AMERICAN
TELEPHONE PRACTICE
BY
KEMPSTER B. MILLER, M. E.
SECOND EDITION.
NEW YORK
AMERICAN ELECTRICIAN COMPANY
^120 LIBERTY STREET--
TABLE OF CONTENTS.
CHAPTER I.
PAGE
HISTORY AND PRINCIPLES OF THE MAGNETO TELEPHONE, i
Early Knowledge of Electromagnetism Work of Oersted, Ampere,
Arago and Davy, Sturgeon, Faraday, and Henry Transformation
of Eleptric into Magnetic Energy Transformation of Magnetic
into Electric Energy Field of Force Morse's Telegraph Reis :
Telephone Sound Waves Bell's Telephone House's Electro
Phonetic Telegraph.
CHAPTER II.
HISTORY AND PRINCIPLES OF THE BATTERY TRANSMITTER, 10
Gray's Variable-Resistance Transmitter Berliner's Transmitter Elec-
trodes in Constant Contact Carbon Electrodes Demonstration of
Advantages of Loose Contact by Hughes Hughes' Microphone
Running's Granular Carbon Transmitter Induction Coil with
Transmitter Advantages of the Local Circuit.
CHAPTER III.
THE TELEPHONE RECEIVER, . . . . . . . .18
Considerations in Designing Mechanical and Electrical Efficiency
Single-Pole Receivers Bipolar Receivers Adjustment between
Magnet and Diaphragm Material for Shells Faults of Imitation
Hard Rubber Commercial Types of Receivers Receiver Cords
Details of Cord-Tip Supports for Receiver Cords.
CHAPTER IV.
CARBON TRANSMITTERS, 32
Action of the Transmitter Single-Contact Transmitters Multiple-Con-
tact Transmitters Granular Carbon Transmitters Commercial
Types of Transmitters Packing : Its Remedy Unusual Forms of
Transmitters Types of Carbon Electrodes.
CHAPTER V.
INDUCTION COILS, . ......... 53
Advantages of the Induction Coil Primary Current Secondary Cur-
rent Design of Induction Coils Methods of Making Comparative
Tests Results of Comparative Tests Selection of Coil for a Trans-
mitterCommercial Coils Varley Method of Winding Mounting
of Induction Coils.
COPYRIGHT, 1899, BY
AMERICAN ELECTRICIAN COMPANY.
PREFACE.
THE intended scope of this book is set forth in its title. To
those interested the writer has endeavored to present in as clear
a manner as possible the general principles of telephony, the
design and construction of commercial apparatus, the circuits
connecting such apparatus into operative systems, and .the
methods used in the construction, operation, and maintenance of
these systems. No attempt whatever has been made to treat
the subject from its purely mathematical standpoint, that being
beyond the scope of this work. - The apparatus and methods
of both Bell and Independent companies have been given im-
partial attention.
The writer sincerely thanks his friends, Mr. Wm. H. Donner
and Mr. Wm. R. Mackrille, for their many suggestions and
untiring labors in proof reading, and also Mr. W. D. Weaver,
editor of the Electrical World and Engineer, for his interest
and assistance throughout the entire preparation of this book.
KEMPSTER B. MILLER.
VI TABLE OF CONTENTS.
CHAPTER VI.
PAGE
BATTERIES, 62
Simple Cell Direction of Current Positive and Negative Poles
Materials Best Suited for Electrodes The LeClanche Cell The
Fuller Cell Specifications for Standard Fuller Cell The Gravity
Cell Local Action in Batteries Amalgamation of Zinc Storage
Batteries The Setting up and Operating of Storage Batteries-
Determination of Positive and Negative Poles of Charging Circuit-
Density of Electrolyte Advantages of the Storage Battery.
CHAPTER VII.
CALLING APPARATUS, 75
Battery Calls The Magneto-Generator Action of Magneto-Generator
The Polarized Bell or Ringer Design of Magneto-Generators
Construction of Armature Core Winding of Armature Permanent
Magnets Form of Current Wave Design of Polarized Bells Iron
for Ringer Frames Length of Hammer Rod.
CHAPTER VIII.
THE AUTOMATIC SHUNT, 86
Necessity for the Automatic Shunt Commercial Types.
CHAPTER IX.
THE HOOK-SWITCH AND CIRCUITS OF A TELEPHONE, 90
Functions of the Hook-Switch Simplified Telephone Circuits The
Warner Hook-Switch Other Forms of Hook-SwitchesCircuits of
a Series Telephone Circuits of a Bridging Telephone Battery Call
Instruments Desk Telephone Wiring of 'Telephones.
CHAPTER X.
COMMERCIAL CALLING APPARATUS, 104
Types of Generators and Ringers Resistances of Armatures and Ringer
Magnets for Different Kinds of Work Constantly Driven Genera-
tors Methods of Driving Generators Motor Generators.
CHAPTER XI.
THE TELEPHONE RELAY OR REPEATER, uS
Simple Relay Circuit Difficulties in Producing a Two- Way Repeater
Circuits of Two-Way Repeater The Erdman Repeater The
Stone Repeater.
TABLE OF CONTENTS. vit
CHAPTER XII.
PAGE
SELF-INDUCTION AND CAPACITY I2 4
Ohm's Law Field of Force about Conductor Electromagnetic Induc-
tionAction between Turns of the Same Coil Impedance Effects
of Self-induction on Undulatory Currents Charge of Electricity
Action between Like and Unlike Charges Electrostatic Induction
Condensers Capacity Specific Inductive Capacity of Dielectrics
Specific Inductive Capacity in Telephone Cables Effect of Con-
denser Bridged across Circuit Effect of Capacity on Varying Cur-
rents Trans-Oceanic Telephony.
CHAPTER XIII.
TELEPHONE LINES, . . .136
Grounded Circuits Noises on Grounded Circuits Causes of Line Dis-
turbancesElectromagnetic Induction Electrostatic Induction
Carty's Experiments Cross-TalkElimination of Cross-Talk
Transpositions Ground-Return Systems Common-Return Sys-
tems Location of Common-Return Wire Size of Common-Return
Wire Connection of Metallic and Grounded Circuits The Repeat-
ing Coil Elimination of Local Induction.
CHAPTER XIV.
SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES, 153
Manual and Automatic Switch-Boards Switch-Boards for Grounded or
Common-Return Systems Types of Drops and Jacks Switch-
Boards for Metallic Circuits Tubular Drops.
CHAPTER XV.
LISTENING AND RINGING APPARATUS FOR SWITCH-BOARDS, . . .163
The O'Connell Key The Cook Key The American Key Plug Listen-
ing and Ringing Devices Plug-Socket Listening Key.
CHAPTER XVI.
SELF-RESTORING SWITCH-BOARD DROPS, 173
Electrically Restoring Drops Mechanically Restoring Drops Com-
mercial Types.
CHAPTER XVII.
COMPLETE SWITCH-BOARDS FOR SMALL EXCHANGES, . . . .183
Arrangement of Switch-Board Parts Description and Operation of
Boards in Common Use.
CHAPTER XVIII.
LAMP-SIGNAL SWITCH-BOARDS 192
Advantages of Incandescent Lamps for Signals Lamp Directly in Line
Circuit Lamp in Local Circuit Controlled by Relay Pilot Lamp-
Life of Lamp Signals.
Vlll TABLE OF CONTENTS.
CHAPTER XIX.
PAGE
THE MULTIPLE SWITCH-BOARD, 200
Limitations of the Simple Switch-Board General Arrangement of
Apparatus in the Multiple Board Tests for Busy Lines The Series
Multiple Board: Its Disadvantages The Branch Terminal Multi-
ple Board Spring-Jacks for Multiple Boards The Kellogg Divided
Multiple Board.
CHAPTER XX.
TRANSFER SYSTEMS, ...... ..... 216
Complexity of Multiple Switch-Boards The Sabin and Hampton
Express System Incoming and Outgoing Trunk Lines Details of
Subscribers' Circuits Details of Trunking Circuits The Clearing-
out Signals Western Telephone Construction Company's Transfer
System The Cook-Beach Transfer System The American Trans-
fer System.
CHAPTER XXI.
COMMON BATTERY SYSTEMS 236
Advantages Series Systems for Grounded and Metallic Circuits The
Stone System The Hayes Repeating-Coil System Supplying Cur-
rent over Two Line Wires in Multiple Storage Battery at Sub-
scriber's Station. Dean's Thermopile System Carty's Multiple-
Transmitter Circuit Series-Transmitter Circuit Detailed Descrip-
tion of Dean's Common Battery System Of Scribner's Common
Battery System Hayes' Systems as Applied to Multiple Board
Scribner's System as Applied to Multiple Board The St. Louis
Bell Exchange.
CHAPTER XXII.
HOUSE SYSTEMS, 265
General Plan of House or Intercommunicating Systems Common Bat-
tery House Systems The Ness Automatic Switch Circuits of
Holtzer : Cabot System.
CHAPTER XXIII.
PROTECTIVE DEVICES, 27^
Static Arresters Fusible Arresters Thermal Arresters Heat Coils
Combined Thermal and Static Arresters The Rolfe Arrester.
CHAPTER XXIV.
DISTRIBUTING BOARDS, 281
Functions of the Distributing Board The Hibbard Board The Ford
and Lenfest Board Jumper Wires.
TABLE OF CONTENTS. ix
CHAPTER XXV.
p
- 294
PAGE
PARTY LINES NON-SELECTIVE.
Classification The Series Party Line Generators and Ringers for
Party Lines Carty's Bridged Party Line Generators and Ringers
for the Bridging System Induction Coils for Bridged Lines Con-
nection of Switch-Board Drops to Party Lines Lock-out Systems.
CHAPTER XXVI.
PARTY LINES STEP-BY-STEP SELECTIVE SIGNALING, .... 308
General Operation Early Systems of Dickerson, Anders, and Lock-
wood Bridging System of Reid and McDonald.
CHAPTER XXVII.
PARTY LINES SELECTIVE SIGNALING BY STRENGTH AND POLARITY, . 318
Success of this Method in Telegraphy Early work of Anders Sabin
and Hampton's Three-Station Line Hibbard's Four-Station Line
McBerty's Four-Station Line Dean's Eight-Station Line The
Barrett-Whittemore-Craft System.
CHAPTER XXVIII.
PARTY LINES HARMONIC SYSTEMS OF SELECTIVE SIGNALING, . . . 338
Principles Involved Early Experiments Lighthipe Bridged System
Harter System Unsatisfactory Development of Harmonic Systems.
CHAPTER XXIX.
WIRE FOR TELEPHONE USE, ......... 347
Iron, Copper, and Aluminum for Conductors Tensile Strength Con-
ductivity Resistance Weight per Mile-Ohm Circular Wire
Gauge Micrometer Gauge The Brown and Sharpe Gauge Other
Wire Gauges Manufacture of Iron Wire Galvanizing Tests for
Galvanizing Grades of Iron Wire Specifications for Iron Wire
Advantages of Copper for Wire Specifications and Data for Copper
Wire.
CHAPTER XXX.
POLE-LINE CONSTRUCTION, 360
Woods for Poles Treatment of Poles Sizes of Poles Number of Poles
per Mile Height of Poles Pole Guards Data Cpncerning Poles
Creosoting and Vulcanizing Poles Cross-Arms Attaching Cross-
Arm to Pole Insulators Construction Tools Setting Poles Guy-
ing Terminal Poles Anchors Pole Braces Methods of Tying
and Splicing Transpositions.
X TABLE OF CONTENTS.
CHAPTER XXXI.
PAGE
OVERHEAD CABLE CONSTRUCTION, 389
Necessity for Cables Comparative Cost of Cable and Bare-Wire Con-
struction Rubber Cables Objections to Rubber Cables Saturated
Core Paper Cables Dry-Core Paper Cables Lead Sheaths for
Cables Data Concerning Cables Supporting Strand for Cables
Cable Hangers Method of Suspending Cables Directions for
Splicing Cable Terminals Directions for Making Pot-Head
Terminals.
CHAPTER XXXII.
UNDERGROUND CABLE CONSTRUCTION, 409
Requirements for Conduits Economy of Space Wooden Conduits
Clay Conduits Cement-Lined Pipe Cement Arch Conduit
Methods of Laying Conduits Man-Holes Rodding Drawing in of
Cables Steam Power for Drawing in Gas in Man-Holes Elec-
trolysis Determination of Danger Points Prevention of Elec-
trolysis.
CHAPTER XXXIII.
TESTING, 424
Classification of Tests Rough Tests Magneto Testing Set Tests for
Grounds and Crosses with Magneto Receiver Test for Grounds and
Cosses Continuity Test Quantitative Measurements Measure-
ment of Resistance The Wheatstone Bridge Directions for Operat-
ing Bridge The Thomson Galvanometer The D'Arsonval Gal-
vanometerAdvantages of the D'Arsonval Galvanometer The Gal-
vanometer Shunt Taking of Galvanometer Constant Insulation
Tests Capacity Tests Location of Faults The Varley Loop Test.
AMERICAN TELEPHONE PRACTICE.
CHAPTER I.
HISTORY AND PRINCIPLES OF THE MAGNETO TELEPHONE.
THE history of the telephone, from its inception to its present
state of perfection, is interesting in the extreme, and affords a
striking example of the fact that great inventions are almost in-
variably the result of long and careful study on the part of many
workers, rather than the sudden inspiration of a single genius,
It is of even greater interest from a scientific standpoint, for in
no way can one obtain a better idea of the fundamental princi-
ples involved in telephony than by following their development,
step by step, noting the contributions made by each of the many
scientists and inventors whose names are closely connected with
electrical progress.
These steps were made in logical order, the knowledge con-
tributed by each investigator making possible a deeper insight
into the subject on the part of his successors. It is best, there-
fore, to follow this order in obtaining primary ideas of the
subject.
The history of the knowledge of electromagnetism begins with
July 20, 1820, and with this date very properly begins the history
of the electric telephone. On that day Oersted, a professor in
the University of Copenhagen, discovered that a magnetic needle
tends to place itself at right angles to a wire carrying a current
of electricity. Ampere immediately took up the subject, and in
a very short time developed the laws upon which present electro-
magnetic theory is based.
In the following year Arago and Davy discovered that if a
current be caused to flow through an insulated wire wrapped
about a rod of steel the latter would exhibit magnetic properties.
It was William Sturgeon, however, who in 1825 made an electro-
magnet as we know it to-day, and called it by that name. To
these three men, therefore, belongs the credit of one of the greatest
discoveries in the history of science. Joseph Henry also made
2 AMERICAN TELEPHONE PRACTICE.
his classic experiments on the electromagnet, and to him must
be accredited a large amount of our knowledge regarding it.
Henry showed how to build a magnet capable of being operated
over a great length of wire, a most important step.
In 1831 Faraday and Henry, independently, discovered the con-
verse of these laws of electromagnetism that if the intensity of
a magnetic field inclosed by a conductor be in any wise
changed, a current of electricity will flow in said conductor.
This current will flow only while such change is taking place,
and its strength will depend directly on the rate of the change.
These two laws concerning the transformation of electric
energy into magnetic, and its converse, the transformation of
magnetic energy into electric, are certainly the most important
in the whole realm of electrical science ; as singly or together
they form the foundations not only of the telephone and tele-
graph, but of electric lighting, electric power transmission, and
B
Fig. i. Sturgeon-Electromagnet.
of every other achievement by which electricity has revolution-
ized the methods of life throughout the whole civilized world.
As these laws form the very root of all telephone practice, a
few illustrations directly in line with the principles of the tele-
phone will not be amiss, even though they are very generally
understood ; for they will give a clearer understanding of the
developments made by subsequent inventors. If, as shown in
Fig. i, a coil of wire be wrapped around a rod, J?, of iron or
steel, and a battery, B, placed in circuit with the coil, the rod
becomes a magnet upon the closure of this circuit, and will
attract an iron armature, A, in the vicinity of either of its poles.
Any variation in the strength of this current will cause corre-
sponding variations in the attractive power of the magnet. If
the rod be of steel, and permanently magnetized, it will exert an
attractive force of its own on the armature, and the current will,
according to its direction, increase or diminish this attractive
force.
About every magnet there exists a field of force ; that is, a
region in which any body capable of being magnetized (such as
7 '//A' MAGNETO TELEPHONE. 3
iron) has exerted on it, by the magnet, an influence of attraction
or repulsion. This field of force is usually graphically repre-
sented by closed curves, radiating from the poles of the magnet,
and the strength of the magnet is commonly measured in terms
of the number of such lines radiating from one of its poles. A
magnet may be made to map out its own field of force by plac-
ing it in a horizontal position and directly over it a sheet of
Pig. 2. Lines of Force of Bar-Magnet.
paper or cardboard. If iron filings are then dropped from a
height of a few feet, on the paper, they will arrange themselves
in the direction of the lines of force. Fig. 2 shows such a map
produced by the bar-magnet, N S.
If now a galvanometer, G, or other current-indicator (Fig. 3)
be placed in circuit with a coil, C, and a magnet, N S, moved in
Fig. 3. Faraday and Henry Magneto-Electricity.
the vicinity of the coil, or the coil in the vicinity of the magnet,
in such manner as to change the number of lines of force passing
through the coil, a current is generated in the coil and is indi-
cated by the galvanometer. This current will flow only while
4 AMERICAN TELEPHONE PRACTICE.
the magnet is being so moved. Its direction will depend on the
direction of the lines of force threading the coil and on whether
their number is being increased or diminished. Its strength will
depend on the rate at which their number is changing.
If a mass of iron be brought within the field of a magnet, the
field becomes distorted by virtue of a larger number of lines find-
ing their path through the space occupied by the iron than
through the same space when filled with air. Therefore, if
a closed coil be placed about a pole of the magnet and a body of
iron be moved to and from the pole, the intensity of the field in
which the coil lies will vary, and currents of electricity will flow
in the coil.
In 1837 Professor Page of Salem, Mass., discovered that a rod
of iron, suddenly magnetized or demagnetized, would emit certain
P K-
f
i .
Fig. 4. Morse Electromagnetic Telegraph.
sounds due to a molecular rearrangement caused by the changing
magnetic conditions. This phenomenon is known as " Page's
effect."
Late in the thirties Professor S. F. B. Morse placed at one end
of a line Sturgeon's electromagnet, M (Fig. 4), with a pivoted
armature, A, and at the other end a battery, B, and a key, K, for
making and breaking the circuit. By manually closing and open-
ing the key, the core of the magnet became magnetized and de-
magnetized, thus alternately attracting and releasing the arma-
ture. By this means signals were sent and recorded on a strip of
paper, carried on a roller, R, in front of the armature, and thus
intelligence was practically conveyed by electrical means be-
tween distant points.
In 1854 a Frenchman, Charles Bourseul, predicted the trans-
mission of speech, and outlined a method correct save in one
particular, but for which error one following his directions could
have produced a telephone of greater efficiency than that sub-
sequently devised by Bell. His words at this date seem almost
prophetic: " Suppose a man speaks near a movable disk suffi-
THE MAGNETO TELEPHONE. 5
ciently flexible to lose none of the vibrations of the voice, and
that this disk alternately makes and breaks the current from a
battery ; you may have at a distance another disk which will sim-
ultaneously execute the same vibrations."
Philip Reis, a German inventor, constructed a telephone in
1861, following very closely the path outlined by Bourseul. He
mounted a flexible diaphragm, D (Fig. 5) over an opening in a
Fig. 5. Reis' Make-and-Break Telephone.
wooden box, and on the center of the diaphragm fastened a small
piece of platinum, P. Near this he mounted a heavy brass spring,
s, with which the platinum alternately made arid broke contact
when the diaphragm was caused to vibrate. These contact points
formed the terminals of a circuit containing a battery, B, and
the receiving instrument. His receiver assumed various forms,
prominent among which was a knitting needle, N t wrapped with
Figs. 6. and 7. Reis' Telephone Transmitter and Receiver.
silk-insulated copper wire and mounted on a cigar box for a
sounding board. Its operation was as follows: The sound waves
set up by the voice struck against the diaphragm of the trans-
mitter, causing it to vibrate in unison with them. This made and
broke the circuit at the contact points, and allowed intermit-
tent currents to flow through the receiver. The currents,
6 AMERICAN TELEPHONE PRACTICE.
which exactly synchronized with the sound waves, caused
a series of sounds in the knitting needle by virtue of " Page's
effect." The sounding board vibrated in unison with the mo-
lecular vibrations of the needle, and the sound was thus greatly
amplified. Reis' transmitter and one form of his receiver are
shown in Figs. 6 and 7 respectively.
Reis' telephone could be depended upon to transmit only
musical sounds, but it is probable that it did actually transmit
articulate speech. The cause of this partial failure will be under-
stood from the following facts :
A simple musical tone is caused by vibrations of very simple
form, while sound waves produced by the voice are very complex
in their nature. These two forms of waves are shown graphically
in Fig. 8.
Sound possesses three qualities: pitch, depending entirely on
the frequency of the vibrations; loudness, depending on the
Fig. 8. Sound Waves of Voice and Simple Musical Note.
amplitude of the vibrations, and timbre or quality, depending on
the form of the vibration. The tones of a flute and a violin may
be the same as to pitch and loudness and yet be radically different.
This difference is in timbre or quality.
Reis' transmitter, as he adjusted it, was able only to make
and break the circuit, and a movement of the diaphragm barely
sufficient to break the circuit produced the same effect as a much
greater movement. The current therefore flowed with full
strength until the circuit was broken, when it stopped entirely.
The intermediate strengths needed for reproducing the delicate
modulations of the voice were entirely wanting. This apparatus
could therefore exactly reproduce the pitch of a sound, but not
its timbre and relative loudness.
For the next fifteen years no great advance was made in the
art of telephony, although many inventors gave it their careful
attention.
In 1876 Professor Alexander Graham Bell and Professor
Elisha Gray almost simultaneously invented successful speaking
THE MAGNETO TELEPHONE. 7
telephones. Bell has, however, apparently reaped the profit, the
U. S. Patent Office having awarded priority of invention to him.
Bell possessed a greater knowledge of acoustics than of electri-
cal science, and it was probably this that led him to appreciate
wherein others had failed. His instrument consisted of a per-
manent bar-magnet, B (Fig. 9), having on one end a coil of fine
wire. In front of the pole carrying the coil a thin diaphragm,
D, of soft iron was so mounted as to allow its free vibration close
Fig. 9. Bell-Magneto-Telephone.
to the pole. Two of the instruments are shown connected in a
circuit in Fig. 9.
Two points will be noticed which have heretofore been absent :
that no battery is used in the circuit, and that the transmitting
and receiving instruments are exactly alike. When the soft-iron
diaphragm of the transmitting instrument is spoken to, it vibrates
in exact accordance with the sound waves striking against it.
The movement of the diaphragm causes changes in the magnetic
field in which lies the coil, which changes, as shown above, cause
an alternating current to flow in the circuit. This current varies
in unison with the movements of the diaphragm. The waves of
this current are very complex, and represented graphically are
similar to those of the voice shown in Fig. 8. Passing along the
line wire, these electrical impulses, so feeble that only the most
delicate instruments can detect them, alternately increase and de-
Figs. 10. and ii. Bell's Early Receiver and Transmitter.
crease the strength of the permanent magnet of the receiving in-
strument, and thereby cause it to exert a varying pull on its soft-
iron diaphragm, which, as a result, takes up the vibrations and
reproduces the sound faithfully. Bell's earlier instruments, ex-
hibited in 1876 at the Centennial in Philadelphia, are shown
in Figs. 10 and n, the former being his receiver, the latter his
8 AMERICAN TELEPHONE PRACTICE.
transmitter. The receiver consisted of a tubular magnet com-
posed of a coil of wire surrounding a core, and inclosed in an
iron tube. This tube was closed by a thin iron armature
or diaphragm as shown. The transmitter consisted of an
electromagnet, in front of which was adjustably mounted a
diaphragm of gold-beater's skin carrying a small iron armature in
its center.
Bell's instrument, in a modified form, is the standard of to-day.
It is now used as a receiver only, a more efficient transmitter, de-
pending upon entirely different principles, having been invented.
In speaking of Bell's invention Lord Kelvin has said : " Who
can but admire the hardihood of invention which devised such
very slight means to realize the mathematical conception that if
Fig. 12. Royal E. House's Electro-Phonetic Telegraph.
electricity is to convey all the delicacies of quality which dis-
tinguish articulate speech, the strength of its current must vary
continuously as nearly as may be in simple proportion to the
velocity of a particle of air engaged in constituting the sound ?"
A very interesting fact, and one which might have changed
the entire commercial status of the telephone industry is that in
1868 Royal E. House of Binghamton, N. Y., invented and pat-
ented an " electro-phonetic telegraph," which was capable of
operating as a magneto-telephone, in the same manner as the
instruments subsequently devised by Bell. House knew nothing
of its capabilities, however, unfortunately for him. The in-
strument- is shown in Fig. 12, and is provided with a sound-
THE MAGNETO TELEPHONE. 9
ing diaphragm of pine wood stiffened with varnish, mounted in
one end of a large sound-amplifying chamber so formed as to
focus the sound waves at a point near its mouth, where the ear
was to be placed to receive them. The electromagnet adapted
to be connected in the line circuit had its armature connected by
a rod with the center of the wooden diaphragm as shown. By
this means any movements irriparted to the armature by fluctua-
ting currents in the line were transmitted to the diaphragm,
causing it to give out corresponding sounds ; and any movements
imparted to the diaphragm by sound waves were transmitted to
the armature, causing it to induce corresponding currents in the
line. Two of these instruments connected in a circuit as shown
in Fig. 9 would act alternately as transmitters and receivers in
the same manner as Bell's instruments.
CHAPTER II.
HISTORY AND PRINCIPLES OF THE BATTERY TRANSMITTER.
IT has been shown that in order to transmit speech by electric-
ity it is necessary to cause an undulatory or alternating current
to flow in the circuit over which the transmission is to be effected,
and that the strength of this current must at all times be in exact
accordance with the vibratory movements of the body producing
the sound.
Bell's transmitter was used as the generator of this current ; as
a dynamo, in fact, the energy for driving which was derived from
the sound waves set up by the voice. The amount of energy so
derived was, however, necessarily very small and the current
correspondingly weak, and for this reason this was not a practi-
cal form of transmitter, except for comparatively short lines.
Elisha Gray devised a transmitter which, instead of generating
the undulatory current itself, simply served to cause variation in
the strength of a current generated by some separate source.
B
Fig. 13. Gray's Variable Resistance Transmitter.
He accomplished this by mounting on his vibrating diaphragm,
D (Fig. 13), a platinum needle, n, the point of which was im-
mersed in a fluid of rather low conductivity, such as water. The
variable distance to which the needle was immersed in the fluid,
due to the vibration of the diaphragm, caused changes in the re-
sistance of the path through the fluid, and corresponding changes
in the strength of the current set up in the circuit by the bat-
tery, B. Instead of making and breaking the circuit, as did the
transmitter of Reis, this instrument simply caused variations in
the resistance of the circuit, and thereby allowed a continuous
but undulatory current to pass over the line. The variations in
this current conformed exactly with the sound waves acting upon
the diaphragm, and were, therefore, capable of reproducing all
THE BATTERY TRANSMITTER.
II
the delicate shades of timbre, loudness, and pitch necessary in
articulate speech.
Gray embodied in this apparatus the main principle upon
which all successful battery transmitters are based, but it was
not long before a much better means was devised for putting
it into practice.
In 1877 Emile Berliner of Washington, D. C., applied for a
patent on a transmitter depending upon a principle previously
pointed out by the French scientist, Du Moncel, that if the pres-
sure between two conducting bodies forming part of an electric
circuit be increased, the resistance of the path between them will
be diminished, and conversely, if the pressure between them be
decreased, a corresponding increase of resistance will result.
Berliner's transmitter is shown in Fig. 14, which is a reproduc-
tion of the principal figure in his now famous patent and in which
Fig. 14. Berliner's Transmitter.
A is the vibratory diaphragm of metal, against the center of which
rests the metal ball, C, carried on a thumb-screw, B, which is
mounted in the standard, d. The pressure of the ball, C, against
the plate, A, can be regulated by turning the thumb-screw. The
diaphragm and ball form the terminals or electrodes of a circuit,
including a battery and receiving instrument. Figs. 15 and 16
show two different views of an exact duplicate of Berliner's original
model as filed in the patent office. This was very roughly con-
structed as shown. The diaphragm was a circular piece of ordi-
nary tin and the contact-piece a common blued-iron wood screw.
12
AMERICAN TELEPHONE PRACTICE.
Figs. 15. and 16. Berliner's Patent-Office Model.
THE BATTERY TRANSMITTER. 13
The action of this instrument is as follows: when the dia-
phragm is vibrating, the pressure at the point of contact, a, be-
comes greater or less, thus varying the resistance of the contact
and causing corresponding undulations in the current flowing.
Soon after this Edison devised an instrument using carbon as
the medium for varying the resistance of the circuit with changes
of pressure. EdisonXfirst iype of carbon transmitter consisted
simply of a button of compressed plumbago bearing against a
small platinum disk secured to the diaphragm. The plumbago
button was held against the diaphragm by a spring, the tension
of which could be adjusted by a thumb-screw.
A form of Edison's transmitter, devised by George M. Phelps
in 1878, is shown in Fig. 17. The transmitting device proper is
Fig. 17. Phelps-Edison Transmitter.
shown in the small cut at the right of this figure, and is in-
closed in a cup-shaped case formed of the two pieces, A and B,
as shown. Secured to the front of the enlarged head, e, of the
adjustment screw, E, is a thin platinum disk, F t against which
rests a cylindrical button, G, of compressed lampblack. A plate
of glass, /, carrying a hemispherical button, K, has attached
to its rear face another platinum disk, H. This second platinum
disk rests against the front face of the lampblack disk, G, and the
button, K, presses firmly against the center of the diaphragm,
D. The plates, F and //, form the terminals of the transmitter,
and as the diaphragm, Z>, vibrates, it causes variations in the
pressure, and corresponding changes in the resistance of the cir-
cuit, thus producing the desired undulations of current.
Professor David B. Hughes made a most valuable contribution
tending toward the perfection of the battery transmitter. By a
series of interesting experiments, he demonstrated conclusively
that a loose contact between the electrodes no matter of what
substance they are composed, is far preferable to a firm, strong
contact. The apparatus used in one of his earlier experiments,
AMERICAN TELEPHONE PRACTICE.
made in 1878, is shown in Fig. 18, and consists simply of three
wire nails, of which A and B form the terminals of the circuit
containing a battery and a receiving instrument. The circuit was
completed by a third nail, C, which was laid loosely across the
other two. Any vibrations in the air in the vicinity caused vari-
ations in the intimacy of contact between the nails, and corre-
Fig. 1 8. Hughes' Nail Microphone.
spending variations in the resistance of the circuit. This was a
very inefficient form of transmitter, but it demonstrated the prin-
ciple of loose contact very cleverly.
It was found that carbon was, for various reasons, by far
the most desirable substance for electrodes in the loose-contact
transmitter, and nothing has ever been found to even approach
it in efficiency.
Another form of transmitter devised by Hughes, and called by
him the microphone, is shown in Fig. 19. This consists of a
JUULfiJM__
Fig. 19. Hughes' Carbon Microphone.
small pencil of gas carbon, A, pointed at each end, and two
blocks, B B, of carbon fastened to a diaphragm or sounding
board, C. These blocks are hollowed out, as shown, in such a
manner as to loosely hold between them the pencil, A. The
blocks, B B, form the terminals of the circuit. This instrument,
though crude/in form, is of marvelous delicacy and is well termed
microphone. The slightest noises in its vicinity, and even those
incapable of being heard by the ear alone, produce surprising
THE BATTERY TRANSMITTER.
'5
effects in the receiving instrument. This particular form of
instrument is, in fact, too delicate for ordinary use, as any jar or
loud noise will cause the electrodes to break contact and produce
deafening noises in the receiver. Nearly all carbon transmitters
of to-day are of the loose- contact type, this having entirely su-
perseded the first form devised by Edison, which was then sup-
posed to depend on the actual 1 resistance of a carbon block being
changed under varying pressure.
Only one radical improvement now remains to be recorded. In
1 88 1 Henry Runnings devised a transmitter wherein the variable
resistance medium consisted of a mass of finely divided carbon
granules held between two conducting plates. His transmitter
is shown in Fig. 20. Between the metal diaphragm, A, and a
parallel conducting plate, B, both of which are securely mounted
in a case formed by the block, D, and a
mouthpiece, F y is a chamber filled with
fine granules of carbon, C. The dia-
phragm, A, and the plate, B, form the
terminals of the transmitter, and the cur-
rent from the battery must therefore flow
through the mass of granular carbon, C.
When the diaphragm is caused to vibrate
by sound waves, it is brought into more
or less intimate contact with the carbon
granules and causes a varying pressure
between them. The resistance offered by
them to the current is thus varied, and
the desired undulations in the current pro-
duced. This transmitter, instead of hav-
ing one or more points of variable contact,
is seen to have a multitude of them. It
can carry a larger current without heating, and at the same time
produce greater changes in its resistance, than the forms previously
devised, and no sound can cause a total break between the elec-
trodes. These and other advantages have caused this type in
one form or another to largely displace all others. Especially is
this true on lines of great length.
Up to this time all transmitters, together with the receiver and
battery, had been put directly in circuit with the line wire.
With this arrangement the changes produced in the resistance
by the transmitter were so small in comparison with the total
resistance of the circuit, that the changes in current were also
very small, and produced but little effect on the receiver. Edi-
1 6 AMERICAN TELEPHONE PRACTICE.
son remedied this difficulty by using an induction coil in connec-
tion with the transmitter. The credit of this improvement,
however, should be given largely to Gray, for in 1875 he had
used an induction coil in connection with his harmonic telegraph
transmitter, and Edison merely substituted a telephone transmit-
ter in the circuits used by Gray.
The induction coil used then and now is made as follows:
Around a core formed of a bundle of soft-iron wires is wound a
few turns of comparatively heavy insulated copper wire. Out-
side of this, and entirely separate from it, is wound another coil,
consisting of a great number of turns of fine wire, also of copper
and insulated. The inner coil is called the primary, the other
the secondary. In telephone work it is now almost universal
practice to place the transmitter, together with the battery, in a
closed circuit with the primary of the induction coil, and to
place the secondary directly in circuit with the line wire and
receiving instrument. This is shown in Fig. 21, in which T
Fig. 21. Transmitter with Induction Coil.
is a transmitter, B a battery, P and S the primary and secondary,
respectively, of an induction coil, L L the line wires, and R the
receiving instrument. It is well to state here that the usual way
of indicating the primary and secondary of an induction coil, in
diagraphic representation of electrical circuits, is by an arrange-
ment of two adjacent zigzag lines, as shown in Fig. 20. A cur-
rent flowing in the primary winding of the induction coil pro-
duces a field of force in the surrounding space, and any changes
caused by the transmitter in the strength of the current produce
changes in the intensity of this field. As the secondary winding
lies in this field, these changes will, by the laws of Faraday and
Henry, cause currents to flow in the secondary winding and
through the line wire to the receiving instrument. In all good
induction coils the electromotive forces set up in the secondary
coil bear nearly the same ratio to the changes in electromotive
THE BATTERY TRANSMITTER. 17
force in the primary coil, as the number of turns in the second-
ary bears to the number of turns in the primary.
The use of the induction coil with the transmitter accomplishes
two very important results : first, it enables the transmitter to
operate in a circuit of very low resistance, so that the changes in
the resistance produced by the transmitter bear a very large ratio
to the total resistance of the circuit. This advantage is well
illustrated by contrasting the two following cases :
Suppose a transmitter capable of producing a change of resist-
ance of one ohm be placed directly in a line circuit whose total
resistance is 1000 ohms ; a change in the resistance of the trans-
mitter of one ohm will then change the total resistance of the
circuit one one-thousandth of its value, and the resulting change
in current flowing will be but one one-thousandth of its value.
On the other hand, suppose the same transmitter to be placed in
a local circuit as above described, the total resistance of which
circuit is five ohms ; the change of one ohm in the transmitter
will now produce a change of resistance of one-fifth of the total
resistance of the circuit and cause a change of one-fifth of the
total current flowing. It is thus seen that fluctuations in the
current can be produced by a transmitter with the aid of an
induction coil which are many times greater than those produced
by the same transmitter without the coil.
The second advantage is that by virtue of the small number
of turns in the primary winding and the large number in the
secondary winding of the induction coil, the currents generated
in the secondary are of a very high voltage as compared with
those in the primary, thus enabling transmission to be effected
over much greater length of line and over vastly higher resist-
ances than was formerly the case.
CHAPTER III.
THE TELEPHONE RECEIVER.
To construct a receiver capable of reproducing speech is a very
simple matter. In fact, nearly any electromagnet, with a com-
paratively light iron armature, such as is commonly used in
electric bells and telegraph instruments, may be made to repro-
duce, with more or less distinctness, sounds uttered in the
vicinity of a transmitting apparatus with which it is in circuit. It
has proved more difficult, however, to construct a receiving
instrument which will reproduce speech well, and at the same
time be practically successful in everyday use.
The bar-magnet with a thin iron diaphragm in close proximity
to one of its poles, used in the early experiments in telephony,
has until recently been very generally adhered to throughout
this country. The instrument has been made much more sensi-
tive than were the early forms, but this result has been accom-
plished by better mechanical and electrical designs, and the use
of better materials, and not by any departure from the original
principles of its action.
Aside from actual talking efficiency, many considerations of a
purely mechanical nature enter into the design of a good tele-
phone receiver. It should be durable and capable of with-
standing the rough usage to which it will necessarily be subjected
by careless or ignorant users. It should be of such construction
that its adjustment will not be changed by mechanical shocks or
by changes in temperature. Failure to provide against this
latter effect is one of the chief sources of trouble in telephone
work. It should be of such external configuration as to enable
it to be conveniently placed to the ear. The chamber in which
the diaphragm vibrates should be small and of such shape as not
to muffle the sound. The binding posts should be so securely
fastened in as to prevent their becoming loose and twisting off
the wires inside the receiver shell ; and the construction should
be so simple as to render the replacing of any damaged part an
easy matter.
By far the greater number of receivers used in America are of
the single-pole type ; although in a few years this statement will
THE TELEPHONE RECEIVER.
probably not be true. The particular form shown in Fig. 22
has proved efficient, and is now largely used by the American
Bell Telephone Company. Its chief merit lies in its simplicity.
In Fig. 22, M is a compound bar-magnet, composed of two
pairs of separately magnetized steel bars arranged with like poles
together. Between the pairs pf bars is clamped a soft-iron pole-
piece, P, at one end, and>a similarly shaped iron block, <2, at the
other end. These parts are firmly bound together by the two
screws, 5 5. On the end of the pole-piece is slipped a coil of
wire, G. This coil is usually wound
with two parallel No. 38 B. & S.
silk-insulated copper wires, and has
a total resistance of about 75 ohms.
The magnet is incased in a shell
of hard rubber, composed of two
pieces, A and B, which screw to-
gether and clamp between them the
diaphragm, D, of thin sheet iron.
The piece, B, is hollowed out as
shown, to form a convenient ear-
piece. A tailpiece, T, carrying two
binding posts, J J, fits over the end
of the case opposite the earpiece, B,
and is held in place by a screw, E.
This screw engages a threaded hole
in the block, Q, and serves not only
to hold the tail-piece in place, but
to bind the magnet securely to the
shell. Soldered to the binding posts
are heavy leading-in wires, W W,
which pass along the sides of the
magnet and are soldered to the re-
spective terminals of the fine wire
forming the coil.
The diaphragm of this instru-
ment is about Yjy in thickness and 2j" in diameter. The diam-
eter of the free portion is if".
In some single-pole receivers the old style of magnet, consist-
ing of a single cylindrical bar of steel, is still used instead of the
compound magnet formed of several separately magnetized bars,
but with generally inferior results, owing to its weaker and less
permanent magnetic field.
In bipolar receivers, which are now coming into general use,
Fig. 22. Bell Single-Pole
Receiver.
20
AMERICAN TELEPHONE PRACTICE.
the object is to strengthen the field in which the diaphragm vi-
brates, by presenting both magnet poles to the diaphragm. The
length of the path of the lines of force through the air is thus
greatly shortened, and the field of force is concentrated at the
point where it will be most effective.
One form of bipolar receiver is shown in Fig. 23, which illustrates
the receiver manufactured until recent date by one of the large
independent companies. The shell, A, and ear-piece, B, are of a
Fig. 23. Bipolar Receiver.
material resembling hard rubber, and clamp between them the
soft-iron diaphragm, D, as in the instrument described above.
The magnet consists of two pairs of separately magnetized steel
bars, F F and F'F', the separate bars in each pair being laid with
like poles together, so that each pair forms in itself a compound
bar-magnet. These two compound bar-magnets are so laid
together that the north pole of one is opposite the south pole of
the other. The two pairs of bars are held apart at one end by
the adjustment block, H, made of the same material as the shell,
THE TELEPHONE RECEIVER. 21
and at the other end by the soft-iron block, 7. On each side of
the block, H, and between it and the pairs of bar-magnets, are
the soft-iron pole-pieces, P P, on which are wound the coils G G,
having a resistance of 50 ohms each. These coils are wound
with No. 36 B. & S. silk-insulated wire and are connected in
series, so that the total resistance of the receiver is 100 ohms.
The block, H, has two segmental flanges projecting out beyond
the sides of the magnet bars. These flanges are screw-threaded
on their circumferential surfaces so as to engage a thread, g, on
the inner surface of the shell, A. The magnet may thus be
adjusted toward or from the diaphragm by turning it in the
shell, A.
A tail-piece, T, of hard rubber is so shouldered as to fit into the
small end of the receiver shell, and is prevented from turning in
its place by small lugs fitting into notches in the shell. A screw,
E, extends through the tail-piece and clamps the magnet into any
position to which it has been adjusted. To the binding posts,
JJ, are soldered heavy leading-in wires, W W, which pass through
holes in the adjustment block, H, and are soldered to the termi-
nals of the fine magnet wire. These heavy wires, W W, are
firmly knotted after passing through thetblock, H, in order to
prevent any mechanical strain coming on the hair-like wires of
the magnet coils when the tail-piece is removed. Sufficient
slack is left in the leading-in wires to allow the removal of the
tail-piece a short distance, to give access to the end of the
magnet for purposes of adjustment.
In many forms of receiving instruments much trouble is
experienced in keeping permanent the adjustment between the
magnet and the diaphragm. This is due to the fact that steel
and hard rubber differ widely as to their amounts of expansion
or contraction under changes in temperature. In instruments
where the magnet is rigidly secured to the shell only at a point
at considerable distance from the diaphragm, the unequal expan-
sion or contraction of the magnet and the shell causes the dis-
tance between the pole-piece and the diaphragm to vary with
every change in temperature. A sudden change will thus often
render a receiver inoperative.
This defect is seen to exist without any attempt at a remedy
in the single-pole receiver shown in Fig. 22. The point of
support of the magnet is as far removed from the diaphragm as.
possible, being at the screw, E, and therefore the full benefit
(which is of course negative) of all the differences in contraction
and expansion between the hard rubber and the steel is obtained.
22 AMERICAN TELEPHONE PRACTICE.
In the receiver shown in Fig. 23 an attempt was made to
remedy this defect by securing the magnet to the shell at a point
close to the diaphragm, so that the differences in expansion and
contraction between the shell and magnet will be reduced to a
minimum. This, however, in this particular case introduced a
defect quite as serious, because the shell was also bound to the
magnet by the screw, E. The contraction and expansion thus
tended to loosen the screw-thread on the block, H, making fre-
quent readjustment necessary. Moreover, a good screw-driver in
the hands of an ordinary repair man or of a subscriber often sub-
jects the screw-thread on block, //, to such a strain as to strip the
thread, thus rendering the receiver useless.
Several important lessons may be and have been learned from
the behavior of these two forms of receiver in actual and long-
continued service :
First : It is poor construction to secure the magnet in the
shell at the end farthest from the diaphragm.
