone installation
is a line to each end of which is connected a subscriber's telephone
set, Fig. 105. It is evident that other telephones may be bridged
(
JT A
-~> i
r
oo
o.
OO
o
FIG. 105.
across the line anywhere between the two ends, or that a branch
line may be run from some point on the main line and one or more
telephones connected to the branch line, as shown in Fig. 106.
FIG. 106.
Telephone lines connected so that more than one subscriber can
be called on the same line are known as party lines.
155. Classification of Party Lines. When telephone service is
supplied to a few subscribers scattered over a comparatively
15 137
138 PRINCIPLES OF THE TELEPHONE
large area, as in country districts, party lines are invariably used.
Within cities, where all lines run to a central office, few party lines
are used, and where they are used seldom more than four sub-
scribers are connected to the same line. Party lines can then be
classified in accordance with the number of subscribers con-
nected to the same line, but it is undoubtedly preferable to
classify them in accordance with the calling system used. We
thus have two classes of party lines: namely, code ringing and
selective ringing.
156. Code Ringing. The most simple party line system, and
the one which was first used, employs the code system. In this
system all ringers are bridged or connected across the line in
parallel, and all must be of the same resistance in order that the
ringing current may be equally divided between them. Any
number of telephones, up to about 20, may be connected to the
line, and as all the bells ring when ringing current is sent over
the line, a code system of ringing is used. A code system with
which everyone is more or less familiar consists of a system
of short and long rings. Below is a code system for 14 stations,
with their corresponding numbers.
Station No. 1 - 11 - 21 - 31 -
, \.t Zi ' O^ " ' "
3 - - 13 - - 23 -
4 - - 14 -
5
The central office is always given ring one. It will be noticed
that the dashes which symbolize long rings represent tens, and
that the short dashes represent units. Of course in the telephone
directory only the number is given. This scheme of ringing is
not often used in towns or cities, but is usually used on country
party lines.
On local battery party lines it frequently happens that sub-
scribers fail to restore their receiver to the hooks or several
parties may be listening at the same time. In either case, the
receivers and induction coils connected across the line being
of low resistance, the ringing current passes through them and
not through the ringer coils, thus preventing the central opera-
tor's calling the desired party. To remedy this condition and
to permit the receiving of a call if receivers are left off the hooks,
a condenser is often connected in the receiver circuit, Fig. 107.
This condenser prevents the passage of the low-frequency ring-
ing currents and causes them to pass through the ringer coils, but
PARTY LINES
139
it does not offer any considerable opposition to the high-fre-
quency voice currents. Hence, this condenser has the same use
as in the central-battery system. It is the practice on code party
lines for one subscriber to call another by giving the code ring
without the call going through the central office. Some com-
U
FIG. 107.
panies, however, desire to have all calls originating on magneto
party lines come into the central office in order to have a record
of all calls made on the line, and at the same time relieve the
subscribers of the necessity of ringing parties by code. When
FIG. 108.
this is the case, the instrument is provided with a push button
which may be used to connect the generator to ground, and
thus use only one side of the line for signalling purposes. The
operation of such a device will be readily understood from Fig.
108. When the switch is closed to ground the ringing-current
140 PRINCIPLES OF THE TELEPHONE
circuit is through the sleeve side of the line to the drop at central,
then to ground and back to ringer. The other bells on the line
are not affected.
Another plan is to have a direct- or pulsating-current generator
in each subscriber's instrument. This current has no effect on
the ringers of the instruments, but operates the signals at the
central office. The pulsating-current generator is merely the
ordinary magneto generator equipped with a commutator and a
push-button switch for making connection with the commutator.
When the line is connected to the commutator and the generator
is turned, the current in the line flows continuously in one
direction. It fluctuates in value, as shown in Fig. 45, but does
not reverse in direction. Such a current will operate the drop at
central, but will not operate the ringers of the other subscribers.
FIG. 109.