Second : It is also poor construction to secure it rigidly near
the diaphragm and also at the opposite end.
Third : It is extremely poor construction to use any of the
materials imitating hard rubber in vital portions of the instru-
ment. These materials, so far produced, are without exception
subject to some or all of the following faults to a greater extent
than hard rubber, viz.: They are not sufficiently tough, and are
usually very brittle. They absorb moisture. They soften when
exposed to heat, and gradually give way under pressure, causing
them to retain a permanent set when again cooled. They are
capable of having threads molded upon them, and as a rule
these molded threads do not fit. Threads in hard rubber are
cut, and may therefore be as accurate as desired. They are liable
to have seams or " cold shuts " formed in molding which will
cause cracks and fractures; and lastly: They are not as good
insulating materials as hard rubber. Some of these materials
may not possess all of these objections, but all possess some of
them. Hard rubber therefore is, so far as materials are at present
developed, the only thing to use in the insulating portions of
receiver shells.
A way of obviating the expansion and contraction difficulty,
used largely in European countries and to an increasing extent
in this country, is to construct the shell holding the diaphragm
of some metal having nearly the same coefficient of expansion
as the c steel magnets.
Fig. 24 shows one of the early forms of bipolar receivers.
THE TELEPHONE RECEIVER. 23
This was devised in 1881 by Clement Ader of Paris, France, and
is with some modifications largely used in France and other
European countries to-day. This embodies the results of one of
the few successful attempts at increasing the electrical efficiency
of the telephone receiver. The magnet, B, is ring-shaped, and
has fastened to its poles two L-shaped pole-pieces carrying coils,
C C. The box, R, inclosing the pole-pieces and coils is of brass
and is secured to the magnet by screws, E E. It is screw-
threaded at G, so as to engage a corresponding screw-thread on
the inner surface of the cap, 5, which has a flaring portion, H,
Fig. 24. Ader Bipolar Receiver.
forming an ear-piece. The diaphragm, D, is clamped between
the pieces, R and 5, as in the American instruments described
above.
Surrounding the opening, leading from the diaphragm to the
ear-piece, is a ring, m, of soft iron, and in this ring lies the chief
point of Ader's invention. The additional mass of iron placed
near the poles of the magnet affords a more ready path for the
lines of force, and their number is thus increased. The dia-
phragm, therefore, moves in a stronger field of force, and the
power of the receiver is said to be correspondingly augmented.
Practice in this country has not, however, shown any perceptible
gain of efficiency by the use of this ring.
24 AMERICAN TELEPHONE PRACTICE.
Fig. 25 shows the form of receiver now manufactured by
the Western Telephone Construction Company of Chicago. In
this the shell is of hard rubber, composed of three pieces ; the
diaphragm being clamped between the shell and ear-piece in the
ordinary manner. The magnet is of horseshoe form and carries
a block of brass grooved on each side to partially inclose the
magnet limbs. The lower portion of this block is screw-threaded,
as shown, so as to engage the corresponding thread turned in
the receiver shell. The upper flange on the block rests on a
corresponding flange on the interior of the shell when the magnet
is screwed home. The pole-pieces are secured to the outside of
the magnets by a bolt passing entirely through the brass block,
each limb of the magnet, and each pole-piece. This bolt is pro-
vided with a nut at each end, so that either pole-piece may be
taken off without removing the other. The heads of the magnet
Fig. 25. Western Telephone Construction Co.'s Receiver.
spools are of brass pressed into position on the pole-pieces.
After being insulated, the spools so formed are wound in a
machine having a special chuck for holding the pole-pieces. The
two coils are for standard work, wound to a resistance of 50 ohms
each with No. 36 silk-insulated wire and connected in series,
thus making the total resistance of the receiver 100 ohms.
The novelty in this receiver is in the method of attaching the
receiver cord to the terminals leading from the coils. These
terminals, as shown, are composed of heavily insulated wire pass-
ing through the brass block into the coil chamber. The other
ends of these wires are twisted together and pass through a cen-
tral opening, where each is soldered to a connector held in place
against the shell by a small machine screw. The cord is provided
THE TELEPHONE RECEIVER.
with similar connectors, which may be slipped under the screw-
heads, thus completing the circuit between the cord and the
wires of the receiver. The connection of the cord is, of course,
made before the tail-cap is screwed in place. An enlargement in
the covering of the cord effectually prevents any strain ever
coming on the cord terminals when the receiver is dropped.
Another feature secured^ by this construction is that no metal
parts are exposed on the outside of the shell, thus insuring
immunity from electric shocks while handling the receiver, this
being considered very desirable by some.
This receiver, except for the method of connecting the cord,
which was designed by the writer, is almost identical with that
used by The American Bell Telephone
Co., in their long-distance work, and
also in most of their common-battery
exchanges. In the Bell receiver the
tail-cap is not provided, and instead two
flanged binding posts are screwed directly
to the hard rubber fronr; the outside.
To these binding posts the ordinary
receiver cord is attached in the usual
way, and the wires leading from the
receiver magnets pass through the shell
and are soldered to an extension of the
binding posts. These forms of receiver
are very efficient, very easily adjusted,
and subject to little or no trouble from
the source of expansion and contraction,
it being seen that the magnet is sup-
ported at a point near the diaphragm
without being bound to the shell at any
other point.
The Stromberg-Carlson receiver is a
very powerful one, and has stood the test of time well. It is
shown in Fig. 26, in which a is a casing of brass, forming a
framework upon which all other parts of the instrument are
supported. This is screw-threaded on its outer surface to
receive the internally screw-threaded cap, b, and lock-ring, b\
One unique feature of this receiver is the method of supporting
the diaphragm, which is held in place in the cap, b, by the
clamping ring b'.
Upon the cap, b, is screwed an ear-piece, &\ The lock-ring, b\ is
adapted to be screwed against the cap, b, to lock it in any adjusted
Fig. 26. Stromberg-Carl-
son Receiver.
26
AMERICAN TELEPHONE PRACTICE..
position. Upon the rear of the casing is provided a projection,
a', against the faces of which rest the soft-iron cores, c l <: 3 , which
extend through the bottom of the casing and carry upon their
ends the telephone coils, c 3 c\ The ends of the permanent
magnet, d, rest upon the cores, c l f, and a screw or bolt, e\ passes
through the ends of the magnet, the cores, and the projection, to
maintain them in position. The ends of the magnet, d, are cut
away as shown to permit the cores to be set flush with the inner
faces of the magnet.
Between the limbs of the magnet, d, is provided a block, d ', of
Fig. 27. American Electric Telephone Co.'s Receiver.
fiber upon which are mounted two binding posts, d*, the binding
posts being connected to the coils, c* c\ by heavy insulated
wires, d* d\ To the binding posts, d*, are also attached the ends
of the receiver cord. Upon the rear of the casing, a, is provided
a threaded flange upon which the insulating casing,/, is screwed,
this latter being provided with an opening at the end through
which the receiver cord passes.
The magnet, d, is mounted rigidly upon the casing, , the cas-
ing, /, being entirely independent so that it may be removed by
unscrewing. The diaphragm support or cap, b, may be raised
or lowered to adjust the diaphragm relatively to the magnet
THE TELEPHONE RECEIVER. 27
cores, c c\ the ring, b\ serving to lock the diaphragm in its ad-
justed position.
This receiver does away entirely with the troublesome effects
due to expansion or contraction. The insulating casing forms
a handle and serves as a protection to the cord terminals, but
forms no part of the working structure itself.
In Fig. 27 is shown the bipolar receiver of the American
Electric Telephone Co. The permanent magnet is formed of two
pieces which clamp between them, at the end farthest from the
diaphragm, a cast-iron block on which is mounted a hard-rubber
disk carrying the binding posts. This block, therefore, serves
the double purpose of completing the magnetic circuit between
the ends of the magnets and of a support for the binding posts
and connections. The lower ends of the magnets carry a screw-
threaded disk upon which is screwed the metal cup containing
the coils and against which the diaphragm rests. Upon, this
block are also mounted the straight pole-pieces carrying
coils similar to those shown in Fig. 23. The cup forming the
chamber for the coils is of pressed brass,, nickel-plated and screw-
threaded to engage the threaded disk carried by the magnet.
Over this entire structure is slipped the case of hard rubber, the
diaphragm being clamped between the earpiece and the brass cup.
In this receiver, adjustment is obtained by turning the cup on
the magnet, the screw-threads producing a longitudinal move-
ment of the latter in respect to the former, thus moving the
pole-pieces toward or from the diaphragm, according to the
direction of the rotation. After the desired adjustment has
been obtained, the cup may be clamped in the position desired
by two screws projecting through flanges carried by the circular
disks of the magnets and extending into the interior of the cup.
These screws, when set, engage the bottom of the cup in such
manner as to hold it from turning.
A single-pole receiver, manufactured by the Holtzer-Cabot
Electric Co., and embodying features of decided merit, is shown
in Fig. 28. In this the magnet is composed of four bars of steel
separately magnetized and clamping between them at one end
an iron block drilled and tapped to receive the screw passing
through the end of the shell. A pole-piece flanged, and screw-
threaded as shown,, is clamped between the other ends of the
magnets. The cup for inclosing the coil and carrying the
diaphragm is of brass, having a hole through its center, screw-
threaded to engage the threaded portion of the pole-piece.
When screwed in position, the flat portion of the cup abuts the
28 AMERICAN TELEPHONE PRACTICE.
flange on the "pole-piece, thus binding the two rigidly together.
The ear-piece is of hard rubber, as usual, and screws to the
brass cap, thus holding the diaphragm in position. The inclos-
ing shell of hard rubber slips over the magnet as shown, and
carries the binding posts, which are connected by heavy leading-
in wires to the receiver coil. No form of adjustment is pro-
vided for this instrument, and this, by the way, is a feature
Fig. 28. Holtzer-Cabot Receiver.
which is meeting with considerable favor and is being adopted
by several manufacturing companies. Great care is taken by
the manufacturers to adjust the instrument properly before it
leaves the factory, after which, with an instrument properly con-
structed, no need for adjustment should exist.
Still another form of receiver, and one of the non-adjustable
type, is shown in Fig. 29: this is manufactured by the Erics-
son Co. of Sweden, and is being imported into this country to
a considerable extent. This is of the bipolar type presenting
to the diaphragm two coils and two pole-pieces very similar
in shape to those shown in Fig. 23. The magnets are
secured to the metal cup by means of two screws shown in the
Fig. 29. Ericsson Receiver.
figure, each extending transversely through the case and into
the magnets. The holes in the case through which these screws
project are slotted so that a certain amount of adjustment can
be obtained if it is absolutely necessary, although the idea of the
manufacturers is to bind it so tightly that no adjustment will
ever be needed. The inclosing tube for the magnets is of brass
covered by a thin layer of insulating material, usually hard rubber,
UNIVERSITY
^CALIF L
THE TELEPHONE J&CE1 V^R. 29
but sometimes of leather. This tube is also held in position by
the screws before mentioned. A piece of hard rubber projects
between the two binding posts of the instrument as shown, the
object of this being to prevent the tips of the receiver cords
from twisting the posts in their sockets until they touch each
other, thus short-circuiting the instrument. This same feature
will be noticed in the t receiver shown in Fig. 27. This
receiver is extremely well made, very handsome in appearance
and very efficient, and probably would have come into very large
use were it not for its high cost.
In the receivers shown in Figs. 26, 27, 28, and 29 the evil
effects due to contraction and expansion of the various parts are
avoided by the use of metal cups for securing the diaphragm.
This method, as before stated, is coming into increasing favor in
this country, although it had long been used in Europe. The
first receiver built with a metal cup which came into anything
like extensive use in this country, was designed by Messrs.
Stromberg & Carlson, and is of substantially the same form as
that shown in Fig. 26. The advocates of the hard-rubber shell
claim that the exposed metal portion of the cup is a source of
great danger in lightning storms, or in case the line has become
crossed with some high-potential wire. This idea has been
carried to its extreme in Fig. 25, where not even the binding posts
are exposed. When it is remembered, however, that many other
parts of a telephone, such as the line binding posts, generator
crank, magneto-gongs, transmitter, and transmitter arm, have
exposed metal surfaces some of which are directly in connection
with the line, it is somewhat doubtful whether this objection is a
very valid one.
The diaphragms used for receivers are made of very soft thin
sheet iron ; the ferrotype plate formerly used for tin-types in
photography being as good material as can be found for this pur-
pose. Some companies, however, notably the Ericsson, the
Stromberg & Carlson, and the American, are using tinned dia-
phragms, which give equally good results.
The diaphragms for the various receivers here described vary
from 2 to 2 T 5 inches in diameter, the free portions that is, the
portion not clamped by the supports ranging from if to 2^-
inches. The usual thickness is from .009 to .01 1 of an inch. The
thickness of a diaphragm, to produce the best results with the given
receiver, must be obtained by experiment, as it depends on the
diameter of the portion free to vibrate, and also on the strength
of the magnetic field due to the permanent magnet. It has been
30 AMERICAN TELEPHONE PRACTICE.
shown that with a very thin diaphragm and a very powerful magnet
the iron in the diaphragm becomes saturated so that it is not
responsive to changes in the strength of the existing field. Of
course, the thicker the diaphragm is the less likely is this to occur.
Many manufacturers aim at making the magnets of their receivers
extremely powerful, but it is very doubtful if much or any
increased efficiency results therefrom.
The question of receiver cords is one of a good deal of impor-
tance, as a faulty cord is one of the most prolific sources of trouble
of any part of a telephone instrument. If the conductors in a
cord are not properly insulated, so that they may come into con-
tact, or if a break occurs in one of the conductors, the instrument
will be short-circuited in the one case, or the circuit left open in
the other. In either event the receiver is rendered completely
useless. These faults are frequently very elusive, as a slight
movement of the cord may cause them to appear or disappear.
The conductors in receiver cords are usually composed of tinsel
woven or twisted into strands, and a few strands of fine cop-
Fig. 30. Details of Receiver Cord Tip.
per wire are frequently added to give greater strength. These
tinsel conductors are then tightly braided, or wrapped with
cotton, silk, or linen, sometimes in several layers, and in some
cases inclosed in a spiral wrapping of spring-brass wire which
incloses each conductor of a cord separately, these two spirals
being laid side by side and braided over with the familiar colored
worsted braid. It is probably better, in putting on the first cover-
ing over the tinsel, to make it a wrapping instead of a braid, as
the former tends to bind the tinsel strands together more securely
than the latter, thus preventing any short ends of the conductor
from piercing the covering and short-circuiting the cord.
The question of tips for receiver cords is one which has received
much attention, as faulty tips are a great source of trouble.
These are necessarily subject to rather rough usage, as it fre-
quently happens that a receiver is dropped, thus allowing a heavy
strain to come on the cords, which is usually most severe where
the tip joins the cord proper. The connection shown in Fig. 30
is one which has become very popular.
THE TELEPHONE RECEIVER. 31
This is formed by inserting a needle or pin, B, of No. 14 brass
wire tapered to a long point, into the hollow of a braided tinsel
cord or spiral spring, as the case may be, for one-half inch, when
the end is passed out through the conductor and covering and
bent backward, forming a hook, as shown at D and A, thus
combining the strength of the conductor and covering. Before
the pin is put in, the conductor is bared for a short distance and,
after the pin is inserted, is wound with fine wire and soldered.
The tip is then finished with a spiral of white wire as shown at,
or with a shell as shown at C.
In order to prevent an undue strain on the conductors when
receiver is dropped, it is best to have the cord provided at each
Figs. 31. and 32. Supporting Loop and Hook for Receiver Cord.
end with an auxiliary loop, A, as in Fig. 31. This loop is usually
a continuation of the braiding of the cord, and may be fastened
to an eyelet in the receiver, or to a small link on one of the bind-
ing posts. Another way of accomplishing this same result is by
means of a hook (Fig. 32) sewed to the braiding, just at the fork
of the cord, which may be closed by a pair of pliers around a
screw-eye or one of the screws in the binding post.
CHAPTER IV.
CARBON TRANSMITTERS.
MANY vain attempts have been made to discover a satisfactory
substitute for carbon as the variable resistance medium in tele-
phone transmitters, the patents on the use of carbon electrodes
having, until a fe\v years ago, formed one of the mainstays of
the American Bell Telephone Company's great monopoly.
The theory of the action of carbon in the transmitter has
been the subject of much discussion. As previously pointed out,
any motion of the diaphragm increasing the pressure between
the electrodes lowers the resistance between them, thus allow-
ing the passage of a greater current. A decrease of pressure
produces the opposite result.
Four different explanations for this action have been put forth,
and are as follows :
First, that the electrical resistance of the carbon itself is caused
to vary by the changes in pressure.
Second, that a film of air or gas exists between the electrodes,
and that the thickness of this film is varied by the changes in
pressure, thus varying the resistance. This theory is apparently
still adhered to by Mr. Berliner.*
Third, that the peculiar property possessed by carbon of lower-
ing its resistance with increased temperature is in the following
way accountable for the action, in part at least: that an increase
of current (due to increased pressure and diminished resistance
between the electrodes) causes a slight heating at the point of
contact ; that this heating causes a still further diminution of
resistance with an additional increase of current ; and that con-
versely a momentary decrease of current causes a decrease of
temperature with a corresponding additional increase of resist-
ance and diminution of current.
Fourth, that change in resistance is due to the variation in the
area of contact between the electrodes that is, the variation in
the number of molecules in actual contact. This change in area
is perfectly apparent in the liquid transmitter of Gray, and in the
* " Microphonic Telephonic Action," by ]mile Berliner, American Electrician^
March, 1897.
32
CARBON TRANSMITTERS. 33
case of solid electrodes may be well illustrated by the following
well-known experiment :
If -a billiard ball be gently pressed on a plain marble slab
coated with graphite, the area of contact of the ball with the slab
will be indicated by a small dot of graphite on the ball. If, now,
the ball be dropped from a considerable height, it will be noticed
that the spot of graphite on the ball is much larger, showing that
the ball has flattened out to a considerable extent, owing to the
greater pressure exerted. This demonstrates clearly the varia-
tion in area of contact between two bodies, due to variations of
pressure between them. Of course, if the two bodies are con-
ductors of electricity, the resistance between them will vary
inversely and the current directly as the area of contact.
It seems most probable to the writer that of the above explana-
tions, the fourth is the true one, and that none of the others aid
in any perceptible degree in producing desirable effects in the
microphone.
As to the first explanation, that the resistance of the carbon
itself changes under pressure, experiments have been made with
long carbon rods ; and with measuring instruments of ordinary
sensibility no difference whatever could be detected in the resist-
ance of a rod when the pressure on it was varied from zero up to
the crushing point, care being taken that all contacts in circuit
were not subjected to the change in pressure.
As to the layer of air theory, Professor Fessenden has thrown
some light upon it,* by showing that if the layer of air were in the
ordinary gaseous state, its resistance would be almost infinite,
while if it existed in some peculiar condensed state of which we
know little, but in which air might be conceived to be a conduc-
tor, then the law of change of resistance between the electrodes
would be different from what it has actually been found to be.
On the other hand, the curves plotted with resistances as ordi-
nates and with distances as abscissae have been found by Pro-
fessor Fessenden and by Messrs. Ross and Dougherty to exactly
agree with the form obtained from theoretical considerations
on the basis that the change in resistance is due to area of sur-
face contact alone.
As to the third explanation, it may be said that the very fact
that the increase of current is needed to cause the rise of tempera-
ture seems to preclude the supposition that the rise of temperature
should cause the diminution of resistance with its corresponding
*" Microphonic Telephonic Action," by Professor R. A. Fessenden, A merican
Electrician, May, 1897.
34
AMERICAN TELEPHONE PRACTICE.
increase of current in time to do any good. The heating effects in
carbon are comparatively slow, and it would seem that the changes
in temperature would lag slightly behind the changes in current
producing them, in such a manner as to be detrimental to tele-
phone transmission.
This property of carbon, of lowering its resistance with in-
creased temperature, is, however, important in that when the
transmitter becomes warm from constant use its resistance as a
whole is decreased. When the transmitter is heated the total
resistance of the circuit is lowered, and the changes in resistance
produced by the sound waves therefore bear a greater ratio to
this total resistance with corresponding increase of efficiency.
It is certainly most fortunate that in one substance carbon
should be found all of the qualifications which make it particu-
larly desirable for microphonic work. It produces the change in
resistance with changes in surface contact, all things considered,
better than any other known substance, possesses the desirable
ig- 33. The Blake Transmitter.
property of lowering its resistance when heated, and is elastic,
non-corrosive, non-fusible, cheap, and easily worked.
The form of transmitter almost universally used in this country
up to within a few years ago, and still largely used, is that de-
vised by Francis Blake of Boston. This instrument is shown in
Fig- 33, in which B represents a metal ring or frame for holding
the mechanism of the instrument. It is screwed to the cover, A',
of the box A, and has two diametrically opposed lugs, B' B*. On
this ring is mounted the diaphragm, C, of rather heavy sh~~<-
CA KBO.V TRA NSMITTERS.
35
iron, supported in a rubber ring, r, stretched around its edge, and
is held in place by two damping springs, D D, each bearing 'on a
small block of soft rubber, a, resting on the diaphragm at a point
near its center. The object of these damping springs is to pre-
vent too great an amplitude of vibration of the diaphragm, and
also to keep it from vibrating in separate parts instead of as a
unit.
Opposite the center of the diaphragm is the orifice, , in the
cover, A', so hollowed out as to form a mouthpiece. The adjust-
ing lever, F, is attached to the spring,/, secured to the lug, B\ of
the ring, B. The lower end of this lever rests upon an adjusting
screw, G, in the lug, B\ which is drilled and slotted as shown
to prevent the screw from working loose. On the back of the dia-
phragm and at its center is placed the front electrode, consisting
of a small bar, e, of platinum ; one end of the bar rests against
the diaphragm, while the other end is brought to a blunt point
and is in contact with the back electrode, e. The electrode, e, is
supported independently upon a light spring, c, mounted on the
lever, F t but insulated from it. This spring tends to press away
from the diaphragm and toward the back electrode. The back
electrode is formed of a block of carbon, e, set into a brass block,
g, of considerable weight, mounted on a spring, d, supported on
the adjusting lever, F. This spring, d, has a tension in the oppo-
site direction to that of the spring, c, and being stronger than the
latter it keeps the electrode, e, in contact with the diaphragm.
It is seen that instead of having one of the electrodes held in
fixed position while the other is pressed against it with greater or
less force by the vibration of the diaphragm with which it is con-
nected, both electrodes are supported in such manner as to move
freely with the diaphragm, but the outer electrode is so weighted
that its inertia will offer enough resistance to the slight and rapid
vibrations of the diaphragm to give a varying pressure between
the electrodes and consequent changes of the resistance of the
circuit. By this means the initial pressure between the two elec-
trodes will not be affected by changes of temperature, and the
adjustment will therefore be more nearly permanent.
This transmitter is very delicate, and transmits the quality of
the voice in a manner unexcelled by others. It is, however, lack-
ing in power, especially when compared with instruments of later
design. Besides this, it has a tendency to rattle or break contact
when acted on by loud noises.
Fig. 34 illustrates the Crossley transmitter, introduced into
Europe early in 1879. This well illustrates the class very
AMERICAN TELEPHONE PRACTICE.
appropriately termed "multiple-electrode." Transmitters de-
vised by Johnson, Govver, Ader, D'Arsonval, Turnbull, and
many others are of this type, and differ merely in the arrange-
ment and number of electrodes. They give much more powerful
results than the transmitters having a single pair of electrodes,
but most of them are subject to the grave defect of breaking
the circuit entirely when subjected to loud noises.
In this figure,/ represents a diaphragm formed of a thin piece
of pine board about y thick and mounted on a supporting ring,
Fig. 34. The Crossley Transmitter.
K. Fastened to this diaphragm are four carbon blocks, F G H
and /, in the relative positions shown. These are hollowed out
to receive the conical ends of the carbon pencils, ABC and D,
which are supported loosely between them. The blocks, H and
G, form the terminals of the transmitter. The current divides at
the block, H t and passes through the pencils, A and C, in multi-
ple to the blocks, Fand 7, and thence through the pencils, B and
D, to the other electrode, G. Vibrations of the diaphragm cause
variations in the' intimacy of contact between the eight points of
Fig. 35. The Turnbull Transmitter.
support of the four rods, and thus produce the desired fluctua-
tions in resistance. It is seen that this is merely a modification
of the Hughes microphone, the principles being the same, but
the multiple contact allows a greater current to pass through the
transmitter, and at the same time produce greater changes in this
current than in the original form, where a single pencil was used.
Moreover, the liability of " rattling" is greatly reduced.
CARBON TRANSMITTERS. 37
Fig. 35 shows the Turnbull transmitter, which has been used
to a considerable extent in this country, even until recently. In
this figure, A is the diaphragm of thin wood, on the back of which
is mounted the bracket, B. Pivoted on a rod, b, carried by this
bracket, are several carbon rods or pendants, a, which rest at
their lower end against a carbon rod, c, carried on a bracket, C,
also mounted on the diaphragm. The rods, b and C, form the
terminals of the transmitter, and the current passes from one of
them through the carbon pendants in multiple to the other.
The variable resistance contact is mainly between the rod, C,
and the pendants, a, although by making the rod, b, of carbon
also an additional effect is obtained between it and the pendants.
Fig. 36 shows still another form of the multiple-electrode
transmitter, using carbon balls instead of pencils or pendants.
A represents the vibratory diaphragm of carbon ; B a plate of
carbon having a number of cylindrical cavities, t t, upon one
side. Fitting loosely in each cavity is a ball of carbon. The
Fig. 36. The Clamond Transmitter.
depth of the cavities is a little less than half the diameter of the
balls, and the diaphragm is so placed in front of the plate that
the balls, following their tendency to roll out of the cavities, will
rest against its inner surface and also upon the edges of the cavi-
ties. Many other forms of instruments have been devised using
one or more balls held in various positions between carbon plates.
Some are used to-day, but all the transmitters so far described
are being rapidly replaced by the Runnings form of instrument,
which, as has already been stated, uses carbon " dust " or granules
for the variable resistance medium.
Among the earlier forms of the granular transmitter is a very
efficient one designed by Emile Berliner, and called the " Ber-
liner Universal." In this the diaphragm, D (Fig. 37), is of car-
bon, and is mounted horizontally in a case formed of the two
pieces, A and B, of hard rubber, a brass ring, R, being clamped
above it to insure good electrical contact. Secured to the en-
larged head, f, of the screw, s, mounted on the block, B, is a
cylindrical block of carbon, on the lower face of which are
3S AMERICAN TELEPHONE PRACTICE.
turned several concentric V-shaped grooves. The points formed
between these grooves almost touch the diaphragm. The finely
divided carbon, c, rests on the diaphragm, and is confined in the
space between it and the carbon block by a felt ring, F, which
Fig- 37- -The Berliner Universal Transmitter.
surrounds the latter and bears lightly against the diaphragm.
To the center of the back plate a soft-rubber tube, r, is fixed
which is of sufficient length to make contact with the diaphragm,
its function being that of a damper to the vibrations of the dia-
phragm. The mouthpiece, M, is so curved as to conduct the
sound waves against the center of the diaphragm. This trans-
ig. 38. Details of Solid Back Transmitter.
mitter was used to a considerable extent by the American Bell
Telephone Company, and has now been entirely replaced for
long distance work by the White transmitter.
The White, or " solid back," transmitter, as it is called, is
CA RB ON TRA NSM1 T TERS.
39
shown in Figs. 38 and 39, the latter giving a clear idea of the
construction of the working parts of the transmitter, the back
casing being removed. The upper portion of Fig. 38 shows the
section of the complete instrument. The sections of Figs. 38
and 39 are taken on planes at right angles to each other. The
separate parts of the " resistance button " of the instrument are
shown in the small cut at the oottom of Fig. 38. This instru-
ment has proven remarkably successful in practice, it being able
to stand a very heavy current without undue heating. Besides
this, the tendency of the granules to settle down in a compact
mass, commonly called " packing," is greatly diminished.
F is of cast brass turned to form the front piece of the trans-
mitter case, and is held, as shown, in the hollow shell, C, the two
pieces forming a complete metallic casing for the working parts
of the instrument. The sound-receiving diaphragm, D, of alu-
ig. 39. Sectional View Solid Back Transmitter.
minum, is encased in a soft-rubber ring,
J
o
C 0)
o
4
i
ui
V,
6^
r
"3
s u
5
'55
c
0>
rt
C
i-
rt
S
S :
v
c
C
r?
c o
c
P ! "^
u
i
61
24
25
1956
35
IOO
3
9
9
.0
3 7
7
.8 1 .:
! .9
2
62
24
25
3191
35
1 80
7
9
o
.1
.0 o
i t.
3| '
7 .O
3
62
24
25
4080
35
250 || .9
q
3
.9 o
.3! (
3 .0
4 116
24
5
3952
35
250
1.5
7
5
3; 5
*l -S
5
230
24
I. 00
3865
35
250
1-3
i .0
3i -2 .1 3
5 '. (
si -3
6
232
24
i .20
4420
35
300
i .5
. .9
6 -9-7 3
. 6
5 ! -5
7 I 295
24
1.50
4278
35
300
i .3
9
5
9
.1 . 1
4
3 -3
8 368
24
2.OO
4735
35
350
1-3
i .0
5
q
.1 .0
4 [ .
r .2
9
368
21
I.I7
4735
29
130.2 1
i .0
.6
9
7 -4
.6, 1.7 -3.
10
135
2 4
10.00
3950
35
400
3
3
3
5
3 -3
4 1
1
sity and clearness of the Blake transmitter with a standard coil
was taken as unity, and the results are expressed in terms of this
standard. The resistance of the primary wire of this coil was
1.05 ohm and that of the secondary 180 ohms. It will be no-
ticed from the results that coils Nos. 4, 6, and 9 were, all things
considered, the best, while coils Nos. i and 10 were very inferior.
The table also shows in general that a coil that was good for a
short distance was also good for a long distance, and this is per-
haps the most instructive lesson to be gained from these tests.
It is hard to draw any definite conclusions from the performances
of the various coils as to their relative merits and to point out
why coils Nos. 4, 6, and 9 should give better results than the others
or why coils Nos. I and 10 should be so much inferior. It shows,
moreover, that good results maybe obtained with the same trans-
mitter and with coils differing widely as to their characteristics;
this being shown particularly in the case of coils Nos. 4 and 9,
the former having a secondary of 250 ohms and a primary of
| ohm, while the latter had a secondary of 130 and a primary
of 1.17 ohm. The coil adopted for the Blake transmitter in this
country has a primary winding of ^ ohm and a secondary of 250
ohms, which, it will be seen, corresponds exactly to coil No. 4 in
this table, which gave the best results. The tendency, however,
among the manufacturing concerns whose practice may be con-
sidered the best, is to reduce the ratio of transformation by
making the secondary windings very much lower than was form-
INDUCTION COILS. 57
erly the case. As an extreme example of this, it may be cited
that the coil used to a large extent with the solid-back trans-
mitter on the long-distance lines of the American Telephone and
Telegraph Co. has a primary of .3 ohm and a secondary of
but 14 ohms resistance. This coil is provided with a very large
core composed of a bundle of soft-iron wires, and its total length
between the cheeks is six inches. 1 The results obtained leave no
doubt as to the efficacy of this construction.
The particular coil to be used with any style of transmitter
and battery should be carefully determined experimentally at
the start, and having once been decided upon, should not be
changed except for very positive evidence that the change is for
the better.
The determination of the proper size and dimensions of a
standard coil is no easy matter, and probably the best way is by
a process of elimination. When carried out properly, however,
even this is a somewhat expensive and tedious operation. Hav-
ing decided on the general dimensions of the core, about a
dozen of them should be made up and wound with primary coils,
using conductors ranging from, say, No. 18 to No. 30 B. & S.
gauge, using in each case two or three layers of these wires only.
This will give a set of cores all alike, having primary coils rang-
ing from perhaps \ of an ohm to 8 ohms. After this a number
of secondaries should be wound on spools, adapted to slip over
the primaries. These may be wound to resistances ranging from
10 to 500 ohms, always using a large enough wire to approxi-
mately fill the available wire space. This will make available a
larger variety of induction coils than would probably be obtained
in any other way, for it is evident that each primary may be
used with each one of the secondaries.
In conducting the experiments, one of the primary coils
should be chosen, and the results tested by using each one of
the secondaries successively in connection with that primary.
The best of these combinations should be noted, and then
another primary should be tried in a similar manner with all of
the secondaries. In like manner all of the primaries should be
tried with all of the secondaries, note being made of the best
combination in each case. In this the best secondary for each of
the primary coils chosen will be known, and, in order to arrive at
the final result, a comparative test should then be made in a
similar manner with each of these combinations. This process
may be carried out with as great a degree of refinement as time
and patience will permit ; and after the best combination has
5 AMERICAN TELEPHONE PRACTICE,
been found for any particular size of core, the entire operation
may be repeated as many times as is desired, using different sizes
of core.
Fig. 53 shows a sectional view, and Fig. 54 a perspective
view, of a coil the dimensions of which were determined by a
method not unlike that just described. This is the coil used
with the transmitter of the Western Telephone Construction
Co., shown in Fig. 43, and has proven itself to be adapted for
almost any variety of work. The core, C, is formed of a bundle
of about 500 strands of No. 24 B. & S. gauge Swedish iron
wire, and is 4 inches in length and -f$ of an inch in diameter.
The spool is formed of a thin fiber tube, 7^, over the ends of
Figs. 53 and 54. Section and Perspective View of Induction Coil.
which are slipped the heads, E, of similar material, the parts
being glued together. On this core are wound about 200 turns of
No. 20 single silk-covered wire. This is two layers deep, so that
the ends of the primary both emerge from the same end of the
coil. Over the primary winding are wrapped several layers of
oiled paper, after which the secondary is wound, this consisting
of about 1400 double turns of No. 34 wire, two in parallel.
These two wires are wound side by side throughout their length,
and give the equivalent area of one No. 31 wire. The resistance
of the primary coil is .38 ohm and that of the secondary 75
ohms. The terminals of the secondary coil are shown at a b
and a b. in Fig. 53. After the coil is wound, the small wires
INDUCTION COILS.
59
of the secondary are attached to larger wires inside of the spool-
head, so that the danger of breakage will be diminished. These
leading-out wires should be coiled in a tight spiral, in order to
avoid breakage and also to give a considerable length of wire in
making connections where it is needed.
A coil constructed on somewhat, radical principles is shown in
Fig. 55, this being manufactured by the Varley Duplex Mag-
Fig- 55- Varley Induction Coil.
net Co. The core consists of a bundle of small soft-iron cables,
each' cable being composed of seven strands of rather fine
Swedish iron wire. On this the primary, consisting of three
layers of cotton-covered magnet wire, is wound. The secondary
is wound in two sections, as shown, and the right-hand head of
the spool is made removable, so that each section may be slid on
or off, as needed, in making repairs. The most unique feature in
this coil is the fact that bare wire is used in winding the sec-
ondary. These coils are wound by special machinery, and the
Fig. 56. Manner of Winding Varley Coil.
adjacent convolutions of the wire are held apart by a fine thread
of silk wound alongside and parallel with the wire, as shown in
Fig. 56. A layer of paper is introduced between each layer
of wire, and in this way the insulation is made complete. The
machines for winding in this manner have been perfected with
such nicety that the layer of paper is automatically introduced
60 AMERICAN TELEPHONE PRACTICE.
between each winding without stopping the machinery, which is
run at a very high speed. Considerably more wire can be placed
on a coil in a given space than with the ordinary method of
winding ; and, of course, the fact that bare wire is used, renders
the coil cheaper.
This same company has recently carried the idea of sectional
Fig. 57. Transmitter Mounted on Ann.
windings throughout the entire field of telephone work. They
construct their spools in such manner that the heads may be
readily removed and a coil replaced without the necessity of re-
winding. A comparative test made by the writer, using an
induction coil wound in the ordinary manner with silk-covered
wire and another coil wound with bare wire on the same size of
core and with the same resistance of primary and secondary,
Fig. 58. Induction Coil in Base of Arm.
showed a very slight advantage in favor of the latter, although
the experiment was not carried far enough to warrant the con-
clusion that this would be true in every case.
It is now quite common to mount the induction coil in the
base of the arm on which the transmitter itself is mounted, such
INDUCTION COILS. 61
construction being shown in Figs. 57 and 58. This base and
arm are made of cast iron joined as shown in such manner as to
allow a considerable vertical movement of the transmitter, in
order to accommodate it to the heights of different users. The
coil is sometimes mounted upon the back- board of the telephone,
but a more desirable method is to mount it in the arm-base, as
shown, the various terminals being brought out to binding posts
on the front of the base. This ^construction, however, is bad,
unless well carried out, and great pains should be taken in insu-
lating the various posts and wires from the conducting base. A
considerable advantage has been claimed, due to the presence of
the iron case about the coil, thus rendering the magnetic circuit
more complete. This, however, is a point of doubtful validity, as
it may be claimed with equal force that the presence of the case
gives rise to eddy currents which would have a detrimental
effect. As a matter of fact, the presence of the case has little
appreciable effect one way or another on the quality of the
transmission.
CHAPTER VI.
BATTERIES.
IF a sheet of zinc and one of carbon be separated from each
other and immersed in a liquid capable of chemically attacking
the zinc, a difference of potential will at once be formed between
the two plates. If the two plates are then connected together
by a wire, a current of electricity will flow from one to the other
through the wire, and while the current is so flowing the zinc will
be eaten away by the solution with more or less rapidity. Such
a combination is called a voltaic cell, and two or more of such
cells may form an electric battery. Of course other substances
than zinc and carbon may be used, it only being necessary that
both plates be of conducting material and that one of them
shall be of such a nature as to be chemically attacked by the
fluid. The two plates of the cell are called electrodes, and the
solution in which they are immersed the electrolyte.
The current is assumed to flow from the plate which is at-
tacked through the electrolyte to the one which is not, and
therefore in the cell under consideration from the zinc to the
carbon plate. The plate which is attacked is therefore always
called the positive plate or electrode, and the one which is not
attacked the negative.
Starting from the surface of the zinc, where the chemical
action is taking place, the current flows through the electrolyte
to the surface of the carbon electrode, thence by means of the
wire back to the zinc electrode.
It will be noticed that the current flows from the carbon to
the zinc in the wire, outside the electrolyte ; and therefore in
order to make the terms positive and negative correspond to
ordinary usage, the carbon terminal is called the positive pole
and the zinc terminal the negative pole. It seems at first a little
confusing to have a positive pole on a negative plate, and a
negative pole on a positive plate ; but if the direction of the
current be kept in mind as being always from positive to
negative, no confusion will arise.
The part of the circuit outside of the battery connecting the
two poles is called the external circuit. The internal circuit is
62
BA TTERIES. 63
of course through the two electrodes and the electrolyte, and
the resistance of this latter path is called the internal resistance
of the battery.
Zinc forms the active or positive element of the great majority
of primary batteries, while the negative electrode is usually of
carbon or of copper. No matter, however, of what materials
the electrodes are formed, that which is attacked by the elec-
trolyte while the battery is in action forms the positive plate of
the cell, the current flowing always from it in the electrolyte.
In nearly all cases hydrogen is liberated from the electrolyte
at the negative plate that is, at the plate which is not attacked.
This forms a film over the surfaces of the negative electrode
which, unless removed or destroyed, tends to greatly weaken the
strength of the battery, for two reasons : First, the film of gas is
of very high resistance and therefore raises the internal resist-
ance of the battery enormously, thus causing a correspondingly
small flow of current ; and second, the gas is itself attacked by
the electrolyte, hydrogen having almost as great an affinity for
the oxygen in the latter, as has the electrolyte itself for the zinc.
This causes a counter-electromotive force to be set up which to
a large extent neutralizes that set up by the action of the elec-
trolyte with the zinc. The phenomenon of the collection of
hydrogen on the negative electrode in a cell is called polariza-
tion ; and it is necessary to adopt some means to prevent it to
as great an extent as possible, as otherwise a cell would become
useless after a very short period of use.
The LeClanche type of battery, which has been and still is used
to the greatest extent for telephone work, consists of a carbon
negative electrode, a zinc positive electrode, and an electrolyte
of a solution of sal ammoniac. The sal ammoniac attacks the
zinc, forming zinc chloride and liberating hydrogen and also
ammonia gas on the surface of the carbon. In order to get rid
of the polarizing effects due to the hydrogen, black oxide of
manganese, usually in small lumps, is in some way closely asso-
ciated with the carbon. This oxide of manganese is exceedingly
rich in oxygen, which slowly unites with the free hydrogen to
form water. In use, cells of this type polarize rather quickly,
but as soon as the external circuit is opened the cell slowly re-
covers, owing to a combination of the hydrogen with the oxygen
as described above. This cell is therefore suitable only for cases
where the circuit will be closed for a few minutes at a time ; and
as this is exactly the condition which is met in telephony, it has
been found particularly suitable in this line of work.
6 4
AMERICAN TELEPHONE PRACTICE.
The cell used almost exclusively by the Bell companies is
shown in Fig. 59-
The zinc electrode is in the form of a rod, while the carbon
electrode is imbedded in a porous pot which is immersed with the
zinc in the electrolyte. Around the carbon within the porous
pot is packed a mixture of black oxide of manganese and broken
carbon, the former to act as the depolarizer and the latter to give
greater conductivity to the mixture and to give a greater surface
to the carbon electrode. One of these cells is almost invariably
found in connection with the Blake transmitter.
A better form of cell than this is one using practically the
same materials for its various parts, designed by Mr. M. M.
Hayden of the Globe Carbon Works, Ravenna, O. This cell is
shown in Figs. 60 and 61, the latter being a
sectional view. The carbon electrode is in
'the form of a corrugated hollow cylinder, I
(Fig. 61), which engages by means of an
internal screw-thread a corresponding thread
on the under side of a carbon cover, 2.
Within this cylinder is a mixture, 10, of broken
carbon and black oxide of manganese, the
latter serving as a depolarizer.
The zinc electrode, 6, is in the form of a
hollow cylinder almost surrounding the car-
bon electrode, and separated therefrom by
means of heavy rubber bands stretched around
the carbon. The rod forming the terminal
of the zinc passes through a porcelain bushing
on the cover plate, so that a short-circuit can-
not take place. The terminal pin, 8, is imbedded in a hole, 4,
in the carbon cover, by first heating the cover to a high degree
and then pouring in melted lead, as shown. This forms, with
the nut, 7, and the washer, 6, a very secure form of con-
nector for the positive pole. Unless some such precaution as
this is taken, corrosion soon sets in around the metallic con-
nection to the carbon, thus causing a poor connection. The
Hayden cells are used to a very large and increasing extent by
the independent telephone companies. They have an electro-
motive force of about 1.55 volts, and recuperate very quickly
after severe use.
Many other forms of sal-ammoniac cells are in common use.
Some of these consist merely of a zinc rod hanging in the center
of a carbon cylinder, no depolarizer being furnished. In other
Fig. 59. LeClanche
Cell.
BA TTERIES. 65
forms the carbons have molded with them the manganese de-
polarizer and are in various forms, but all act in the same
general way.
The advantages of the LeClanche type of cell for telephone
work are many. They are inexpensive in first cost and in
renewals. They are very cleanly, giving out no noxious fumes
and containing no highly- corrosive chemicals. They require
almost no attention, the addition of a little water now and then
to replace the loss due to evaporation being about all that is
generally required. They give a rather high electromotive
force and have a moderately low internal resistance, so that
they are capable of giving a considerable amount of current for
Fig. 60. Hayden Cell.
a short time, and lastly, if properly made they recuperate
quickly after polarization due to heavy use.
To set up and maintain cells of the LeClanche type place not
more than four ounces of prime white sal ammoniac in the jar.
Fill the jar one-third full of water and stir until the sal ammoniac
is all dissolved. Then place the carbon and zinc elements in
place. A little water poured in the vent-hole of the porous-pot
forms will tend to hasten the action. Unless a cell is subject to
very severe use, it will require but little attention if it is a good
one. Water should be added to supply loss by evaporation. If
the cell fails to work, examine its terminals for poor connections.
If the zinc is badly eaten, replace it with a new one. If this fails
to improve it, throw out the solution and refill as at first. If
66
AMERICAN TELEPHONE PRACTICE.
now the cell does not work properly, the porous pot or carbon
element may be soaked in warm water, and if this gives no
better results they should be replaced. In the Hayden cell, the
depolarizer may be removed by unscrewing the carbon from the
cover.
The Bell Company is now using in its long-distance work, in
connection with the solid-back transmitter, another form of cell
known as the " Standard " Fuller. In this the positive electrode
Fig. 61. Sectional View Hayden Cell.
is a heavy block of zinc molded into conical form around a heavy
copper wire, which forms the negative pole. The negative elec-
trode is a block of carbon hanging through a slot in a wooden
cover. The separate parts are shown in Fig. 62. The zinc rests
in the bottom of a porous cup when in place. The electrolyte
for this cell is made as follows :
Sodium bichromate,
Sulphuric acid, .
Soft water,
6 ounces
17 ounces
56 ounces
BA TTERIES. 67
Dissolve first the sodium bichromate in the water and then add
slowly the sulphuric acid. (Never pour the water into the acid.)
The mixture should be made in an earthen vessel, or if in a
glass jar the jar should be placed in cold water in order to pre-
vent overheating.
Another solution called electropoin fluid may be used as the
electrolyte in this cell. It is made with bichromate of potash
instead of bichromate of sodium.^
The cell is set up according to the following directions :
Place the quantity of solution made by the above formula in
the glass jar.
Put one teaspoonful of mercury in the bottom of the porous
kjfc_ * ; .
Fig. 62. Parts of " Standard " Fuller Cell.
cup, add two teaspoonfuls of common salt, place the zinc in the
bottom of the cup, and fill to within two inches of the top with
soft water.
Place the porous cup in the jar and put on the cover, passing
the wire from the zinc through the hole provided for it. The
cell is then ready for use.
The active element in the electrolyte in this cell is the sul-
phuric acid, which of course attacks the zinc. The bichromate of
sodium or of potash serves as a depolarizer, the oxygen in it com-
bining with the hydrogen, liberated at the positive pole, to form
water.
68
AMERICAN TELEPHONE PRACTICE.
The specifications for this cell, as used by the New York
Telephone Company and some other large Bell concerns, are in
substance as follows :
One cell of Standard Battery shall consist of the following parts:
I glass jar ; I wooden cover ; I carbon plate with binding post
,1 L
1
"LINE OF PARAFFINE
: I
f
..L
Fig. 63. Carbon Plate for " Standard " Fuller Cell.
and locknuts ; I cast zinc ; I porous pot all as hereinafter
specified.
Glass Jar: The glass jar shall be of first quality flint glass,
cylindrical in form, 6 inches in diameter and 8 inches in depth.
Wooden Cover: The cover shall be of clear kiln-dried white-
wood. It shall be thoroughly coated with two coats of asphalt
pamt, and be of such dimensions as to form a proper cover for
the jar.
Carbon Plate : The carbon plate shall be of the form and
BA TTERIES.
69
dimensions shown in the drawing (Fig. 63). It shall be of good
quality, homogeneous and free from flaws, cracks, and other de-
fects, and completely carbonized. Each carbon shall be provided
with a clamp of the form and dimensions shown in the drawing
(Fig. 64). The parts of the clamp shall be of bronze, and shall be
nickel-plated. Before attaching the clamp to the carbon, the
carbon shall be heated to a temperature of at least 250 degrees
Fahrenheit, and the top portion of it, to the extent indicated in
the drawing, shall be immersed in paraffin at a temperature of
about 250 degrees Fahrenheit, the immersion to continue until
the immersed portion of the carbon is saturated. After the clamp
is attached to the carbon, but before the locknuts atre in place,
Fig. 64. Details of Clamp for " Standard " Fuller Cell.
the carbon shall be immersed in melted paraffin at a temperature
less than 170 degrees Fahrenheit. The carbon plate is then to
be completed by attaching the locknuts.
Cast Zinc : The zinc shall be of the form and dimensions shown
in the drawing (Fig. 65). It is to be made of Rich Hill spelter.
Cast into the zinc shall be a soft copper wire .1018 of an inch in
diameter (No. 10 B. & S. gauge). The zinc and the copper wire
shall be amalgamated to a height of 4 inches.
Porous Pot : The porous pot shall be cylindrical in form, 3
inches in diameter and 7 inches deep.
The " Standard " Fuller cell made according to the above speci-
fications gives an E. M. F. of 2.1 volts, and is exceedingly well
adapted for heavy telephone service. A still more powerful
7 AMERICAN TELEPHONE PRACTICE.
cell, and one somewhat more convenient to handle, is shown
in (Fig. 66).
In this the zinc is very heavy, and in order to present a greater
surface to the electrolyte has a horizontal cross-section in the
form of a cross. The carbon electrode is in the form of a hollow
cylinder completely inclosing the porous pot. The carbon cylin-
NO.IOWIRE
COMPLETE ZINC
TO WEIGH NOT LESS
THAN 18 OZ.
Fig. 65. Zinc for " Standard " Fuller Cell.
der has a flaring top provided with a flange which fits over the
upper edge of the glass jar, thus forming a very complete cover
for the entire cell.
The following are the data given by the Globe Carbon Co.
concerning the main points of this form of Fuller cell :
E. M. F., 2.1 volts.
Current, about 8 amperes.
BATTERIES. 7 1
Carbon, 4! inches diameter by 8| inches over all.
Carbon surface exposed to solution, 156 square inches.
Zinc weighs 2 pounds ; 2-J inches across ; total length, 8 inches.
Zinc surface exposed, 54 square inches.
Porous cup, 3 inches diameter, 7 inches long.
Jar, 6 inches diameter, 8 inches deep.
Solutions same as " Standard " Fuller cell.
Cell, complete, weighs 8 pounds 12 ounces.
The internal resistance of Fuller cells is very low, especially in
the cylindrical carbon type. They will stand for several months
on open circuit with but little local action.
Formerly three cells in series, giving six volts, were used with
the solid-back transmitter, but it has been found that two cells
give, all things considered, as good or better results.
Still another form of battery, of entirely different type, is shown
in Fig. 67. This is known as the gravity battery, and is used to a
Fig. 66. Parts of Globe Fuller Cell.
very large extent in telegraph service, and also in telephone
work where it is necessary to have a small but constant current
always flowing. In this cell the negative electrode is of sheet
copper, 3 strips of which are riveted together at their centers,
after which the ends are bent outwardly, so as to present a large
surface to the electrolyte. The zinc is in the form of a " crow
foot," cast with a lug adapted to hook over the edge of a glass
jar. In setting up this battery the copper is first put in place in
the bottom of the jar. Sulphate of copper, or blue vitriol, as it is
called, is then filled in around the copper to a height almost suffi-
cient to cover it. The jar is then filled with water and the zinc
put in place.
In this battery sulphuric acid is formed, which attacks the zinc
72 AMERICAN TELEPHONE PRACTICE.
to produce zinc sulphate. This fluid is lighter in weight than
the solution of copper sulphate and therefore occupies the upper
portion of the cell. The fact that the two solutions in this
battery are kept apart by gravity instead of by the use of a
porous pot, as in the Fuller cell, is accountable for the name,
" gravity cell." As the zinc sulphate is colorless, while the copper
sulphate is of a dark-blue color, the separating line between the
two liquids is easily distinguished. This line is termed the "blue
line," and should be kept about midway between the copper and
the zinc. If the blue line rises too high, so as to come in contact
with the zinc, it should be lowered. This can be done by short-
circuiting the battery for a short time, or by drawing off some of
the blue fluid with a siphon and filling in with water or with zinc
sulphate from another battery. In cases, however, where the
battery is in constant use, it very rarely happens that the blue
Fig. 67. Gravity Daniell Cell.
line reaches too high a level, and the reverse is more likely to
take place. If the blue line reaches the upper portion of the
copper, more crystals of bluestone should be dropped in, and if
this does not remedy the difficulty some of the zinc sulphate
from the top of the cell should be siphoned out and replaced by
clear water. These batteries are very satisfactory for closed-
circuit work, but are not well adapted for telephone work in gen-
eral on account of their high internal resistance.
When a battery is on open circuit there should be no action
between the electrolyte and the zinc. This would be the case
BATTERIES. 73
were it economical to use perfectly pure zinc, but inasmuch as
commercial zinc always contains impurities, frequently consisting
of other metals, a local galvanic action is set up, the impurities
forming with the zinc minute galvanic couples. In order to re-
duce this action to a minimum, it is advisable, especially in such
cells as the Fuller, to amalgamate the zinc that is, to coat it with
mercury. This seems to -form'a perfectly homogeneous surface
to the zinc, which prevents local action. The fact that this local
action takes place on account of impurities in the zinc makes
it very clear that the quality of metal used is a matter of very
considerable importance.
STORAGE BATTERIES.
If two plates of lead are immersed in a weak solution of sul-
phuric acid, no difference of potential will be established between
them, because the acid, if it acts on them at all, does so to an
equal extent on each plate. If now an electric current, as from
a battery or a direct-current dynamo, is sent through the two
plates and the solution between them, a redistribution of ma-
terials will take place in the cell. The electrolyte will be decom-
posed, the oxygen in it forming, with the plate to which the
positive terminal of the charging source is connected, lead per-
oxide ; while hydrogen is liberated at the plate to which the
negative terminal is connected. On disconnecting the source of
current, the cell, which was before incapable of producing a
difference of potential, is found able to drive a current through a
circuit formed by connecting its poles together by a wire or any
other conductor. The combination has become a voltaic couple.
The current from this couple always flows in a direction opposite
to that of the charging current.
The cell, consisting of two lead plates in a solution of sulphuric
acid, was devised by Gaston Plante, and is the prototype of all
modern storage batteries or accumulators. Nearly all commer-
cial cells, of which there are many good ones, have the plates
coated with some compound of lead, rich in oxygen. This is
changed by the charging current into lead peroxide on the posi-
tive plate, and to spongy lead on the negative.
In storage cells of considerable size it is customary to use
more than two plates, all the positive plates being connected
together by a heavy strip of lead, and likewise all the negative
plates by another similar strip. There is usually one more of the
negative than of the positive plates, the arrangement being such
that the plates are alternately positive and negative.
74 AMERICAN TELEPHONE PRACTICE.
The setting-up and operating of storage batteries is a very
simple matter, yet there are a few mistakes to be guarded against,
which if made are liable to injure or ruin the battery. The elec-
trolyte is usually formed of four or five parts of water to one of
acid. These should be mixed in an earthenware vessel by slowly
pouring the acid-into the water, and not the water into the acid.
In charging storage batteries the positive terminal of the
dynamo or other source of current is connected to the positive
pole of the battery, and the negative terminal of the dynamo to
the negative pole of the battery. A simple test, and the most
reliable one, for determining which is the positive pole of any
source of current is to dip wires leading from both terminals into
a small vessel containing slightly acidulated water. Bubbles of
gas will be given off from each wire, but at a very much higher
rate from the wire leading to the negative pole than from that
leading to the positive. The poles of the charging dynamo
should always be determined with absolute certainty before con-
nection is made to the terminals of the storage battery, for a
reversal in the connections is very likely to ruin the battery.
The manufacturers of storage batteries usually furnish direc-
tions concerning the proper rate of charge and discharge for a
battery of a given size. These should be followed as closely as
conditions will allow.
The most accurate method of determining the condition of a
cell is by the use of a hydrometer for measuring the density of the
electrolyte. It is usual to have the normal density of the solution
about i . 1 80 ; when it becomes as low as 1 . 1 70 the cell may be con-
sidered fully discharged, and when as high as 1.250 fully charged.
These figures will vary somewhat with different forms of battery.
Water only should be added to replace loss by evaporation,
while spilled solution must be replaced by the regular acid solu-
tion according to formula.
The extremely low internal resistance of storage batteries, and
the fact that their voltage is high (2 volts) and constant and
that they are not subject to polarization, make them, all things
considered, the ideal source of current for telephone work. They
are being largely used for supplying the operators' transmitters
in large central offices. They are much more economical in
operation than any form of primary cell, inasmuch as there is
practically no consumption whatever of the materials in the cell
itself, it depending of course for its energy on some outside
source. Their ease of manipulation and general cleanliness and
reliability are also strong points in their favor.
CHAPTER VII.
CALLING APPARATUS.
So far we have dealt solely with the apparatus by which the
actual transmission of speech is accomplished. While these are, of
course, the most vital parts of a complete telephone, they would
be of little use were not means provided whereby one party might
call the attention of another in order to bring about a conversa-
tion. Many attempts have been made to devise telephone in-
struments capable of reproducing speech so loudly that one has
only to call into the transmitter in order to attract the attention
of a party at the other end of the wire. Such attempts have so
far resulted practically in failure, and this is perhaps fortunate,
as one of the most convenient features of telephones to-day is
that a conversation can be carried on in secrecy, at least so far
as the receiving is concerned.
Ordinary vibrating bells, using current derived from a battery,
were at first used for calling, and as the battery for operating the
transmitters could also be used for this purpose, this plan seemed
to offer many advantages. It was found, however, that the
amount of energy furnished by a telephone battery was insuffi-
cient to operate call-bells at great distances. Of course, practi-
cally as high voltage as was desired could be obtained, by using
induction coils and causing induced currents from the secondary
to pass out over the line. This, however, reduced the current in
the same, proportion as it raised the voltage, leaving the amount
of energy the same.
What is known as a " magneto-generator " is now almost uni-
versally used among the independent companies, and until re-
cently by the Bell Company. It is the simplest known form of
the dynamo, and consists of an armature of the Siemens type,
wound with many coils of fine wire, and so mounted as to enable
it to be rapidly revolved between the poles of a permanent horse-
shoe magnet. Its theory of action is very simple and depends
on the principles of magneto-electricity discovered by Faraday
and Henry, and pointed out in a previous chapter that if the
number of lines of force passing through a closed coil be varied,
currents of electricity will be generated in this coil, the direction
7 6
AMERICAN TELEPHONE PRACTICE.
of these currents depending upon the direction of the lines of
force and on whether their number is decreasing or increasing.
In Fig. 68 is shown a simple loop of wire, a, which may be re-
volved about a horizontal axis in the field of force of the per-
manent magnet. The horizontal arrows represent the direction
of the lines of force set up by the magnet through the loop.
Suppose the loop to be turned in the direction of the curved
arrow. When it is in the horizontal position no lines of force
will pass through it. As it approaches the position shown by
the full line it will include a larger and larger number of these
lines. The current induced in the coil will then be in the direc-
tion indicated by the arrows, x, and will so continue until the
loop is in its vertical position. The number of lines passing
X' ' \
\ ,/ X
a
Fig. 68. Field of Force in Magneto-Generator.
through the loop then begins to decrease, and the current there-
fore takes the opposite direction, as indicated by the arrows, s.
The current increases in strength in this new direction until the
coil is horizontal. At this point the rate at which the number of
lines through the coil is changing is greatest, and the current
is therefore a maximum. As the coil passes through the
horizontal position the number of lines passing through it begins
to increase again. This would cause another change in the
direction of the current, were it not for the fact that the direc-
tion of the lines of force through the coil also changes. The
same events take place during the next half-turn, when the coil
is dn the position from which it started.
We thus see that the current generated is an alternating oe,
changing its direction twice during every revolution.
CALLING APPARATUS.
The armature, instead of having a single turn of wire, as in
Fig. 68, has a great number of turns of fine wire wound on a cast-
iron core of the form shown in Fig. 69. In this figure, A repre-
Fig. 69. Armature of- Magneto-Generator.
sents a shuttle-shaped core of cast iron, on which the coils of
wire, w, are wrapped. One end of the wire forming the coils is
fastened to the pin, /, which is fastened to and is in metallic
connection with the core, A. The other end is fastened to the
pin, p ', which is insulated from the core, but connects with the
pin, Cj projecting from the end of the armature shaft and is in-
sulated therefrom by the fiber bushing, b. Projections, a a, in-
tegral with the core, are turned down to form bearings for the
armature. A pinion,/, is carried on the end of the shaft, in order
^Ss^
Fig. 70. Diagram of Generator and Bell.
to transmit to the armature the motion, received from a large
driving-gear wheel with which it meshes.
A magneto-generator in connection with a call-bell is shown
diagrammatically in Fig. 70. To the poles of the permanent
magnets, N 5, of the generator are attached cast-iron pole-
pieces, P P, bored out so as to allow the armature, A, to turn
78 AMERICAN TELEPHONE PRACTICE.
freely between them. The bearings of the armature are usually
mounted on brass plates firmly attached to the ends of the pole-
pieces, but not shown in this figure. By means of a crank
attached to a suitable gear wheel engaging a pinion on the
armature shaft, the armature may be made to turn rapidly.
As the currents generated are alternating, a polarized bell or
ringer is needed. C C are the two coils of an electromagnet.
Pivoted in front of the poles of this magnet is a soft-iron arma-
ture, A', carrying a hammer, //, on the end of a thin rod extend-
ing at right angles from its center. A permanent magnet, N S,
is so mounted as to magnetize by induction the armature, A', and
the cores of the coils, C C.
The two poles of the electromagnet will thus have a given
Fig. 71. Complete Magneto-Generator.
polarity, say, north, while the two ends of the armature will
have an opposite polarity, south. As a result, the armature will
have a tendency to stick to one pole or the other of the magnets.
The two coils are oppositely wound, and when a current passes
through them it strengthens the magnetism of one pole and
weakens that of the other. The next instant the current reverses,
and the strong pole becomes the weaker, and vice versa. As a
result the armature vibrates with each reverse of current and
causes the hammer, H, to strike the bells, B B. A complete
magneto-generator and call-bell, mounted in a box, is shown in
Fig. 71. The magnets of the call-bell are mounted on the inside
of the lid, the hammer extending through a hole therein to
CALLING APPARATUS.
79
strike the gongs, on the outside. Fig. 72 shows one of the
commercial forms, and a very efficient one, of call-bell mechanism.
The forms of ringers used by different manufacturers differ
widely ; but all depend on the same principles for their mode of
action.
The armatures of ordinary hand generators are usually
wound to resistances varying from 300 to 650 ohms. The resist-
ance of the ringer coils is usually from 75 to 100 ohms, but is
sometimes as high as 5000 ohms, varying according to their use.
The standard generator and ringer for ordinary exchange
work are so wound that the generator will ring its own bell, or
another like it, through a resistance of 10,000 ohms. Such an
outfit is spoken of as a io,ooo-ohm magneto, and the 10,000 re-
Fig. 72. Polarized Ringer.
fers not to the resistance of the bell magnets or the generator arma-
ture, as is often supposed, but to the external resistance through
which they will successfully work.
With a given magnetizing force, as for instance that set up by
a permanent magnet, the number of lines of force extending
from one pole of the magnet to the other will depend on the
material between the poles and also on the distance between
them. Certain substances, if placed in a magnetic field of force,
will have set up in them a vastly greater number of lines of
force than would air when subjected to the same magnetizing
force. Such substances, in which a given magnetizing force will
produce a high degree of magnetization, are said to possess a
high degree of permeability. The permeability of a substance is
expressed numerically by the ratio of the number of lines of force
set up in a given area of it by a given magnetizing force to the
number set up in the same area in air by the same magnetizing
8o AMERICAN TELEPHONE PRACTICE.
force. Thus, if the given magnetizing force sets up 50 lines of
force per square inch in air and 20,000 lines per square inch in a
piece of wrought iron, the permeability of the iron would be
- = 400. The permeability of air is always taken as unity.
Iron, in all of its forms, is by far the most permeable of all
metals, and even among the various grades of iron there is a
great difference in this respect. Soft wrought iron is much more
permeable than cast iron, and cast iron much more so than hard
steel.
The great point in the design of magneto-generators, as in
fact in dynamo design in general, is to cause as great a number
of lines of force as possible to pass through the core of the
armature. In the design of ordinary dynamos, where the field
is composed of electromagnets, the magnetizing force can be
varied almost at will by subjecting the field to the influence
of a great number of ampere-turns.
In the design of magneto-generators, however, the strength of
the field, when once determined, is, for all practical purposes, in-
variable, as the strength of the magnets is in no wise dependent
on the current generated. Obviously, therefore, the only re-
course, in bettering the efficiency of the machine in this respect,
is to use as fine a grade of iron as possible in the armature, and to
so design it as to present a path of as small resistance as possible
to the flow of the magnetic lines. Not enough attention has
been given to this point, and often a poor grade of cast iron
which was allowed to chill after casting and thus become ex-
ceedingly hard, has been used in constructing generator arma-
tures. Fortunately, however, a very hard grade of iron is very
difficult to turn in a lathe, especially in this particular form, and
this has, indirectly, made some manufacturers seek fairly soft, uni-
form iron for this purpose.
A cast armature, even though soft, is subject to another ob-
jection, in that eddy currents are generated in the core, which,
of course, interfere greatly with the efficiency of the machine.
In order to do away with both of these objections, some com-
panies are now building laminated armatures, composed of soft
sheet-iron punchings about T 1 ^- of an inch or less in thickness,
clamped together on a central shaft which forms the spindle of
the armature. These laminated armature cores are, when com-
pleted, of about the same shape as the cast core, and the wire is
wound on them in the ordinary way.
After the armature core, however formed, is complete, it
CALLING APPARATUS. 81
should be thoroughly insulated by paper and cotton cloth, held
in place by some insulating adhesive such as shellac, after which
it is placed in a winding machine and wound with the required
number of turns.
The winding should be of the largest size of wire that will give
the desired number of turns, but the wire space should not be so
completely filled as to cause the 1 wire to bulge out and strike the
pole-pieces of the generator in its rotation, thus wearing away the
insulation and frequently breaking the wire itself.
It is a commonly expressed opinion that the turns of wire
near the center of the armature coil are of little or no value in
producing electromotive force. This, however, is not the case,
for the permeability of iron is so much greater than that of
air that nearly all of the lines of force due to the permanent mag-
nets of the generator will pass through the shank of the core in-
stead of leaking around through the air space. Of course, in
order to pass through this shank, they must also pass through the
inside turns as well as those nearer the surface.
The question of permanent magnets is a puzzling one, prin-
cipally because very little seems to be known concerning the
kind of steel best adapted for this purpose. Makers of steel can-
not or will not reproduce the quality of samples of steel given
them, even after careful chemical analyses. It may be said,
however, that a few makers of this steel are able to turn out year
after year large quantities of very uniform steel for this purpose,
which is capable of giving very satisfactory results. If, however,
a sample of one maker's steel is given to another to analyze and
reproduce, the result is usually failure. It has been the experi-
ence of the writer that the only way to procure a good magnet
steel is to test all of the samples obtainable, and, having found a sat-
isfactory steel which the manufacturer is able to produce in large
quantities, to stick to that particular grade. It may be said
further that the more expensive grades of steel are not by any
means capable of producing the best magnets; and frequently
where a manufacturer is paying ten to twelve cents per pound
for magnet steel, a little experimenting would enable him to find
something which would give as good or even better results at
from two and one-half to five cents per pound.
The usual method of treating steel for making permanent mag-
nets is to cut it in the desired lengths and, if the cross-section be
not too heavy and the form not too complicated, to bend it in a
special former while cold. It is then heated to a light cherry-
red in a rather slow fire and then grasped by a special pair of tongs
82
AMERICAN TELEPHONE PRACTICE.
iii such manner that it will not bend from the desired shape,
and plunged into a tank of cold running water, being kept in vio-
lent agitation during the entire time of cooling. All parts of the
tongs which come in contact with the magnet during this process
should be bored as full of holes as the required strength will per-
mit in order that the water my have free access to all portions of
the steel. After the bar has hardened it is magnetized by strok-
ing it several times across the poles of a very powerful electro-
magnet. The pole-pieces of this magnet should be sufficiently
close together to allow one leg of the horseshoe magnet to rest
upon the north pole and the other upon the south pole. In
some cases the bars are magnetized by inserting them in a sole-
noid, but probably the best results are obtained by the method
of stroking.
Until recently cast iron was the only material used for pole-
pieces in magneto-generators, and many good generators are
now constructed with that material. A number of generators,
Fig. 73. ---Detail of Generator Pole-Pieces.
however, have recently been produced using soft sheet-iron pole-
pieces stamped and formed into the desired shape. This forms
a cheaper pole-piece than can be procured by the use of cast iron,
because the latter must necessarily be subjected to a consider-
able amount of machine work, such, for instance, as the boring of
the concave cylindrical surfaces between which the armature re-
volves. A point in favor of the cast-iron pole-pieces is that the
air gap may be made much smaller because of the greater accuracy
of this bore than can be secured by the use of punched sheet-metal
pole-pieces. An argument in favor of the latter, however, is that
the quality of iron is much better, and this is, of course, of some
advantage, but not so great as it would at first appear, because the
flow of magnetic lines through these pole-pieces is always in the
same direction ; and the loss, therefore, due to a lack of permea-
bility in the cast-iron pole-pieces is probably fully offset by the
greater cross-section of iron available for the lines to traverse
and also by the smaller air gap.
In the construction shown in Fig. 73 the pole-pieces are cf
CALLING APPARATUS. 83
cast iron firmly secured together by shouldered brass rods.
After being thus fastened together they are bored out with a
special tool, after which the magnets are put in place and
clamped by any suitable means. This is a very good, although
somewhat expensive, construction when properly done.
The efficiency depends to a considerable extent upon the form
of the current wave generated [ by the machine. This is gov-
erned largely by the relation between the width of the pole of
the armature and the distance between the flat surface of the
generator pole-pieces. In Fig. 73 the best relation between
these dimensions is illustrated quite clearly. It will be noticed
in the figure at the left that the curved portion of the armature
pole exactly corresponds to the concave portion of the -pole-
pieces, while in the figure at the right, which shows the armature
in a different position, the poles of the armature are just suffi-
cient in width to bridge across the space between the pole-pieces
without overlapping.
The sine wave has been found to be most efficient in the
ringing of magneto-bells, especially on lines of considerable
length and possessing a high degree of self-induction and capacity ;
and the relation between the armature poles and the pole-pieces
shown in the above figure gives the nearest approximation to
this form of wave. Where the armature poles do not fill the
space between the pole-pieces, the current- curve will have four
distinct humps in each complete cycle. There will be a break in
the magnetic circuit just as the armature pole leaves the pole-
piece on one side, which will cause a sharp fluctuation in the
electromotive force ; and another sharp fluctation will occur im-
mediately after, when the opposite points of the armature poles
approach the corners of the pole-pieces. These two fluctuations
will occur twice in each cycle. When the armature poles are so
wide as to overlap, when in the position shown in the right-hand
portion of Fig. 73, the wave is flattened unduly and does not,
therefo?'e, give as high an electromotive force as could otherwise
be obta'ned.
The effective pressure of the ordinary magneto-generator, when
rotated at the ordinary speed by hand, is from 65 to 75 volts, and
it may be made, of course, higher or lower to meet certain re-
quirements by winding with a greater or less number of turns or
by gearing the armature so as to rotate with greater or less
speed.
Some telephone linens, as for instance party lines, using a large
number of instruments in series, require magneto-generators
84 AMERICAN TELEPHONE PRACTICE.
capable of producing a very high electromotive force in order to
successfully overcome the great resistance offered. Inasmuch as
all of the bells are in series, the current required is not large. In
a bridged line, however, where all of the ringer magnets are con-
nected across the line in parallel, the current required is heavy,
while the voltage need not, as a rule, be so high. In long lines
of this latter type using a high-resistance wire, it becomes neces-
sary to develop enough pressure to overcome the resistance of
the line wire in order to ring the bell at the farthest end, and also
a sufficient current to pass in multiple through all of the ringers.
In this case a rather high voltage is required and a heavy current,
so that the total amount of energy is large and cannot be effect-
ive merely by winding the instrument to a higher resistance. In
generators of this type it is customary to use heavy and very
powerful permanent magnets and to exercise the greatest care in
the construction to produce the highest efficiency.
The construction of the polarized call-bell, or ringer, is a mat-
ter requiring no less attention to detail than that of producing an
efficient generator. The old form of ringers, using a cast-iron
frame polarized by small electromagnets, was subject to very
grave defects. The frame became readily polarized in one di-
rection or the other, due to the passage of a heavy current
through the magnets, and would thus give the armature a set to
one side or the other, which frequently succeeding currents of a
weaker nature could not overcome. This, with the fact that
with every reversal of the current the entire magnetic field set
up through this heavy mass of poor-quality iron had to be com-
pletely reversed, was a point rendering the construction of an
efficient ringer almost an impossibility. The tendency in the
present form of ringers is to make a magnetic circuit which is
subjected to the changes due to the magnetizing force as short
as possible and to make the magnetic circuit of the very best pos-
sible material. Swedish or Norway iron, cold drawn and an-
nealed, has been found to meet these requirements most per-
fectly. The sticking of the armature to one pole or the other is
further prevented by the interposition of a thin sheet of non-
magnetic material, usually copper, between the faces of the arma-
ture and the pole-pieces. Sometimes this is accomplished by
inserting a small rivet either into the center of the pole-piece or
into the armature face itself.
The length of the rod carrying the hammer plays a consider-
able part in the sensitiveness of the bell. A long rod will secure
for the hammer a long, and therefore powerful, stroke, but the
CALLING APPARATUS. 85
sensitiveness is correspondingly reduced. On the other hand, a
short rod will produce a short and comparatively weak stroke, but
the bell will be more sensitive than with the long rod.
Other points in the design of magneto-generators and ringers
will be taken up in a subsequent chapter on Commercial Forms
of Magneto-Bells. Before considering these in detail, however,
certain other accessories must be described.
CHAPTER VIII.
THE AUTOMATIC SHUNT.
ON account of the high resistance of the generator armature
and its great retarding effects, it is desirable to have it shunted
out of the line when the generator is not in use. Especially is
this desirable on party lines where two or more instruments arc
used on a single line. To accomplish this many devices have
been used, both automatic and manual. The automatic devices
have now almost entirely supplanted the manual, as the latter
were never satisfactory, owing to the inability of ignorant and
careless persons to properly manipulate them. Many styles of
these automatic shunting devices have come into general use, the
ones shown and described being typical. Referring to Fig. 74,
Fig. 74. Western Electric Armature Shunt.
which shows the shunt used by the Bell Company, the gear-wheel,
G, is mounted on the crank-shaft, S, and is free to turn thereon
through a small portion of a revolution. Terminals, a a', are
connected to the terminals of the armature winding.
When the generator is at rest a current coming over the line
will pass from a through the crank-shaft and out through the
spring, o, to the terminal, a; this path being of almost no resist-
ance, while that of the armature winding is large. When the
crank is turned, however, the pin,/, rides out of the notch in the
hub of the gear-wheel and in so doing pulls the shaft out of con-
tact with its spring, o, thus breaking the low resistance path or
shunt around the armature, and leaving the latter effectively in
the line.
86
THE AUTOMATIC SHUNT. 87
In Fig. 75, A is the core of the armature, G' its pinion, and w
a diagrammatic representation of the winding. While at rest
current from the line, instead of passing through the coil, w, will
take the path . from #' through the core, A, to the spring, 5,
thence to pin,/, on which 5 normally rests and thence out through
pin, c } to the terminal, a. When^he armature is revolved the cen-
Fig. 75- Centrifugal Armature Shunt.
trifugal force of the end of spring, 5, causes contact to be broken
between it and pin, /, which opens the shunt around the arma-
ture winding.
In Fig. 76, G is the large gear-wheel and G' its pinion on the
armature shaft. The low-resistance path around the armature is
from point a through spring, 5', screwed on the inside of the
generator box, through spring, 5, gear, G, to the frame of the
machine and out at a '. When the crank is turned, the collar, c,
Fig. 76. Western Telephone Construction Co. Armature Shunt.
which is loose on the shaft, z, but rigid with the crank, forces
the spring, 5, away from the spring, 5', by virtue of the pin, p y
mounted on the shaft, z, engaging the spiral slot in the collar, c.
88
AMERICAN TELEPHONE PRACTICE.
In the later forms of this shunt, which is that of the Western
Telephone Construction Company, a disk collar pressed from
sheet brass is interposed between the springs, 5 and 5', thus
affording a better contact surface for the spring, S'.
A shunt device recently put on the market by the Sterling
Electric Company is shown in Fig. 77. The shunt mechanism is
operated by the crank-shaft, which carries the large gear-wheel.
This shaft turns in a hollow sleeve, 24, which is journaled in the
brackets, 25, mounted on the end plates of the machine. A pin,
27, fixed in the crank-shaft, 23, engages a diagonal slot, 26, in
the sleeve, 24. This pin is held at one end of the slot by the
spring, 28, which is coiled around and fastened to the sleeve, 24.
Fig. 77- Details of Cook Armature Shunt.
When, however, the crank-shaft is rotated, the pin, 27, rides against
the side of the slot, 26, until it assumes the position shown in the
upper right-hand portion of the figure, after which the sleeve and
gear turn with the shaft. This causes the crank-shaft, 23, to break
contact with spring, 29, and thus break the shunt around the
generator.
Another shunt, dependent on an entirely different principle
very ingeniously applied, is that used by the Holtzer-Cabot Com-
pany. A small cylindrical case of brass is mounted directly on the
projecting portion of .the armature shaft, and is in metallic contact
therewith. The insulating pin with which the other terminal of
the armature winding is connected projects into the chamber
formed by this case ; but does not come into metallic contact with
the casing nor the armature shaft itself. The chamber is then
THE AUTOMATIC SHUNT.
89
partially filled with small bits of metallic wire which normally
form a connection between the central pin and the casing itself,
thereby forming a short circuit or shunt around the armature
winding. When, however, the armature is rotated, the centrif-
ugal force, due to the rotation acting upon the bits of wire, causes
them to fly to the outer portion of the casing, thus breaking the
contact with the central pin and removing the shunt from the
armature. This is probably the^ simplest shunt on the market,
and should prove reliable. In order to prevent corrosion of the
parts, the casing and the small particles of wire are silver-plated.
Still another shunt of new design is that used by the Williams
Electric Company on their new generators. It is shown in
Fig. 78.
In this figure the crank-shaft, s, is tubular and incloses the shaft,
s' t on which the gear-wheel is mounted. The crank-shaft is
Fig. 78. Williams Armature Shunt.
capable of a slight rotation before the gear-shaft is moved, such ro-
tation, or what may be termed lost motion, being consumed in de-
pressing two springs, s s', which actuate levers, pp', within the cup-
shaped piece, c, mounted on the crank shaft. One or both of these
levers makes contact with the arched metallic strip, A, insulated
from the frame of the generator, while the crank is at rest. As
soon, however, as it is turned the lost motion is taken up in remov-
ing the levers from contact with this arch. This breaks the shunt
which is formed from the frame of the machine, which is in contact
with one terminal of the armature winding, through the levers
to the arched strip, which is connected by a wire, to the spring
at the right-hand portion of the generator, which bears upon
the armature spindle.
CHAPTER IX.
THE HOOK SWITCH AND CIRCUITS OF A TELEPHONE.
So far we have considered the talking apparatus and the call-
ing apparatus separately. It is obvious that inasmuch as these
are used alternately, some means is necessary for switching one
or the other into the circuit. As an instrument must, when not
in use, be ready to respond to a call, the call-bell must, of neces-
sity, be normally left in the line ; and further, as the resistance
and self-inductance of the call-bell magnets would be detrimental
to the transmission of talking currents, the call-bell must in most
cases be switched out of the line when the talking instruments
are in use.
At first, hand switches were used to accomplish this result, and
even before the adoption of the battery transmitter the instru-
ments were provided with ordinary two-point hand switches, so
arranged as to alternately close the line circuit through two
branches one containing a call-bell and generator, and the
other the magneto-telephone. It was soon found necessary to
make this switch as nearly automatic as possible, as careless or
ignorant users would frequently leave it in the wrong position.
To attain this end, the switch lever was so designed as to be
held by the weight of the receiver in contact with a terminal of
the calling circuit, but when released therefrom to be moved by
a spring into contact with the talking circuit terminal. Soon
after, battery transmitters having come into general use, it
became necessary to provide means for opening and closing a
local circuit containing the local battery, the primary of the
induction coil, and the microphone transmitter. This was done
in order to have the battery in use only when the telephone
instrument was being used, and was accomplished by the addition
of a single contact point with which the hook made contact
when released from the weight of the receiver.
Fig. 79 shows the circuit of an ordinary telephone instrument.
The hook, H, is shown in its depressed position as though under
the weight of the receiver. In this position all talking circuits
are inoperative, being open at the points, I and 2, and are for that
reason represented by dotted lines. A calling current from some
go
THE HOOK SWITCH AND CIRCUITS OF A TELEPHONE. 91
other station coming over line wire, Z, would pass through wire,
a, to the generator, G, thence through the windings of the call-
bell magnet, C, to the contact point, 3, through the lever of the
hook switch and out through line wire, L ', or to ground, in case
r F
Fig. 79. Telephone Circuits, Hook Down.
no return wire is used. This current will ring the bell. To ob-
viate the necessity of this current passing through the armature
winding of the generator, a shunt should be provided, as described
in the last chapter. When the instrument is used for sending a
call the crank of the generator is turned, automatically breaking
the shunt around the armature and sending the current out over
Fig. 80. Telephone Circuits, Hook Up.
the line through the call-bell magnets of this instrument to those
of the distant station.
In Fig. 80 the hook is shown in its raised position, as when
released from the weight of the receiver. The circuit through
the generator and call-bell is inoperative, being open at the
92 AMERICAN TELEPHONE PRACTICE.
point, 3, and is therefore shown dotted. The local circuit con-
taining the primary winding, P, of the induction coil, the battery,
B, and the transmitter, T, is closed by the switch lever making
contact with the points, I and 2. Current therefore flows in this
circuit, and variations in the resistance of the microphone, T,
cause corresponding variations in this current, which induce cur-
rents in the secondary winding, S, of the induction coil. These
currents pass from the secondary coil to the point, 2, thence
through the switch lever to line, L r , to the instrument at the other
end of the line, back by line, Z, and through the winding of the
receiver, R, to the secondary coil, 5. An incoming current from
Fig. 81. Telephone Circuits, Hook Down.
a distant station follows the same path, and causes the diaphragm
of the receiver to reproduce sound.
In Figs. 8 1 and 82 are shown circuits and apparatus for accom-
plishing the same results, but in a slightly different way. It will
be seen that the circuit through the generator and call-bell, and
that through the receiver and secondary winding, are perma-
nently closed, and are unaffected as to their continuity by the
position of the hook switch. In Fig. 81 the hook is depressed,
thus rendering operative the calling apparatus. The circuit
through the instrument is now from line, L, to the generator, G,
thence through the call-bell magnets, C, through the switch lever
to the point, 3, and by way of wire, , and the two upper contacts of the button, to binding post, 2.
The talking circuits are closed in the ordinary manner by the
raising of the hook lever.
Fig. 91 shows a type of telephone set which is becoming
Fig. 91. Desk Telephone Set.
very popular for business men who do not desire to leave their
desks in order to use the telephone. The particular set illus-
trated is made by the Holtzer-Cabot Company. The transmitter
is of the granular-carbon type mounted on a handsome, nickel-
plated and polished wood stand, as shown. One terminal of the
transmitter is formed by the frame itself, while the other terminal
is carried down the inside of the tube, or standard. The lever of
102
AMERICAN TELEPHONE PRACTICE.
the hook switch is pivoted at its end in an enlargement of the
standard and actuates a slender rod which passes down into the
base, where it engages the switch lever. This lever is pivoted on
a separate bar, mounted on a fiber block and acted upon by a
spiral spring in such a manner as to press upward against the
vertical rod, thus tending to raise the hook. In this position a
knife edge, carried on the switch lever in the base, presses against
the two springs, which form, respectively, terminals of the pri-
mary and secondary talking circuits. When the hook is sub-
jected to the weight of the receiver the switch lever is depressed
by the rod against the tension of the spiral spring, thus breaking
connection with the two springs of the talking circuit and making
connection with a spring forming the terminal of the calling cir-
cuit. The induction coil is mounted in the base. This apparatus
Fig. 92. Circuits of Comple Desk Set.
may be used in connection with a magneto-generator and call-
bell, which are usually placed under the desk or at some point
where the generator crank is within easy reach of the user, or in
connection with a battery-call outfit, in which case the circuits
would be similar to those shown in Fig. 90.
Fig. 92 shows the circuits of an apparatus similar to this, manu-
factured by the Western Telephone Construction Company.
Seven binding posts are arranged in this set cfn the upper side of
the magneto-box, to which all terminals from the various pieces
of apparatus are run. i and 2 form the line binding posts, and 6
and 7 the battery binding posts, the other terminals being con-
THE HOOK SWITCH AND CIRCUITS OF A TELEPHONE. 103
nected as shown. Inasmuch as the induction coil in this set
is mounted in the generator box, it becomes necessary for five
different conductors to run from the generator box to the desk
stand. These conductors are constructed in the same manner as
an ordinary receiver cord, there being five strands instead of two.
In this latter set the hook switch is of the same type as previously
described in connection with Fig. 88, but is so modified as to
enable it to be placed in the vertical standard supporting the
transmitter. It is platinum-pointed and, as recently modified,
should prove reliable.
The connections in a telephone cannot be too carefully made.
All possibility of two wires coming into accidental contact should
be carefully avoided. All joints should be soldered, without the
use of acid. Where connection is made under the head of a
screw passing through the wood of the box, means must be taken
to prevent the loosening of the connection due to shrinkage of
the wood. If possible, the connection should be soldered ; if
not, a spring washer may be used.
CHAPTER X.
COMMERCIAL CALLING APPARATUS.
THE combination of a magneto-generator and a polarized bell
or ringer, mounted in a suitable box, is usually termed a magneto-
bell. The hook switch, from the fact that it is usually mounted
in the generator box, is often counted as a part of a com-
plete magneto-bell. Owing to the lively competition between
various manufacturers, and also to the increasing demands for
good service on the part of the public, great improvement has
been made in this line of work during the last few years. This
chapter will be devoted to a consideration of some of the more
approved forms of this very important part of telephone
equipment.
The details of the Western Telephone Construction Company's
large generator for heavy work are shown in Figs. 93,94, and 95.
Figs. 93 and 94. Details Western Telephone Construction Co. Generator.
This instrument is very similar to the one used by the Bell
companies. In it the pole-pieces are of cast iron, riveted together
by means of the shouldered brass rods, B B. After this they are
bored by a special tool to the required internal diameter to re-
ceive the armature. The core of the latter is of cast iron and is
shown in Fig. 95, being accurately turned to fit between the pole-
COMMERCIAL CALLING APPARATUS. 105
pieces. The bearing plates are of cast brass with a shoulder also
turned to fit between the pole-pieces so as to be self-centering when
secured in place. They are each fastened to the ends of the pole-
pieces by four screws, as shown. The gears are cut from heavy
cast brass, the large gear being mounted on a shaft journaled in
the same bearing plates as the armature itself.
The magnets are bent cold from I" x I" magnet steel, and are
secured in place by clamping plates, C C, and screws, 5 5, the
latter passing between the magnets and into the pole-pieces.
This generator, while it embodies no new or radical features, is
well made and generally satisfactory. It would give better
results, however, were the armature of smaller diameter. The air
gap in machines of this type may be reduced to less than 1 -J- Tr of
an inch without endangering the smooth running of the arma-
ture. The automatic shunt already referred to is shown to better
advantage in these figures.
The polarized bell used with the later types of this instrument
is shown in Fig. 88. The two coils, />, are parallel, and attached
. 95- Armature Core of Generator.
at one end to a soft-iron yoke, the ends of which extend beyond
the coils, to receive the two round bar-magnets, which polarize
the frame of the bell.