157. Selective Ringing. In selective ringing the N ringers are so
arranged that only the bell of the person wanted at the telephone
is rung. Selective ringing is accomplished by two principal
methods. One is by the use of a biased bell with pulsating
ringing currents; and the other is by making use of bells which
will respond only to a given frequency of an alternating
current, this latter method being known as harmonic ringing.
A common method of selective ringing, for use where only two
parties are connected to a single line, is to connect one sub-
scriber's ringer between one side of the line and ground, and the
other subscriber's ringer between the opposite side of the line
and ground, Fig. 109. In order to ring either party, then, it is
only necessary for the central operator to send ringing current
PARTY LINES
141
over that side of the line to which the desired subscriber is con-
nected, which will ring his bell but will not call the other sub-
scriber. The talking circuit is connected across the two sides of
the line, as usual. In the four-party selective system, biased
bells are used.
A biased bell is a polarized ringer designed to operate with
pulsating current ; that is, current which flows in one direction but
is interrupted from time to time. The biased bell shown in Fig.
110 is essentially a polarized ringer with a spring attached to
the armature in such a manner as to hold the clapper in the ex-
FIG. 110.
treme left or right position when no current is flowing. When
the clapper is in the extreme left position, the right end of the
armature is near the right core of the magnet. It is evident when
the armature is in this position that it can be affected only by
current flowing through the coils in such a direction as to cause
the left core of the magnet to become a S. pole, when it will
attract the left end of the armature and overcome the strength of
the spring, causing the clapper to move to the right and strike
the right gong. As soon as the current ceases to flow, the arma-
ture will be returned to its original position by the spring, and
the clapper caused to strike the left gong. The rapidity with
which this operation is repeated will depend upon the frequency of
142
PRINCIPLES OF THE TELEPHONE
the pulsations, or, in other words, the number of times the current
is interrupted per second. In order to ring properly, the bell
must be in selective adjustment; that is, the spring must be
strong enough to pull the armature back to its original position
during the time that no current is flowing; yet the spring must not
be so strong that the force of the magnet can not overcome it.
Ringers not in selective adjustment can be operated only by
alternating currents. In ringing biased bells, selection between
four stations on a party line may be had by connecting two biased
bells, one of each polarity, between each wire and the ground,
four bells in all, as shown in Fig. 111. When the pulsating
TIP SIDE.
FIG. 111.
generator is connected so that current flows out along the tip
side of the line, the ringer at A is operated. When the con-
nections are reversed so that the current flows out through
ground, it will operate the ringer at station B. In the same way
the ringers at stations C and D may be operated by connecting
alternately the positive or negative terminal of the generator to
the line and the other terminal to ground.
158. Harmonic Ringing. In a harmonic system alternating
current of four different frequencies is used for ringing purposes,
the bells being so arranged that each one will ring only when
supplied with current at one of the four frequencies. In order
that a bell may ring for a given frequency of current, its clapper
must swing from one extreme position to the other during the
period that the current reverses. Bells used in harmonic ringing
PARTY LINES
143
have a spring which holds the clapper in its middle position when
no current is flowing. In order that the ringers may operate at
different frequencies, the strength of these springs and the weights
FIG. 112.
of the clappers are different. If the ringer is properly adjusted
for the given frequency, a small ringing current will cause the
clapper to vibrate violently enough to strike the gongs, in the
FIG. 113.
same manner that a very small force at the right time causes the
pendulum of a clock to swing. Just as a considerable force is
required to cause the pendulum of a clock to swing at any but
144
PRINCIPLES OF THE TELEPHONE
its natural period, so it is necessary that a heavy ringing current
be required to cause the tuned ringer to ring at any other than its
natural frequency.