On the projecting ends of these two permanent magnets is
mounted a second yoke bar of soft iron, on which is adjustably
mounted the bracket in which the armature is pivoted. The two
magnets have their forward ends of one polarity and their rear
ends of the other ; and, together with the two yokes, form a
rectangle, one of the yokes being of positive polarity, the other
negative. In this rectangle are mounted the coils and armature
of the ringer, which operate in the usual manner by the alternating
ringing currents.
The Holtzer-Cabot Electric Company are manufacturing an ex-
cellent magneto set, the generator and ringer of which are shown
in Figs. 96 and 97. The end plates in which the generator arma-
ture is journaled are of cast brass and are screwed directly to the
cast-iron pole-pieces, the ends of which are flanged and machined
so as to fit accurately. The armature core is of soft sheet-iron
io6
AMERICAN TELEPHONE PRACTICE.
laminations. The punchings forming the core are clamped
together on a steel rod, which therefore serves as the armature
shaft. The drive-wheel is mounted in a long bearing, adjustably
secured to an extension on the right-hand end plate. This com-
pany uses two forms of driving gear, one the regular gear-
Fig. 96. Holtzer-Cabot Generator.
wheels and the other the chain and sprocket mechanism shown.
It seems to prefer the latter, which, it must be said, when provided
with a steel chain, runs very easily and noiselessly. This genera-
tor is provided with the centrifugal shunt described in Chapter
VIII., mounted directly on the armature shaft, the whole forming
a very efficient combination. The generators for bridging-bell
service manufactured by this company are of a similar type, but
provided with four magnets instead of three, and a correspond-
- 97- Holtzer-Cabot Ringer.
ingly long armature. It is one of the most powerful generators
for this purpose ever tested by the writer.
The ringer furnished with this generator is of the same general
type illustrated in Fig. 72. The gongs are mounted on adjust-
able standards pivoted at their upper ends, and each held by
a screw engaging a slot in their lower ends.
COMMERCIAL CALLING APPARATUS.
107
The Williams-Abbott magneto-bell is shown complete in
Fig. 98 and in detail in Figs. 99, 100, and 101. In this the pole-
Fig. 98. Williams-Abbott Magneto-Bell Complete.
pieces of the generator are stamped from soft sheet iron and
fastened to the circumferential edges of the end plates by four
screws at each end. The edges of the end plates are of the
proper curvature to maintain the inner surfaces of the pole-pieces
in their proper relation to the armature. The lower edges of the
Figs. 99 and ioo,-End and Side Views Williams-Abbott Generator,
pole-pieces are bent back and up so as to form a seat for the per-
manent magnets, which are secured in place by two bolts passing,
respectively, between the two outside and the center magnets.
The gear-wheels in this instrument are cut with a very wide face
to prevent wear.
io8 AMERICAN TELEPHONE PRACTICE.
The ringer shown in Fig. 101 is unique and embodies several
desirable features: The two ends of the U-shaped permanent
magnet are of the same polarity say, south and its middle is
of the opposite polarity. (Hence the name tripolar, frequently
applied to this ringer.) The two coils are mounted on the iron
cross bar, extending between the legs of the permanent magnet.
The poles of these electromagnets therefore partake of the
polarity of the permanent magnet ends, that is, south, while
the armature, supported from the center of the permanent magnet,
becomes of north polarity. The action of this ringer is usually
misunderstood at first sight, the natural supposition being that
the limbs of the permanent magnet are of opposite polarity.
Besides being a very efficient ringer, this has the advantage of
having its working parts inclosed by the rigid permanent
magnet, which serves to protect them from mechanical injury.
In Figs. 102, 103, 104, 105, and 106 are shown the details
Fig. 101. Williams-Abbott Ringer.
of a unique magneto-generator and ringer recently put on the
market by the Williams Electric Company of Cleveland. This
apparatus is the design of Mr. J. A. Williams, and embodies such
radical departures from the details of the ordinary magneto as to
warrant a somewhat minute description.
The magnets of the generator are formed of heavy bars of
steel, | of. an inch thick by ij inch wide. They are placed
close together, thus covering the entire length of the pole-pieces
and securing a maximum magnetic density across the pole faces.
The pole-pieces are punched from sheet iron, and are ac-
curately formed to inclose the space in which the armature turns.
The end plates, which hold the crank-shaft and armature bear-
ings, are punched from heavy sheet brass, and have riveted into
them long brass bearings, which is an important feature in secur-
ing good wearing qualities.
COMMERCIAL CALLING APPARATUS.
109
The end plates and the pole-pieces are secured by screws, as
shown in Fig. 104, to horizontal brass plates above and below the
armature. The method of affording a path for the magnetic
lines from the permanent magnets to the pole-pieces is ingenious.
Two sheet-iron contact plates are, provided, one for each pole-
piece. Each of these has two ears turned inwardly to hold it
in position on the pole-piece, and also four ears turned out-
wardly to hold the permanent magnets in their proper place on
the generator.
Three portions of each contact plate are left flat or straight, so
as to make a good contact surface on the permanent magnets,
and two portions are formed so as to make curves coincident
Figs. 102 and 103. Williams Generator and Magneto-Bell.
with the outside curvature of the pole-pieces. This arrangement
is for the purpose of securing good contact between the perma-
nent magnets and pole-pieces, and also for taking the magnetism
from the permanent magnets, at points about opposite the center
of the armature, thus securing a somewhat longer effective
magnet.
The gear and pinion are on the left-hand side of the machine
instead of the right, as is usual, the object being to get them as
far away as possible from the uneven strain on the shaft due to
HO AMERICAN TELEPHONE PRACTICE.
the turning of the crank. The feature of the gear, however, is
that it is radially corrugated, as shown in Figs. 103 and 104, the
object of this being to secure a uniform rate of wear between
Fig. 104. Details of Williams Generator.
the teeth on the pinion and on the gear. The corrugations on
the gear cause its teeth to play over a width of pinion about five
times as wide as the face of the gear. This, in view of the fact
that the gear has about five times as great a circumference as the
pinion, renders the wearing surface on the two about equal. In
order to prevent the large gear plowing a rut in the small one by
always traveling in the same path, the ratio of the teeth is
made uneven, there being 134 teeth on the large gear and 27 on
N
Figs. 105 and 106. Williams Ringer.
the pinion ; thus the armature must make 134 revolutions before a
tooth, on the gear engages the same tooth on the pinion twice.
The pinion is attached to the armature shaft by means of a key
and machine screw, and can be easily taken off and replaced
COMMERCIAL CALLING APPARATUS. 117
without driving, a feature of great convenience in making
repairs.
The magnets are clamped to the frame of the machine by two
brass bolts, these being long enough to pass through the bottom
of the generator box, so as to receive nuts for securing the
generator in the box. The automatic shunt of this machine has
already been described in Chapter VIII.
Not less unique than the generator of this machine is the
ringer. It has but a single core, which is parallel with the vibrat-
ing armature. The core heads or end pieces are formed up of
Swedish iron, and are swaged onto the core, forming a very per-
fect magnetic joint.
The permanent magnet of the ringer is U-shaped and of the
consequent-pole type, the central portion being of one polarity
and the extremities of the other. The armature is suspended
from the central and consequent pole.
The magnetic circuit of this ringer is well shown in Fig. 106
by the iron filings which cling to the poles. It will be noticed
that the core of the coil really forms no part of the magnetic
circuit of the permanent magnet, as each of its ends is magnet-
ized to an equal extent and with the same polarity.
The action of this ringer is as follows : The two pole-pieces,
which form at the same time the heads of the spool, are mag-
netized by the ends of permanent magnet, with the same
polarity we will say, north. The armature, attached to the
center or south pole, will therefore have the opposite polarity-
south. When a current traverses the coil, it gives one end of
the core and the corresponding head a north polarity and the
other end and head a south. This strengthens the magnetism
already possessed by the former and weakens that of the latter,
thus causing the armature to be attracted by the stronger pole.
The next instant the current reverses, and the opposite end of
the armature' is attracted.
The design of this ringer is certainly good. The length of
magnetic circuit through which the magnetism set up by the cur-
rents in the coil acts is reduced to a minimum. The ends of
the core are subjected to a normal magnetic stress, yet the core
forms no part of the normal magnetic circuit. It is therefore
rendered very sensitive to changes in the magnetizing force due
to the coil, because the normal magnetic flux through the core is
nil.
Fig. 102 shows the complete instrument provided also with a
long-lever gravity switch hook, with Craig silver contact-pieces
112 AMERICAN TELEPHONE PRACTICE.
attached to a hard-rubber block, which, together with the hook,
is mounted in the bell box in such a way as to make the switch
hook self-contained.
The box is wired throughout with tinned copper wire, which
rests between tinned spring washers, secured by entering the
binding posts, thus forming spring contacts, all other contacts
being soldered.
The regular io,OOO-ohm magneto set usually has an armature
wound with No. 35 or No. 36 B. & S. silk-covered wire, to resist-
ances varying from 400 to 550 ohms. The ringer magnets are
usually wound with No. 31 B. & S. wire and have a resistance of
from 75 to 100 ohms. The ordinary construction of ringer mag-
nets is to drive the fiber heads directly onto the cores, and after
insulating the surface of the latter to wind the spools thus formed
with silk-insulated wire of the desired size. Fig. 107 shows one of
Fig. 107. Varley Ringer Coil.
the sectional coils of the Varley Duplex Magnet Company. Their
coils are wound separately, with bare wire separated by silk thread.
After winding, the coils may be slipped on the core and locked by
the end washer or head. The advantages of this construction are
in the ease of replacing burned-out or otherwise injured coils, and
also in the perfect uniformity of the winding. Bare-wire winding
will probably play an important part in telephony of the future.
Where many instruments are to be placed in series on a party
line the ringer magnets should be made as low as 40 ohms, in
order to reduce the amount of self-induction through which it is
necessary to talk. This is a practice little followed ; but good,
nevertheless, when instruments must be placed in series.
For bridging work the conditions are changed. The generator
must have greater current capacity, and for that reason a larger
wire in the winding is necessary. This necessitates fewer turns
for the same winding space and a consequent loss in voltage.
In some long lines the voltage must necessarily be maintained,
and in order to make up for the loss in this respect, due to fewer
COMMERCIAL CALLING APPARATUS. 113
turns, the field magnets may be made stronger and the armature
longer. A good generator for bridging work may be wound
with No. 33 wire to a resistance of 350 ohms.
The ringer magnets for bridging work must possess a very high
degree of self-induction. This should be obtained by winding
them to a high resistance with a comparatively coarse wire, so as
to obtain a large number of turns in the winding. The length
of the cores is increased for the double purpose of getting more
iron in the magnetic circuit, and therefore a higher retardation,
and also for affording a greater amount of room for the winding.
The Western Electric Comp;uiy wind their coils to a resistance
of lOOOohms, using No. 33 single-silk magnet wire. Many other
companies use No. 38 wire and wind to a resistance of 1200 or
1600 ohms. This does not give such good results, however, as
using the coarser wire and the lower resistance and long cores.
Some companies wind, or once wound, their bridging-bell mag-
nets partly with German-silver wire, in order to make a high
resistance at a low cost. They should learn, however, that resist-
ance in itself is not the thing desired, but a great number of turns
in the winding, which, of course, incidentally produces a high
resistance.
CONSTANTLY DRIVEN GENERATORS.
In telephone exchanges, constantly driven generators are em-
ployed for ringing purposes, in order that the operators may be
Fig. 108. Western Power Generator.
saved the labor of manually turning a crank every time a subscri-
ber is called up.
114 AMERICAN TELEPHONE PRACTICE.
Fig. 108 shows one type of machine for this purpose. It is
merely a magneto-generator provided with very long and powerful
Fig. 109. Holtzer-Cabot Belt-Driven Magneto.
permanent magnets. The machine is mounted on a slate base
and driven by means of a grooved pulley, mounted on a separate
Fig. 1 10. Motor-Generator.
shaft connected to the armature shaft by a short flexible rubber
coupling.
Machines of this type are suitable for small exchanges, and
may be driven by any available source of power. They are
COMMERCIAL CALLING APPARATUS. 115
frequently placed in an electric light or power plant, where they
may be constantly driven. Wires are then run from them to the
exchange, from which the current is sent out over the various
subscribers' lines as desired.
Another convenient method of obtaining power for switch-
board generators is by the use ( of small water motors, run from
Fig. ill. Motor-Generator for Battery-Charging.
city water mains. These are specially built for this purpose,
and consume but little water.
A more convenient source of power is an electric motor ; and
Fig. 109 shows a Holtzer-Cabot magneto belted to a small direct-
current motor, also manufactured by that concern. Convenient
forms of alternating-current motors are also made for running
generators for this purpose.
All the machines so far mentioned are suitable for exchanges
of 500 subscribers and under. For large exchanges, and also
many smaller ones, the motor-generator, types of which are
shown in Figs, no, ui,and 112, are being commonly used.
The one shown in Fig. no is manufactured by the Holtzer-
Cabot Co., and is provided with a double winding, on a single
armature core, the same field serving for both windings. The
n6
AMERICAN TELEPHONE PRACTICE.
side having the commutator is the motor side, and may be
wound for any desired direct voltage. The right-hand side is
provided with collector rings for supplying alternating current
for ringing purposes at a pressure of 75 volts.
Fig. 1 1 1 shows a machine of similar type manufactured by
the Cre.cker- Wheeler Co. This furnishes alternating ringing
current, and besides is provided with an extra winding for sup-
plying low-voltage direct current, for the purpose of charging
storage batteries.
In Fig. 112 is shown a similar machine for charging storage
batteries in telephone work, and provided with an automatic
Fi
>
-I- +
A'
/
Fig. 123. Electrostatic Disturbances.
makes it easier for most of the current to pass through the trans-
position wires.
Theoretically, the currents set up in a metallic circuit by
electrostatic induction from another circuit can be eliminated
only by making an infinite number of transpositions. Practi-
cally, however, it is found that on long circuits transpositions
every quarter- or half-mile are amply sufficient to render them
unnoticeable.
The scheme of transposition used by the American Telegraph
and Telephone Company on the New York-Chicago telephone line
is shown in Fig. 124. It will be seen from this figure that trans-
# /300/+-
f-jjoo/e 1
Y 1300ft-
'S/ipercr
QSS Jlrn
'V
j
(^
/
\
\
/
\
m
ver- Cr
?5s Arr
1
Ji.
)
)
(
i
a '
Jto
^
Fig. 124. Diagram of Transpositions.
positions are made on this line practically four times in every
mile, that is, upon every tenth pole; and while this involves the
placing of transposition insulators on poles a quarter of a mile
144 AMERICAN TELEPHONE PRACTICE.
apart, it does not follow that every circuit is transposed at each of
these intervals. The reason for this arrangement is that if two
lines running side by side were transposed in exactly the same
manner throughout their lengths, the desired non-inductive condi-
tion would not be secured for the relation between the corre*
spending wires in the two circuits would then be the same as if no
transpositions whatever had been made. In order to overcome
this difficulty, transpositions on the second circuit should be
made twice as often as those on the first. This is the scheme
adopted in Fig. 124, where it will be seen that the center pair of
wires on each set of cross-arms is transposed every mile, while
the pair immediately adjacent to it on each side is transposed
twice as often. The outside pairs on each cross-arm are trans-
posed only once per mile, but these transpositions are staggered
with respect to those on the center pair. The same scheme is
followed out on every cross-arm, but the transpositions on the
top set of cross-arms are staggered with respect to those on the
set immediately below this being the case throughout the
entire number of cross arms on a pole ; the 1st, 3d, 5th, /th, and
9th being transposed according to the scheme shown in the upper
part of Fig. 124, while the circuits on arms Nos. 2, 4, 6, 8, and 10
are transposed according to the scheme in the lower part of this
figure.
A very perfect transposition is effected by twisting two sides of
a circuit together, and this idea is followed out in the English
pole-line construction, where the two sides of the circuit are not
only transposed laterally, but also pass successively over and under
each other several times in each mile, thus effectually giving the
circuit a number of complete twists. This method, however,
involves several disadvantages in the stringing of wires, and
increases the liability of crosses between them. It is not, there-
fore, adopted to any considerable degree in this country.
The twisted pair of insulated wires used so largely in inside
wiring, and also in cable work, accomplishes the transposition of
circuits very thoroughly, it in fact amounting to a complete
transposition for every twist of the wires. This method is now
depended upon entirely in the construction of telephone cables,
with so great a degree of success as to absolutely prevent all
induction between the circuits. This will be discussed at greater
length in the chapter on cables.
Where a number of lines radiate from a central point to a
number of subscribers' stations the cheapest way of arranging
the circuits, if expense alone is to be considered, is to make each
TELEPHONE LINES. 145
a grounded circuit. This is done by grounding each line at the
subscribers' station after it has passed through the telephone
there, and also at the central office after it has passed through
the coil of the annunciator or signaling device. Such an arrange-
ment is shown in Fig. 125, and it may be assumed that the lines
there shown run in the same direction on the same poles or in
the same cable to the various subscribers whom they serve. In
each case D is the line drop at the central office and R repre-
sents the entire telephone set at the several subscribers' stations.
It is evident that, with such an arrangement, disturbances in
the receivers may be produced by any one or all of the causes
already considered. An electric light or power wire, carrying a
CENTRAL
OFFICE
Fig. 125. Ground-Return Systems.
heavy current, may cause trouble by electrostatic or electro-
magnetic induction, or by leakage ; and, moreover, each telephone
wire, when in use, may be a disturbing wire to all of the others.
As has been pointed out, the proper remedy for these disturb-
ances is to make each line a separate metallic circuit, and to prop-
erly transpose the two sides of each circuit at frequent intervals
where the lines are long. This course is followed in most large
telephone exchanges, and many small ones ; but it frequently
happens that commercial considerations will not allow it in
smaller installations. Where this is the case a system called the
common-return or McCluer system is frequently used, with ex-
cellent results. The layout is the same as that of the grounded
system, with the exception that the return of every circuit is
made through a heavy wire common to all of the circuits. A
clear conception of the common-return system may be had by
considering a heavy wire, Fig. 126, to take the place of the earth
in a grounded system ; each line wire being connected to it at or
near the subscriber's station after passing through the telephone
i 4 6
AMERICAN TELEPHONE PRACTICE.
instrument, R, and at the exchange after passing through the
switch-board drop, D.
It is quite evident that the common-return system will, if
properly installed, remove all trouble due to leakage or earth
currents ; for the entire system of wiring may be kept highly
insulated from the ground and from other conductors. Practice,
however, differs to a large extent in this respect, as some com-
panies ground the common-return wire at the exchange, and also
at several other points along its length, others at the exchange
only, while others keep it entirely insulated from earth. Prob-
ably the reason for placing several grounds on the return wire is
to effect a reduction in the resistance of the return path, but this,
if needed, should be brought about in another way. Probably
the best practice in most cases is to keep entirely free from
grounds, although there are many who claim to have effected a
marked improvement by heavily grounding the common return
at the central office. At any rate, this is an easy experiment to
try. The location of the common-return wire with respect to the
COMMON RETURN
Fig. 126. Common-Return Systems.
other wires on the poles is a question concerning which there
is much difference of opinion. Undoubtedly the best way of
disposing it, so far as purely electrical considerations are con-
cerned, is to place it on brackets between the two middle cross-
arms on the poles, for then it bears a more symmetrical position
with respect to all of the line wires and is, therefore, better
adapted to neutralize induction from outside sources. It is, how-
ever, often put on other parts of the pole, sometimes above all
the wires, sometimes on a bracket just below the top cross-arm,
and frequently below the lowest cross-arm. The latter is prob-
TELEPHONE LINES. 147
ably the most convenient place, as wires led off to buildings will
usually stand clear of the other wires on the pole.
There is even more difference of opinion as to the proper size
of the common-return wire than as to its location. An analysis
of the inductive action from neighboring wires may perhaps
throw some light upon the subject. If we assume that the entire
system is insulated from the ground and from other conductors,
it will be safe to say that all disturbances in the telephones due
to leakage or earth currents will be eliminated. The only way,
therefore, by which neighboring wires can affect the telephone
wires is by induction, and this may be either electromagnetic or
electrostatic, or both. The ideal arrangement of the common-
return wire with respect to the line wires would be that in which
all were at an equal distance from the disturbing wire. This con-
dition can only be roughly approximated in practice, but in Fig.
127 it will be assumed that the disturbing wire, which may be a
CENTRAL
OFFICE
u I
A
r c 2
==!*
D
IAAA/ c. o. DROP
1 <
S.R. b
C'
DISTURBING WIRE
Fig. 127. Induction on Common-Return Lines.
trolley or electric light line, is at approximately the same dis-
tance from the line wires, I and 2, from the return wire, C R. The
lines, i and 2, are connected at central by the cord circuit,
C, a clearing-out drop being bridged between the cord and the
common return, and the line drops, D D, cut out as in ordinary
practice.
It is evident that if no sounds are to -be produced in the
receiver of Line I, the points, A and B, must be of equal potential.
Similarly the points, a and b, must be of equal potential for no
disturbance to be produced in the receiver of Line 2, and the
points, C and C\ for no current to flow through the clearing-out
drop. It matters not if C, a, and A are all of different poten-
tials, the one requisite for silence in the receivers being that the
148 AMERICAN TELEPHONE PRACTICE.
two points at the terminals of each shall be of the same potential
The question, therefore, becomes, what size of wire shall be used
for the common return in order to bring about these conditions ?
Considering the induction from the disturbing wire to be elec-
tromagnetic, it is evident that the electromotive force set up in
the several wires will be proportional only to their lengths, as
their distances from the, disturbing wire is assumed to be the
same in each case. The sizes of the wire will have nothing to do
with the pressure developed, any more than the size of the wire
per se affects the electromotive force developed in the armature
winding of a dynamo. In the case of the wire, i, the E. M. F.
generated in the length, C A, will equal that generated in the
length, C l B, of the common-return wire. This means that the
points, A and B, have the same potential as have also C and C 1 t
and no current will flow through the receiver of that line or the
clearing-out drop. By the same reasoning the pressure devel-
oped in length, C a, of Line 2 will equal that in the length, C 1 b,
of the common return, and no current will flow through the
receiver of Line 2. So far as electromagnetic induction is con-
cerned, therefore, the size of the common-return wire is im-
material.
Considering the question from the standpoint of electro-
static induction, it will be seen that for a given charge on' the
disturbing wire charges of the same potential will be induced on
each of the line wires and on the common-return wire. There
will, perhaps, be a tendency for the small wires to assume charges
of slightly higher potential than the larger wire, on account of
their smaller radius of curvature, but this would be so slight as
to be negligible under the conditions assumed, and would be
eliminated were the common-return wire made of the same size
as the line wires.
So far as inductive disturbances from outside sources are
concerned, it seems that the size of the common-return wire is
practically immaterial, with perhaps a slight theoretical advan-
tage to be gained by making it of the same size as the line wires.
This view seems to be at direct variance with nearly all written
statements on the subject.
The same reasoning will show that when any one of the tele-
phone lines is considered as the disturbing wire, the same conclu-
sions are reached.
So far no valid reason has been shown for making the com-
mon-return larger than the line wires. There is, however, a
good reason why this should be done, and this is the fact that
TELEPHONE LINES. 149
the return circuit of any one wire is made, not only through the
common-return wire, but also through all of the other line wires
in multiple. Referring to Fig. 126, it is evident that currents
generated in line, B C, may find a return path through the com-
mon-return wire, and through all of the other wires in multiple.
In fact, all of these return paths will be chosen, the current
dividing among the other line wires and common return, in-
versely as to their respective impedances.
The currents flowing through these other lines would produce
cross-talk were they of sufficient magnitude to do so, and the
only way of preventing this is to make the common-return wire
of such low resistance that practically all of the current will pass
through it. The fact that the common-return wire may be made
to possess practically no self-induction, and, therefore, only the
impedance due to its ohmic resistance ; while the line wires all
include in their circuits either the receiver and induction coils or
the bell and drop coils, serves to divert nearly all of the current
through the common-return wire, where it belongs. On account
of the marvelous sensitiveness of even poor receivers, a compara-
tively small resistance in the common return will shunt enough
current through the various instruments to cause cross-talk to a
considerable extent.
There is undoubtedly a large amount of copper wasted in
common-return wires, and it is probable that a No. 8 B. & S.
gauge copper wire will in most cases answer the purpose. It too
frequently happens that larger common-return wires are used in
the hope of remedying a difficulty which is due to another cause
entirely. Cross-talk in switch-boards and office cables is often
attributed to the smallness of the return wire, which in this
case might be increased indefinitely without improving the
service.
Of course, the ideal conditions assumed for the disposition of
the wires cannot be attained, and in so far as they are not
attained, induction is likely to occur.
It frequently becomes desirable to connect a grounded line
with a metallic line, and for this purpose what is known as the
repeating coil forms the most ready solution. A repeating coil is
merely a special form of induction coil, usually constructed in the
form shown in Fig. 128. The primary and secondary coils are
wound upon a heavy core formed of a bundle of soft-iron wires,
after which the ends of the cores are bent around the outside of
the coil, thus completely inclosing it in a casing of wire. The
coil is then clamped to the base, usually by metal straps, as
15 AMERICAN TELEPHONE PRACTICE.
shown in the figure. The terminals of the primary are brought
out to binding posts at one end of the base, while those of the
secondary are similarly brought out to binding posts at the op-
posite end. The wire forming the core should be of No. 24
Fig. 128. Repeating Coil.
annealed iron formed into a bundle about f of an inch in diameter.
The two coils are usually made equal in resistance and in number
of turns, 200 ohms for each coil being perhaps the most common
figure. No. 31 B. & S. gauge silk-covered wire is a suitable size
for a coil adapted to ordinary work. Of course these figures may
be departed from to almost any degree in order to design coils
for special work.
In Fig. 129 is shown in diagram a metallic circuit line con-
nected with a grounded line through a repeating coil. The two
METALLIC CIRCUIT /~5&\ GROUNDED LIN
Q
Fig. 129. Connection of Metallic and Grounded.
terminals of the metallic circuit line are merely brought to the
two binding posts of one of the coils, while the terminal of the
grounded line is brought to one of the binding posts of the other
coil ; the remaining binding post is then grounded. Any vary-
ing currents set up in one of the circuits will act inductively on
the other circuit through the windings of the coil, each of which
may thus be called upon to act alternately as a primary and a
TELEPHONE LINES. 151
secondary. By the use of the repeating coil in this manner,
two lines may be connected for conversation without grounding
one side of the metallic circuit, which would be necessary were
the repeating coil not used.
There is a very common impression among the independent
telephone users that a repeating coil is the one panacea for all
of the evils connected with grounded lines. It is perhaps well to
correct this impression, by saying that no number of repeating
coils will render a noisy grounded line quiet. A repeating coil
will, however, prevent the unbalancing of a metallic circuit line,
and therefore in many cases insure a degree of quietness on
two connected lines which would otherwise be unattainable.
It sometimes happens that a long grounded line is paralleled
throughout a portion of its length only by some disturbing wire,
DISTURBING WIRE
\
Fig. 130. Eliminating Local Induction.
such, for instance, as an electric-light line. Where it is not pos-
sible from commercial considerations to make the entire line a
metallic circuit, much relief may sometimes be had by resorting
to the plan shown in Fig. 130, which consists in making only
that portion of the line a metallic circuit which is within the
direct influence of the disturbing wire. The two ends of the
grounded line may then be connected with the intermediate
metallic portion by means of the repeating coils, R R, as shown.
By this arrangement the disturbing wire produces no effect on
the metallic circuit between the repeating coils, if proper precau-
tions are taken in the way of transposing its two sides. Tele-
phonic communciation may be had over the entire length of line,
the currents undergoing two transformations at the repeating
coils.
Much trouble is often had where it is necessary to ring
through repeating coils, especially if the lines are very long. It
is therefore advisable that repeating coils should always be
placed at a central station if possible, and such arrangements
152 AMERICAN TELEPHONE PRACTICE.
made that it will not be necessary to ring through them. How-
ever, a coil properly constructed with a magnetic circuit com-
pletely closed should serve as a very efficient transmitter, even
for the slowly alternating currents of a magneto-generator, and
good results may be obtained with such coils on good lines even
when it becomes necessary to ring through them.
CHAPTER XIV.
SIMPLE SWITCH-BOARD^ FOR SMALL EXCHANGES.
THE object of a telephone exchange is to afford means for
placing any telephone user (subscriber) into communication with
any other subscriber in the same system. The lines leading to
the telephone instruments of the various subscribers radiate from
a central point where they terminate in an apparatus known as
a switch-board.
Switch-boards may be divided into two classes, manual and
automatic. In the manual switch-board, operators girls are
employed to make the connections called for, while in the au-
tomatic, the operation of connecting or disconnecting lines is
performed by the subscriber desiring the connection. The
manual switch-board only will be considered at present, as it is
in almost universal use, the automatic, owing to its great and
necessary complexity, having proven successful only in rare cases.
The simplest form of switch-board, one typical of the kind
used in small exchanges and designed for use on grounded or
common-return systems, will first be considered.
Each line entering the exchange terminates in what is known
as a spring-jack. Spring-jacks are sockets containing or asso-
ciated with simple switching devices and are mounted on the
face of the board within easy reach of the operator. In order
to make a connection with any line, plugs are provided which
may be inserted into the jacks, and thus continue the electrical
path from the line wire terminating therein, to and through a
flexible conducting cord attached to the plug.
Fig. 131 shows a simple spring-jack with a connecting plug
inserted. The metallic base, a, of the jack, usually of brass, is
drilled from its forward end to receive the shank of the plug, P.
A forwardly projecting sleeve on this base fits snugly into a
hole bored in the front board, A, of the switch-board, to which
it is fastened by the shoulder and small wood-screw as shown.
Firmly secured to the rear end of the piece, a, is the line spring,
c, formed with a rearwardly projecting tongue, to which the
wire, /, leading from the line is soldered. The forward end of
the spring, c, rests normally against the pin, /, carried by, but in-
153
154 AMERICAN TELEPHONE PRACTICE.
sulated from, the base, a. A wire, g, leads from this pin, and
through the coil of the line annunciator or drop to ground.
When the plug is inserted in the jack its conducting tip makes
contact with the tip of the line spring and at the same time
forces it out of engagement with the pin, /. Normally, therefore,
Fig. 131. Spring-Jack.
the line wire is connected to the ground through the wire, /,
spring, c, pin,/, wire, g, and line-drop to the ground connection.
When the plug is inserted in the jack, however, the line is dis-
connected from the branch leading through the drop, but is con-
nected through the medium of the plug to the flexible cord.
Fig. 132 shows a common form of switch-board drop. The
purpose of the drop is to attract the attention of the operator
whenever any subscriber wishes a connection.
The coil of the electromagnet is mounted on
the back of the front plate, c, of the switch-
board, as shown. To the armature, a, pivoted
at its upper end, is attached a rod, #, passing
forward through a hole in the front plate and
provided with a hook on its forward end,
Fig. 132. Switch- adapted to engage the upper portion of a
Board Drop. pivoted drop-shutter, s, and to hold it in its
raised position. The attraction of the arma-
ture due to a current passing through the coil causes the hook
to rise, thus releasing the shutter, which falls to a horizontal
position and displays to the operator the number by which that
line is designated.
In order to attract the attention of the operator at night or
at such times as she may not be in sight of the board, a night-
alarm attachment is provided on each drop, which serves to close
the circuit through a battery and vibrating bell whenever the
shutter is down. The small cam surface on the lower portion of
the shutter, s, forces the light spring, /, into contact with the
SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. 155
pin, /', when the shutter is down, thus accomplishing the above
result.
Fig. 133 shows diagrammatically the circuits of such a switch-
board. But two subscribers' lines with their spring-jacks and
Line. Jacls
Line JJrops.
Fig- 133- Switch-Board for Grounded Lines.
drops are shown. These, it will be noted, enter by the line
spring of the jack, and thence when the plug is not inserted their
circuits pass through the contact pin of the jack through the
electromagnets of their respective drops and to ground at G.
In the lower portion of the figure, R represents the operator's
receiver, T her transmitter, B the transmitter battery, s and p
respectively the secondary and primary windings of the opera-
tor's induction coil, Pand P a pair of plugs, and K, K' y and K"
keys connected therewith, the purpose of which will be described
later. When one of the line drops falls, indicating that the
subscriber ^n thp- f .!"* desires a connection, the operator takes
156 AMERICAN TELEPHONE PRACTICE.
up the plug, P , and inserts it into the jack bearing the cor-
responding number, say, No. 20. She then moves the lever of
the key, K" , into the position shown that is, so that the spring
of this key makes contact with the stop below. This movement
connects the operator's telephone set with the telephone of the
subscriber calling, the circuit being traced from ground at the
subscriber's station through his instrument to his line wire, from
the line wire to the line spring in the jack, thence to the plug,
P', cord, c', to the lever of the key, K', through the upper contact
of this lever to the lever of key, K' ', thence through the
operator's receiver and the secondary of her induction coil to
ground, G.
She now ascertains from the subscriber the number of the line
with which he desires connection, which we will say is No. 63.
She thereupon takes up the other plug, P, of the pair and inserts
it into jack 63. In order to call subscriber No. 63, she presses
the key, K, into contact with its lower stop. This completes
connection from the ground at the central office, through the
operator's generator, through key, K, cord, c, plug, P, jack No. 63
to subscriber No. 63, and through the ringer magnet of his in-
strument to ground. All keys being in their raised position, the
two subscribers may converse with each other over the following
path: line wire No. 20 to jack No. 20, plug, P, cord, c , key, K' y
through the upper contact of this key, through the coil of the
clearing-out drop to key, K, thence through cord, c, plug, P, jack
63 to subscriber 63. In case at any time the operator wishes to
" listen in " to ascertain if the parties are through talking, she
may do so by depressing key, K", which throws her telephone
into a branch or derived circuit of the circuit between the two
subscribers. The key, K', may be used to connect the generator
with the line to which the plug, P', is connected.
The clearing-out drop is placed in the circuit between the two
plugs to indicate to the operator when either of the subscribers
turns his generator to ring off.
But a single pair of plugs with their corresponding keys and
clearing-out drop are shown, for simplicity's sake. It is usual to
place ten of such pairs of plugs for each one hundred sub-
scribers in the system, it being found that this number is
sufficient to meet the requirements at the busiest periods of
the day.
The drops in a board of this type are usually wound to a resist-
ance of about 80 ohms, unless designed for multiple or bridged
telephone lines, in \^hich case the resistance of the drops is the
SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. 157
same as that of the ringer coils of the telephone instruments on
that line, usually 1000 ohms.
This switch-board has not been described because it is a fail-
example of modern, up-to-date apparatus, but because, stripped
of all complicated devices for facilitating the work of the opera-
tor, it can be more easily comprehended by the beginner. A
large number of such switch-boards are, however, in use, and for
small exchanges may give as good service as it is possible to
obtain with grounded or common-return lines.
It has already been pointed out that in order to avoid induc-
tion and other sources of trouble, metallic circuits are rapidly
superseding ground circuits in telephone exchanges. The switch-
boards in common use for small metallic-circuit exchanges are
built on the same general principles as those for grounded
circuits just described, differing from them only in such details
as to render possible the connections of the two branches
of one line with those of another line through the cord cir-
cuits. For this purpose two separate contacts are provided in
each jack forming the terminals of the two branches of the line.
The plugs also have two separate contact-pieces adapted to
register with the contact-pieces in the jack when a connection is
made. Each contact on the plug is connected to a similar con-
tact on the other plug of a pair through the medium of a double-
conductor flexible cord.
One form of metallic-circuit jack is shown in Fig. 134. Here
the tubular portion, a b, forms a terminal for one side of the line,
Fig. 134. Metallic-Circuit Jack.
while the flexible spring, d, forms the terminal for the other side
of the line. The terminal,^, connected with the pin upon which
the spring, d, normally rests, forms one terminal for the coil of
the line-drop. The other terminal of this coil is attached to the
terminal, a, so that when the spring, d, is in contact with its pin
the circuit is complete from one side of the line to the other
through the drop coil. The tubular frame of this jack is made in
two pieces, a and b. The front portion, b, is a hollow screw,
158
AMERICAN TELEPHONE PRACTICE.
threaded to engage a tapped hole in the front of the piece, a.
By this arrangement any jack may be readily removed from the
board by unscrewing the piece, b, until it disengages the rear
portion, a. A slot for receiving a screw-driver is provided on the
front of the piece, b, to accomplish this.
A metallic-circuit plug in common use is shown in Fig. 135.
The tip conductor is formed of a rod of brass slightly enlarged at
t b
Fig. 135. Metallic-Circuit Plug.
its forward end. This is encased in a bushing, b, of hard rubber,
and over this is slid a tube, s, of brass forming the sleeve of the
plug. A second bushing, #', covers the rear portion of the
sleeve, j, and the rear portion of this latter tube is in turn cov-
ered by the tube, b", of hard rubber, forming the handle of the
plug. The tube, s, forming the sleeve has a portion which pro-
jects rearwardly into the handle and is there provided with a
connector, , to which the terminal of one conductor of the flex-
ible cord is attached. The other connector, c', is attached to
the rear portion of the tip piece, t, and forms the terminal for
the other conductor of the cord.
In Fig. 136 is shown a form of jack and plug manufactured
by the American Electric Telephone Co. The jack is self-
Fig. 136. American Jack and Plug.
Fig. 137. Keystone Telephone
Co.'s Jack and Plug.
contained and is mounted on the board by means of a screw-
threaded thimble, in much the same manner as the jack shown
in Fig. 134. The two springs are secured rigidly to the frame of
the jack, but are insulated from it and from each other by strips
of hard rubber and by insulating bushings for the screws. The
plug differs in its details from that shown in Fig. 135, but its
SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES.
'59
contacts perform the same functions. The entire metal portion
of the plug, including the tip and sleeve contacts, are screw-
threaded into the hard-rubber bushing, forming the handle, and
make contact with the terminals of the flexible cord in such
manner as to bind it firmly in place without the use of other
connectors. The screw-threaded thimble of the jack is provided
with a long shank so as to adapt it to fit almost any thickness of
panel board. This jack and plug -are made with special reference
to use upon boards already installed, when it is desired to in-
crease their capacity.
In Fig. 137 is shown another form of jack and plug, manu-
factured by the Keystone Telephone Co. of Pittsburg, Pa. The
construction and operation of this are evident from the cut.
In Fig. 138 is shown in diagrammatic form the circuits of a
switch-board of this class. Here the line wires, / J and / 2 , forming
C.O.
XT*
iJSD
Fig. 138. Circuits of Metallic Switch-Board.
the two sides of a metallic circuit, enter the spring-jacks, e, e 1 , and
e'\ in the manner described in connection with Fig. 133. It will
be noticed that while the tip-spring, d, is in its normal position,
circuit is traced from the line, /', through the coil of the drop,/,
and back to line, / 2 , so that current sent from the subscriber's
station will actuate the drop, thus indicating a call. When one
of the plugs, P or P', is inserted into the jack spring, d is raised
from its normal resting-place and breaks contact with the termi-
nal leading to the drop-coil, thus cutting the drop out of the
circuit. At the same time, the connection is continued from the
two line wires, /' and / 2 , to the two strands of the cord circuit.
When an operator notices that a drop has fallen she inserts the
answering plug, P, into the jack corresponding to that drop and
by pressing the button, K, belonging to that cord circuit, bridges
her telephone set, 7", across the two strands, I and 2, of the cord
circuit. This enables her to communicate with the subscriber call-
160 AMERICAN TELEPHONE PRACTICE.
ing, to ascertain his wants. She then inserts the calling plug, P,
into the jack of the called subscriber and presses the button, K ,
thus connecting the terminal of the generator, G, with the two
sides of the line of the subscriber called.
It will be noticed that when the key, K , is in its normal posi-
tion the conductors from the tip and sleeve of the answering plug
to the tip and sleeve of the calling plug are made continuous by
the springs of the calling key resting against their inside anvils.
When the key is depressed the springs break contact with the
inside anvils, thus severing the connection between plugs, P
and P', and immediately afterward connect with the outside
anvils forming the terminals of the generator, G, thus sending
current over the called subscriber's line.
The clearing-out drop, C O, is permanently bridged across the
cord circuit as shown, in order to indicate to the operator when
either subscriber rings off. In order that the efficiency in talk-
ing may not be impaired, this drop is made of high resistance
and high impedance.
The line-drops are usually of the ordinary type described in
connection with the grounded-circuit switch-board. The clear-
ing-out drops, however, must be made to meet more difficult
requirements than the line-drops. As they are always bridged
across the circuit of two connected subscribers, it is found that
unless special precautions are taken much trouble will be expe-
rienced from cross-talk due to induction between two adjacent
drops. This difficulty cannot be overcome, as in the line-drops,
by cutting them out of the circuit whenever two subscribers are
connected, inasmuch as the very purpose for which they exist
requires them to be always in such circuits. Neither can it be
overcome by placing the drops at such a distance from one
another that this induction will not be felt, for the limited space
on switch-boards requires that they be put as close together as
mechanical conditions will allow.
It has thus been found necessary to design drops which would
neither affect nor be affected by any similar drop in its imme-
diate vicinity. This has been accomplished in several ways, but
the best example is that shown in Fig. 139, which illustrates
what is known as the " Warner Drop." In this the coil is wound
in the ordinary manner on a soft-iron core and is then encased in
a tubular shield, c, also of soft iron. The armature, d, is pivoted
at points, e, in a bracket,/, mounted directly on the rear portion
of the tubular magnet. From this armature a rod, #, extends
forward through a notch in the front plate, b, in such manner as
SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. l6l
to engage the upper portion of the shutter and thus hold it in its
raised position. A screw, /, passing through the front plate, />,
serves not only to hold the magnet in place, but to hold the
core in its place within the shell. The terminals of the coil are
led out through two small holes in the armature and are con-
nected with the terminals,// Amounted on an 'insulating strip,
carried on the bracket,/.
These drops should be so nicely made that the armature, d,
will fit closely against the end of the tube, c, in such manner as
Fig. 139. The Warner Drop.
to almost completely close the magnetic Circuit in which the coil
is placed. The lines of force generated by the passage of a cur-
rent through the coil follow almost entirely the path provided for
them by the shell and the core of the magnet, thus not only pro-
ducing a very efficient electromagnet, but also preventing any
of the lines of force from extending beyond the limits of the
shell. These drops are usually wound to a resistance of 500
ohms and may be mounted as closely together as desired with-
out producing perceptible cross-talk. The impedance due to the
great number of turns in the coil, and to the perfect magnetic
circuit surrounding the same, is so great that practically no
diminution in the strength of speech transmission is felt due to
its being bridged across the two sides of the line.
162
AMERICAN TELEPHONE PRACTICE.
Another form of tubular drop is shown in Fig. 140. This is
manufactured by the American Electric Telephone Co., and
needs but slight description. The tubular magnet is mounted
on a brass bracket extending from the rear plate of the switch-
board, upon the front of which is pivoted the shutter. The
armature is pivoted at its lower edge in the brass bracket, and
carries on its upper side a forwardly projecting rod which serves
as a catch for the shutter. This drop gives excellent service in
Figs. 140 and 141. American and Keystone Tubular Drops.
practice, but is probably not quite so sensitive as the Warner
drop, because the armature in its backward movement must
necessarily pull the shutter back slightly before it can release it.
This is but a slight objection, however, and does not, as stated
above, seriously impair its efficiency.
In Fig. 141 is shown the tubular drop of the Keystone Tele-
phone Company, the arrangement of the parts being evident
from the cut.
CHAPTER XV.
LISTENING AND RINGING APPARATUS FOR SWITCH-BOARDS.
IN order to accomplish the changes of circuit by which the
operator is enabled to connect her telephone with the line of any
subscriber, and to send calling current to actuate the bells at any
subscriber's station, many forms of circuit-changing switches
have been devised. One of these, shown in Fig. 142 and known
h-
Fig. 142. O'Connell Key.
as the O'Connell key, has been in use in this country, either in
the form shown or with slight modifications, for many years.