The frequencies usually used for harmonic ringing are 16%,
33%, 50, and 66% cycles per second. Since two alternations
are required to complete one cycle, the number of alternations
per minute, corresponding to the above, are 2,000, 4,000, 6,000,
and 8,000. (For example, 16% X 2 = 33% alternations per
second; and 33% X 60 = 2,000 alternations per minute.) In
Fig. 112 is shown a Western Electric ringer for harmonic party-
line service; and the clapper rods for ringers operating at four
different frequencies mentioned above are shown in Fig. 113.
FIG. 114.
Eight-party service may be given by connecting four harmonic
ringers of different frequencies between each side of the line and
ground, if such service be desired. The ordinary method,
however, is to bridge the ringers directly across the line, as shown
in Fig. 114, making only four stations on the line.
159. Extension Bells. Many times a telephone ringer can not
be heard as far from the instrument as the subscriber desires,
in which case an extension bell can be used. As the extension
bell is always connected to the same line as the ringer of the
telephone, the extension bell must be of the same resistance and
have the same adjustment as the other ringers of the line.
QUESTIONS
1. Into what classes are party lines divided?
2. What is meant by code ringing?
PARTY LINES 145
3. What are the advantages and disadvantages of code ringing?
4. Of what use is a condenser in a receiver circuit, in party-line service?
5. What is meant by selective ringing?
6. In what way is a biased bell different from an ordinary polarized ringer?
Explain its operation.
7. What kind of ringing current is used with biased bells?
8. Explain the connections and operation of a four-party line using biased
bells. Show how each bell can be rung without ringing the others. Show
connections by diagram.
9. What is meant by harmonic ringing? What kind of current is used for
harmonic ringing?
10. How are harmonic ringers different from other ringers which have
been discussed? What is the difference between ringers designed for
different frequencies of ringing current?
11. Explain the operation and connections of four- and eight-party har-
monic lines.
CHAPTER XV
INTERCOMMUNICATING TELEPHONE SYSTEMS
160. Definition. An intercommunicating telephone system
is the arrangement of several sets of telephones such that any
station can call any other station without the assistance of a
central operator. Such systems are extensively used in factories,
offices, apartment buildings, stores, and large private dwellings
as they afford a ready means of communication between different
departments.
Telephone systems for intercommunication may be operated
either by a local battery for the talking circuit and a magneto
for signalling, or they may be operated entirely from a common
battery. When the common battery type is used, two sets of
batteries are invariably employed.
The most simple system of the local battery type is one in
which two telephone sets are connected by a single line. Such a
system needs no further discussion. However, when more than
two stations make up the system, the arrangement is more com-
plex. Of course, all the instruments could be connected to a
single party line, but this would necessitate code ringing. The
usual arrangement of intercommunicating systems is to have a
separate line run from each instrument to every other one of the
system. For magneto ringing the circuits are quite simple and
easily designed. Each station is provided with a panel upon
which are mounted as many jacks as there are stations, and
lines running from any one station connect the jacks into as
many parallel groups as there are stations. At each station the
ringer is bridged across one line. This line is designated at all
other stations as belonging to the station at which the bells are
bridged. The talking and ringing circuit at each station is pro-
vided with a terminal plug which is used to make connection
with the jack of any other station. Fig. 115 is a simplified dia-
gram of such a system. When a person at station A wishes to
call some one at station D, he inserts the plug into the jack con-
nected to the J) line and turns the magneto. As the only ringer
146
INTERCOMMUNICATING TELEPHONE SYSTEMS 147
that is bridged across this line is at station D it is the only station
that will hear the call. As soon as the person at station D inserts
his plug in jack D, the talking circuit with station A is complete.
Although such a system is extremely simple, owing to the con-
venience of automatic signalling provided by the common battery
system, the latter is displacing it.
161. Common Battery Interphone Systems. Most of the
manufacturers of standard telephone apparatus also manufacture
STATION A I STATION & STATION C
STA TIO'N D
iflnf
^
FIG. 115.
intercommunicating telephone apparatus. In general the prin-
ciples of operation of the different makes are the same, but each
has some distinctive method of connection for ringing.
At each station is a telephone set, either a wall set containing
the keys and talking set, or a desk stand with a separate key box.
Each wall set, or desk set key box has a series of buttons, each
one numbered or lettered to indicate the line it controls. Typical
C.B. intercommunicating sets are shown in Figs. 116, 117, and
118. A person at one station wishing to talk to one of the other
stations presses the corresponding button down to the ringing
position, and the desired station is signalled. When this button
is pushed down, any other button in the set which might happen
148
PRINCIPLES OF THE TELEPHONE
to be depressed is automatically restored, thus clearing the
station of any previous connection. When the pressure is re-
moved, the button comes back to a halfway or talking position,
so that as soon as the called station receiver is removed the
talking connections are complete.
The wiring of an intercommunicating system appears com-
plicated, but this is due to the multiplicity of wires at each
telephone. As a matter of fact the circuits are quite simple.
FIG. 116.
Diagrams of the circuits of two stations involved when one calls
the other of a Western Electric interphone system is shown in
Fig. 119. The diagram shows that two sets of batteries are used,
one for ringing and one for talking.
162. Western Electric Intercommunicating System. In the
diagram shown the station at the left is supposed to be ringing
the station at the right. In doing this the push button d is
depressed as far as it will go. This closes both the ringing
INTERCOMMUNICATING TELEPHONE SYSTEMS 149
FIG. 117.
FIG. 118a.
FIG. 1185.
150
PRINCIPLES OF THE TELEPHONE
circuit at d, and the talking circuit at the lower contact. The
ringing current then passes from the ringing battery to the bell
c, which it rings, at the station called, through the back contacts
STATION NO. G LlA/S'
CALLINQ STATION N&4
FIG. 119.
n of the switch hook at that station, over the wire s of the line
and through the lower contact of the button d at the calling
station, whence over the other wire t back to the ringing battery.
TALKING STATIOH N0.4
AN3IV/r//Hr ST/lT/0/f #0. 6.
FIG. 120.
When button d is released, it springs part way back opening the
circuit at 1 but leaving it closed at 2 and at the lower contact.
This condition is shown in Fig. 120. As soon as the subscriber
INTERCOMMUNICATING TELEPHONE SYSTEMS 151
152
PRINCIPLES OF THE TELEPHONE
CIRCUIT DIAGRAM FORA FULL METALLIC SYSTEM
RINGING BAT.
STATION ;
TALKING BATTERY
RINGING BATTERY
OUR SETS ARE WIRED FOR FULL METALLIC
SYSTEMS.
TO ADAPT THtM FOR COMMOH RETURN
SYSTEMS-MAKE THE rctlCWlH* CMAHit*
(0 STRAP TOGETHER THE TWO TALKING
BATTERY TERMINALS (DO NOT counter
THE TALKING BATTERY WIRES IN THE
CABLE TO THESE TERMimAL*)
(Z) STRAP TOGETHER THE LOWER
TERMINALS OF ALL LINES.
(3) CONNECT THE BLACK TKAHSPO&IYICM
WIRE TO POSITIVE (*) RlNlf4CBATTUl
TERMINAL*..
<4) CONNECT THE RED TRANSPOSITION
WIRE TO THE UPPER TCRMINAUOF
THE HOME STATION LINE.
te)COMNECT THE NEGATWE(-) TALK I MS,
BATTERY CABLE WIRE TO LOWL*
LINE NO.I
RINGING BAT.
TALKING BAT.
CIRCUIT DIAGRAM FOR A COMMON RETURN SYSTEM
REOTKAN*) i^g ?^ FEPTRAMt-4 Vl__
,mOHl*4^_ J POSOWHLM^ ^J
STATION^ STATION*3
&%X~tr^C
TALKING BATTERY RINGING BATTERY
FIG. 1216.