Six different contact-springs are so mounted and formed as to be
acted upon by a wedge, b, of insulating material adapted to slide
vertically among them. This wedge is mounted upon a rod,
carrying at its upper end a button, by which it may be raised or
lowered by the operator. The two springs, 3 , form the terminals
of the two strands of the flexible cord leading respectively to the
tip and sleeve of the calling plug. These springs are provided
with rollers, b\ in order to reduce friction when acted upon by
the wedge, b. The springs, b\ form respectively the terminals
for the strands of the cord leading to the tip and sleeve of the
answering plug. The two springs, b z , are connected each to one
terminal of the operator's set, while the pins, b'\ are connected
each to one terminal of the calling generator. The normal posi-
tion of this apparatus is when the wedge is raised to its highest
position. In this position the springs, #*, rest against the smallest
portion, b\ of the wedge, $, and are not in engagement with the
springs, # 3 . The springs, tf, however, rest against the springs, b\
thus making complete the connection from the tip and sleeve of
the answering plug to the tip and sleeve, respectively, of the call-
163
i6 4
AMERICAN TELEPHONE PRACTICE.
ing plug. In this position two subscribers may converse without
being heard by the operator.
When the button is depressed one notch the springs, 2 , ride
upon the second portion of the wedge, b, thus forcing them into
engagement with the springs, 3 , without causing these latter to
break contact with the springs, b\ In this position the circuit
between the two plugs is not broken, but the operator's tele-
phone set is connected across the two strands of the cord, thus
allowing the operator to listen in and to communicate with either
of the two subscribers who are talking. In its third position,
which is that shown in Fig. 143, the springs, 3 , break contact
Figs. 143 and 144. O'Connell Key.
with both springs, b 1 and b\ and come into contact with the
pins, b\ which are connected with the terminals of the generator.
This sends calling current to the called subscriber without affect-
ing in any manner the circuits leading to the calling subscriber.
When the button is depressed to its utmost extent the springs,
//, are pressed outwardly as is shown in Fig. 144, until they not
only make contact with pins, b\ but also with pins, b\ These
pins, b\ are each connected to pins, 8 , against which the springs,
b\ are now resting. This completes a circuit from the generator
terminal, 5 , through the springs, b\ to the pins, //, thence to the
pins, b*, and thence through the springs, b\ to the sleeve and tip
of the answering plug and to the line of the calling subscriber.
Thus, in this final position of the key, calling current is sent not
only to the subscriber to be called, but also to the one who
originated the call. Of course, this is necessary only when for
some reason the calling subscriber has left his instrument.
LISTENING AND RINGING APPARATUS.
165
In later and better keys, arrangement is made whereby either
the calling or the called subscriber may be called without dis-
turbing the other.
The combined listening and ringing key shown in Fig. 145
is the invention of Mr. Frank B. Cook, of the Sterling Electric
*?
Fig. 145. Cook Key.
Co. This key is quite extensively used by some of the licensees
of the Bell Company, and also in all of the switch-boards manu-
factured by the Sterling Company. The springs in this key
are arranged in duplicate sets, the two sets being divided by a
hard rubber partition, 18, as shown in the sectional view at the
bottom of the figure. The two sets of springs are shown sepa-
rated in the upper portion of the figure, and their various circuit
connections clearly indicated. The springs are acted upon by
the cam, 12, of hard rubber pivoted in the metal frame, and adapted
i66
AMERICAN TELEPHONE PRACTICE.
to be turned through a small arc by the handle, 15. The springs
23, on each side of the partition bear against the left-hand portion
of the cam, and form the terminals of the operator's talking
circuit including her receiver and the secondary of her induction
coil. On the opposite side of the cam are the two springs, 24,
Fig. 146. American Key.
forming the terminals of the clearing-out drop, E, one of them
being placed on each side of the partition. The springs, 28,
form the terminals of the tip and sleeve strands of the calling
plug, while the springs, 29, form the terminals of the two corre-
sponding strands of the answering plug. These springs normally
rest against the two contact strips, 36, so that the tip of the calling
plug is normally connected through one of the strips, 36, to the
tip of the answering plug, and similarly the sleeve of the calling
plug with the sleeve of the answering plug through the other
strip, 36. When the cam is rotated in one direction the listening
LISTENING AND RINGING APPARATUS. 167
springs, 23, are pressed into engagement with the two strips, 36,
thus bridging the operator's telephone across the two sides of the
cord circuit. When the cam is in its opposite position the two
clearing-out springs, 24, are pressed into engagement with these
strips, thus bridging the clearing-out drop across the cord circuit.
This latter position is the normal position of the cam. Two
outside terminal strips, 37, are provided, one on each side of the
partition, these forming the terminals of the switchboard
generator,/. By means of pressure on one of the buttons, 16
or 17, the contact springs, 29 or 28, may be pressed out of
engagement with the strips, 36, and into engagement with
the generator terminals, 37, thus disconnecting the strands of
the cord from the rest of the circuit and at the p~i
same time connecting them with the genera-
tor terminals. Upon releasing the button the \
springs resume their normal position, completing
the circuit between the two plugs and cutting
out the generator. These circuit changers have
the advantage of being entirely self-contained,
thus rendering the removal of any one of them
from the switch-board a comparatively easy matter
when repairs are necessary. They are entirely in-
closed, and are therefore quite free from dust, which Fig. 147. Section-
causes much trouble in the way of poor connec- al View Ameri-
tions in many otherwise efficient circuit changers.
A new key, just put upon the market by the American Electric
Telephone Co., is shown complete in Fig. 146 and in sec-
tion in Fig. 147. In this all of the listening and ringing opera-
tions are performed by the manipulating of a single lever, no
buttons being required to p-erform the ringing, as is usually the
case. As in the Cook key, two sets of springs are provided,
being separated by a partition, A, of hard rubber. The springs
are mounted in slits cut in hard-rubber blocks, B, these blocks
being clamped between two brass side-plates forming the frame
of the circuit changer. The front one of these side-plates is
removed in Fig. 147 in order to better show the construction.
The circuits of this apparatus are shown in Fig. 148, the two
sets of springs being separated in order to render their action
clearer. It should be remembered, however, that the cam, C, acts
in the same manner on each set of springs, so that the two sets
always occupy similar positions. The pair of springs, I, form the
terminals of the operator's set, and are adapted to make contact
with the springs, 2, when the lever is pressed to the right. The
i68
AMERICAN TELEPHONE PRACTICE.
springs, 2, normally bear against the springs, 4, which form the
terminals of the tip and sleeve strands respectively of the answer-
ing plug, P. The springs, 3, make normal contact with the springs,
5, which form the terminals of the tip and sleeve strands of the
calling plug, P' . As the springs, 2 and 3, on each side are perma-
nently connected together, it follows that in the normal position
of the circuit changer the tip strand is made complete through
spring, 4, spring, 2, spring, 3, and spring, 5, and the sleeve strand
Fig. 148. Circuits American Key.
is made complete through the same springs on the other side of
the partition. When the cam lever is thrown to the right, the
springs, I, are, as before stated, pressed into engagement with
the springs, 2, thus bridging the operator's telephone across the
combined cord circuit. When the lever is pressed still further
to the right, the rubber plate, D, carried upon it, presses the
springs, 5, into engagement with the springs, 6, at the same time
breaking the contact with the springs, 3. As the springs, 6, form
the terminals of the switch-board generator, this sends calling
current over the line with which the calling plug, P', is connected.
No current is sent to the line with which the other plug is con-
nected, because the circuit is broken between springs, 3 and 5, on
each side of the partition. In a similar manner a pressure of the
lever to the extreme left causes the springs, 4, to break engage-
LISTENING AND RINGING APPARATUS.
169
ment with the springs, 2, and to come in contact with the springs,
7, which also form terminals of the calling generator. This sends
calling current over the line with which the plug,/*, is connected.
The combined listening and ringing key of the Western
Telephone Construction Co. is operated entirely by one lever,
this lever being normally held in ( its central position by the con-
tact springs against which it operates. In order to connect the
operator's telephone across the terminals of the cord circuit with
this key, the lever is pushed straight down without rocking it at
Fig. 149. American Listening Key.
all, this action pressing the light operator's springs into contact
with the tip- and sleeve-springs of the cord circuit. When the
lever is rocked toward the operator, the tip- and sleeve-springs
of the calling plug are pressed into engagement with the gener-
ator contacts, thus at the same time cutting off the connection
with the tip-spring of the answering cord. A rocking motion
in the other direction presses the tip- and sleeve-springs of the
answering plug into engagement with the generator contacts, at
the same time cutting off the tip-springs of the calling cord.
Fig. 149 shows a simple key manufactured by the American
Electric Telephone Co., and typical of a large number of keys
designed for the purpose of either listening or ringing. In this
the plunger, A, is of hard rubber, and is normally pressed into
1 70 AMERICAN TELEPHONE PRACTICE.
its upper position by the spiral spring below it. It may be
depressed, however, by means of the lever, in an obvious
manner. The outside springs form the terminals of the cord
circuit, while the inside shorter springs may form the terminals
of the operator's telephone set or of the calling generator,
according as to whether the key is to be used for listening, or for
ringing purposes. When the plunger is depressed, the outside
springs fall into the depression on the plunger, while the shorter
springs ride upon its enlarged portion, thus pressing the two pairs
of springs into contact and bridging the telephone or generator
across the cord circuit.
To facilitate the manipulation of switch-boards it is, of course,
desirable to make the number of motions necessary to effect
a connection as few as possible. If, therefore, some act which
must necessarily be performed by the operator in inserting a
plug in or withdrawing it from a jack can be made use of to
bring about some of the other changes of circuit, a decided
advantage is gained. In Fig. 150 a device for accomplishing
Fig. 150. Plug Listening Device.
this is shown. The line springs of a jack are represented by
a and b; c and d are two springs arranged adjacent to the line
springs and forming terminals of the operator's telephone
set. The plugs are formed with alternate depressions and
enlargements, which are so spaced that when the plug is
partially inserted into a jack the two line springs ride upon the
two enlargements, thus pressing the line springs into engage-
ment with the operator's terminals, c d. This places the
operator into communicative relation with subscriber. After
the operator has learned the connections desired, she inserts
the plug fully into the jack, thus allowing the two line
springs to fall into the recesses of the plug. This maintains
the connection between the line and the conductors of the plug,
but breaks the connection between the operator's telephone and
the line. The apparatus is then in the position shown in Fig.
150. If at anytime the operator wishes to listen in without
breaking the connection between two subscribers, she may do
so by partially withdrawing the plug. If she finds that they
LISTENING AND RINGING APPARATUS.
171
are through talking, the movement is continued and the plug
replaced in its seat. Where this device is used an ordinary
ringing key is required to connect the generator across the
terminals of the calling plug.
In Figs. 151 and 152 is shown a device in common use, de-
signed by Mr. W. O. Meissner, which accomplishes the con-
Fig. 151. Meissner Ringing Device.
nection of the calling generator with the line of the subscriber
called for by inserting the plug to its utmost extent into the jack.
The illustrations show a jack and plug for common-return or
grounded lines. In Fig. 151 the plug is shown partially inserted
into the jack, in which position the line spring, C, makes contact
at the point, C 1 , with the conductor, j5 2 , of the plug, thus com-
pleting the connection between the line, C*, and the strand, B\
of the cord. In this position the circuit is continuous between
two connected subscribers or between the operator and one sub-
scriber, as the case may be. When it is desired to ring out on
the line, C*, the plug is pressed to its fullest extent into the jack,
Fig. 152. Meissner Ringing Device.
as shown in Fig. 152. In this position the spring, C, rides upon
the insulated sleeve, J3 l , of the plug, thus breaking connection be-
tween the spring and the contact, B 1 , and at the same time
pressing the tip of the spring into contact with the strip, C 9 ,
which is connected by wire, C\ to one terminal of the generator.
Current from the generator thus flows to line until the plug is
released, at which time it is forced outward by the action of the
spring and again resumes the position shown in Fig. 151.
Where this device is used, listening in by the operator is accom-
plished by an ordinary listening key.
172
AMERICAN TELEPHONE PRACTICE.
In Fig. 153 is shown another device for listening in. Pis the
calling plug of any pair, and is shown in its normal socket on the
Fig. 153- Plug Socket Listening Key.
key table. By tilting it in its socket until it assumes the posi-
tion shown iii the dotted lines, the spring, S, is forced from
its normal position, and thus presses the springs, q and r, into
engagement with terminals, q l and r 1 . As is shown by the
diagram, this act connects the operator's telephone set across
the two strands of the cord circuit, TT 1 .
The knob, Q, upon the spring, 5, may be used to connect the
operator's telephone across the cord circuit, in case it is desirable
to listen in after the plug, P, has been removed from its socket.
Calling is done by pressing the key, K. This affords a very
rapid means for connecting the operator's telephone into circuit
with any line, for after having inserted an answering plug into
the jack of a calling subscriber, she can, by part of the move-
ment which withdraws the calling plug, P, from its socket,
connect her telephone with the calling subscriber's line. A
continuation of this movement completes the connection with
the called subscriber, and at the same time cuts the operator's
telephone out of circuit.
CHAPTER XVI.
SELF-RESTORING SWITCH-BOARD DROPS.
IT is generally considered of great advantage to have switch-
boards so arranged that it will be unnecessary for the operator to
manually restore the drops. The reason for this is that every
movement on the part of the operator, in establishing a con-
nection between two subscribers, requires a certain amount of
time, and that in the busier portions of the day an opera-
tor is worked almost to the extremity of her endurance, and
therefore that the saving of any movements in handling
these connections will be a great gain in the rapidity with which
the board can be operated. Such saving of the work of the
operator not only insures a quicker and therefore a better service,
s~
o
- \
/ o
a
iz
Fig. 154. Self-Restoring Drop.
but also may reduce the cost of the operation of the exchange
by enabling fewer operators to handle the system. There are,
however, many who contend that the greater part of an operator's
time is necessarily taken up in talking or listening to the sub-
scriber in order to ascertain his wishes, and that while she is doing
this she may restore the drops by hand without loss of time.
Notwithstanding this, however, the number of exchanges using
self-restoring drops is rapidly increasing, and many inventions
have recently been made and put into practice to bring about
this result.
Brief mention has already been made of the electrically re-
storing switch-board drops used to a large extent by the American
Bell Telephone Company. The details of such a drop are shown
in Figs. 154, 1 5 5, and 156. In Fig. 154, a is a tubular electromag-
174
AMERICAN TELEPHONE PRACTICE.
x^f
Fig. 155. Self-Re-
storing Drop. Sec-
tional View.
net, carrying on its rear end an armature, C, pivoted at c, which
armature carries an arm, c\ which projects forward and is pro-
vided with a catch, c\ on its extremity. So far the arrangement
is almost identical with that of the Warner tubular drop already
described. A second tubular electromagnet,
d, is secured to the front of the plate, b, which
also supports the magnet, a. This second
magnet has its poles facing the front of the
board. An armature, transmitter, a, primary coil, c\ and to the line wire, 3.
LAMP SIGNAL SWITCH-BOARDS. 195
The transmitter, a, will therefore receive current from the bat-
tery, z, sufficient to operate it, and yet it will be operating with
all the advantages to be derived from a local circuit and induc-
tion coil ; for, although the current operating it comes from
the central office, any fluctuations in this current caused by the
transmitter, a, will pass through the low-resistance battery, d,
which will act in this case very much in the same manner as, a
condenser. This system is the invention of Mr. C. E. Scribner
of Chicago.
To return now to the luminous signal feature, we find our-
selves confronted with several rather serious objections; in the
first place, the resistances of no two subscribers' circuits are the
same, owing to the differences in the lengths of these circuits and
other causes, and therefore either the resistances, g or h, or that
of the bell magnets, e, will have to be varied in each case in
order to insure the proper amount of current passing through
the lamp, /. This is a feature easy to overcome, and a much
more serious one is that arising from crosses between two line
wires from any source whatever, such a cross, of course, always
subjecting the lamp to an undue amount of current, and there-
fore burning it out. This latter objection has proved so serious
as to cause the abandonment of the plan of including the lamp
directly in the line circuit in nearly every case where it has been
tried. Of course, for underground systems this objection is not
such a serious one.
Passing now to the second method of associating the lamp
signal with the line circuit, reference will be made to Fig. 173.
This shows the circuits of three subscribers' stations, S, S, 1 and
5 2 , these circuits being connected with the central office by the
metallic circuits, I, 2. The line wire, I, in each case passes to the
sleeve-spring, <^ 2 , of the spring-jack, y, and thence through
the relay contacts, c\ to the ground. The line wire, 2, passes in
a similar manner to the tip-spring, d, thence through the relay
contacts, 2 , the winding of the relay, b, and the battery, a, to
the ground. The signal lamp, e, is in each case included in a
local circuit containing the contact, 4, of the relay, b, belonging
to its line, and a battery,/", common to all lamps. The relay, c,
of each line, which controls the contacts, c 1 and c*, is known as
the cut-off relay, and is included in a local circuit through the
jack-thimble, d\ and the plug contact, m, with the battery, n,
whenever a plug, I, is inserted into a jack for making connection
with the line. The two sides of the line at the subscriber's sta-
tion are permanently closed through a high-resistance bell and a
io6
AMERICAN TELEPHONE PRACTICE.
condenser, the latter having a capacity of about .75 microfarad,
so as to allow the alternating currents from the calling generator
Fig. 173- Lamp Relay System.
at i\\G central office to pass through it and operate the call-bell
in the usual manner. The high impedance of the call-bell mag-
nets, however, prevents the short-circuiting of the voice currents
LAMP SIGNAL SWITCH-BOARDS. 197
when the receiver is removed from its hook. The call-bell circuit
therefore presents an open circuit to the direct current from the
battery, a y thus normally insuring a condition of no current upon
the line wire. When, however, the receiver at the subscriber's
station is removed from its hook, a path of comparatively low
resistance is formed between the two line wires, and a cur-
rent proceeds from the battery, a, through the relay, b, contact,
<: a , line wire, 2, to the subscriber's station, back by line wire,
I, through relay contact, c\ and by ground to the opposite termi-
nal of the battery, a. This current is sufficient to operate
the relay, b, and, unlike the case where the lamp was used
directly in the line wire, a considerable variation in the amount
of this current is allowable. The operation of the relay, b, closes
the circuit of the lamp, e, through the following path : from the
battery,/, through the relay, g, wire, 5, common wire, C, lamp,
e, wire, 3, relay armature, 4, and back by ground to the opposite
pole of battery, f. For the purpose of clearness the relay, g,
need not be considered at all at present, as it has nothing to do
with the operation of the lamp, e, and we may therefore consider
one pole of the battery,/, to be connected directly to the com-
mon wire, C. The closure of this circuit illuminates the lamp, e.
The next step in the operation of the system is the insertion of
the plug, /, into the jack of the line on which the signal is dis-
played. The insertion of this plug closes the circuit through the
relay, c, over the following path : from the battery, n, wire, 10,
plug sleeve, m, thimble, d l , wire, 9, relay coil, c, to ground and
back to the opposite pole of the battery, n. This causes
the relay to attract its armature and . break both of the
contacts, c l and c*, thus accomplishing a double purpose, the first
of which is to break the circuit through the relay, b, and thus
cause it to release its armature, 4, breaking the circuit through
the lamp, e ; and the second of which is to cut off both sides of
the line circuit, I, 2, beyond the spring-jack, J. This latter
feature is a very important one, since it removes all difficulty
from cross-talk and other troubles in the auxiliary circuits of the
central office.
The circuits illustrated in Fig. 173 are substantially those in
common use by the Bell Company in their lamp-signal exchanges,
so far as the circuits of the relays and lamps are concerned. Sev-
eral different modifications of the circuits at the subscribers' sta-
tions have, however, been used. The subscribers stations may
or may not include local batteries, and the tendency of prac-
tice is now to do away with these entirely and to supply current
198 AMERICAN TELEPHONE PRACTICE.
from central office for the operation of the transmitters as well as
for all signaling purposes. This feature, however, will form the
subject of a subsequent chapter.
The relay, g, and its associated apparatuses form a very inter-
esting addition to the system as outlined. It is found desirable
to use what are termed pilot lamps for certain groups of line
lamps, for the purpose of attracting the operator's attention more
surely to a signal on her board. If two or more signals are
displayed at once, but one of them may attract the attention
of the operator, who might therefore neglect the other.
Pilot lamps are used in such connection that they remain
lighted as long as any one of the line lamps in their group is
lighted, and as they occupy a very conspicuous position and
are as a rule brighter than the others they cannot escape
the operator's attention. It is not desirable to put a relay
for operating such a pilot lamp in the common wire, C,
of the local circuits of the lamps, ^, for the reason that
the fall of potential or drop through such a relay would vary,
according to the amount of current passing through it, and if
several of the lamps, e, were operating at the same time, they
would probably not therefore receive enough current to properly
illuminate them. The relay, g, is therefore included in circuit
with battery, y, and means are provided whereby its resistance
will be short-circuited the instant it is operated. To accomplish
this, the armature, g 1 , makes contact with the point, g 3 , as soon
as it is attracted, thus short-circuiting the resistance of the coil,
g. At the same time it breaks contact with the point, g*, and
thus allows the current to flow from the battery,/", through the
lamp, i t and resistance, k, thus illuminating the lamp. In order
that the armature, g, may not fall back against the contact point,
g*, as soon as the coil, d, is de-energized by being short-circuited,
a small dash pot, k, is provided in connection with the armature
to render its movements sluggish. Thus before the armature, g\
has time to move very far away from the point, ^ owing to its
sluggish action, it will be at once attracted again, and the inter-
val during which the resistance coil, g, is in circuit with the
common wire, C, and the lamps, e, is so small that it does not
have time to affect these lamps. This system is also due to Mr.
Scribner, and both it and the one shown in Fig. 172 form inter-
esting examples of the highly skillful manner in which he always
solves his telephone problems.
Incandescent lamps for signaling purposes are commonly built
for 10 or 20 volts pressure, the tendency being rather to increase
LAMP SIGNAL SWITCH-BOARDS. 199
the voltage than to decrease it. At first lamps of 2 and 4 volts
were used, but for various reasons, not the least among which
was the trouble of securing proper contacts at the relay and
switch points for such low voltages, the voltage was gradually in-
creased to the above-mentioned figures.
Mr. A. V. Abbott of Chicago has recently given some interest-
ing figures concerning the life of incandescent lamps in switch-
board work, and mentions one case where a lamp was flashed
over a million times without showing serious signs of deteriora-
tion. His tests seem to indicate that for general service in
switch-board work the average lamp will live for a period of about
1 200 hours, although in laboratory tests a much longer life has
proved possible. He points out, as a result of his observations,
that according to theory, the lamps used in subscribers'-line
circuits should last about twenty-five years, and those in the cord
circuits used as " supervisory " and clearing-out lamps, from
one to two years. He also says that such a life has already been
obtained in the cord-circuit lamps, but that it is doubtful if the
theoretical limit for the line lamps will ever be closely approxi-
mated.
CHAPTER XIX.
THE MULTIPLE SWITCH-BOARD.
WHEN the number of subscribers in an exchange exceeds 400
or 500, the switch-boards so far considered become inadequate ;
for in order to afford room for the number of operators needed
to properly handle all the connections, the board must be made
of considerable width, and is thus too wide for the operators to
reach across with their cords.
The multiple switch-board, which is designed to enable each
operator to make any connection required without the aid of any
other operator, and without the use of unduly long cords, is
used in most of the large exchanges in this and other countries.
The idea underlying the construction of multiple boards is very
simple. In practice, however, the greatest complexity is met,
but this is due entirely to the great number of repetitions of
one comparatively simple circuit.
The boards are divided into sections, each section usually
affording working room for three operators. Each line, instead
of being provided with a single spring-jack or terminal, as on
the boards used in small exchanges, is provided with a spring-
jack on every section of the board, and with a drop or other visual
signal on one section only. Each section therefore contains a
spring-jack for every line entering the exchange, and also a
number, usually 200, of line-drops. Suppose an exchange to
have 3000 subscribers. The multiple board would then prob-
ably have 15 sections, each containing 3000 jacks, that is, a jack
for each line. Each section would also contain 200 drops belong-
ing to the 200 lines whose calls would always be received on that
particular section. An additional jack, called an answering jack,
is usually provided for each line on the particular section at
which that line's drop is located. These answering jacks are
placed in a separate panel at the lower part of the switch-board.
Before considering any particular form of multiple board
it is probably well to describe in a general way the opera-
tion of the multiple board. When a subscriber calls the at-
tention of the operator at whose section his drop is located,
the operator plugs into the answering jack of that line with an
THE MULTIPLE SWITCH-BOARD. 2oi
answering plug, and having switched her telephone into the cord
circuit of that plug, ascertains the number of the subscriber
desired. She then completes the connection with the subscriber
called for by inserting the calling plug into the multiple jack of
that subscriber's line, one of which is, of course, on her section.
As each section contains one multiple jack for every line in
the exchange, it is evident that an operator will always be able
to complete a connection with any subscriber who may be called
for by any of the 200 subscribers whose drops are located at her
section. During the least busy portions of the day one operator
at each section usually suffices to handle all of the calls originat-
ing at that section. As the number of calls increases two
operators may be placed at each section, and during the busiest
part of the day three are usually required.
When three operators are seated at a section, the center one
can reach all of the jacks on the section at which she works.
The operator at her right cannot well reach the jacks on the
extreme left-hand portion of that section, but she has within her
reach a similar portion of the section at her right into which she
may plug when necessary. In a similar manner, the operator at
the left cannot well reach the jacks on the extreme right of her
of her own section, but can reach with her left hand the jacks on
the extreme right of the section at her left. Thus every opera-
tor has a multiple jack for each of the entire number of lines
within her reach ; the right-hand operator controlling the right-
hand two-thirds of her own section, and the left-hand one-third
of the section at her right, the center operator controlling her
entire section, and the left-hand operator controlling t the left-
hand two-thirds of her section, and the right-hand one-third of
the section at her left.
In order to prevent two or more connections being made to one
line at different boards, some sort of a test system is necessary.
It is therefore usually so arranged that when a line is "busy"-
that is, when it is connected to some other line for conversation
an operator at some other board than the one at which the
connection is made, in trying to connect another party to that
line, will in some simple way be notified of the fact that that line
is already busy. This is known as the " busy test " and on its
efficiency to a great extent depends the operativeness of the
multiple board.
In Fig. 174 are shown diagrammatically three lines passing
through three separate sections in a multiple board. One
side of each line for instance, of line I passes in multiple
202
AMERICAN 7'ELEPHONE PRACTICE.
to all the contact rings, b, of a jack on each section. It then
passes to one terminal of the line-drop, d. The other side of
the line passes to the spring, a, in the jack belonging to that line
at section I. This spring rests against an anvil, , to which a
wire is connected which runs to spring, a, of the jack belonging
to that line on the second section. The anvil from this jack is
connected to the line spring, a, of the jack on the third board,
and so on the connection of the line is continued through a jack
M
2,vne, 2..
Line 3.
.c
a\
\
>
b
j
[
S
\>
\
HJ fij fij
o" o" o" a" o" a:
fi
r
a 1
i
Fig. 174. Simplified Diagram of Series Multiple Board.
on every section, and finally to the other terminal of the drop, d.
Lines 2 and 3 pass successively through the sections in a similar
manner.
When a subscriber operates his generator, the current passes
over the line wire through all of the contacts, a and ^, in series,
through the drop-coil, and back over the other side of the line.
When an operator inserts a plug into a jack, the spring, a, is
lifted from contact with the anvil, c, by the tip of the plug. The
sleeve of the plug makes connection with the test-ring, b, and
thus the tip and sleeve strands of the plug are connected,
respectively, into the metallic circuit of the line, while the circuit
through the drop is cut off at the anvil, c.
The operator's telephone, T, may be then bridged across the
cord circuit in order to enable the operator to converse with the
THE MULTIPLE SWITCH-BOARD.
203
subscriber who has called. Means for connecting the operator's
telephone in the circuit in this manner are not shown in
Fig. 174, the details of the cord circuit being described later
in connection with another figure. This telephone in Fig. 174 is
shown connected in a ground branch from the tip side of the
cord circuit, in order to better illustrate the principles of testing
in this system.
The sleeve strand of each cqrd circuit is grounded through a
battery, B, and in order that this ground may not produce serious
175. Cord Circuit of Series Multiple Board.
effects in unbalancing or crossing the circuit of two connected
lines, an impedance or reactance coil, /, is placed in this circuit.
Whenever any plug is inserted into a jack, one side of the test-
battery, B, is thrown on to all of the test-rings, b, of the line to
which that jack belongs. If now an operator at another board
desires to make a connection with that line, she touches the tip
of her answering plug to the test-ring, b, of that line. This will
connect the test-ring, b, to ground, through her telephone, T, and
a click will be heard, due to the passage of the current from
battery, B. The operator will therefore know that that line is
busy, and will refrain from making the connection.
In Fig. 1 74 the three lines have their drops located at section
3. It must be remembered that other lines would pass through
jacks on the various sections in a similar manner, but would have
their drops located on sections I or 2. The operator at any sec-
tion will, of course, answer calls on lines terminating or having
drops on her section only, but she may be required to connect
one of these lines to any other line in the exchange by means of
the multiple jack.
The details of the cord circuit for this system are shown in
204 AMERICAN TELEPHONE PRACTICE.
Fig. 175. K and K' are ringing keys for connecting the genera-
tor, G, with either of the plugs, P or P. The circuit between
the plugs is normally maintained continuous, through the tip
and sleeve strands, as can be readily seen. When the listening
key, K' ', is depressed, the condenser, C, is looped into the tip
strand and at the same time the operator's telephone circuit is
bridged between the tip and the sleeve strand. The center point
of the coil of the operator's receiver, R, is grounded, and the
secondary coil is split into two parts, 5 and S', one part on each
side of the receiver. This arrangement is to prevent the unbal-
ancing of the line by the ground on the receiver coil. The test
is made when the key, K", is depressed, the test circuit then being
from the the tip of the plug, P', through the tip strand to the
right-hand spring of the key and through its anvil, the part 5 of
the secondary coil, and one-half of the receiver coil to ground.
The condenser, C, is for the purpose of preventing disturbances
in the line with which the plug, P, is connected from giving a
false busy test.
The arrangement of circuits here shown is that used in what is
termed the series-multiple board, the name series being derived,
of course, from the manner in which the line passes through the
contact-springs and anvils of the multiple jacks. This system,
although once widely used, is subject to grave defects, and is
being rapidly replaced by another form of multiple board known
as the ''bridging" or "branch terminal multiple." In the series-
multiple an open circuit may be caused in any one of the jacks
by a particle of dust or other foreign insulating matter becoming
lodged between the line-spring and its anvil, or by virtue of one
of the springs becoming weak and failing to bear upon its anvil.
The liability to open circuits, therefore, is very great, especially
in large exchanges.
Another serious objection to the series board is that when a
plug is inserted into a jack, one side of the line is cut off at the
anvil of that jack, but the test side is not cut off, and is continu-
ous through the drop of that line and back to the anvil of the
jack which is plugged. This, in a large exchange, means that to
one side of the line is attached an open branch, perhaps several
hundred feet long and containing the drop-coil. This destroys
to a certain extent the balance of the line, and is liable to pro-
duce cross-talk.
The branch-terminal system was designed to remedy the
defects inherent in the series system, and possesses many advan-
tages over it, chief arnon^ which are the facts that when a con-
THE MULTIPLE SWITCH-BOARD.
205
nection is made with any line the balance of that line is in nowise
affected, and that the liability of open contacts in the jacks,
which is such a serious defect in the series system, does not exist.
The branch-terminal system, moreover, lends itself more readily
to the use of self-restoring drops, as will be described later.
In Fig. 176 is represented one type of the branch-terminal
system, sometimes called the three-wire system. In this figure
Fig. 176. Diagram of Branch Terminal Multiple Board.
three distict line circuits are shown passing through three sections
of board. The wires, k and k' , of each line have branch wires
leading off to a jack on each board ; the branches from wire, k,
leading to the contact-thimbles,/", and the branches from wire,
k\ leading to the short springs, c, in the same jacks. Bridged
across the two wires of each line is the line coil, n ', of the indi-
vidual annunciator belonging to that line. This coil is high-
wound in order that it may be left permanently bridged across
the'line without materially affecting the efficiency of the system
in talking.
A third wire, /, passes through the board in parallel with each
line. From this wire branch wires are run to the test-thimble,^,
and to the spring, b\ in each jack belonging to that line. The
test wire, /, after passing through all of the boards, runs through
a low-resistance coil, 2 , on the drop of the line to which that
particular test wire belongs, and then passes to ground through
a battery, o, common to all test wires. These test wires are
represented by dotted lines in the figure in order to distinguish
206 AMERICAN TELEPHONE PRACTICE.
them more readily from the line wires. The remaining spring, b,
in each jack is permanently connected to a ground wire, G, com-
mon to all of the jacks.
Each plug in this system is provided with two contacts, h and/,
which form terminals respectively of the sleeve and tip strands of
the cord circuit. The tip, //, registers with the spring, c, when
the plug is inserted into the jack (see Fig. 177), and the sleeve,/,
Fig- 177. Three-Wire Plug and Jack.
registers with thimble, /. A conducting ring, t, entirely insulated
from all other portions of the plug, registers with the springs, b
and &', in the jack, and connects them together electrically.
Three general results are accomplished by the insertion of a
plug into a jack. The tip and sleeve strands of the cord circuit
are connected respectively with the sides, k and k, of the line,
thus continuing the line circuit to the cord circuit. The con-
necting of springs, b and b ', by the ring, i, completes the circuit
of the battery,
THE MULTIPLE SWITCHBOARD. 209
through the coil, ri, of the line annunciator, n, at section 2 of the
board. The operator, noticing this signal, inserts plug, /, into
jack,-/'. This completes the circuit from ground, through bat-
tery, o, coil, ft*, of the line annunciator, thence to spring, b',
through the ring, z, on the plug to spring, b, and to ground.
The front armature of the annunciator is therefore attracted and
the drop restored.
The operator then connects her telephone across the cord cir-
cuit by raising the key, u, and communicates with Subscriber No.
I, in order to ascertain his wishes. Having found that he desires
a connection with Subscriber No. 2, she takes up plug,/', of the
same pair and tests to find out whether line No. 2 is con-
nected to at some other board. If it is busy a current will pass
from battery, o, through one-half o f the. coil of her receiver, and
one part of the secondary to the spring, u ', in the listening key~
From this spring it passes to the tip strand, q, of the cord circuit,,
and to the tip, h, of the testing plug. As the test-thimble to
which the plug is applied is grounded by the insertion of a plug
at another board, the current will pass through it to ground.
This will produce a click, which will indicate to her that the line
is busy, and she will not complete the connection called for. If ?
however, she finds the line to be free she thrusts the plug entirely
into the jack, in which position it is shown in the figure, and
depresses the key, r\ in order to throw current from generator
Pj G, upon the line of Subscriber No. 2.
The two subscribers are now connected for conversation.
When either rings off the current passes through the coil, x' y
bridged across the cord circuit, and actuates the clearing-out drop.
The operator, noticing this, again listens in, by raising the key,
//, in order to find out whether they are through talking, or
whether one of them desires another connection. The act of
listening in closes spring, & 3 , against anvil, d, and thus restores the
shutter of the clearing-out drop. If the subscribers have finished
talking, the plugs are removed and placed in their normal resting-
place.
If, while Subscribers I and 2 were connected together at sec-
tion 2, as above described, someone at section I had desired con-
nection with, say, line No. 2, the operator at section i, in applying
the tip of her plug, / 3 , to the test-thimble, g, as shown, would
receive a click in her receiver for the reason, as pointed out above,
that contact, g, is connected to the ground by a short circuit by
the plug inserted in jack, / s . No difference of potential would,
therefore, exist between thimble, g, and the ground, and hence a
210
AMERICAN TELEPHONE PRACTICE.
current from the battery, o, would pass through the telephone of
the operator making the test.
Success in practical telephone working can be attained only by
the greatest attention to matters of detail. Nowhere is this fact
better illustrated than in the design of the various parts which go
to make up a multiple board. In the construction of large boards
Fig. 179. Plan View of Multiple Jack-Strip.
of this type, the possible capacity of the board is limited by the
number of spring-jacks that can be placed within the reach of a
single operator. It is evident, therefore, that space must be
economized to the last degree, and yet the jacks must be sub-
stantial, in order to resist the wear and tear of years of service ;
must be made easily removable so as to be accessible for repairs ;
must perform their electrical functions with absolute certainty,
and at the same time be so arranged as to facilitate the orderly
Fig. 1 80. Front and Rear View of Multiple Jack-Strip.
and systematic connection of the wires leading from the line
cables.
Moreover, when we consider that a multiple board with a
capacity of 5000 subscribers will have in the neighborhood of 130,-
ooo spring-jacks, we can easily realize that the cost of produc-
ing these jacks must be seriously considered. It is well to state
here, however, that any economy in the construction of a switch-
board that will tend to decrease its durability and reliability of
action is poor economy indeed.
As an illustration of modern spring-jack construction", we will
consider the spring-jacks used in the branch-terminal multiple
THE MULTIPLE SWITCH-BOARD.
211
board just described. It has become common practice to mount
the jacks in strips of twenty, and to so arrange each strip that it
may be removed from the board by the removal of two screws,
which bind it firmly to the framework. Figs. 179 to 182, inclu-
sive, show the details of the construction of one of these jack-
strips.
The hard-rubber strip, a, forms the framework for each strip
of twenty jacks. The projections at its ends provide for attach-
Fig. 181. Bottom View of Multiple Jack-Strip.
ment to the switch-board. In this strip are milled, on its upper
side, the transverse grooves, a 1 a\ and on its lower side similar
grooves, a* a*; these being best seen in the right-hand portion of
Fig. 182.
Perforations are drilled from the front of the strip, one per-
foration to each pair of grooves, having its axis centrally located
1J1
S3'
d
\
Fig. 182. Details of Jack Parts.
and parallel with respect to the grooves. A small portion of the
hard rubber is removed from between the grooves so as to leave
a rectangular opening, # 6 , shown in Figs. 181 and 182, through
the strip connecting the two grooves at those ends which are
nearer the front of the jack. In the grooves, a l t upon the upper
surface of the rubber strip are mounted springs, b and c. The
spring, b, is the longer of the two, so that its curved extremity is
presented close to the end of the perforation through the front
portion of the strip, a. The springs are insulated from each
other by a strip or tongue, d, of hard rubber, thin and flexible
enough not to impede the flection of the two springs. In the
212 AMERICAN TELEPHONE PRACTICE.
under groove, a" 2 , is mounted another spring, b\ similar to spring,
b, and of equal length.
The three springs, b, c, and b\ are firmly secured to the strip, a,
by a bolt, e, passing through them and the body of strip, a. The
bolt is insulated from the springs, b l and , by rubber washers
and bushings. In the perforations, a 3 , in front of the strip, are
inserted short tubes, / of brass. Each tube or thimble, / is pro-
vided with a shoulder, which bears against a corresponding ledge
in the perforation, a*, so as to prevent the tube from being
thrust backward toward the rear of the jack by the insertion of
the operators' plugs. The thimble, f, is provided with an exten-
sion, /', to afford electrical connection with it from the rear of
Fig. 183. Details of Plug.
the jack. This strip,/ 1 , extends through an oblique duct,/ 2
shown in dotted lines in Fig. 181 and thence through a trans-
verse slot or saw-cut, a\ to the rear of the strip.
In front of the thimbles,/ in the perforations, a s , are placed
the test-rings, very short tubes of brass, g. These are forced into
place against other ledges in the perforation. The ring,^, is also
provided with an extension, g\ projecting to the rear of the strip
of spring-jacks. These extensions, g l , are of wire and pass
through another duct, g", in the front portion of the strip, a, into
a saw-cut, cf, thence to the rear of the strip, where they are con-
nected with the spring, b.
The springs in these jacks are of hard German silver, which has
been found the most desirable material for this and similar
purposes.
The detail of one of the plugs used with these jacks is shown
in Fig. 183. The tip, //, of brass is secured by the rod, 7/ 1 , to
the block, ff, also of brass. Insulated from the tip portion by a
rubber bushing is the sleeve contact, h\ of the plug, which pro-
jects rearwardly and forms the main body of the plug. Over this
portion is slipped a shell, // 6 , of hard rubber or fiber, which forms
a handle for the plug. Between the tip and sleeve, and insulated
from each, is the ring, h\ which, as was described before, is for
short-circuiting the springs, b and b l , when the plug is inserted in
the jack.
Screw connectors, k* and // 6 , form convenient terminals for
THE MULTIPLE SWITCH-BOARD. 213
attaching the strands of the cord to the tip and sleeve, respect-
ively, of the plug. These connectors are always readily acces-
sible for inspection or repair by the removal of the sleeve, h\
This brings us to the subject of flexible cords for switch-board
use. The matter seems at first thought to be a simple one, .but
much thought and time have been spent in perfecting this branch
of the equipment. Even at tliis late day the best cord is not
good enough. As near a perfect cord as has up to the present
time been made is constructed as follows : each conductor in the
cord is composed of strands of tinsel and a few fine copper wires,
to add strength and conductivity. Around each of these con-
ductors are wrapped tightly, in opposite directions, two layers of
floss silk. These should be wrappings and not braids, as a wrap-
ping serves to keep any broken ends of the tinsel or wire down
in the bunch better than a braid. Around these wrappings of
silk is a braid of linen, which adds strength to the conductor. If
the cord be for a metallic circuit, the two conductors thus formed
are then wrapped with two more layers of floss silk, and the whole
is then encased in a strong spiral wrapping of hard spring brass
wire. The wire in this spiral is stiff enough to retain its shape,
but the spiral as a whole is so flexible as to allow free bending
of the cord. An outer covering of polished cotton is tightly
braided over the spiral, and after the ends are tightly anchored
to prevent the conductors sliding back and forth in the spiral, the
cord is complete. An extra layer of outside linen braid is often
put on the cord for a distance of about one foot back of the plug,
to prevent the sharp bending or kinking of the cord by the
operator in inserting the plugs into the jacks.
For handling very large exchanges Mr. Milo G. Kellogg of
Chicago has invented a number of divided multiple-board
systems, some of which will probably prove important factors in
the telephone industry of the immediate future. In these he
divides the lines into four classes and the switch-boards into
corresponding divisions, one division for each class of lines.
Each line has four polarized drops and four answering jacks, one
drop and jack being located on each division of the boards. In
addition to this each line has a multiple jack on each section of
one of the divisions, the arrangement being the same in this respect
as in the ordinary multiple-board. Two of the polarized drops
are of opposite polarity and are connected in series between one
side of the line and ground. The other two are also of opposite
polarity and are connected in series between the other side of the
line and ground. By sending a current of ene polarity or the
214
AMERICAN TELEPHONE PRACTICE,
other over one or the other of the line wires and ground, a
subscriber may thus signal any division of the exchange.
In operation, if a subscriber in class A desires to call one in
class D he sends, for instance, a negative current over the test
side of the line, which operates his drop at division D of the
exchange. As the called subscriber belongs to the D class he
will have a multiple jack upon each section of the D division of
boards, and the operator will answer the call from the A line, by
inserting a plug in the answering jack, which is always on the
same section as the drop, and complete the connection by
Fig. 184. Kellogg Divided Multiple Switch-Board.
inserting the other plug of the pair in one of the multiple jacks
of the D subscriber called for. The calling of any division of
the exchange is done by an arrangement of push-buttons at the
subscriber's station, the subscriber always pressing the button
bearing the letter of the class to which the subscriber called
belongs.
In Fig. 184 is shown diagrammatically the circuits of one of
the Kellogg divided board systems. The four divisions of the
board are represented by A, B, C, and D ; A 1 A 2 , B 1 B 2 , etc.,
representing the various sections of board in each division.
THE MULTIPLE SWITCH-BOARD. 215
Each jack, 5, is composed of a tip contact,^, a sleeve contact,/,
a test contact, d, and two auxiliary contacts, a and b, adapted to
be pressed into engagement with each other and with the test
contact by an insulated portion carried on the connecting plugs.