INTERCOMMUNICATING TELEPHONE SYSTEMS 153
at the station called takes the receiver off the hook he depresses
the answering button K which operation connects the two
transmitters TI and T 2 directly across the line which is composed
of the two conductors s and r. The talking battery is also
bridged across the line through the two windings x and y of a
retardation coil. The function of this coil is to prevent inter-
ference or cross-talk from other stations which might be con-
nected together for conversation at the same time, as the same
talking battery is used for all the telephones in the system.
The receivers R\ and R% are each connected in a local circuit
which includes the secondary of an induction coil at each station.
The connection between the talking battery and the ringing
battery is necessary to prevent cross-ringing, that is, the ringing
FIG. 122.
of a bell at a station other than the one called. Figs. 121a and
1216 show the arrangement of a typical Western Electric system.
163. The Kellogg Intercommunicating System. The Kellogg
intercommunicating telephone which is shown in Fig. 118
operates on the same principle, but the connections differ some-
what. Instead of employing the same button for connecting
the circuit and ringing, separate buttons are provided. Thus
to call a station the button corresponding to the station desired
is depressed. This closes the ringing circuit but not the talking
circuit for the closing of which a separate green button is pro-
vided. In answering a call at any station all that is necessary is
to press the red or home button and remove the receiver from the
hook in the regular manner.
16
154
PRINCIPLES OF THE TELEPHONE
164. The Monarch Intercommunicating System. The Mon-
arch intercommunicating system is also of the push button
type, but a modification is made in the manner of connecting
the battery to the talking and ringing circuits. The operation
u u
NORMAL- POSITION
O O O O O
TAUK1NG POSITIOH
FIG. 123.
O O O OO
RINGING POSITION
of the system will be readily understood from an examination
of Figs. 122, 123, 124, and 125. Fig. 122 shows the method of
wiring for two stations. Two batteries are employed, one for
ringing and one for talking, as in the Western Electric system.
The ringing circuit is permanently connected to the talking
circuit at the sleeve side of /the line at
S; from there it leads through the buz-
zer or bell, the lower contacts of the hook
switch to the ringing battery, and to
the calling key, a, for station 2 at station
1. When this key is depressed the cir-
cuit is closed through conductor Ri to S.
To call station 2, the person calling de-
presses the calling, key 2 which closes
the calling circuit as shown at a, Fig.
123, completing the circuit and ringing
the bell at station No. 2. When the
calling key is released it springs back
part way opening the ringing circuit at
a, and when the person called at station No. 2 takes his receiver
off the hook the ringing circuit is also opened at the hook switch.
The talking circuit is controlled partly by the calling key at sta-
tion No. 1 and also by the home key at station No. 2. The switch
controlled by the home button is shown in Fig. 124. In the
FIG. 124.
INTERCOMMUNICATING TELEPHONE SYSTEMS 155
156 PRINCIPLES OF THE TELEPHONE
normal position of the home button, the switch points at d are
closed. This corresponds to the lower contact at d, Fig. 122.
When the home button is depressed at the station called, the
switch points at d } Fig. 124, are opened and those at g are closed.
This corresponds to the point g at station 2, Fig. 122. Normally
only one side of the battery is connected to the talking circuits.
When one station wishes to communicate with another station,
the station calling leaves his home button in the normal position,
but the station called depresses his button, thus transferring
the battery connection at his station to the other side, bridging
the battery across the talking circuit through two retardation
coils. The circuit is not complete, however, until the receiver
is removed from the hook. A complete diagram of connections
for ringing and talking between two stations is shown in Fig.
125. An examination of this diagram will make clear the
operation of the system.
QUESTIONS
1. (a) What is meant by an intercommunicating telephone system?
(6) What is the difference between an intercommunicating system and a
party line?
2. Show by diagram the connections between two stations for the Western
Electric intercommunicating telephone system.
3. Explain the operation of the Western Electric system from your
diagram.
4. Diagram the connections between two stations for the Monarch
system.
6. Explain the operation of the Monarch system from your diagram.
INDEX
Action of a condenser, 93
Alternating currents, 49
American wire gage, 11
Ammeter, 15
Ampere, 15
Annealed copper wire, table of, 14
Arresters, carbon block, 122
lightning, 122
self-cleaning, 124
Artificial magnets, 24
horseshoe, 25
Automatic switch, 68
B
Bar electromagnet, 31
Batteries, electric, 6
primary, 6
storage, 7
Battery, 4
resistance for parallel connec-
tions, 21
Bell or ringer, 69
extension, 144
Bipolar receiver, 50
Bridging telephone, 78
connections of, 79
Brown and Sharpe gage, 11
Capacity of a condenser, 89
unit of, 89
Carbon block arresters, 122
electrodes, 45
transmitter, 40
Cells, dry, 8
in parallel, 20
in series, 20
Circuits, closed, 16
electric, 15
grounded, 17
Circuits, local battery, 78
magnetic, 28
of C. B. subscribers 7 telephones,
112
open, 16
series and parallel, 16
short, 16
signalling, 63
Circular mils, 11
Closed circuit, 16
Code ringing, 138
Coil, heat, 129
induction, 4, 59
retardation, 98
Common battery interphone sys-
tem, 147
telephone, 87, 113
C. B. wall set, 100
desk set, 101
hotel set, 100
Complete telephone, 72
Condenser, 88
action of, 93
analogy for, 92
and ringer in series, 95
capacity of, 89
manufacture of, 90
Conductors, lightning, 120
and insulators, 8
Connections of bridging telephone,
79
Construction of electromagnets, 32
Current, alternating, 49
direct, 49
electric. 14
sneak, 128
D
Direct current, 49
receiver, 57
Dry cells, 8
Dynamo, Faraday's, 63
157
158
INDEX
E
Electrical pressure, 7, 15, 48
unit of, 13
resistance, 9
unit of, 13
Electric batteries, 6
circuits, 15
current, 14
Electrodes, 6
carbon, 45
Electrolyte, 6
Electromagnet, 30
bar, 31
construction of, 32
horseshoe, 32
ironclad. 32
tubular, 32
Electromagnetism, 29
Entrance holes, 132
Excessive voltage, 117
Extension bells, 144
Farad, 89
Faraday's dynamo, 63
Faults, localizing, 110
in C. B. telephones, 113
in L. B. telephones, 110
on telephone apparatus, 107
Function of condenser in telephone
circuit, 95
Fuses^ 126
enclosed, 126
G
Gage, American wire, 11
Birmingham, 13
Brown and Sharpe, 11
New British Standard, 13
numbers, 11
Standard wire, 13
steel wire, 13
Generator, 4, 63
telephone, 66
Grounded circuit, 17
Ground wiring, 134
II
Harmonic ringing, 142
Heat coil, 129
Heating effect of current, 117
Hook switch, 4, 72
Kellogg, 73
Stromberg-Carlson, 73
Western Electric, 72, 74
Horseshoe electromagnet, 32
magnet, 25
Impedance, 59
Induced electric pressure, 48
Induction coil, 4, 59, 96
electromagnetic, 58
magnetic, 24
mutual, 58
self-, 57
Inside wiring, 133
Installation, 132
Instruments, telephone, 3, 80
Insulators and conductors, 8
Intercommunicating telephone sys-
tem, 146
definition of, 146
Kellogg, 153
Monarch, 154
Western Electric, 148
Interphone system, 147
Western electric, 148
Ironclad electromagnetic, 32
K
Kellogg intercommunicating tele-
phone, 153
receiver, 53
Laws of magnetic attraction and
repulsion, 25
Leading-in wires, 132
Le Clanche cell, 7
Lightning arresters, 121
location of, 125
conductors, 120
INDEX
159
Lightning phenomena, 118
Lines, magnetic, 27
party, 137
telephone, 137
Local battery circuit, 78
systems, definitions of, 76
Localizing faults, 110
Locating faults in C. B. telephones,