L and L are two subscribers' lines. Line L is an A line, and
therefore has a jack on each section of division A, and one jack
on one section only of each of the other divisions. Line L
belongs to class C, and therefore has a jack on each section of
the C division and on one section of each of the other divisions.
Line cut-off relays, x x, are provided, one for each line. Each
of these is included in a local circuit containing the common
battery, B ', the ground, and the springs, a and b, of each jack
belonging to its particular line. These relays when operated cut
off both taps to ground, containing the line-drops, w, of which
there are four for each line. The insertion of a plug into a jack,
therefore, by pressing together contacts, a and b, operates the
relay, x, to cut off all the drops from the line. It also connects
the test contact, d, with the ground at G through the spring a ;
and as all the test contacts, d, of the jacks on one line are per-
manently connected together, all are grounded. This ground-
ing of all test contacts of the jacks belonging to a busy line is to
enable an operator to determine the condition of the line. The
tip side of her cord circuit is while making the test grounded
through an impedance coil and battery. If the line tested is
busy, she obtains a click due to the closed circuit from the tip of
her plug to ground through the test contact and spring, a. If
the line is free the test contact is not grounded, the circuit
remains open, and no click is obtained.
The advantage claimed for this division of multiple-boards is
the enormous saving of multiple jacks. The limit to the number
of subscribers in a single multiple board is found to be about
6000, a greater number rendering the boards so cumbersome
that an operator cannot reach all of the multiple jacks on her
section. By thus dividing the exchange into four divisions it
becomes possible to place as many as 24,000 subscribers in
a single exchange.
CHAPTER XX.
TRANSFER SYSTEMS.
person of intelligence can visit one of the large exchanges
of the Bell Company, equipped with a modern multiple switch-
board, without being deeply impressed by the magnificence of
the equipment, the perfection of the system in its entirety and in
its minutest detail. If he is conversant with telephone matters,
he must also be impressed by the fact that while the multiple
switch-board gives the subscriber what he needs, quick reliable
service, it gives it only at a great cost to the operating company.
There is no question but that at the present stage of telephonic
development the multiple-board systems represent the highest
type of central-office equipment, and while one form or another of
them are in use in nearly all the really large exchanges the world
over, there are a few notable exceptions. The success of some
of these, coupled with the enormous expense necessarily entailed
in the installation of large multiple boards, leads the writer to
believe that the coming system, while it may in some degree
embody the plan of multiple jacks, will depend for its action on
other ideas already developed to a considerable extent.
In the multiple switch-board the cost of installation increases
approximately as the square of the number of the subscribers.
When a new section containing, say, 200 spring-jacks, annuncia-
tors, and other apparatus, is added to an exchange, the increase
does not end there. Two hundred multiple jacks must be added
also to each of the sections already existing in the exchange.
This is clearly an objection which increases in seriousness as the
exchange grows. A multiple board, of the type so far consid-
ered, having a capacity for 6000 lines, would probably be divided
into 30 sections of 200 lines each. On each section would be
placed 6000 multiple jacks, plus the 200 answering jacks belong-
ing to the particular 200 lines whose annunciators were located
at that section. This would make a total of 6200 spring-jacks on
each section, or 186,000 in all. The first cost is, therefore, large;
increasing the board is excessively expensive, and cost of main-
tenance is necessarily heavy.
The enormous multiplying of jacks in the multiple system is
for one purpose to enable each operator to have within her
216
TRANSFER SYSTEMS. 217
reach a terminal of every line in the exchange, to the end that
she may be able herself to complete any connection called for
over any one of the lines under her immediate supervision :
that is, that she may be able to answer any call arising at her
section, and, without requiring the aid of any other operator,
make the connection called for r In other words, in a 6000 ex-
change, 186,000 spring-jacks would be used, instead of only
6000, in order to accomplish this result.
Clearly, if multiple jacks are not used, two operators or more
will have to be instrumental in making a connection between
two subscribers whose lines terminate on different portions of the
board. It would seem at first thought that this would be a con-
siderable disadvantage, and would result in a slower system. On
further consideration, however, why should it be more of a dis-
advantage to divide the labor of manipulating a switchboard be-
tween several operators, than it is to apportion the labor of mak-
ing a pair of shoes or any other manufactured article among a
large number of operators, as is now done in all large factories?
The loss of time required by one operator having to repeat an
order to another, or, as is the case in some systems, of the sub-
scriber having to repeat his own order, is at least partially com-
pensated for by the gain in simplicity over the multiple system.
Systems depending for their operation on the transfer of a
connection from one portion of a board to another are termed
transfer systems, and one of the most successful of these is the so-
called " express system" of Messrs. Sabin & Hampton of San
Francisco. This system has been used for several years in San
Francisco, and has, according to reports, demonstrated its capa-
bility of handling with success an exchange having over 6000
subscribers.
The system is so radically different from anything so far de-
scribed that its consideration in detail should be a matter of
much interest. One striking feature in it is that no magneto-
generators are used at the subscriber's station, and when it is con-
sidered that approximately one-third of the cost of a complete
telephone set of the ordinary type is in the magneto, it will be
seen that this is a saving of considerable moment. The doing
away of the magneto, however, is not an essential feature of the
express system, but is one of the advantages incident to its use.
Briefly stated, the underlying ideas of the express system are
as follows: The boards are divided into two classes, termed for
convenience " A " and " B." Similarly, the operators at the re-
spective boards are termed "A" operators and "B" operators.
2i8 AMERICAN TELEPHONE PRACTICE.
There is but one line jack for each line in the exchange, anct
these are divided into groups of one hundred each and are placed
only at the " B " board. At the " A" boards, which are entirely
removed from the " B " boards (they may even be in another
exchange), are placed plugs which form the terminals of other
trunk lines leading from the various "B" boards; and also jacks
forming the terminals of other trunk lines leading to the "B"
boards.
The former trunk lines that is, those terminating in plugs at
the "A" boards also terminate in plugs at the "B" boards.
These are termed "A" trunk lines. The latter trunks that is,
those terminating in jacks on the "A" boards terminate in
plugs on the "B" boards, and are termed "B" trunk lines.
When a call is received it attracts the attention of one of the
" B " operators by displaying an annunciator in the ordinary man-
ner. The "B" operator at whose board the call is received
pays no further attention to it than to insert one of the plugs of
an "A" trunk into the jack of that line, thus transferring the
call to an " A " operator, who answers it with a listening key in
the ordinary manner. No means whatever are provided for a
" B" operator to listen in on a subscriber's circuit, this duty be-
ing confined solely to the "A" operators.
The "A" operator, having learned that the subscriber calling
desires to be connected with a certain other subscriber, conveys
this information, by means of a special order wire, to the " B "
operator at whose board the called-for subscriber's line termi-
nates. The " B " operator then tells the " A " operator what " B "
trunk line to use, and the "A" operator then inserts the plug of
the "A" trunk line used into the jack of the "B" trunk line
thus designated. This brings the connection as far as the board
of the second " B " operator ; that is, the " B " operator at whose
board the called-for subscriber's line terminates. This operator,
in order to complete the connection, simply inserts the plug of
the " B " trunk used into the jack of the called-for subscriber's
line, and presses a ringing key in order to call that subscriber.
It will be seen that the connection has really been handled by
three different operators, but that the first of these operators does
no more than to insert a plug into a jack, giving the matter no
further attention.
All signaling between the subscribers and the operators and be-
tween the various operators, whether it be for establishing or
clearing out a connection, is entirely automatic, and therefore not
dependent upon the volition of the parties concerned.
TRANSFER SYSTEMS.
2 19
Fig. 185 shows the arrangement of the circuits at the sub-
scriber's station, and also the arrangement of the spring-jacks and
annunciators on the " B " boards. One side of the line wire at
the subscriber's station is normally grounded through the polar-
ized ringer, R. This means that calling a subscriber from the
central office must be accomplished over one limb of the line
C^fft^j^B artery
Fig. 185. Subscribers' Circuits Express System.
wire and ground, instead of over a metallic circuit, as in case of
talking and other signaling in this system. The other circuits
and apparatus at the subscriber's station are of the ordinary ar-
rangement and type, the only difference being that the magneto-
generator is omitted entirely. The line wires of each subscriber
terminate in two springs, a and b, of their spring-jack,/". These
springs normally rest on two anvils, c and d, one of which con-
nects through an annunciator, e, with a heavy wire leading to one
pole of the calling battery, and the other of which leads to a
similar wire connecting with the other pole of this battery.
This annunciator, e, has a shutter which is simply lifted by the
attraction of the armature, and again dropped into its normal
position when the armature is released. It is, therefore, the
simplest type of self-restoring drop. The circuit of the call-bat-
220 AMERICAN TELEPHONE PRACTICE.
tery is normally open only at the subscriber's station. It is auto-
matically closed through the receiver and secondary winding of
the induction coil at the subscriber's station whenever the sub-
scriber removes his receiver from its hook. This allows enough
current from the calling battery to pass through the drop, e, to
raise its shutter, and thus attract the attention of the " B " opera-
tor at that board.
The shutter remains raised until the operator inserts the plug
of one of the "A " trunk lines in order to transfer the call to the
" A " operator. The insertion of this plug, however, lifts the
springs, a and b, from the anvils, c and d, thus cutting off the
battery and allowing the shutter of the annunciator, e, to drop to
its normal position, and the " B " operator therefore pays no
more attention to it.
A single battery is made to serve for actuating the signals of
every line in the exchange, no matter how great their number
may be. Storage cells are used for this purpose, ten cells being
connected in series so as to give a pressure of about twenty volts.
It is said that the average flow of current from this battery is
about one and one-half ampere, and never exceeds two amperes,
in the San Francisco exchange of approximately 6000 subscribers.
It will be thus seen that the cost of maintenance of these batteries
is trifling.
Another good feature of this arrangement is that, should a
" B " operator by mistake withdraw a plug from a jack before a
subscriber has finished talking, that is, before he has hung up his
receiver, she will be at once notified of her mistake by the dis-
play of a signal belonging to that line.
In Fig. 186 is shown a simplified diagram of the express sys-
tem. At the bottom and top of this figure are shown the sub-
scribers' lines, leading in each case from the subscriber's telephone
apparatus to the drop and jack at the central office. This part of
the apparatus is the same as that shown in Fig. 185, but in this
figure the details of the local circuit at the subscribers' stations
have been omitted for the sake of clearness.
The jacks and drops belonging to these lines are, as has already
been stated, stationed at the " B " boards of the exchange. The
subscriber's indicator battery is represented at vS / B and the
indicators at /. Leading from the section of the " B " board
shown at the top of the figure is a trunk line leading to a plug on
the second section on the " A " boards. It will be noticed that
this trunk line terminates in a plug at each end, and is termed
the " A " trunk. An intermediate jack and plug are shown in the
TRANSFER SYSTEMS.
221
circuit on this " A " trunk, but these at present need not be
considered. Suffice it to say that the plug at the " B " board is
connected by a metallic circuit to the plug at the second section
of the " A " board. Leading from a certain jack on the second
section of the " A " board is a trunk line extending to a plug on
another section of the " B " Boards. This is termed a " B "
Fig. 1 86. Simplified Diagram Express System.
trunk. Only one " A " trunk and one " B " trunk are shown,
but it must be remembered that a number of " A " trunks
lead from each of the " B " boards to the " A " boards, and
that from the " A " boards a number of " B " trunks lead
back to each of the " B " boards.
When a subscriber, as for instance the one shown at the top of*
the figure, removes his receiver from its hook, his indicator, /,
is displaced automatically, and the operator at the particular
" B " board at which this indicator is located extends the circuit
of his line to one of the " A " boards over an " A" trunk. This
222 AMERICAN TELEPHONE PRACTICE.
she does by inserting the plug of an " A " trunk into the jack of
the calling subscriber. The operator at the ''A " board, having
learned the desire of the calling subscriber, extends the circuit
to that subscriber's line by means of the " B " trunk, still
further on to the particular " B " board at which the jack of the
called subscriber is placed. This the "A" operator does by in-
serting the plug of the " A " trunk used into the jack of a " B "
trunk at her board. The " B " operator at whose board the
calling subscriber's jack is placed then completes the connection
between the two subscribers by inserting the plug of the " B "
trunk used into the jack of the calling subscriber's line. The two
subscribers' lines are shown connected in Fig. 186 by the proc-
ess and over the circuits just described.
In order to facilitate matters it is evidently necessary that a
most complete and elaborate set of signals must be .provided be-
tween operators. The first in this series of signaling operations
is to notify the " A " operator that her attention is desired on a
certain " A " trunk. This must always occur just after the " B "
operator has inserted one of the " A " trunk plugs into the jack
of the calling subscriber's line. It will be noticed in Fig. 1 86 that
the relay, R, operating signal lamp, L, is bridged across the tip
and sleeve strands of the " A " trunk circuit, and this bridge may
be traced from the tip strand through the balance coil, B C,
thence through the clearing-out indicator battery, C O B, to the
battery wire, thence through the coil of the relay at the '" A "
board, and thence to the sleeve strand of the " A " trunk. Re-
membering that the calling subscriber at the top of the figure
has removed his receiver from its hook, then the insertion of the
plug of the " A " trunk will restore the line drop, /, and at the
same time will close the circuit from the clearing-out indicator
battery, COB, and the relay, R, through the subscriber's line
and telephone instrument. This will operate the relay and cause
it to close the circuit of the signal lamp, Z, thus calling the atten-
tion of the " A " operator to the fact that an unanswered call is
upon the trunk line to which that lamp belongs.
The apparatus of the " A " operator is shown more clearly in
Fig. 187. The sleeve and tip strands of the " A " trunk are shown
at the extreme left of this figure. When the armature of the re-
lay, R, is attracted, as described above, the circuit from the
local battery and white lamp is completed at the point, b, of the
relay. It will be noticed that this local circuit extends through
two of the springs, e and f, normally closed, on the listening key
of the " A " operator, and also through a pair of contacts, / and c,
TRANSFER SYSTEMS. 223
held closed by the weight of the plug of the " A " trunk in its
socket. The white lamp will therefore remain lighted until one
of the following three things happens : until the operator lis-
tens in, which causes the local circuit to break at the listening
key ; or until she removes the plug of that trunk line from its
socket, which would break the local circuit at the point, c; or until
the calling subscriber hangs urj his receiver, which would cause
the relay to let go of its armature, and thus break the circuit
at point, b. The white lamp remains lighted, therefore, as long
as the call on its " A " trunk is unattended to.
The first act of the " A " operator on seeing this light is to
throw her lever corresponding to that light into its horizontal
position, thus connecting her telephone to the terminals of the
" A" trunk in the usual manner. This enables the " A " opera-
tor to communicate with the calling subscriber in the ordinary
manner. It should be noted that these keys on the " A " opera-
tor's boards are the only means afforded to any operators for
communicating with subscribers. The operation of this key breaks
the circuit of the white lamp at the points, e f. As soon as the
listening key is thrown again into its normal position the white
lamp is again lighted, thus calling the operator's attention to the
plug to be used in making the connection.
This precaution is a wise one, for before making the next move
in the connection, the "A" operator must communicate with
the " B " operator at whose board the called-for subscriber's line
terminates. The " A " operator does this by depressing her or-
der-wire key, shown in Fig. 186, but omitted from Fig. 187, which
act connects the " A " operator's telephone directly with the
telephone set of the outgoing " B" operator. The "A" opera-
tor then tells the " B " operator the number of the line with
which connection is desired, and the " B " operator in return
tells the " A " operator the number of the trunk line she is to
use in making the connection.
Herein lies one of the greatest defects of the system. That is,
the necessary waiting by the " A " operator for the reply of the
" B" operator. This is a loss of time on the part of the "A "
operator, for it often occurs that the " B " operator may have
several other connections under way which she could not well
leave in order to reply to the " A " operator by designating the
number of " B" trunk to be used.
It will be noticed that the trunk jacks of the " B " trunks are
in reality arranged on the plan of the multiple board. This is
for the purpose of placing within the reach of every " A " opera
224
AMERICAN TELEPHONE PRACTICE.
tor a jack belonging to every " B " trunk line. No test system,
however, is required on these jacks, as an " A " operator always
first learns from an outgoing *' B " operator which " B" trunk to
use, and of course a " B " operator would never designate any
trunk which was already in use, or " busy."
It would seem that a " busy " test, or preferably a visual " busy "
signal for the multiple jacks on the "A" boards could be used
CfiB.
Fig. 187. Table of " A " Board.
with advantage. This would enable the " A " operator to at
once select an idle " B " trunk and at the same time inform the
proper "B" operator of the connection to be made and the
trunk line plug to use in making it. For instance, the " A "
operator could simply say: " 1504 on 10," meaning by the first
number the number of the subscriber called for, and by the
second number the trunk line to be used,
We have now carried the connection, or extended the circuit of
the calling subscriber's line, as far as the trunk line plug at the
outgoing "B" board. The outgoing " B " operator then com-
pletes the connection by inserting this plug into the jack of the
TRANSFER SYSTEMS. 225
called subscriber. The " B " operator then depresses her ringing
key, shown in a simplified form in Fig. 186, which sends calling cur-
rent from the generator, G, over the sleeve strand of the plug
cord, thence to line and to ground through the polarized call-bell
at the subscriber's station.
The next feature to considers that of the automatic clearing-
out signals. As the connection between the two subscribers is
made by three operators, it is evident that three distinct clear-
ing-out signals should be given, one at each of the boards of the
operators who help establish the connection. Turning again to
Fig. 187, it will be seen that the raising of the "A" trunk plug
from its socket changed the circuit of the local battery from the
white lamp to the red lamp, by moving the selecting lever, /, from
the point, c, to the point, d. Remembering now that as long as
the calling subscriber's receiver is off its hook, the circuit from the
clearing indicator battery is closed through the relay, R, at the
" A " board, thus attracting its armature. As soon, there-
fore, as the calling subscriber finishes his conversation, he
hangs up his receiver, and thereby breaks the circuit through the
relay at the " A " board, thus closing the circuit through the red
lamp. This lamp will therefore be lighted as a notification to
the " A " operator that disconnection on that trunk is desired.
The replacing of the plug in its socket opens the circuit of the
red lamp at the point, d, thus extinguishing the lamp. This
apparatus at the "A" board is very ingenious, and deserves
special attention. It should be noticed that should an operator
by mistake remove one of the " A " plugs, and replace it in its
socket before the subscriber connected had hung up his receiver,
the white lamp would be relighted, thus calling the attention of
the operator to the error.
The clearing-out signal is given to the incoming " B" operator
in much the same way as that on the " A " board, the clearing indi-
cator, or relay, on the " A " trunk of the " B " board being wired
in multiple with the relay on the " A " board.
The clearing-out signal on the outgoing " B " board is accom-
plished by much more complicated means, and will be explained
by reference to Fig. 188. In this figure the ringing key,/, is shown
in more detail than in Fig. 186. The " B " trunk line jack on the
" A " board is represented by a. In the normal position of the key
the two strands of the " B " trunk are connected to the tip and
sleeve of the corresponding plug on the outgoing " B " board.
When, however, the key is depressed, the sleeve strand of the cord
is connected with the calling generator, the other terminal of
226
AMERICAN TELEPHONE PRACTICE.
which is grounded. When the operator depresses this ringing
key, a secondary pair of contacts, i i l , are closed, thus actuating the
lower magnet, /, of the compound relay, and causing it to attract
its armature, /'. When the operator allows the key,/, to rise, the
armature, /, falls back, but is caught by the hook, m\ of the upper
coil, ;//, of the relay. The hook, m\ and the tip of the armature
a
Fig. 188. Table of
are platinum-pointed, and their con-
tact causes the signal lamp, n, to be
lighted. This lamp remains burn-
ing until the called subscriber takes
his telephone off the hook, which act
closes the circuit through the com-
bined clearing-out relay and signal, c,
in exactly the same manner as the relays on the incoming " A "
trunk line were operated. The operation of this relay therefore
closes a circuit at the points, f c\ through the upper magnet,
m, causing it to raise the hook, m\ and allow the armature, /', to
drop back. This extinguishes the lamp, n, and shows the opera-
tor that the subscriber has responded. The armature, c\ of the
relay, c, remains attracted until the called subscriber hangs up his
receiver, which de-energizes the magnet, c, and allows the signal
carried by the armature to resume its normal position. This is
the clearing-out signal for the outgoing "B" operator, and she
accordingly pulls out the plug.
To review the action of the indicators at the outgoing " B "
board, the releasing of the key for transmitting a calling signal to
the subscriber lights the lamp, , and shows the operator that
this part of her work has been attended to. The response of the
subscriber is indicated by the going out of the lamp, and by the
raising of the signal, k. The clearing-out signal is given by
the lowering of the signal, k.
TRANSFER SYSTEMS. 227
We have now traced through the operation of all the signals
between the subscribers and the operators, and between the
operators themselves, which were necessary to establish a connec-
tion between two subscribers ; and also the subsequent signals
between the subscribers and the operators, indicating that a dis-
connection is desired. The striking feature of all this elaborate
system of signaling is that each signal is automatically given with-
out the volition of the operator or subscriber, inasmuch as it is
brought about by some action necessary in the actual connection
or disconnection.
In order to reduce the work of the operators to the last degree,
two phonographs are placed in connection with the exchange, one
of which is constantly and politely repeating the sentence, " Busy.
Please call again," while the other repeats with equal regularity,
" Subscriber called for does not reply." Each of these phono-
graphs speaks to a transmitter arranged in connection with an
induction coil and battery in the ordinary manner. The terminals
of the secondary of the induction coil of the " busy " phonograph
terminate in a jack on each section of the " A " boards. In like
manner the " does not reply " phonograph is connected with a
jack on each section of the " B " boards. When, therefore, an
"A " operator learns that a line called for is busy, she inserts the
plug of the " A " trunk to which the calling subscriber is connected
into the phonograph jack, and the familiar but disappointing mes-
sage, " Busy. Please call again," is automatically conveyed to the
calling subscriber.
In a similar manner the outgoing " B " operator may inform
the calling subscriber that the subscriber called for does not
respond. The use of the phonograph for this purpose may seem
at first thought to be carrying the labor-saving idea to an extreme,
but it enables an operator to attend to another subscriber while
she is telling the first subscriber that his line is busy or that his
party does not respond. It moreover insures that the wrath of
the calling party will produce no evil effects on the nerves of the
operator, which at busy times is no unimportant consideration.
The writer is indebted to an able article by Mr. George P. Low,
in the Electrical Journal, for much information concerning this
very interesting exchange system.
Fig. 189 is a schematic representation of the system used in
the larger exchanges of the Western Telephone Construction
Company of Chicago. This system has proved very successful
in practice for exchanges up to fifteen hundred subscribers,
although with a larger number certain difficulties are met in
228
AMERICAN TELEPHONE PRACTICE.
the disposal of the transfer plugs. In this figure I, 2, 3, 4, etc.,
represent different sections of the board, each section having
one hundred combined drops and jacks of the type shown in
Figs. 158 and 159 together with a complete operator's equipment.
One answering plug, A, together with one calling plug, B, is shown
at each section. These are connected together in pairs through
clearing-out drops, O, by ordinary flexible cords which contain the
JO
11
$
5 k.ii
LL L
ILL
i_a
Trrr
Fig. 189. General Scheme of Western Transfer System.
necessary switching apparatus for enabling an operator to listen
in and to ring out over either cord as desired. Connected with
each of these sets of plugs is a trunk line to which is connected
at every third section a transfer plug, C, as shown. Thus, a
pair of plugs, A and B, shown at section i, is connected by
means of a trunk line to a transfer plug at section 4, another at
section 7, another at section 10, and so on. A careful considera-
tion of this figure will show that the same is true for each pair
of plugs, A and B, at the other boards. Fig. 190 shows in
greater detail one pair of plugs, A and B, connected by a trunk line
to the several transfer plugs according to this system. The plugs,
A and B, are in this case at section 4, while the transfer plugs,
C, C, C, are at sections I, 7, and 10, or in other words, at every third
section on each side of section 4. The form of circuit-changing
lever L is here shown for convenience only, and serves to illus-
TRANSFER SYSTEMS.
229
trate the principle, but not the actual connections in this
system. By throwing this lever to the left, its two springs are
connected with an operator's circuit through the secondary wind-
ing of the 'induction coil, as shown, while when the lever is
thrown to the right the terminals of the generator, G, are con-
nected with the plug circuit. The circuit-changer actually used
in this cord circuit is the one of the Western Telephone Con-
struction Company, described in 1 Chapter XV. The operator
immmm
L-vwJ L
^yiiiJ
t VWV [^
Ir^THJJ]
Fig. 190. Transfer and Instruction Circuits.
pulls the lever forward to ring the called subscriber, pushes it
from her to ring the calling subscriber, and presses it downward
to listen in.
The system can now be more readily understood by describ-
ing its operation. If a subscriber whose line terminates in
section 4 calls up, the call is answered by the operator at that
board by inserting one of her plugs, A, the insertion of this plug
restoring the shutter mechanically. The operator then throws
the lever Z, to the left, connecting her telephone set, E, with
the line of the subscriber calling. Having learned the want
of the subscriber, who we will say is 1001, the operator at 4
depresses the key, K, which connects her telephone set with an
instruction circuit, /, terminating in the telephone set of the
operator at section 10. The operator at section 4 is thus en-
abled to communicate with the operator at section 10 over this
circuit, and the former informs the latter of the number desired
and of the particular transfer plug, C, she is to use in making this
connection. The operator at section 10 then takes up the plug,
C, designated and inserts it into the jack of the called sub-
230 AMERICAN TELEPHONE PRACTICE.
scriber, the operator at section 4 meanwhile holding the lever,
Z, of the particular plugs used, into the ringing position. As
soon as the connection is completed at section 10 the first
operator is informed of the fact by the operation of a buzzer
placed in the cord circuit so that she knows that the signal has
been properly transmitted to the line of the subscriber 1001.
After the conversation is completed, one or both subscribers
ring off, which throws the clearing-out drop, O, and informs the
operator at section 4 that a disconnection is desired. She there-
fore removes her answering plug and places it in the socket, in-
forming the operator at section 10 to do likewise. It will be
seen that, in addition to the transfer lines extending from the
answering and calling plugs at each board to transfer plugs at
each third board therefrom, a system of instruction circuits is also
provided, each circuit terminating in an operator's set at one
board and connected with push-buttons at every third section
therefrom, so that an operator is enabled to communicate only
with those operators located at every third section from her own
board. This peculiar arrangement serves several advantageous
purposes, among which is the reduction of plugs necessary for the
successful operation of the board and also the reduction of the num-
ber of operators talking over any one instruction circuit. It more-
over enables any operator to reach, by means of her own calling
plug or a transfer plug handled by another operator, any portion
of the board. For instance, if a subscriber calling on section 4
desires connection on section 3 or section 5, the operator at
section 4 will complete the connection herself by the use of the
calling plug, B, as she can readily reach any jack on her own
section or on that at her right or left. We have seen how a
connection is made between section 4 and some section at which
one of the transfer plugs of that section is located. If, how-
ever, the subscriber on section 4 had called for a subscriber at
section 9 the operator at 4 would have signaled the operator
at 10, who would then have completed the connection, using
transfer plug, C, with her left hand. If the called-for sub-
scriber had been upon section 8, operator No. 4 would have
signaled No. 7, who would have used a plug, C, at her section
with her right hand to complete the connection. Ten pairs of
calling and answering plugs are furnished for each section of
100 drops, each pair being connected by trunk line with transfer
plugs distributed through the system as already described.
A system of lamp signals for facilitating the w<5rk upon these
boards has been devised and used in many of the later exchanges.
232 AMERICAN TELEPHONE PRACTICE.
In this a white light is so arranged in connection with the night-
alarm circuit as to be illuminated, upon each board, whenever a
drop is thrown. A similar lamp in series with this is also ar-
ranged to be displayed on the chief operator's table, thus serving
as a telltale to call the attention of the chief whenever a
drop remains unattended on any section. A colored lamp
is arranged in connection with each set of transfer plugs and
controlled by normally open contact points in the plug seats
of the transfer plugs and normally closed contact points in the
plug seats of the answering plugs. Two lamps are arranged
in series in each circuit, one at the set of transfer plugs to which
it belongs and the other at the set of answering plugs with which
these transfer plugs communicate. Whenever an operator
raises an answering plug in order to establish a connection,
the lamp circuit is opened at that point by the operation of the
contacts in the plug seat. When another operator removes the
transfer plug to complete the connection this same lamp circuit
is closed at that point by the operation of the contacts in the
transfer plug seat. The circuit, however, still remains open at the
answering plug seat. When a calling-out signal comes, and the
operator removes the answering plug to disestablish the con-
nection, the lamp circuit is closed at its only open point, which
lights the lamp in front of each operator. This shows the trans-
fer operator that a disconnection is desired, and also shows the
answering operator that the disconnection has not yet been
made. The cycle of events is completed when the transfer
operator removes the transfer plug and replaces it in its seat,
which act opens the lamp circuit at that point, thus putting out
both lamps.
The switch-board of the Delmarvia Telephone Company at
Wilmington, Del., is shown in Fig. 191. This board embodies
all the features mentioned above and is undoubtedly representa-
tive of the best exchanges of the Western Telephone Con-
struction Co.
What is known as the Cook-Beach transfer system has been in
long use among some of the Bell exchanges of medium size, and
the large switch-boards now manufactured by the Sterling Elec-
tric Co., Chicago, are operated upon this plan. The subscriber's
lines terminate in drops and jacks on the various sections of the
board, no multiple connection whatever being used between
them. A set of transfer jacks is also provided on each
section, these jacks being connected by trunk lines extending
to transfer plugs located at the several sections. When a
TRANSFER SYSTEMS. 233
call is received at any section, the operator answers it by
inserting one of her answering plugs into the corresponding
jack. Having learned the number of the subscriber called for,
she inserts the corresponding connecting plug into the transfer
jack connected by a trunk line with a plug at the board where
the line of the subscriber called for terminates. She then com-
municates with the operator at that board, who picks up the
transfer plug designated and inserts it into the jack of the called
Fig. 192. 8oo-Line Sterling Switch-Board.
subscriber. The connection between any two subscribers is
thus made complete by the use of three plugs. This style of
transfer system has proven its adaptability to good telephone
service by long-continued use in both Bell and independent
exchanges.
A board embodying this plan of operation is shown in Fig.
192. The line-drops may be seen in the upper portion of the
panel of each section, and immediately below them the corre-
sponding jacks. The drops are of the type already referred to
in Chapter XIV., and are provided with the restoring feature by
which a whole vertical row of shutters are restored by the pres-
sure upon a button below that row. Immediately below the line
jacks are shown the transfer jacks, these being of such a nature
that when a plug is inserted into any one of them, a signal is
234
AMERICAN TELEPHONE PRACTICE.
automatically displayed at the other end of the trunk line to
which it belongs, notifying the operator at that section that a
connection is desired on that line. The listening and ringing
keys are of the type shown in Fig. 145. Two trunk lines are
provided from each loo-drop section of this board to every other
section, this number being found to give an ample number of
trunk lines at the busiest portions of the day.
The larger exchanges equipped by the American Electric Tele-
phone Co. use a transfer system somewhat similar to that of
the Cook-Beach type, composed of trunk lines extending be-
tween the various sections of the board and terminating in jacks
at one end and plugs at the other. Such a board is shown in
Fig. 193. There are two such trunk lines extending from each
Fig. 193. 1200-Line American Switch-Board.
ioo-drop section to each other section, thus giving four trunk
lines between each two sections two outgoing and two in-
coming from any position. These trunk lines terminate in jacks
at the outgoing ends, and plugs at the incoming ends. Each
operator has, besides the set of regular listening keys, a set of
instruction keys, one for each of the other operators, the de-
pression of any one of which connects her telephone set with
the set of another operator corresponding with that key. In
this way, a call received for a number not within the reach
of the answering operator is transmitted to the operator in
whose section the line called for terminates, being given by
means of the instruction key just mentioned. The calling line is
then connected by a pair of cords and plugs to the jack of one of
the two transfer lines reaching to the section in which the called-
for subscriber's line terminates. The establishment of this con-
TRANSFER SYSTEMS. 235
nection causes a lamp to light at the other end of the trunk line
and shows the operator there which of the two lines is to be
used. The drops used are the same as those illustrated in Figs.
161, 162, and 163, the shutter being restored by the insertion of
the plug, and again, after it has been operated as a clearing-out
signal, upon the withdrawal oi { the plug from the jack. Calling
is accomplished merely by pressing the calling plug to its fullest
extent into the jack of the called subscriber.
CHAPTER XXI.
COMMON-BATTERY SYSTEMS.
IT is an obvious disadvantage to have a separate source of
current at every subscriber's station in an exchange ; and it is
not to be wondered at that many efforts have been made to
centralize not only the transmitter batteries, but the calling cur-
rent generators as well. By bringing about such a centralization
of the sources of energy many desirable results are attained.
The idle capital represented by the local batteries and the calling
generators is done away with no small consideration in large
exchanges, because the magneto-generator is in itself the most
expensive part of an ordinary telephone set. The labor of visit-
ing or inspecting the subscribers' apparatus is greatly reduced ;
that necessary to repair and renew batteries, together with the
expense of material for such renewal, being rendered nil. The
subscribers' instruments are made neater and more compact.
The electrical efficiency of the plant is greatly increased by
having a few large units in operation practically all of the time,
instead of a great number of small units in operation but a small
portion of the time. Lastly, no freezing of the local batteries
occurs ; there is no spilling of the acids or other chemicals, and
no corrosion of the various parts by fumes therefrom.
As indirect advantages attained in the most modern exchanges
wherein all sources of energy are centralized, may be mentioned
the fact that the labor on the part of the subscriber in obtaining
a connection or a disconnection is reduced to a minimum, and
the labor on the part of the operator has been so greatly lessened
as to enable her to handle with success about twice as many
subscribers as with the old system. Most of the advantages
enumerated were appreciated by telephone men long ago, and
many attempts were made at an early date to realize them in
practice. The first attempts involved a return to first principles,
doing away with the induction coil and placing the transmitters
and receivers of two connected subscribers directly in series in
the circuit of the two line wires. In one of these, made in 1881,
by George L. Anders, the transmitter batteries were placed in a
loop used to connect the circuit of two line wires. In this the
236
COMMON-BATTERY SYSTEMS. 237
switch-board was of the old cross-bar type, and, while it used no
cord circuits, the batteries were placed in series in the connecting
wire corresponding to the cord circuit in later exchanges.
This general method, as applied to a board having plugs and
flexible cords, is illustrated in Fig. 194, where A and A represent
Fig. 194. Series Common-Battery System Grounded.
two subscribers' stations connected at the central office, C, by a
pair of plugs, P and P', having a battery, B, included in circuit
between them. The transmitter and receiver of each subscriber's
station are placed in series in the line wire, and each transmitter
when operated serves to vary the resistance of the entire circuit
formed by the two connected lines, and to thereby vary the
strength of the current flowing from the battery, B, in such
manner as to produce the desired effects in the receivers.
In Fig. 195 the same principle of operation is applied to
n
T\
Fig- 195. Series Common-Battery System Metallic.
metallic-circuit lines, two of which are shown connected at the
central office, C, by the pair of metallic-circuit plugs, P and P '.
In both of these cases, in which the battery is included in the
cord circuit in series with the combined circuit of the two lines,
the use of a separate battery for each cord circuit is, under
ordinary circumstances, necessary. This is always true of the
grounded system shown in Fig. 194, and is also true of the me-
tallic-circuit system shown in Fig. 195, unless the battery, B, is
made to have a very low internal resistance. This fact was
pointed out by Mr. Anthony C. White, who, in 1890, showed that
it was possible to supply all of the cord circuits from a single
battery by connecting them in the manner shown in Fig. 196.
This involves the bunching together of one side of each of the
cord circuits, the battery supplying current in multiple to the
238 AMERICAN TELEPHONE PRACTICE.
various pairs of lines in use at one time. This figure shows four
stations, A, A ', A", and A'", connected in pairs by two cord cir-
cuits and pairs of plugs. Fluctuations set up by the transmitter
in the line of subscriber, A, will circulate in the combined circuit
of the lines of subscribers, A and A'. Similar fluctuations set up
by the transmitter at A" will flow through the circuit of the
Fig. 196. Single Battery Series System.
lines, A" and A'". The battery, B, and the wire, a and a, in
which it is included, are common to both of these line circuits,
and if the resistance from the point, a, to the point, a ', through
the battery is considerable in amount, a part of the fluctuations
flowing in the circuit of subscribers, A and A', will be shunted by
this resistance through the combined circuits of the subscribers,
A" and A'". If, however, the resistance ffom the point, a, to the
point, a', is made extremely small, practically all of the current
changes will flow through the battery instead of being shunted
around through the circuit of the subscribers, A" and A'", owing
to the comparatively high resistance and impedance of that cir-
cuit, with its included instruments. The desired reduction in
the resistance between the points, a and a', may be accomplished
by making the battery, B, of extremely low resistance and by
shortening the wire, a.a', which is common to all of the circuits.
The former result is accomplished by using a storage battery of
rather large capacity, and the latter by joining the various cord
circuits directly to the bus-bars with the battery, so as to practi-
cally eliminate all resistance in the wire, a a'.
The common-battery arrangement shown in Fig. 197 is one
which has come into extensive use and was designed by Mr.
John S. Stone in 1892 and 1893. In this figure, A and A' are,
as before, two subscribers' stations connected by metallic circuit
lines with the central office at C. The transmitter and receiver
at each station are connected in series in the line circuit. The
COMMON-BA T TER Y SYS TEMS.
2 39
battery, B, however, is connected between the two sides of the
cord circuit, terminating in the plugs, P and P. On each side
of the battery is placed an impedance coil, / and /', as shown.
The action in this case is as follows : the current from the
positive pole of the battery, B, flows through the impedance
coil, /, to the point, a, where it divides, a part passing through
(
A C
A a .A
Fig. 197. Stone Common-Battery Arrangement.
the receiver and transmitter of each of the subscribers' stations.
The two parts of the current, after flowing back to the central
office through the opposite sides of the lines, unite at the point,
a ', and flow through the impedance coil, /', to the negative pole
of the battery. Inasmuch as the impedance coils are of low
ohmic resistance, they offer but little obstruction to the passage
of this current. If now the transmitter, T, at station, A, is
caused to lower its resistance, the difference of potential between
the points, a and a, will be lowered. This will result in a
Fig. 198. Stone Common-Battery Arrangement.
diminution in the current flowing in the line of subscriber, A ' .
On the other hand, if the resistance of the transmitter, T, is
raised, the difference of potential between a and a' will be
raised, thus causing a greater current to flow through the instru-
ment of subscriber, A ' . Every fluctuation in the resistance of
the transmitter, caused by sounds at either station, will thus
cause corresponding fluctuations in the current flowing through
the receiver at the other station, thus causing them to reproduce
the sounds. The same battery, B, is used to supply a large
240 AMERICAN TELEPHONE PRACTICE.
number of cord circuits, the arrangement being then as shown in
Fig. 198, each side of the various cord circuits being con-
nected to the poles of the battery through impedance coils, as
before. The fluctuations set up in the circuit of the two sub-
scribers, A and A', while perfectly free to pass through these two
particular lines, cannot find a path to any other lines, as, for in-
stance, those of subscribers, A" and A'", without passing through
the impedance coils, / and /', and also /", and /". It is said that
by means of this system a direct-current generator can be used
in place of the battery, B, the impedance coils serving to
effectually weed out all of the fluctuations in the generator cur-
rent which have always been found so annoying in telephone
work. Notwithstanding this, however, the storage battery is
always used in practice in systems embodying these principles.
Early in 1892 Mr. Hammond V. Hayes devised the method
of supplying current to transmitter batteries shown in Fig. 199,
Fig. 199. Hayes Common-Battery Arrangement.
this having come into very extended use in the Bell Companies,
and it has formed the basis of some of the most successful common-
battery systems in the world. The apparatus at the subscribers'
stations, A and A', is arranged as in all of the preceding sys-
tems. At the central office, K K' are repeating coils each
having two windings, k and k '. The two windings of the coil, K,
are connected together at the point, a, which is connected with
the positive pole of the battery, B. The other ends of these
two windings are connected with the upper contacts of the plugs,
Pand P, as shown. In an exactly similar manner the two
windings of the repeating coil, K ', are connected together at the
point, a', which is connected with the negative pole of the
battery, B, the other two ends of these coils being connected
with the lower contacts of the plugs, P and P'. By this arrange-
ment the battery is included in a bridge conductor between the
sides of the circuit formed by the two connected lines, and one
limb of each line includes one of the windings of one of the re-
peating coils. The current from the battery, B, will, when the
subscribers' receivers are removed from their hooks, divide at
COMMON-BA TTER Y S Y STEMS.
241
the point, a, and pass in multiple through the two windings of
the repeating coil, K, thence the two portions of the current will
pass through the transmitter and receiver of the two subscribers'
stations respectively and back to the repeating coil, AT', through
the windings of which they pass to the negative terminal of the
battery. (
Any changes in the current in either circuit, produced by the
operation of one of the transmitters, will act inductively through
the repeating coils upon the other circuit, causing corresponding
fluctuations in current to flow through that circuit and actuate
its receiver. Thus when the subscriber at station, A, is trans-
mitting, the windings, k k, will operate as a primary coil of an
induction coil of which the secondary is formed by windings,
k k , When the subscriber, A, is transmitting, this action is re-
i
G G
Fig. 200. Dean Common-Battery Arrangement.
versed, k k serving as a primary and k k as a secondary coil.
The transmitter at any station is compelled to vary the resistance
of its own line circuit only, and in this way some of the advan-
tages of a local circuit are gained. The two helices of each repeat-
ing coil are, under ordinary circumstances, of the same resistance
and number of turns, and wound side by side on the same core.
The resistance of each helix is usually made rather low, being in
the neighborhood of five ohms.
All of the systems so far described have contained the sub-
scriber's talking apparatus directly in series in the line wire. Mr.
J. J. Carty is responsible for the broad idea of supplying current
to the transmitter of the subscriber's station over the two sides of
a metallic line circuit in parallel, using the ground as return.
This method, as worked out by Mr. Carty, has been improved
upon by Mr. W. W. Dean, who has produced an extended series
of inventions embodying this feature. One of them is shown,
stripped of details, in Fig. 200, in which A and A are two sub-
scribers' stations and C the central office. / is an impedance
coil bridged across the two sides of the cord circuit of the plugs,
242 AMERICAN TELEPHONE PRACTICE.
Pand P. The center point, a, of this coil is grounded through
the talking battery, B. The receivers at the subscribers' stations
are connected serially with the secondary coil, s, of an induction
coil in the metallic circuit formed by the two sides of the line
wire. /' is an impedance coil bridged between the two sides of
the line circuit at each subscriber's station, the center point, /, of
this coil being connected with one side of a primary circuit con-
taining the transmitter, T, and the primary coil,/, of the induc-
tion coil. The opposite side of this primary circuit is grounded
at the point, g. Current from the battery, B, flows to the center
point, a, of the impedance coil,/, in the cord circuit; thence
through the two sides of this coil in multiple to the points, b and
Cj on the opposite sides of the cord circuit. From these points
the current flows over the two line wires in multiple to the points,
, carried on the pawl, thus holding it out of engagement
with the ratchet-wheel until the rotation of the lever is nearly
completed. At this point a cam on the under side of the ratchet-
wheel pushes the dog, y, out of engagement with the pin, /, and
thus allows the pawl to drop into position against the ratchet-
wheel. It will be seen that this device accomplishes with cer-
HOUSE SYSTEMS. 271
tainty what the memory of the telephone user could not be relied
upon to do. This entire mechanism is well constructed, all of
the parts subject to wear being of hardened steel. The diagram
of circuits given in Fig. 218 shows the system wired for four
stations operated with common calling battery, and with local
batteries at each instrument for talking purposes. This company
also manufactures these instruments arranged for the ordinary
magneto system, in which case the wiring may be substan-
tially the same as that shown in Fig. 214, but without its dis-
advantages.
CHAPTER XXIII.
PROTECTIVE DEVICES.
THE matter of protecting telephone apparatus from the damag-
ing effects of currents other than those which properly belong
on telephone lines, is not such a simple one as might be at
first supposed. The "lightning arresters" found on nearly all
telephone instruments are for the purpose of protecting the
instruments only from what may be called high-tension
currents, such as those produced by lightning. The usual
form of this arrester is shown in Fig. 219, in which A and B
Fig. 219. Usual Form of Telephone Lightning Arrester.
represent the two line plates carrying two binding posts and
forming the terminals of the instrument. The plate, C, is con-
nected with the ground. These three plates are not in contact,
the idea being that a charge of lightning will jump across the air
gap to the ground plate before it will pass through the high
resistance and impedance of the instrument coils within. They
do some good, but are by no means infallible, as lightning has
too many freaks to be kept out by any such simple device.
The holes, e, f, and g, are for the reception of a metallic
plug, which if placed in e short-circuits the instrument, and
if in / or g connects either one side or the other of the line
to ground. The placing of the plug in the hole, e, affords a very
efficient means of protecting an instrument during a storm, but
it is subject to the very grave disadvantage that people will for-
get to remove it after the storm. If but one subscriber is served
by a line, no one is hurt but himself, but if it is a party line, con-
PROTECTIVE DEVICES. 273
siructed on the bridging principle, the insertion of such a plug
causes a short-circuit which may disable the whole line. Cases
are very numerous where a repair man has had to drive perhaps
twenty miles in order to tell a subscriber to remove his plug, for,
obviously, the subscriber cannot be called up by telephone.
The next most simple means consists in placing in the tele-
phone circuit a fuse-wire of very small current-carrying capacity.
These wires are usually mounted upon mica strips, which may be
inserted between clips forming terminals of the line and instru-
ment wires. Although largely used, these have not proved at all
reliable, but they often save an instrument when the one shown
in Fig. 219 fails. Considerable difficulty is apparently experi-
Fig. 220. American Lightning Arrester.
enced by the manufacturers of very small fuses in gauging them
to blow at a given amperage. It is frequently found that" ^-am-
pere " fuses carry two amperes without showing any signs of
blowing. Again, inasmuch as these fuses are necessarily very
fine, being not much larger than a hair, it is a very easy matter
to break them, thus causing an open circuit the location of which
may not be at once apparent. In the case of a high-tension cur-
rent these fuses usually blow, but frequently start an arc across
the terminals, which does the damage to the instrument as effect-
ively as if the line wire was continuous. An instrument is some-
times found burned out with its fuse still intact, although this is
uncommon.
Still another form of protector consists of two carbon blocks
held apart by a thin disk of mica, one block forming the terminal
of the line, the other being grounded. These blocks are usually
arranged to slip in pairs between rather strong springs, so that
they may be easily removed when desired. One form of these,
shown in Fig. 220, represents the double-carbon lightning arrester
of the American Electric Telephone Company. The two bind-
274 AMERICAN TELEPHONE PRACTICE.
ing posts at the top of the figure are attached to the two branche^
of the line, which are not cut, but run continuously to the tele-
phone instrument, or whatever it is that is to be protected. The
third binding post is in connection with the ground plate upon
which the two lower blocks rest. The idea in this is that a cur-
rent coming in over the line will jump across the very small
space between the carbons and pass to the ground without harm-
ing the instruments.
The arrester shown in Fig. 221 is that used by the Western
Fig. 221. Western Lightning Arrester.
Telephone Construction Company, and is a combination of the
-carbon and the fusible arresters. In this the two line wires of a
metallic circuit enter the two binding posts at the right of the
cut, from which each circuit passes through the fuse-wires
mounted on the mica strips, and then to the vertical springs
bearing against the right-hand carbon block. These springs are
respectively in connection, by metallic strips underneath the
porcelain block, with the two binding posts at the extreme left
of the figure. The single binding post is in connection with the
two vertical plates holding the carbons, and is grounded. In this
the fuse is supposed to blow for any current considered too great
for the carrying capacity of the instrument, while, if the current
is of a high enough tension to form an arc, it will jump to the
ground between the lightning arrester plates. Some advise con-
necting the fuse on the instrument side of the carbon arrester in-
stead of on the line side, but this is not best in most cases ; for,
if there is a cable in the line, and a cross occurs at some point
beyond it, the current which would flow through the telephone
instrument would, owing to the high ohmic resistance of the
latter, not be strong enough to injure the cable ; but, if the cur-
PROTECTIVE DEVICES.
275
rent jumps to ground through the carbon arrester a practical
short-circuit is formed which might allow a very heavy current
to flow through the cable to the ground and thereby damage the
conductor. For this reason it is better to have the fuse on the
line side of the circuit.
The devices so far described have been designed to cut off all
currents from the apparatus to be protected, above a certain
maximum value. It frequently happens, however, that a very
small current, due perhaps to a cross on the line, will not be suf-
ficient to blow the fuse, and will yet, by reason of a long-con-
tinued flow, store up enough heat in a switch-board or ringer coil
Fig. 222. Hayes Thermal Arrester.
to char the insulation or burn it out entirely. These currents
are the telephone-exchange manager's worst enemies, and are
very appropriately termed " sneak currents." They frequently
pass all the protective devices placed in the circuit to arrest
them, without producing any effect whatever; but, on reaching
a close coil of wire, they, by slow degrees, develop enough heat
to burn out the coil, or, as has frequently happened, to burn up
the whole exchange.
A device to afford protection against such currents as these,
types of which have come into almost universal use by Bell com-
panies, is termed a heat coil, and was, so far as I am aware, first
introduced by Mr. Hammond V. Hayes of the Bell Company in
Boston, Mass. This device is illustrated both in its assembled
state and in its various details in Figs. 222, 223, and 224. In
Fig. 223 are shown the details of the heat coil proper. A bobbin
is formed of the two disks d, and d l , in the thin flat space, x,
between which is wound about 10 ft. of No. 32 B. & S. German-
silver wire. On the side, d\ is carried a metallic shoulder, e, and
a flange, 4, forming a deep groove,/, between them. A hole, 8,
2 7 6
AMERICAN TELEPHONE PRACTICE.
is formed through the bobbin, through which projects a hard-
rubber pin, s. The pin, s, is fixed in place by a small amount of
easily fusible solder, which normally holds it in the position
shown in the left-hand portion of Fig. 224. One terminal
of the German-silver wire is attached to the flange, e, while the
other terminal is attached to an inclosing ring, d? 2 , of brass.
Fig. 223. Hayes Heat Coil.
The flange, 4, and the ring, d 1 , are thus insulated from each
other, except for a path through the German-silver wire, w.
Three springs, a, b, and r, are mounted as shown, upon a base
plate, A. The spring, a, which forms the terminal of the line, I,
is slotted at a\ in such manner as to receive the neck or groove,
/", of the heat coil, as shown in Figs. 222 and 224. When in
place, the spring, b t which forms the terminal of the instrument
to be protected, rests against the ring, d\ so that the circuit is
Fig. 224. Details of Hayes Arrester.
complete from the line wire, I, through the spring, a, flange, 4,
German-silver wire, w, ring, ^ 2 , and spring, b, to wire, 2, to the
instrument. When a current stronger than a certain predeter-
mined value passes through the coil, a sufficient amount of heat
is generated in the wire, w, to melt the solder. This allows
the spring, c, which is connected by the wire, 3, to the
ground, to push the pin, s, entirely through the coil, so that con-
tact is made between spring, c, and flange, 4, as shown in the
right-hand cut of Fig. 224. This at once grounds the line with-
out leaving any air-gap whatever in the circuit, as in the previous
PROTECTIVE DEVICES.
277
arresters. It has been found advisable, however, to use these
heat coils in connection with carbon arresters, and also with
comparatively heavy fuses, as will be described later.
Fig. 225. McBerty Thermal Arrester.
Another device somewhat similar to this, but adapted to open
the circuit instead of connect it with the ground, is shown in
Figs. 225, 226, and 227. This is an invention of Mr. F. R. Mc-
Fig. 226. Parts of McBerty Heat Coil.
Berty of the Western Electric Company, Chicago. The con-
struction of the coil itself is best illustrated in Fig. 226, in which
b is a small hollow rivet of conducting material, upon which the
Fig. 227. Heat Coil of McBerty Arrester.
coil, a, of German-silver wire is wound. One end of this coil is
attached to the shank of the rivet, as shown, and the other end
to a metallic plug or button,/". The hook, c, is soldered into
278 AMERICAN TELEPHONE PRACTICE.
the hollow rivet, b, and the whole is inclosed in a hard-rubber
bushing, d, as is clearly shown in Fig. 227. A hard-rubber plug, e,
is forced into the cavity in the bushing after the rivet has been put
in place, and this serves to insulate the head of the rivet from
the metallic plug,/". A forked standard, g, is mounted, as shown
in Fig. 225, so as to form a support for the heat coil when in
place. A leaf-spring, i, insulated from the support, forms one
terminal of the line and presses firmly against the plug,/". The
hook, c, of the heat coil is engaged by a spring, /, which is held
thereby under a considerable tension. This spring forms the
terminal of the instrument wire, so that the circuit from the line
passes through the spring, z, the plug,/, the coil, a, the hook, c,
and the spring, /, to the instrument. When a current of suffi-
Fig. 228. Combined Carbon and Heat Coil Arrester.
cient strength to melt the solder passes through the coil, the plug,
<:, is pulled out and the spring, /, is thus allowed to assume its
normal position. This produces a wide opening in the line, so
as to prevent arcing across the gap.
Heat coils may be so adjusted as to be operated by extremely
small currents, and they show great uniformity in their opera-
tion. By varying the length of the resistance wire or its size,
they may be made to respond to a given current in almost any
length of time desired. The times of operation for coils con-
structed in the same manner will seldom vary over I per cent, from
each other. These coils are usually adjusted to act when sub-
jected to a current of one-quarter ampere for thirty seconds ;
they are, however, sometimes adjusted for currents as low as .15
ampere. In later coils about 30 ins. of No. 31 B. & S. German-
silver wire is used. Heat coils of types similar to these are now
built in several different forms and are generally combined with
carbon arresters. In protecting switch-boards, it is of the utmost
importance that arresters be provided for each side of each drop,
and, as economy of space is a very important item in telephone
PROTECTIVE DEVICES.
279
exchanges, it becomes necessary to arrange them in as compact
a manner as possible. Fig. 228 shows combined carbon arresters
and heat coils mounted on long strips of iron for this purpose.
The principles of operation are the same as those already de-
scribed, although the structural details are somewhat different.
The line wire enters at spring, U, and thence passes to spring, F,
through the heat coil, B, to the spring, G, and thence to the
switch-board wire through terminal, E. The spring, F, rests in a
groove of the carbon block, A, which is separated from a similar
block by a small strip of mica, shown in detail at the right-
hand portion of this figure. This second block rests on a
ground plate. An added feature of protection is provided by
Fig. 229. Rolfe Arrester.
inserting in one of these carbon blocks a small drop of fusible
metal. If an arc occurs between the two blocks, this metal will
melt, thus establishing a perfect connection between the two, and
grounding the line. In case, however, a smaller current comes
in over the line, it operates the heat coil and allows the central
rod, which is of metal in this case, to press the light spring at-
tached to the lower side of F into connection with the ground
plate. The other side of the line is connected to the other side
of the switch-board coil in the same manner ; the line entering
at the terminal, C, passes by means of an insulated bolt through
the iron fram e on which the apparatus is placed, to and through
the corresponding heat coil on the lower side of the plate. It
passes to the switch-board wire by the terminal, E.
280 AMERICAN TELEPHONE PRACTICE.
Still another type of arrester which is coming into increasing
use among the independent companies is that shown in Fig. 229.
In this, which is the invention of Mr. C. A. Rolfe of Chicago,
the two binding posts which are connected respectively to the
clips, D, form the terminals of the line wire, and of the wire lead-
ing to the instrument to be protected. On an insulating strip, C,
usually of fiber, are provided the metal ends, c, which are adapted
to be held firmly between the clips, D. A fine-wire coil, E, of
German silver is connected between the metal end pieces, c, its
terminals being attached thereto by small screws and washers.
The coil, E, is imbedded in a mass, G, of some easily fusible
substance resembling plumbers' wax ; the smaller portion of
which extends through an eye, //, on the plate, C. This eye is
arranged to support G, and to provide a stop against which the
head of the button is normally held by the tension of a spring, /,
secured to the, upper portion of the plate, C, as shown. A is a
coiled spring mounted upon the base, K, and provided with an
arm, a, which may be held by a catch, B. The relative positions
of the springs, /and A, are such, that if the spring, /, is released,
it will strike the arm, a, of the spring, A, and cause it to disen-
gage the catch, B. The spring, A, will then, in its attempt to
rise, as indicated in the dotted portion of Fig. 229, strike the
under side of the plate, C, and lift it entirely out of the clips, D.
As the coil, E, forms a part of the circuit, a current in
excess of that which it is adapted to carry will develop enough
heat to melt the wax, and this will allow the head of the coil
to pull through the ring, H. This in itself usually breaks the
wire, E, and thereby opens the circuit ; but, as an additional
protection, the spring, A, gives a violent kick, which is sufficient
to throw the entire plate, C, and its mechanism high into the
air. This affords a very effective break between the line ter-
minals so much so that is almost impossible for an arc to form.
This arrester is sometimes termed the grasshopper cut-out, on
account of its peculiar action.
CHAPTER XXIV.
DISTRIBUTING BOARDS.
IN every central office some means must be provided for dis-
tributing the various line wires which enter the exchange to their
proper numbers on the switch-board and to enable changes to be
made in this distribution as required. If such provision were not
made, and the line cables were run directly to the switch-board,
the wires in one one-hundred-pair cable, for instance, being led
to the No. I section, and those of another to the No. 2 section
of the switch-board, and so on, it would be necessary, at any time
when a change in a subscriber's number was desired, to open the
cable, take out the proper wire and fasten it alongside of one of
the other cables leading to the proper section of the board.
The changing about of wires from one part of a board to another
is a very frequent occurrence, and to do it in the manner above
suggested would be entirely impracticable. To do it in any
manner without a proper regard to systematic arrangement
would lead to endless trouble, by producing a tangle of wires,
commonly and well termed a u rat's nest."
In order to provide means for the systematic arrangement of
the wires, what is called a distributing board or frame is used.
These assume a great variety of forms, but the principle on which
they are designed is as follows : on one side of the distributing
board are placed clips, suitably arranged, in which wires of the
line cables may terminate. On the other side of the distributing
board is arranged another set of clips or connectors, in which the
separate wires of the switch-board cables may terminate. Sup-
pose, for convenience, that the cables entering an exchange
are twenty in number, each consisting of one hundred pairs of
wires. These wires would pass through suitable office cables to
the various terminals on the line side of the distributing board.
Suppose further that the switch-board was arranged in twenty
sections of one hundred drops each. Then twenty switch-board
cables, of one hundred pairs each, would lead from the switch-
board terminals to the terminals on the switch-board side of the
distributing board. This brings connections from the line cables
and also from the switch-board cables, in a permanent manner, to
282
AMERICAN TELEPHONE PRACTICE.
the various connectors on the respective sides of the distributing
board.
The gap between the terminals of any pair on the line side of
the distributing board and that of the corresponding pair of wires
leading from the switch-board is filled by means of " bridle " or
"jumper " wires. Suppose that the line circuit terminating in
terminal No. 101 on the line side of the distributing board is to
be connected with drop and jack No. 599 on the switch-board ;
Fig. 230. End View Hibbard Distributing Board.
then a bridle wire is run from terminal No. 101 on the line side
to terminal No. 599 on the switch-board side, thus completing the
circuit of that line between the switch-board drop and the sub-
scriber.
One side of the distributing board often carries lightning ar-
resters through which the various line circuits pass before enter-
ing the switch-board. These, however, are sometimes placed on
a separate board between the line side of the distributing board
and the cable heads. Test clips are also often provided on one
side of the distributing board. These are usually simple forms
of jacks, normally maintaining the continuity of the lines. They
are, however, adapted to receive a test plug so that the testing
operator may connect his testing apparatus with the line side of
the circuit, leaving the switch-board side open, or with the switch-
DISTRIBUTING BOARDS.
283
board side of the circuit, leaving the line side open, or he may
merely bridge his testing apparatus between the two sides of the
line without breaking its continuity.
Inasmuch as there are in a large exchange several thousand of
these bridle wires, means are provided for their systematic
arrangement as far as possible. The chief object of distributing
boards is to bring all of the confusion among the wires leading
from the subscribers to the switch-board into one small place,
and then to minimize that confusion as much as possible. This
i U U \ I k V U U i I \\\
I ^^^^ = J^^^ = ^^^^^
Fig. 231. Side Elevation Hibbarcl Distributing Board.
is well done in the Hibbard distributing board, modifications of
which are used to a large extent in the Bell exchanges.
This was designed by Mr. Angus S. Hibbard, and is illustrated
somewhat in detail in the accompanying figures.. The frame is
built up entirely of iron pipes, extending in three directions and
mounted upon a hollow platform, a, shown in Figs. 230 and 231.
These two figures represent respectively the end and side eleva-
tions of the complete framework, a plan view being shown in Fig.
232. Vertical pipes serve as supports for the structure, and are
intersected at short intervals by transverse pipes, z, and longi-
tudinal pipes, e, /, and g, extending the entire length of the
framework. As a result of this arrangement channels or horizontal
runs are formed for the jumper wires between the vertical and
the lateral bars, and vertical channels or falls between the sets of
intersecting horizontal bars. On the ends of the lateral bars, i,
284
AMERICAN TELEPHONE PRACTICE.
are vertical strips, d and d', of insulating material, upon which are
arranged the terminals for the various wires in the cables and
the jumpers.
The general plan by which the wires are led from the cable
heads to the switch-board is shown quite clearly in Fig. 230, where
t
f j
K
r 1 1
f I 1 1 f 1
I I
I
^ -
-^=:
^
=~
<-
____^
__
t
7
/
i
d '
Fig
I
.232
; 11
. p
Ian ^
t |
^ievv
. 1
Hit
; i
bar
i Di
strib
utinc
I
r BO
L I 1 !
arcl.
H represents the cable head carrying the terminals of the line
cable, C. The various wires, w, leading from the cable
head are bunched into a cable, 7 2 , which enters the cable run in
the box beneath the frame, and after passing in a horizontal
direction to the proper insulating strip, d', is led upward and
fanned out, the various pairs of wires being soldered to the outer
ends of the terminals on the insulating strip. The method of
fanning out is shown in Fig. 233, the covering of the cable being
Fig. 233. Method of Fanning Out Cables.
taken off and the various pairs of wires, r r, being led out at in-
tervals corresponding to the distance apart of the terminals on
the strip. After being properly formed the cable is laced and
varnished or coated with beeswax, after which it is strapped into
place and the wires soldered to the terminals on the strip.
The details of these strips and the method of attaching the
wires of the cable are shown in Fig. 234, in which / and /' are
the connectors screwed to the strip, d. These connectors have
outwardly bent lugs, u, to which the wires may be soldered.
The ends of the jumper wires are shown at 1 1'. In a similar
manner the wires leading from the switch-board jack are bunched
DISTRIBUTING BOARDS.
285
into a cable, C* y which is then led through the cable run and to
the proper strip, d, of the distributing board, where it is fanned
out and connected to similar terminals. The vertical portions
of the cables, which are to be fanned out on the distributing board,
Fig. 234. Enlarged Plan Hibbard Board.
are supported by the lateral horizontal rods, i, by being laced there-
to, this being shown quite clearly in the enlarged plan view of
Fig. 235.' The jumper wires, which are usually formed of No. 22
B. & S. gauge tinned rubber-covered wire in twisted pairs, are at-
Fig. 235. Detail of Connection Strips.
tached to the inner ends of the terminals on the line side of the
distributing board and led through a hole in the strip and through
the proper channels in the framework to the desired terminals
on the switch-board side, where they are secured in the same
manner.
This arrangement serves to keep the wires fairly open and easy
286
AMERICAN TELEPHONE PRACTICE.
of access, but for very large exchanges it becomes cumbersome,
and has been supplanted by one designed by Messrs. Ford &
Lenfest, some of the details of which are shown in Figs. 236, 237,
SWITCHBOARD. g , and brought
into contact with the wires, w 1 and w 1 . This operation produces
the twofold effect of switching the main-line circuit through the
appropriate vibrating transmitter reed, B? ', and of reversing
the polarity of the main battery, G, with respect to the line.
The change of the polarity of the main-line current causes the
polarized armature, P, at every substation to be deflected from
its normal position, thus bringing it in contact with the stop, q\
and closing the circuit of the local battery, Q. The closing of
the local battery in this manner will, however, in itself produce
no effect upon the electromagnet, K, of the bell, as the latter is
shunted by the contact between the reed, O, and the bar, R,
which rests upon it. The reed, O, at station 2 being adjusted
to vibrate in response to the reed of the transmitter, j5 2 , will be
set in vibration, and this vibration will cause the loosely pivoted
bar, R, to hop up and down, interrupt the shunt-circuit, and
allow the magnet, K, to become active, thus causing the bell, /,
to ring. The bells at all the other substations, being cut out by
the action of their respective shunts, will remain quiescent.
The bell of any other station is actuated in precisely the same
manner, the only difference being that the reed-armature, O, in
each instance is adjusted to vibrate in harmony with its corre-
sponding transmitter at the central office and to respond only to
currents sent to line by it.
The device of Currier and Rice depended on the vibrating
reed to close the circuit through the call-bell, while in that of
Gray and Pope the reed served only to break a shunt around the
belL In Fig. 266 is shown a system designed by J. A. Light-
hipe of San Francisco, in which the gongs are struck directly by
the reed, without the use of an auxiliary magnet. This will be
PARTY LINES HARMONIC SYSTEMS.
343
understood from the diagram without much explanation. The
reeds, <^ 2 , e*, k?, and / 2 , carry hammers adapted to play between
the gongs at the substations when acted upon by their magnets,
d, e, k, or /. At stations, A and B, these magnets are bridged
directly across the two sides of the metallic line, while at stations,
C and D, on another line, they are bridged between one side of
the circuit and ground. A condenser, d' or e', is in each bridge
wire in the former case, to prevent the leakage of current from
j>
Fig. 266. Lighthipe Bridged Harmonic System.
the signaling battery, b', when the telephones are not in use.
Associated with the cord circuit of a pair of plugs at the central
office are the signal transmitters, each having a reed tuned to
the rates of vibration of the several reeds of the call receivers.
Pressure of the button, //, for instance, closes the circuit of bat-
tery, g, through electromagnet, /^, over the limb, a, of the tele-
phone line through the electromagnets, d and e, at the substations
and back by the limb, a', to the opposite pole of the battery.
The magnet, #*, is thus excited, and attracts the reed, which in
its forward movement completes a short circuit around the
battery. The reed vibrates back and forth, sending current
344
AMERICAN TELEPHONE PRACTICE.
impulses over the circuit, and as its rate of vibration is the same
as that of reed, d*, at station A, these impulses will have the
proper frequency to actuate that reed and sound its bells. Call-
ing central from the subscribers' stations is performed in precisely
the same way as in the Sabin and Hampton and other systems
already described.
As the apparatus in this system is arranged on the bridging
plan, it is of course necessary that the bell magnets should
? a
Fig. 267. Harter Harmonic System.
possess high impedance in conformity with the requirements of
bridged lines.
Fig. 267 shows a somewhat elaborate system, invented by Mr.
William H. Harter, of Great Falls, Mont., and embodying a
lockout mechanism in addition to the signaling devices. In this
figure two substations, I and 2, are shown connected by a metallic
circuit line, with two transmitting devices at the central office.
Instead of this connection being permanently made as shown, it
would be brought about in practice by a spring-jack and plugs, the
transmitting devices being connected across the cord circuit.
The reeds, b and b\ at the substations, I and 2, are tuned to the
PARTY LINES HARMONIC SYSTEMS. 345
same pitch as reeds, a and a 1 , of their respective transmitters at
central.
Upon connecting the cord circuit with the circuit of the line,
the battery, C, is thrown across the two sides of the line, and
current therefrom passes through each of the locking magnets, v,
in multiple, attracting their armatures, s, and locking all of the
receiver hooks in their depressed position. Pressure of key No.
I (for the purpose of calling station No. i) establishes a local
circuit through transmitter magnet,^ 1 , and the back contact of its
reed, thus throwing it into rapid vibration. By means of its
front contact, e, and the reed, a, impulses of current from C are
allowed to flow over the line circuit, through the magnets, B,
of the substations ; and as these are of the right frequency to
actuate the reed at station No. I, this reed is thrown into
vibration, the others remaining at rest.
The reed, b, in its vibration completes a local circuit contain-
ing a magnet, /, and local battery, K, and causes it to attract its
armature, m, against three contacts, n, w, and/. The circuit closed
at the contact, n, allows the impulses of current coming over the
line from the battery, C, to operate the bell, o. The circuit
closed at the contact,/, includes also the contact, n, and contains
the magnet, /, and local battery, k, and thus serves to keep the
armature, m, depressed, regardless of the action of the reed.
The circuit closed at the contact, w, short-circuits the locking
magnet, v y thus releasing the hook-lever at the station being
called.
It will be seen that the act of plugging-in locks all stations, and
the closure of key No. I throws reed, b t at station No. I into
vibration. This operates magnet, /, which closes the bell circuit
and also unlocks the hook-lever at that station.
In practice a modification of the central-office circuits would
be necessary, for, as shown, the contact made between the
vibrating reed at key No. I and its contact, e, simply closes
a circuit from the battery, 6", which is already made at key No. 2.
Each key should, therefore, be disassociated from the other
keys during the transmission of the vibratory currents.
These are only a few of a large number of systems depending
on the general principles outlined. The harmonic idea is
attractive, and may be applied in a great number of ways to the
solution of the problem. It has, however, as before stated, been
productive of but few practical results. In fact, but one harmonic
selective signaling system is, so far as the writer is aware, in
practical operation in the United States. It is in use by the
346 AMERICAN TELEPHONE PRACTICE.
local Bell Company at Sacramento, Cal., and is not an unqualified
success, although it has been used over three years. This
slight use of the harmonic principle should not detract, however,
from the interest in the subject, for a knowledge of the
experience of others is a valuable aid in any branch of work, and
in none more so than in telephony.
CHAPTER XXIX.
WIRE FOR TELEPHONE USE.
THE wires in use in telephone work are, at present, of copper
and iron exclusively. Aluminum will probably, as the price of
its manufacture is cheapened, come into extensive use, and it
will not be surprising if it eventually supersedes both copper
and iron for all except very long distance service. Iron pos-
sesses a slight advantage over copper on account of its tensile
strength, and a very decided advantage in point of first cost, but
in all other respects copper is vastly superior.
The tensile strength of a wire is its ability to resist a pulling
stress and the amount of tensile strength is usually expressed in
the number of pounds necessary to break a given wire. The
breaking stress varies, of course, in the same metal with the size
of the wire, that is, with the area of its cross-section. The
weight of a given wire varies also in the same ratio, and therefore,
in order to have a convenient method for designating the break-
ing strength applicable alike to all sizes of wire of a certain grade,
the breaking stress is frequently expressed in the number of
times the weight per mile of the given wire necessary to break it.
Thus, knowing that a certain grade of wire has a breaking
strength equal to two and one-half times its weight per mile, all
that we have to find out in order to know the breaking strength
of any size of this same grade, is the weight per mile of that size.
For example, a No. 12 iron wire weighs 165 pounds per mile.
This we find out by consulting any table giving the weight of
wire, or by weighing a known length of wire. Knowing that the
breaking strength of this grade of wire is 2\ times its weight per
mile, we may at once arrive at the conclusion that the breaking
strength of this particular size is 2 times 165=412^ pounds.
The most important electrical property of line wire is its
conductivity per unit area of cross-section. A conductor of iron
may be made to have a resistance as low as that of a copper
conductor, by giving it about seven times the cross-sectional area.
In doing this, however, we make its inductive capacity much
greater, and, as has been shown, this is a decided disadvantage.
Besides this, the greater weight of an iron wire of the same
348 AMERICAN TELEPHONE PRACTICE.
conductivity as that of a copper wire, is a very objectionable
feature in that it gives the insulators and poles, or other supports,
a far greater burden than is necessary.
The resistance of a conductor varies, of course, inversely as the
conductivity, and therefore inversely as the cross-sectional area
of a uniform wire. Since the weight also varies with the cross-
section, it follows that the resistance of a wire varies inversely as
its weight per mile. A very convenient method of comparing
the relative resistance of various grades of metals used in making
wire is to take as the standard of conductivity the weight per
mile-ohm. The weight per mile-ohm of a conductor is the weight
of a conductor a mile long, and of such uniform cross-section as
to have a resistance of one ohm. Evidently the better the
conductor, the smaller such a wire would be, and therefore a low
value of the weight per mile-ohm will indicate a high conductivity.
The relative conductivities of any two metals may be determined,
knowing the weight per mile-ohm of each. Thus, if the weight
per mile-ohm of pure copper is 873.5 and that of a sample wire
is 896, then calling the conductivity of pure copper 100 per
*>_-, -
cent, the conductivity of the sample will be ' X 100 = 97
per cent.
In making conductivity tests, the resistance of the sample tested
is measured, and from it is calculated the weight per mile-ohm
for that sample. This value can then be compared with the
weight per mile-ohm of pure copper as in the above example.
By doing this the trouble of calculating the resistance of a pure
copper wire of the same dimensions as that of the sample is
saved.
The diameter of wire for electrical purposes is usually ex-
pressed according to some gauge, and there are, unfortunately, a
number of such. Most of the different gauges have been brought
into existence by various wire manfacturers and used in connec-
tion with their particular products only. In these guages the
sizes of wires are referred to by numbers, and in nearly every
case the smaller numbers refer to the larger wires. A better
way, and one which is coming into more common use, is to refer
to the diameter in thousandths of an inch or in mils, as thou-
sandths of an inch are called. A very convenient way of
expressing the area of a wire is to give its cross-section in cir-
cular mils ; a circular mil being the area of a circle, the diameter
of which is one mil, or ^-^ of an inch. This is better than
expressing the area in square inches, because the area in circular
WIRE FOR TELEPHONE USE. 349
mils is obtained simply by squaring the diameter of the conductor
in mils. This very simple relation between the area in circular
mils and the diameter in mils is true, because the area of two
circles are to each other as the square of their diameters. To
reduce the area expressed in circular mils to square inches,
7T
multiply it by or .7854.
4
It is a matter of importance, when purchasing wire in any
quantity, to measure its diameter accurately, so as to be sure of
Fig. 268. Circular Wire Gauge.
obtaining the size ordered. It is not an uncommon thing to
order a wire in one gauge and have your order filled in another,
and the latter gauge usually happens to be smaller than the
former.
Circular wire gauges, such as is shown in Fig. 268, are
obtainable, and serve their purpose well, but are subject to the
disadvantage that a separate gauge is necessary for each partic-
ular set of gauge numbers. These gauges are used by inserting
the wire into the notches in its periphery until one is found which
it just fits ; the number corresponding to that notch is then the
gauge number of the wire. A far better gauge, although one
which is at first a little puzzling to use, is that shown in Fig. 269
and known as the micrometer. It consists of a yoke of tempered
steel, in one side of which is mounted agraduated thumbscrew. The
wire or other object to be measured is placed between the end of
the thumbscrew and the anvil on which it rests when closed, and
the screw turned until it makes light contact with the object on
both sides. These screws are arranged with forty threads to the
inch, so that one complete turn of the screw in a left-handed direc-
tion will open the micrometer -fa of an inch. The edge of the collar
carried by the screw is divided into twenty-five equal parts, so
that a turn of the screw through one of these divisions will open
the micrometer -fa of -fa, or y^Vrr of an inch. The shaft on which
350 AMERICAN TELEPHONE PRACTICE,
the collar turns is divided into tenths of an inch, and each T ^
is subdivided into four parts. Thus a rotation of twenty-five
divisions on the collar will equal one division on the shaft, or .025
inch. If the collar is turned so as to expose the first division on
the shaft and thirteen divisions on itself, then the distance which
the jaws have opened will be equal to .025 -(- .013 = .038.
The Brown & Sharpe gauge, usually abbreviated B & S., is prob-
ably used more for copper wire than any other gauge, while the
Birmingham Wire Gauge, abbreviated B. W. G., is used to a
greater extent for iron wire.
A decided advantage in the B. & S. gauge over any of the
others is that the areas of the cross-sections of the various sizes
Fig. 269. Micrometer.
of wire diminish according to a geometrical progression as the
gauge number increases. The ratio in this progression is 1.26,
or more accurately the cube root of two. From this it follows
that when we have increased three sizes we have doubled the
sectional area of the wire ; and, on the other hand, when we have
diminished three sizes we have reduced the cross-section one-
half. A very convenient thing to remember in the B. & S.
gauge in connection with copper wire is that the diameter of a No.
10 wire is T V of an inch and that the resistance per thousand feet
of this wire is one ohm. These figures are not perfectly accurate,
but enough so for most practical purposes. If one desires to make
an approximate calculation regarding the size of any wire, he
may do so by remembering these figures, which is readily done
because of the number of times the number ten occurs in them.
For example, suppose it were desired to find the resistance of a
No. 13 B. & S. gauge copper wire. Inasmuch as 13 is three
sizes smaller than 10, the area of a No. 13 wire will be one-half
that of the No. 10, and its resistance per thousand feet double
that of the No. 10, or 2 ohms. If the resistance of a No. 14
instead of a No. 13 were desired, it could be found by finding the
resistance of a No. 13 as before and multiplying by 1.26, thus
obtaining the result 2.52 ohms.
WIRE FOR TELEPHONE USE.
35 [
Table II. gives the relative sizes of various numbers of wire
in several of the gauges which are or have been in use in this
country.
TABLE II.
TABLE SHOWING DIFFERENCE BETWEEN WIRE GAUGES IN DECIMAL PARTS OF
AN NCH.
D
bo
3
c8
O
g
o
** t/5
as *>
in
IH
O-2
OS
x>o
.ss
g
1
Cj
5
.5
Q)
IH
looo feet.
Mile.
At 60 F.
At 75 F.
5
0000
4 6o.
2II600.
641.
3382.
.04811
.04966
000
410.
l68lOO.
509-
2687.
.06056
.06251
oo
365.
325-
133225.
105625.
403-
320.
2129.
1688.
.07642
.09639
.07887
.09948
I
289.
83521.
253-
1335-
.1219
.1258
2
258.
66564.
202.
1064.
.1529
I 579
3
229.
5244I-
I59.
838.
.1941
.2004 ;
4
204.
4l6l6.
126.
665.
.2446
2525
5
182.
33124.
100.
529-
374
3172
6
162.
26244.
79-
419.
3879
.4004
7
8
9
144.
128.
114.
20736.
16384.
12996.
63.
SO-
39-
262!
208.
.491
.6214
7834
.5067
.6413
.8085
10
IO2.
10404.
32-
166.
9785
.01
ii
Q I.
828l.
25-
132.
1.229
.269
12
81.
6561.
20.
105.
1-552
.601
13
72.
5184-
15-7
83-
1.964
.027
14
64-
4096.
12.4
65-
2.485
.565
J5
57-
3249-
9.8
52.
3-133
3.234
16
5i-
26OI .
7-9
42.
3 .914
4.04
17
45-
2025.
6.1
32.
5.028
5.189
18
40.
I6OO.
4.8
25.6
6.363
6.567
19
36.
1296.
3-9
20.7
7.855
8.108
20
32.
1024.
3- 1
16.4
9.942
10.26
21
38.5
8l2.3
2-5
13-
12.53
12.94
22
25-3
640.1
i. 9
10.2
15-9
16.41
23
22.6
510.8
i .5
8.2
19.93
20.57
24
20.1
404.
1.2
6-5
25-2
26.01
a
27.9
15-9
320.4
252.8
97
77
4-
31.77
40.27
32.79
41.56
27
I 4 .2
201.6
.61
3-2
50.49
52.11
28
12.6
158.8
.48
5
64.13
66.18
29
"3
127.7
39
79-73
82.29
3
10.
100.
3
'.6
101 .8
105.1
3 1
18.9
79.2
.24
.27
128.5
132.7
32
8.
64.
i * 19
.02
164.2
33
7.1
50-4
.81
202.
208.4
34
6-3
39-7
.12
63
256-5
264.7
y
5-6
5-
3i-4
25-
095
.076
5
4
324.6
407.2
335-i
420.3
WIRE FOR TELEPHONE USE. 357
Abbott gives the following specifications governing the require-
ments to be made of manufacturers in purchasing copper wire :
COPPER WIRE.
i
1. Finish. Each coil shall l?e drawn in one length and be
exempt from joints or splices. All wire shall be truly cylin-
drical and fully up to gauge specified for each size, and must not
contain any scale, inequalities, flaws, cold shuts, seams, or other
imperfections.
2. Inspection. The purchaser will appoint an inspector, who
shall be supplied by the manufacturer with all facilities which
may be required for examining the finished product or any of
the processes of manufacture. The inspector shall have the
privilege of overseeing the packing and shipping of the samples.
The inspector will reject any and all wire which does not fully
come up to all the specification requirements. The purchaser
further reserves the right to reject on reception any or all lots
of wire which do not fulfill the specifications, even though they
shall previously have been passed or accepted by the inspector.
3. Apparatus. The manufacturer must supply, at the mill, the
necessary apparatus for making the examination called for. This
apparatus shall consist of a tension-testing machine, a torsion-
testing machine, an elongation gauge, an accurate platform scale,
and an accurate bridge and battery. Each of these pieces of
apparatus may be examined by, and shall be satisfactory, to the
inspector.
4. Packing for Shipment. When ready for shipment each coil
must be securely tied with not less than four separate pieces of
strong twine and shall be protected by a sufficient wrapping of
burlap so the wire may not be injured during transportation.
The wrappings shall be placed upon the wire bundles, after they
have been coiled and secured by the twine. The diameter of
the eye of each coil shall be prescribed by the inspector, and all
coils shipped shall not vary more than two inches in the diame-
ter of the eye.
5. Weight. Each coil shall have its length and weight plainly
and indelibly marked upon two brass tags, which shall be secured
to the coil, one inside the wrapping and the other outside.
6. Mechanical Properties. All wire shall be fully and truly up
to guage standard, as per B. & S. wire guage. The wire shall
be cylindrical in every respect. The inspector shall test the size
and roundness of the wire by measuring both ends of each coil,
358
AMERICAN TELEPHONE PRACTICE.
and also by measuring at least four places in the length of each
coil. A variation of not more than \\ mil on either side of the
specified wire-guage number will be allowed, and the wire must
be truly round within one mil upon opposite diameters at the
same point of measurement. The strength of the wire shall be
determined by taking a sample from one end of each coil, 30" in
length. Of this piece, 18" shall be tested for tension and elonga-
tion, by breaking the same in the tension-testing machine. The
samples should show a strength in accordance with the following
table :
TABLE V.
BREAKING WEIGHT OF HARD-DRAWN AND ANNEALED COPPER WIRE.
Size of Wire, B. & S.
Guage.
Breaking Weight of
Hard-Drawn Pounds.
Breaking Weight of
Annealed Pounds.
oooo
997i
5650
000
7907
4480
00
6271
3553
o
4973
2818
i
3943
2234
2
3>27
1772
3
2480
1405
4
1967
1114
5
1559
883
6
'237
700
7
080
555
8
778
440
9
017
349
10
489
277
ii
388
219
12
307
174
13
244
138
14
193
109
15
153
87
16
133
69
J7
97
55
18
77
43
19
61
34
20
48
27
A variation of i per cent, on either side of the tabular limits
will be accepted by the inspector. The elongation of the wire
must be at least three per cent, for all sizes larger than No. i ;
ij per cent, from No. i to No. 10, and i per cent, for sizes less
than No. 10, for hard-drawn copper wire. The remainder of the
sample selected will be tested for torsion. The torsion sample
will be twisted in the torsion-testing machine, to destruction, one
foot in length being placed between the jaws of the machine.
Under these circumstances hard-drawn copper wire shall show
WIRE FOR TELEPHONE USE.
359
not less than 20 twists for sizes over No. I ; from 40 to 90 twists
in sizes from No. I to No 10; and not less than 100 twists in
sizes less than No. 10. Should the sample selected from one
end of each coil show failure to come up to the specifications,
the inspector may take a second sample from the other end of
the coil. If the average of the results from both samples shall
be within the specifications, the coil shall be accepted ; if not
within the specifications, the coil shall be rejected. The weight
per mile shall be determined by carefully weighing 2 per cent, of
the number of coils called for in the contract, and the weight
thus obtained shall correspond, within 2 per cent, on either side.
of the result given in the following formulae :
\\r - u^ -i
Weight per mile = = ~
CM
Weight per 1000 ft. =
330-353
7. Electrical Properties. The electrical properties of the wire
shall be determined by the inspector selecting 3 per cent, of the
coils, and from them taking lengths of 100 ft., 500 ft., or 1000
ft., at his discretion, and measuring the conductivity of the same
with a standard bridge. For soft-drawn copper wire the follow-
ing resistance per mil-foot will be assumed :
TABLE VI.
RESISTANCE OF COPPER WIRE AT VARIOUS TEMPERATURES.
a
a
e
"3
fe
s
'S&i
3
5 tfl
* ?/i
53 ^
- 5
rt 0)
I
i) r^
si
50
2
'53
ss
|!
II
50
^-
c/3
G 3
||
a|
I 1
3
5
10
15
6
10
20
30
i
175
256
452
633
20
40
't
813
25
50
it
994
Rubber cables are often incased in lead, in which case the
rubber is made somewhat thinner and the braid over the indi-
vidual wires and much of that over the entire bunch is omitted,
because the lead affords protection both from mechanical injury
and from the weather.
Rubber-covered cables give excellent results as to insulation
and durability ; but a serious objection to their use for telephone
work is that their electrostatic capacity is very high. This is
due to the fact that while rubber is a splendid insulator, its spe-
cific inductive capacity is much higher than that of some other
insulators. Dry air is the most desirable in this respect, its spe-
cific inductive capacity being lower than that of any other known
392 AMERICAN TELEPHONE PRACTICE.
substance. A great improvement in regard to the electrostatic
capacity of cables has been brought about by use of paper insu-
lation between the individual conductors. In the earlier forms
of cables so constructed the wires were wrapped with paper,
which was afterward impregnated with some insulating material,
such as paraffin, having a low specific inductive capacity. It has
been found by aerating the paraffin thus used with dry carbonic
acid gas that the electrostatic capacity between the conductors
was reduced as much as 15 per cent. In order to still further
reduce the capacity what are known as dry-core cables have been
introduced and have come into extensive use. These are usually
formed by wrapping the separate conductors with two layers of
dry paper loosely laid on. Sometimes only a single wrapping is
used. The two wires which are to form a twisted pair are, after
being separately wrapped, twisted together, the length of a com-
plete twist being about three inches. Another way of forming
a twisted pair is to lay the two wires upon opposite sides of a
strip of paper and twisting the two together with the paper be-
tween them. The pair is afterwards served with a single wrap-
ping of paper, forming a complete tube around it. After the
twisted pairs are formed, by whatever method, the desired num-
ber of them are laid loosely together and covered with a lead
sheath, usually one-eighth of an inch in thickness.
The saturated-core cable may be formed in the same way, the
difference being that the paper is impregnated with some insu-
lating material before the lead sheath is put on.
The saturated cable has the advantage of not being so suscep-
tible to moisture as the dry core, but its electrostatic capacity is
usually 15 microfarads per mile, or higher, while in the dry core
capacities as low as .05 microfarad are said to have been at-
tained. It is doubtful if this latter figure could be reached as an
average, and specifications for dry-core cables usually require an
average capacity of .080 per mile. So long as the lead covering
remains intact no difficulty is experienced with the dry-core
cable, but when a puncture is made moisture enters to a sufficient
extent to greatly lower the insulation of the cable. If the dam-
age is not quickly repaired a considerable length of the cable is
apt to be injured, as the moisture finds its way quickly through
the dry paper. For this reason, in small telephone exchanges
not equipped with the proper means for testing out and repair-
ing cables, the saturated core is most desirable. Where the
requisite means are at hand for frequent testings the dry core is
greatly to be preferred.