113
Location, of lightning arresters, 125
of protector, 133
of telephone set, 135
M
Magnetic action, 25
of receiver, 33
attraction and repulsion, law
of, 25
circuit, 28
field, 27
induction, 24
lines, 27
substances, 24
Magnetism, 23
Magnetite, 23
Magnets, artificial, 24
horseshoe, 25
natural, 24
permanent, 26
temporary, 26
Magnet wire, 33
Manufacture of telephone con-
denser, 90
Measurement of wire, 10
Microfarad, 89
Mil, circular, 11
Monarch intercommunicating sys-
tem, 154
Mutual induction, 58
N
Natural magnets, 24
Nonconductors, 9
Nonmagnetic substances, 24
O
Ohm, 13
Open circuit, 16
Operator's receiver, 55
P
Parallel cells, 20
circuits, 16
resistance of, 17
connections, battery resistance
for, 21
Party line, code ringing, 138
harmonic ringing, 142
selective ringing, 140
lines, classification of, 137
Permanent magnets, 26
Power circuits, protection against,
125
Pressure and resistance of electric
current, 15
electrical, 7
Primary batteries, 6
Properties of sound, 36
Protection, against power circuits,
125
against weak currents, 128
of telephone lines and appara-
tus, 117
Protector, 127
location of, 133
Western Electric, 127
R
Receiver, 4, 50
action, 23
and transmitter in series, 95
bipolar, 50
direct-current, 57
early, 48
Kellogg, 53
Monarch, 57
operator's, 55
sensitiveness of, 55
Western Electric, 51
Reluctance, 28
Resistance, unit of, 13
electrical, 9
of a parallel circuit, 17
of a series circuit, 17
Resistivity, 9
Retardation coil, 98
Ringer, 4, 69
Ringing code, 138
160
INDEX
S
Selective ringing, 140
Self-cleaning arresters, 124
Self-inductance, 58
induction, 57
Sensitiveness of receivers, 55
Series, cells, 20
circuits, 16
resistance of, 17
telephone, 76
Short circuit, 16
Shunt, 16
Side tone, 107
wiring, 112
Signalling circuits, 63
Sneak current, 128
Solenoids, 29
Sound, 35
loudness, 36
pitch, 36
properties of, 36
timbre or quality, 36
velocity of, 36
Sources of excessive voltage, 117
Speech, transmission of, 37
Subscribers' telephones, circuits of
C. B., 112
Substation, 130
instruments, 111
faults in, 111
Switch, automatic, 68
hook, 72
T
Table of annealed copper wire, 14
of resistivities, 9,10
of wire gages, 12
Telephone batteries, 7
bridging, 78
circuit, function of condenser
in, 95
common battery, 87
generator, 66
instruments, 3, 80
Kellogg intercommunicating,
153
lines, 137
locating faults in C. B., 113
operation, 1
Telephone, protection of, 117
receiver, 23, 48
series, 76
set, 72
desk, 83
hotel, 82
location of, 135
wall, 80
subscribers', 112
systems, intercommunicating,
146
troubles, 106
localizing of, 106
Temporary magnets, 26
Tests for telephone troubles, 106
Tone, side, 107
Transmission of speech, 37
Transmitter, 3
carbon, 40
Kellogg, 44
Monarch, 45
new Western Electric, 42
operator's, 45
White solid-back, 40
Tubular electromagnet, 32
U
Unit of electrical pressure, 13
of resistance, 13
Variable resistance, 38
Velocity of sound, 36
Volt, 13
Voltage, 117
Voltmeter, 13
W
Weak currents, protection against,
128
Western Electric protector, 127
receiver, 51
system, 148
Wheatstone's bridge connection, 99
Wire gage, table of, 12
magnet, 33
measurement, 10
Wires leading-in, 132
Wiring, ground, 134
inside, 133
"--,,,,',,
1 t^o*^a*ty
UNIVERSITY OF CALIFORNIA LIBRARY