OVERHEAD CABLE CONSTRUCTION. 393
The locating of faults in cables may be facilitated by specify-
ing that one or two small rubber-covered wires be laid through the
center of the cables, these wires afterward being reserved as test
wires for use in the Varley loop test so often used in locating
leaks.
The size of wire used in telephone cables varies from No. 18
B. & S. gauge to No. 22, No. 19 being probably the most com-
mon. Specifications usually state that the cable sheath shall be
composed of an alloy of lead and tin, the amount of the latter be-
ing not less than three per cent, of the entire mixture. This re-
quirement has been made because it has been found that such an
alloy is not so susceptible to chemical action as lead alone, an im-
portant consideration in underground work. Much difficulty has
been found in manufacturing them, however, to secure an even
mixture of the lead and tin. The Standard Underground Cable
Company are firm advocates of the use of a pure-lead sheath,
afterwards treated with an external coating of pure tin, arguing
that the tin when mixed with the lead makes the sheath brittle
and that the tin will be most effective if all of it is placed on the
outside. Notwithstanding this, it is customary, as stated above,
to specify that the sheath shall be composed of the alloy.
The use of braiding saturated with a moisture-proof compound
placed over the lead sheath is often advocated. Opinions differ
as to the advisability of this, but it is probable that its disad-
vantages outweigh its advantages in either overhead or under-
ground work. The locating of punctures in the sheath is made
.much more difficult by the use of this braid in overhead cables,
for when the sheath is bare they may be often located by mere
external inspection ; moreover, the braiding considerably in-
creases the expense of the cable, and its only advantage is its
prevention of abrasion. This need not occur if the cable is prop-
erly supported. In underground work the braiding affords a pro-
tection for the sheath during the drawing in process and may
afford some protection against chemical action. After it rots,
however, the pieces may so thoroughly clog up the conduit as to
prevent the withdrawal of the cable, thus not only losing that
length of cable, but rendering the duct in the conduit unavailable.
Table XV. gives the outside diameter and the weight per 1000
feet of the various sizes of lead-covered paper cable manufac-
tured by a prominent firm. The conductors are No. 19 B. & S.,
-each being served with two layers of paper.
394
AMERICAN TELEPHONE PRACTICE.
TABLE XV.
AERIAL CABLE.
Number of Pairs.
Outside Diameter,
Inches.
Weights per 1000 Feet
in Pounds.
i
2
i
-
I
214
302
3
-
515
4
i
V
629
5
1
747
6
1
i
877
7
i
i
912
10
]
I
1214
12
i
I
1375
15
1566
18
20
\i
V
1758
1940
25
i~^
5 *
3232
30
35
if
2748
2985
40
i
V
3176
45
i
3365
50
i
3678
55
i
I
3867
60
i
4055
65
i-
f
4241
70
2
4430
80
2
^
4804
90
2-
b
5180
100
2
1 .
5505
Aerial cables are supported on steel rope stretched tightly be-
tween the poles or other supports. This is necessary on account
of the fact that the cable does not possess the requisite strength
to support its own weight. For the heavier cables the messenger
wire, as the supporting strand is called, is usually composed of
seven No. 8 steel wires twisted together into a rope. Table XVI.
gives the common sizes of messenger wire, together with their
weights and breaking strengths :
A special strand may be procured, the various sizes of which
have about double the breaking strength given for the cor-
responding sizes in this table.
Table XVI. is useful in determining the size of cable that any
messenger wire can safely carry for any given length of span.
By referring to the table giving the weights per 1000 feet of cable
and, knowing the length of span, the size of messenger wire is
readily determined.
OVERHEAD CABLE CONSTRUCTION.
395
TABLE XVI.
MESSENGER AND GUY WIRE.
7 Wires.
No.
Approximate \
Diam. in Inches.
Weight per
loo Feet.
Tensile Strength
in Pounds.
8
\
52
8320
9
10
t
42
36
6720
5720
ii
29
4640
12
TS
21
3360
13
3
16
2560
14
ir
12
1920
15
16
A
10
8
1600
1280
17
T 3 *
6
960
18
19
1
3ft
688
528
20
t
2 T%
384
21
A
2
320
TABLE XVII.
SUPPORTING CAPACITY OF GALVANIZED STEEL STRANDS.
5
Spans in Feet.
7 Wires.
i 2
No.
u~
100
no
120
125
130
140
150
175
200
^
WEIGHTS IN POUNDS OF IOOO FEET OF CABLE.
8
i
28l8
2516
2263
2152
2050
1867 ! 1709
I39i
H54
9
if
2520
2247
2O2O
1920
1827
1663
1520
1234
1130
10
TV
2030
1812
1630
1550
1476
1344
1230
1001
9OO
ii
f
1580
1409
1266
I2O4
1146
1043
953
774
640
12
A
1 1 10
SQQ
890
846
805
733
670
544
450
13
A
860
765
680
6 5 2
620
563
513
414
340
15
i
585
521
468
445
423
35
352
280
235
16
A
433-
385
346
329
313
284
260
210
172
17
337
300
270
257
245
223
204
I6 5
137
The messenger wire may be supported in several ways, one of
which is to bolt a piece of angle iron to the pole and suspend one
messenger wire from each of its ends. The wire may be sus-
pended below the angle-iron cross-arm, or it may rest in a slot in
39 6
AMERICAN TELEPHONE PRACTICE.
it. There are also several good forms of supports on the market,
one of which is shown in Fig. 311.
The cable is supported from the messenger wire in several
different ways. Rubber-covered cable is frequently suspended
Fig. 311. Messenger Wire Clamp.
by binding it to the messenger wire by strong tarred marline.
The marline is wrapped around both cable and messenger, usu-
ally in two directions, to give greater security. The method now
most extensively used in supporting lead-covered cables is by
means of metallic clips or hangers, adapted to tightly girdle the
cable sheath and provided with a hook to slip over the support^
Figs. 312 and 313. Cable Hangers.
ing wire. There are several good hangers, two styles of which
are shown in Figs. 312 and 313. In attaching the one shown
in Fig. 312 the metal strip which passes around the cable is first
passed for a distance of one inch through the slot in the lower
part of the hook. The other end is then bent around the cable
OVERHEAD CABLE CONSTRUCTION. 397
and through the slotted key. The key should then be turned
to the left until tight and then locked by driving it endwise until
the ears on it engage in the star-shaped hole. The flexible strip
in this hanger is made of zinc. The hanger shown in Fig. 313 is
of malleable iron and is attached by bending it around the cable
with a special tool.
It is a good plan to place a piece of sheet lead -fa inch thick,
or a piece of leather or rubber hose, around the cable at the
point where the hanger is to be applied ; but if this is to be done
the additional thickness must usually be allowed for in ordering
the hangers.
It is well, in ordering cables, to specify that it shall be placed
upon the reels in such manner that both its ends are accessible
without unreeling it. Where this is done it is an easy matter to
Fig. 314. Running up Cable.
make tests for continuity of the conductors and for insula-
tion resistance and capacity before the cable is unreeled, and
thus any defects which may exist will be known to be the fault
of the manufacturer or the transportation company. When the
cable arrives both of its ends will be sealed to prevent the entrance
of moisture, and, after testing, the ends should be carefully fe-
sealed in a manner which will be described later. The ordinary
method of hanging cables is shown in Fig. 314, which is taken
from Roebling's " Handbook on Telephone Cables," as are several
of the succeeding cuts illustrating the method of splicing. The
end of the supporting strand, after passing over the last clamp
or cable cross-arm, D, is firmly secured to a guy-stub driven in the
ground at A. The reel on which the cable is coiled is placed in
line with the messenger wire, and a few feet beyond the stake, as
398 AMERICAN TELEPHONE PRACTICE.
shown. One or more grooved pulleys, C C, mounted as shown,
are placed between the reel and stake in such manner as to sup-
port the cable as it is paid out. A stout rope, or better a small
wire cable, is previously hung on pulleys or hooks below the cross-
arms of the entire stretch over which the cable is to be drawn.
One end of this is attached to the end of the cable, while the
distant end is attached to a capstan or other form of windlass.
As the cable passes over the rollers, C C, the hangers are at-
tached and are placed one by one upon the inclined messenger
wire as they reach the point, B. As the cable progresses line-
men stationed on each pole lift the hangers over the messenger
wire clamp or cross-arm as they pass. In this way the entire
length of cable is drawn up to and along the stretch without
subjecting any portion of it to an undue strain. The hangers
are usually attached at distances of from twenty-four to thirty
inches, according to the size of the cable. The work is somewhat
expedited if, during the drawing up of the cable, only about every
fifth hanger is hooked over the messenger wire. This reduces
the labor of the linemen in lifting the hangers over the support.
When, however, the forward end of the cable reaches the begin-
ning of the last span, the signal should be given to all linemen
stationed on the poles to hook on all of the hangers as they pass,
and in this way all of the hangers will be secured in place through-
out the entire stretch without going out over the line afterwards.
This method is subjected to one disadvantage in that the slid-
ing of the hanger hooks along the messenger wire tends to loosen
them on the cable, sometimes to such an extent that several of
them become bunched at one point on the cable. A method for
overcoming this disadvantage, and also for expediting the work,
has been devised by Mr. F. S. Viele of the Standard Under-
ground Cable Company. In this, carriers, each consisting of a
small grooved roller with a hook below it for engaging the cable,
are placed upon the messenger wire, and serve to support the
cable at frequent intervals instead of the hangers during the proc-
ess of stringing. At each cross-arm a small switch or side track is
placed upon the messenger wire, which serves to displace the car-
rier rollers far enough to clear the messenger wire, and then to
guide them down under the cross-arm and again up on the messen-
ger wire. These side tracks are about three feet long and may
be readily attached or detached from the messenger wire. When
the forward end of the cable reaches the beginning of the last
span of the stretch a man is sent up each pole to place the hang-
ers on the messenger wire and remove the carriers as they pass,
UNIVERSITY
\0>
OVERHEAD CABLE CONSTRUCTION. 399
thus leaving the entire cable permanently suspended when the
forward end reaches its destination.
It is always well to leave sufficient slack in aerial cables at fre-
quent intervals to allow for subsequent splicing in case repairs are
needed. This slack, moreover, frequently saves a cable from se-
rious injury when the pole line is subjected to some severe strain
which the cable, if unable to give, would not be able to bear. At-
tention to this point will often prevent the necessity of splicing
in a new piece in the middle of a cable, due to insufficient length
for making an ordinary splice.
Where it becomes necessary to splice a cable the greatest care
should be taken that no moisture be allowed to enter while the
splice is being made, and that the splice shall be so thoroughly
sealed at the end of the operation that there will be no possibility
of the subsequent entrance of moisture. A suitable staging
should be erected on the pole where the splice is to be made, if
Fig. 315. Cable Prepared for Splicing.
it is possible to bring the splice within reach of the pole. This
can always be provided for in new cable, but sometimes in repair-
ing a leak it is necessary to make these splices from a car sus-
pended from the messenger wire. When all is ready the lead
sheath of each end of the cable to be spliced should be cut away
for a distance of about twelve inches, the ends of the cable hav-
ing previously been sawed off square. Boiling paraffin, heated
in a large pan on a portable furnace, should then be ladled over
the ends of the wire to prevent, as far as possible, any moisture
from the atmosphere from entering the cable and also to prevent
the untwisting of the paper insulation, which, in a dry-core cable,
often gives considerable trouble during the operation of splicing.
A lead sleeve, consisting of a lead pipe about two feet long, and
of a slightly greater internal diameter than the external diameter
of the cable sheath, should then be slipped over one end of the
cable and back several feet out of the way. A paper sleeve
should then be slipped over each wire of each pair. The pairs in
paper-covered cables are usually colored red and white, and as a
matter of convenience a paper sleeve should be slipped over all
of the red wires on one end of the cable and one of the white
wires on the other. This brings the cable ends into the condi-
AMERICAN TELEPHONE PRACTICE.
tion shown in Fig. 315. The corresponding wires of each pair
are then skinned for a short distance and twisted together, as
shown in Fig. 316, and after a number of pairs are so joined all
Figs. 316 and 317. Splicing a Pair.
joints should be carefully soldered, particular pains being taken
to use no acid flux. Tubular solder provided with a rosin flux
inside is convenient for this, or the grease from a tallow candle is
perhaps better yet. With a rather large soldering iron these
joints may be soldered almost as fast as the iron can be touched
to the wire. After soldering, the twists should be bent down as
shown in Fig. 317, after which the paper sleeves are slipped over
the bare portion of the wires, leaving the completed splice, as
shown in Fig. 318. The joints in each pair should not lie opposite
Fig. 318. Finished Splice on Pair.
each other, and within the space allowed between the ends of the
cable sheaths all joints should be staggered as much as possible
so as to prevent the formation of a large bunch at any one place.
Another point to be remembered is to guard against joining any
good wires in one cable to wires known to be bad in the other.
Before making these splices all wires should be tested out and the
bad ones tagged. Obviously, if a good wire in one length of the
cable is attached to a bad one in another, that wire is unavilable
for use in either cable. After all of the wires are spliced and
Fig. 319. Boiling Out.
covered by paper sleeves the cable should be carefully " boiled
out," this process being shown in Fig. 319 and consisting in
ladling boiling paraffin over the joint until all traces of air bub-
bles in the hot paraffin disappear. This portion of the work
OVERHEAD CABLE CONSTRUCTION.
401
should never be slighted, as it is one of the most important in the
entire operation.
After the " boiling out" process is completed a plain strip of
Fig. 320. Finished Cable Splice.
white cotton should be wrapped over the splicing, after which the
joint should again be boiled out. The joint is now ready for the
Fig. 321. Moon Cable Head.
services of a plumber, and upon his work much depends. The
section of pipe should be slipped over the splice before it has
had time to cool, and the sleeve thoroughly wiped to the cable
402
AMERICAN TELEPHONE PRACTICE.
sheath at each end by the ordinary method used in joining lead
pipes. The finished joint presents the appearance shown in Fig.
320, and when such a joint is properly made that portion of the
cable should be practically as good as any other portion.
Whenever it is necessary to leave a cable end exposed all moisture
should be expelled by boiling out, after which the end of the
sheath should be sealed by a wiped solder joint with as much care
as if it were to be a permanent affair. If it is suspected that
moisture has entered the end of a cable a short length of it should
be cut off and dipped into boiling paraffin, when the presence of
moisture will be indicated by the rising of bubbles in the hot
Fig. 322. Fused Terminal Block for Moon Head.
fluid. If there is room to spare the cable should be cut back, a
short length at a time, until it gives evidence of being dry, but
if this cannot be done the sheath should be heated with a torch,
beginning at a point several feet from the end, and working gradu-
ally toward the end, so as to expel the moisture. After this the
ends should be thoroughly boiled out and a splice made as
already described, or, if this is not to be done, the end should
be sealed.
Where a cable terminates means must be provided for distribut-
ing its various wires and connecting them to the wires forming parts
of the same circuits. For this purpose what are termed cable
terminals or cable heads are used, several forms of which are on
the market. These usually consist of iron boxes, inside of which
are arranged terminals for the wires in the cable. The lower parts
of these boxes are usually provided with a brass tube or sleeve
adapted to fit over the cable sheath, after which it is secured
thereto by a plumber's wiped joint, thus hermetically sealing both
OVERHEAD CABLE CONSTRUCTION.
403
cable head and sheath at that point. The wires of the cable are
fanned out to the terminals within the box, which terminals are
usually connected through water-tight insulating bushings with
their terminals outside of the boxes. After the various connec-
tions are made within the box a cast-iron cover is screwed in place,
the joints being hermetically sealed by a rubber gasket. On the
outside of the boxes are usually provided lightning airresters for
Figs. 323 and 324. Cook Pole Top Terminal.
each line, the circuit being completed from the inner connectors
through these arresters to the outer wires. One of these, known
as the Moon cable head, is clearly shown in Fig. 321, in which
several wires are shown extending from the tube below the box to
one of the terminals within. In Fig. 322 is shown a fused termi-
nal for use with the Moon head. The bushing which serves to
lead the terminal pin through the iron casing also carries a brass
lug between which and the pin is placed the fuse, mounted on a
hard-rubber block as clearly shown. Another form of cable head
44
AMERICAN TELEPHONE PRACTICE,
which is becoming very popular is known as the Cook pole-top
terminal, views of which are shown in Figs. 323 and 324.
In this, which is the invention of Mr. Frank B. Cook of the Ster-
Fig. 325. Pole Equipped with Cook Terminal.
ling Electric Company, the cable is led up within the cast-iron
box, forming the framework for an entire terminal, the various
wires being fanned out to conductors arranged in circular rows
around the inside of the box. Connection is made through suit-
able air-tight plugs with outside circuits, the line wires being
fused at the insulator, as shown at the left-hand lower portion
OVERHEAD CABLE CONSTRUCTION'. 405
of Fig. 324. Before screwing on the cover several lumps of un-
slacked lime are placed within the box, after which the cover is
screwed on, being hermetically sealed by rubber gasket as in
the other form described. This lime absorbs any slight mois-
ture which may be in the box at the time, and the fact that it has
in many cases remained unslaoked for years proves conclusively
that these terminals may be made perfectly moisture-proof. Af-
ter all connections are made a sheet-iron cover is placed over the
entire terminal.
This terminal, as its name implies, is placed at the top of the
pole in a manner shown in Fig. 325.
A much less expensive method of terminating cables than any
so far described consists in the use of what are termed pot-heads,
and while these present a somewhat homely appearance they are
very effective, and are used to a large extent by many of the
Bell and other companies. There is no doubt but that a pot-
head terminal, properly constructed, forms as reliable and service-
able a terminal as any, it having the additional advantage of being
far cheaper than any of the others. The directions for making
these, together with the description of all material, are given
in the following specifications, which are those of one of the
leading Bell companies.
POT-HEAD TERMINALS.
Materials.
Lead Sleeves of unalloyed lead \ inch thick of the following
dimensions :
For loo-pair cable ; length, 24 inches, inside diameter, 3 inches.
HQ " " " 2O " " " 2\ "
2 20 u 2
Drift out the sleeve for one-half its length until its diameter is
increased J of an inch.
Okonite Wire, twisted pair, red and black No. 20 B. & S. gauge,
3 2-inch insulation, without braid or outside covering.
Okonite Tape, f inch wide.
Paper Sleeves, boiled in paraffin just before using.
Brass Tubing thin, inch in diameter, length 2\ inches less
than that of lead sleeves.
Heavy Cotton Twine, or wicking.
Wiping Solder, containing 40 per cent. tin.
Splicing Compound, as furnished for the purpose by the com-
pany. Do not mix the compound with other materials.
406
AMERICAN TELEPHONE PRACTICE.
Directions.
Remove the cable sheath for fifteen inches, slip the lead
sleeve over the cable, splice the cable wires to okonite wire in
the usual manner, join the colored wire of the cable to the red
Fig. 326. Cable Terminal Box with Balcony.
okonite of each pair, cover each splice with a sleeve, and keep the
splices within a limit of thirteen inches from the end of the cable
sheath. Remove all pieces of paper or other debris, bind the
cable wires as they leave the sheath tightly with several layers of
twin6 to prevent the compound entering the cable, tape all the
okonite wires together for three inches in such a manner that one-
half inch of the taped wire will be below the surface of the com-
OVERHEAD CABLE CONSTRUCTION. 4 7
pound. Open up the spliced wires as much as possible to allow
free spaces between, bind the brass tube with twine alongside
the wires with the lower end even with the end of the sheath of
the cable, binding no higher than the wire splices; draw up the
lead sleeve until its lower end laps over the cable sheath
1 1 inch, and wipe it to the sheath. Secure the whole in an up-
Fig. 327. Interior Cable Heads.
right position, warm the lead sleeve until it can barely be touched
with the hand, place a funnel in the brass tube, and slowly
pour in the sealing mixture, previously heated to 350 F. r
until it fills the sleeve to within one-half inch of the top,
remove the funnel and allow the compound to settle and cool.
Test the compound just before using by putting in a short piece
of okonite wire for two minutes. If the okonite is not softened
so as to readily come off the wire the compound is not too hot.
Protect the open end and the wires leading therefrom against the
40 8 AMERICAN TELEPHONE PRACTICE.
weather. On the next day fill with hot compound to make up
for the settlement. Three days later do the same if necessary.
After this, and when thoroughly cold, dress the top of the lead
sleeve into contact with the okonite tape wrapping, which at this
point should consist of at least four layers. Place a cross on the
outside of the lead sleeve at a point opposite the upper end of
the brass tube. Caution : Do not boil out the cable end with
paraffin. If dampness enters, the cable should not be used until
this defect is remedied or the part cut away. Under no circum-
stances must paraffin be used on okonite ends.
The okonite paired wires shall project above the end of the
flexible end for a distance of :
For 100 pairs cables 3 feet.
" 50 " " 2 "
Cable heads, whether of the ordinary iron or the pot-head type,
when placed on poles are usually inclosed in a wooden box of
suitable dimensions for allowing connections with the external
circuits. These boxes should be provided with hinged doors and
made as nearly water-tight as possible. Directly below these boxes
should be erected a balcony upon which the workmen may stand
when making connections for testing the various lines. Such
construction is well shown in Fig. 326.
In Fig. 327 is shown a row of eight cable terminals used in
terminating as many one hundred pairs of cables within an ex-
change. These particular terminals are the product of the Ster-
ling Electric Company, and are provided with combined sneak
current and carbon arresters for each line.
CHAPTER XXXII.
(
UNDERGROUND CABLE CONSTRUCTION.
THE tendency at present is to place all telephone wires in
cities underground, and the primary requisite for this construc-
tion is that a suitable conduit shall be provided in which the con-
ductors may be laid. It is usually necessary to provide conduits
having a suitable number of ducts to meet the requirements for
future as well as immediate use, and much judgment should be
exercised in this respect in planning the system. Suitable
openings are provided for the conduits at frequent intervals,
these being in the form of man-holes, from which sections of the
cables may be drawn into the ducts and withdrawn when occasion
requires for repairs. The principal requirements for a good con-
duit may be outlined as follows :
The material of which the conduit is made must be durable,
and this implies that it must be absolutely proof against decay or
corrosion due to moisture, dry rot, gases, or the liquids present
in the soil. It should, moreover, be fire-proof if possible, al-
though this is a minor consideration.
The conduit should possess both tensile shearing and crush-
ing strength. Severe vertical strains are frequently imposed upon
subway structures, due to the removal of the support from be-
neath them, caused by excavations in the streets or by the
settling of the ground. Side strains are not so likely to occur,
and their effects are usually slight ; therefore, it follows that
the conduit should be, if possible, strongest in a vertical direc-
tion. If the stress imposed upon the structure is such as to cause
a fracture or undue settling, the alignment of the ducts is thereby
destroyed, which may interfere with the drawing in or with-
drawal of cables. Moreover, the grade of the duct is destroyed,
so that the proper drainage cannot be effected. The ducts be-
tween man-holes should be straight if possible, and where curves
are necessary they should be very gradual and present no sharp cor-
ners which would interfere with the drawing in or seriously abrade
the cable sheath. Slight turns in conduits are frequently made
by joining together short straight sections, but where the nature
of the conduit used permits it, it is better to form all bends of
409
410 AMERICAN TELEPHONE PRACTICE.
curved sections. It is desirable that the structure should be com-
posed of insulating material and be moisture-proof. No depend-
ence, however, for insulation of the conductors themselves must
be placed on the conduits, as the cables must in all cases provide
the means for keeping the conductors thoroughly insulated and
free from moisture, even under the most adverse circumstances.
Even the most perfectly constructed conduits cannot be kept
dry, on account of the sweating of their interior walls.
It is very essential that the conduit must contain no chemical
agents capable of exerting a deleterious effect on the cable
sheath. As an example of this may be mentioned certain forms
of wooden conduits, which in the process of decay liberate acetic
acid, which in a short time totally destroys the cable sheath,
changing it to lead acetate. This difficulty has been experienced
with some forms of creosoted wood conduit, but in the later
products in this line this difficulty is said to have been com-
pletely removed by the use of a better grade of creosote oil and
improved methods.
Economy of space is often an important item in the selection
of conduit to be used, and under crowded conditions that con-
duit which will place a given number of ducts within the smallest
space is the most desirable, other things being equal.
The earliest form of conduit used in this country was the
open-box conduit, which consisted merely in a trough made of
inch-and-a-half or two-inch lumber and of sufficient size to
accommodate enough cables to meet the existing demands,
as well as the future growth of the system. These troughs
were laid in a trench, the bottom of which was properly
graded, the sections of the trough being about fifteen feet
in length and butt-jointed that is, laid together end to end.
The joints were held in line by boards nailed on the outside and
overlapping each end about a foot. The cable was laid in these
troughs by driving the reel containing it slowly alongside of the
trench, the cable being carefully laid as it was unwound from the
reel. After all the cables were in place the trough was filled
with hot pitch, when the cover was nailed in position and the
trench refilled. This is probably the simplest form of under-
ground cable construction, with the exception of a method some-
times practiced in Europe, of laying the cable directly in the
ground without any conduit whatever.
The cheapest and simplest form of conduit which permits the
drawing in or withdrawal of the cables is that composed of creo-
soted wood tubes, or " pump logs," as they are commonly
UNDERGROUND CABLE CONSTRUCTION.
411
and appropriately termed. These are usually made in eight-
foot lengths, having a square external section 4^ x 4^ inches, with
a 3-inch bore. A tenon joint one-and-one-half inch long is used
for securing proper alignment of the joint. Several views of
this tube are shown in Fig. 328. The wood is treated with creo-
sote or dead oil of coal tar in the following manner: The lum-
ber is laid on cars and run into a large steel cylinder six feet in
Fig. 328. Creosotecl Wood Conduit.
diameter, which is closed by a heavy iron door. It is first sub-
jected to live steam at a temperature of 250 F. until the timber
is heated through and through, the purpose of this being to co-
agulate the albumen in the sap. A vacuum pump is next applied
to the tank, exhausting all air and steam, the pump maintaining
a vacuum of about twenty-six inches. This evaporates practi-
cally all of the sap and water from the wood, thus seasoning the
timber. The next step in the process is to pump creosote oil pre-
viously heated to a temperature of 100 to 125 F. into the tank
until it is full. This is then placed under a pressure of about
eighty pounds per square inch and the amount of creosote which
is forced in after the filling of the tank is carefully measured,
this being the amount that is taken up by the pores of the
wood. Specifications for the treatment require that from eight
to twenty pounds of the oil shall be absorbed by each cubic foot
of timber. Twelve or fifteen pounds is the average amount re-
quired for electrical purposes. As has been stated before, much
trouble has existed owing to the liberation of acetic acid from
conduit treated with creosote. It is claimed, however, that by
using a proper quality of creosote oil, and by using the method
of impregnation just described, that this trouble has been entirely
eliminated. The life of creosoted wood conduit is, to say the
least, problematical, but there seems to be good reason to believe
that when properly treated and laid it will last an ordinary life-
time, if not longer. In laying this conduit the trench is dug to a
412 AMERICAN TELEPHONE PRACTICE.
sufficient depth, and after its bottom is properly graded so as to
have a gradual slope either from an intermediate point toward
both man-holes or an uninterrupted slope from one man-hole to
the other, a creosoted wood plank two inches thick is laid as a
foundation. The ducts are then laid on this plank side by side
and in as many different layers as are necessary to give the re-
quired number. They should be so laid that the separate ducts
break joints in order to give strength to the entire struc-
ture. Over the upper layer is then laid another creosoted wood
plank two inches thick, after which the trench is filled in with
earth. The great point in favor of this conduit is its cheapness,
this being greatly enhanced by the fact that no concrete is em-
ployed for a foundation.
Conduits of clay or terra-cotta, burned hard and with vitrified
surface, are being extensively used and are giving unqualified
satisfaction. These are made up in a number of forms which
may be divided into two classes, namely, multiple duct and single
duct. The multiple-duct conduit is made up in a variety of
ways, some of which are shown in cross-section in Figs. 329, 330,
Fig. 329. Multiple Duct Conduit.
and in the upper portion of 331. The sections of conduit shown
in Fig. 329 usually have a cross-section lox 10 inches and a length
of three feet. Similar tiles are frequently used having six or eight
ducts, each about 3^- x 3^ inches square, the tiles varying from
three to six feet in length. Frequently the ducts are made large
enough to accommodate several cables, but this has a decided
disadvantage, owing to the fact that much trouble is experienced
in withdrawing cables under these conditions. The successive
lengths of these tiles are joined by wrapping them with burlap
previously dipped in asphalt. This makes the joint tight, main-
tains its alignment, and prevents entrance of dirt during subse-
quent operations.
The conduit shown in Fig. 330 is valuable only where multi-
ples of four ducts are required. Each tile is essentially a trough,
open on top and having three intermediate partitions, thus form-
ing four ducts, each of which is 3f inches wide by four inches
high, the walls being one inch thick. These are made in two-foot
lengths, and are laid in concrete as shown, one section being laid
directly on top of another until the required number of layers are
UNDERGROUND CABLE CONSTRUCTION.
413
formed. The top of the upper section is composed of a sheet of
mild steel of No. 22 B. W. G., having its edges bent down so as
to lap over the sides of the tile.
The tile shown in the upper portion of Fig. 331 was designed
by Mr. H. W. Johnston of St. Louis for the purpose of distribut-
/*
7J7J7/
^
i/> ' '?'!:!
-^ 4#$
'/; - / $
I > ' &
( im
-^/ ^^;;
;4^ i ^^^M5|^
//I ---' ^^x^^r-its? r !~-
Fig. 330. Four-Duct Terra-Cotta Tile.
ing the wires from the main cable, running to a certain section, to
the various lateral ducts. The lower central duct is for the main
cable running to the man-hole at the center of distribution.
From this it branches out through a junction box to the distrib-
uting wires, the two four-inch openings on either side being for
Fig. 331. Johnston Distributing Duct.
these wires or cables, which are led out through holes in the side
wall of the duct to the lateral ducts, consisting of three-inch
iron pipe. The upper central duct is for rodding and drawing in
of additional wires in the side ducts. A runner is drawn through
the central upper duct with arms projecting over the side ducts,
and by means of these projecting arms new wires may be drawn
414 AMERICAN TELEPHONE PRACTICE.
into the side duct without in any way disturbing those already in
place.
The single-duct class of tiles possesses some advantages over
the multiple-duct tiles, chief among which are the greater flexibil-
ity and the increased ease of handling. The form shown in Fig.
332 has come into very wide use and has proven its adaptability
Fig. 332. Single-Duct Conduit.
to meet almost any conditions that may arise. These tiles are
4| inches square by 18 inches long, and have a 3J--inch bore. By
it curves are easily made, short curved lengths being provided, or
curves of long radius may be made with the regular tiles, the
lengths being so short as to form a practically smooth interior
surface. This conduit is laid in much the same way as ordinary
brick, and in order to insure proper alignment a mandrel (Fig.
333), three inches in diameter and about thirty inches long, is laid in
Fig. 333. Mandrel.
the duct and pulled along through it by the workmen as each ad-
ditional section is laid on. The rear end of this mandrel is pro-
vided with a rubber basket a little larger than the diameter of the
conduit, which effectually smooths the inner surface and pre-
vents the formation of lips which might prove injurious to the
cable sheaths in drawing in. On the front end of the mandrel is
provided an eye which may be engaged by a hook carried by the
workmen in order to move it forward. Fig. 334 is a photograph
showing a 48-duct subway in process of construction. In laying
vitrified clay tile of any description the process to be used is as
follows : The trench is dug to such a depth as to allow at least
two feet of earth above the top of the entire structure. Some
UNDERGROUND CABLE CONSTRUCTION.
415
specifications call for as great depth as three feet, but this is nec-
essary only where there is a probability that new ducts may be
added to the conduit in the future. The width of the trench
should be at least eight inches in excess of the actual width of
the number of ducts which are to be laid side by side. In the
bottom of the trench is laid a (concrete foundation to a depth of
from three to six inches four inches is under ordinary circum-
stances sufficient. The tiles are then laid in place in cement
mortar, and as each layer is finished the sides of the trench should
Fig. 334. Forty-Eight Duct Subway, Cleveland, Ohio.
be filled to the top of that layer with the same concrete as that
used for the foundation. The space between the tiles in a layer
and between the layers should be carefully filled with good cement
mortar mixed thin enough to readily fill the interstices. After
the required number of layers are in place the top is covered with
a mass of concrete not less than four inches in thickness.
The concrete used in this work should be composed of one part
of hydraulic cement, two parts of clean sharp sand, and five parts
of broken stone, screened gravel, or broken brick. The size of the
broken stone or brick or gravel should not be larger than one inch
in any dimension. The cement and sand should be thoroughly
mixed while dry, and then enough water added to form a soft
416 AMERICAN TELEPHONE PRACTICE.
mortar, after which the broken stone should be thoroughly mixed
in.
The mortar should be composed of one part of hydraulic ce-
ment and two parts of clean sharp sand, thoroughly mixed to-
gether, and then with water as before. It is a matter of greatest
importance that the ducts should not be moved while the mor-
tar or concrete is setting.
After the entire subway is laid from" one man-hole to the other it
is advisable to draw through it a scraper, thus removing all projec-
tions on the inside walls. The ducts may then be washed out with
a hose, thus removing all grit and leaving a clean, polished tube.
Another style of conduit is the cement line pipe, which has
also proven itself to be thoroughly reliable in all respects. This,
as usually constructed, consists of a wrought-iron pipe of No.
26 B. W. G., with riveted joints, the rivets being set one and
one-half inch apart. This pipe is lined with Rosendale cement,
the thickness of the lining being five-eighths of an inch, and the
interior of the lining being polished. The standard size of this
tube is in eight-foot lengths, with a three-inch bore. It is pro-
vided with cast-iron ball and socket joints at the ends in order to
insure proper alignment and to provide a certain amount of flex-
ibility in making turns.
This conduit is laid in concrete in much the same manner as
the clay pipe, it being common practice to separate the different
pipes in the layer by about one-half of an inch and the various
layers themselves by about one inch.
Another form of conduit, radically different from all those so far
described, is the Sewall cement arch conduit, which is a recent pro-
duction, but which gives great promise of success. The cross-
section of a single duct is shown in Fig. 335, while in Fig. 336 is
Fi g s - 335 and 336. Cement-Arch Conduit.
shown in perspective the method of joining two lengths.
The separate ducts are formed of cement molded around a strip
of wire gauze, as shown in Fig. 335. The arch is made of equal
parts of Portland cement and sand, being strengthened by the
metal gauze, which is made from No. 19 B. & S. gauge iron
wire, woven with a mesh three-eighths of an inch square. The
UNDERGROUND CABLE CONSTRUCTION. 4*7
inside measurement of the standard size is three inches wide
and three inches from the top of the arch to the base. In lay-
ing these arches the trench is dug in the usual manner and a
concrete foundation laid. The top of the foundation is made
smooth to grade, being troweled to a polish. The arches are then
wet and placed on the floor so fbrmed under a templet which gives
them accurate alignment. Joints are made by abutting the sec-
tions end to end. Over the joint is placed a wire gauge bridge, as
shown in Fig. 336, the 'wire being lined inside with cotton cloth,
to which is cemented a similar cloth upon the outside. As
the arches are accurately molded, and as the floor is supposed
to be perfectly plain, the joints so formed present a smooth
interior. As soon as the first tier of arches is in position it is
immediately covered with concrete, which is then smoothed down
to form a second floor, upon which the second tier of arches is
laid. After the required number of ducts are in place, the usual
Fig. 337. Twelve-Duct Cement Arch Conduit.
layer of concrete is placed over the entire structure. In Fig. 337
is shown a sectional view of a 12-duct conduit of this type.
Curved sections of these arches are made for making bends,
these curves usually being made in 6-inch lengths. The cross-
section of the duct in this conduit possesses some advantage over
the round duct. If foreign matter finds its way into the duct
after the cable is in place, or if a piece of the external braiding-
from a cable sheath is torn off, it will not be so likely to bind the
cable in drawing it out as in the circular form of duct, because
the foreign matter will be likely to sink down in the lower cor-
ners of the duct, where considerable room is provided for it.
In laying conduit in city streets numerous obstructions are
418
AMERICAN TELEPHONE PR ACIDIC E.
met, and must be overcome in the manner best suited to the
individual case. It frequently becomes necessary to remove the
support from heavy pipe lines for a considerable distance, as,
for instance, when such a pipe line lies diagonally across the trench.
In all cases suitable supports for these pipes or other structures
should be provided until such time as the trench is again filled.
Fig. '338. Avoiding Obstacles.
The usual means adopted is to place a beam of sufficient
strength across the top of the trench and support the pipe
therefrom by chains or heavy rope. It is frequently necessary
in passing an obstruction to fan out the pipes in one layer so
that they occupy the same level as those of another layer. Such
a construction, and also a rather crooked piece of conduit work,
is shown in Fig. 338, where, on account of obstructions in
the street, the two layers of two pipes were formed into
one layer of four until the obstructions were passed. This par-
UNDERGROUND CABLE CONSTRUCTION.
ticular obstruction was a sub-cellar extending out under the
street.
The man-holes may be built of various forms and dimensions
to meet existing requirements. In the best construction the
foundation consists of a layer of concrete six inches deep, the
concrete being mixed as specified for the laying of tiles, with
the exception that the crushed stone may be considerably coarser.
The walls of the man-hole are then built of good brick-work of
suitable thickness and well plastered on the outside with cement
mortar in order to exclude as much dampness as possible. For
the ordinary man-hole an eight-inch wall is sufficiently thick,
but in building very large underground vaults it sometimes be-
comes necessary to double or triple this thickness. Where these
very thick walls are required it is good practice to allow about
one inch air space between the outer course of brick and the
inner in order to render the interior as dry as possible. A
common-sized man-hole is five by five by five feet, and smaller
sizes down to three by three feet with five feet head room are
also common. As a rule a man-hole should provide at least
enough room for two men to work in conveniently. Of course,
where a great number of ducts enter a man-hole the size must
be increased accordingly.
After the conduits are laid and the man-holes finished the
next step is the drawing in of the cables. In order to accom-
plish this a process called rodding is in most cases first neces-
sary, in order that a rope may be stretched through the duct,
which is afterwards to be used for drawing in the cable itself.
For this purpose a large number of wooden rods about three-
fourths of an inch in diameter and four feet long, and equipped
with screw or bayonet joints at each end, so that they may
readily join together, are necessary. A man stationed in one
of the man-holes inserts one rod into the duct, and, after join-
ing another rod to it, pushes this also into the duct. Suc-
cessive rods are joined and pushed through until finally the
first rod reaches the next man-hole. A rope is then attached
to one end of the series of rods, which is then pulled through,
disjoining the rods as they are taken out of the duct. Where
the ducts are smooth and comparatively straight this process
may be simplified by using a continuous steel wire about one-
fourth of an inch in diameter in place of the rods. It is a good
plan to attach to the forward end of this wire a lead ball, which
will facilitate it in riding over obstructions. The cable reel is then
placed near one of the man-holes in such manner that the cable will
420
AMERICAN TELEPHONE PRACTICE.
pay out from the top of the reel instead of from the bottom.
The end of the cable is then attached to the rope and started into
the duct. In the distant man-hole the rope is led over one or
more sheaves suitably arranged on upright beams placed within
the man-hole to a capstan or other form of windlass by which the
cable may be slowly drawn through the duct. A funnel-shaped
shield should be placed at the mouth of the duct into which the
cable is being fed for protecting the sheath against the sharp
corners at the entrance. This shield, however, is not a sufficient
protection for the cable, and one or more men should be stationed
in the man-hole for guiding the cable into the duct. The best
way to attach the rope to the end is by means of clamps especially
provided by the cable companies for this purpose. However, if
these are not used, a secure grip may be had upon the cable end
by winding several strands of stout iron wire in opposite direc-
tions about the cable sheath for a distance of two feet from its
end. An eye may be formed in this wire opposite the cable end,
to which the rope may be attached. Particular attention should
be paid to the sealing of the cable end before it is drawn into the
Fig. 339. Drawing in by Steam Power.
duct, as ducts are always moist, due so sweating of the interior
walls.
Where a large amount of cable is to be drawn in the method
shown in Fig. 339 maybe employed. Instead of the hand-oper-
ated winch or windlass a three-and-one-half horsepower horizon-
tal engine and capstan mounted on a low wagon is used.
By suitable gearing the engine causes the capstan to revolve
slowly. This method, so far as the writer is aware, has been
used in but one case, that being in the recent extensive under-
ground construction work in St. Louis by the Bell Telephone
Company of Missouri. It is said that with this contrivance
a speed of twenty-five feet of cable per minute is easily attained
without in any way damaging the cable, and the remarkably
UNDERGROUND CABLE CONSTRUCTION. 421
short time in which the enormous amount of cable installed by
that company was drawn in testifies further to the practical value
of this scheme.
Cables, in passing through man-holes, should be laid around
the side of the man-hole and supported on hooks provided for that
purpose. Shields formed of sheet lead or of heavy felt should be
placed under each cable just at the point where it emerges from
the duct, in order to prevent injury of the sheath at that point.
Workmen should be cautioned against needlessly bending cables
while working in ducts, and the use of the cables in place of
ladders for climbing in and out of the man-holes should be
strictly prohibited.
As much slack as possible should be left in the man-hole, in
order to allow room for subsequent splicing when necessary.
Trouble is frequently experienced, due to the presence of gas
in the man-hole, and care should always be exercised before
striking a match or taking a torch into a man-hole, to make
sure that all gas has been removed. There are several methods
of doing this, one of which is to pump the gas out with an
inverted umbrella made specially for the purpose. The um-
brella is lowered into the man-hole while closed and then suddenly
withdrawn, this opening the umbrella and lifting out the gas.
Another way of clearing man-holes from gas is to place a cloth
screen above the man-hole and on the side opposite to that from
which the wind is blowing. The wind on striking the screen
is deflected downward, thus causing an eddy which removes
the gas from the man-hole. Very serious explosions have been
caused by the formation of gas in man-holes, which becomes ig-
nited either by an electric sp:irk or by the torch of a workman.
One of the most serious difficulties in connection with un-
derground cable work is that brought about by electrolysis, due
to the action of stray earth currents, usually due to the ground
return of electric railways. It is found that the electrolysis oc-
curs at points where a current flowing along the cable sheath
leaves the sheath and enters the ground. At this point oxy-
gen is liberated which, with the chemicals in the earth, rapidly
corrodes lead sheaths. Of course, the construction of high-class
conduits, composed of insulating material, has done much to-
ward the alleviation of this trouble. Frequent tests should be
made, however, on all cable systems to determine the polarity
of the cable sheaths with respect to surrounding conductors.
The tests for this purpose may be made as follows : Two brass
rods about six feet long should be provided, each having a steel
422 AMERICAN TELEPHONE PRACTICE.
contact point at one end. Between these two rods should be
connected by flexible wires a portable voltmeter one reading
to five volts will usually be found most suitable. The test
should be made at the man-holes, these being the only avail-
able points for reaching the cable. One of the steel contact
points should then be placed in firm contact with the cable
sheath and the other into contact with any water or gas pipes
which run through the man-hole, and in each case the voltage
should be noted, not only in amount but in direction. Read-
"ing should also be taken between the cable sheaths and the
rails of adjacent electric railroads, and to whatever underground
structures exist in the immediate vicinity. It is evident that
where the cable sheaths are negative to the surrounding con-
ductors no danger will exist, as this would indicate that the
current tended to flow through the other conductors to the
sheath. If, however, the cable is found positive to the sur-
rounding conductors, the matter should be carefully followed
up by taking readings in successive man-holes. By these means
the maximum danger point can be located, it being, as a rule,
the point at which the maximum positive difference of poten-
tial exists. At this point the cable sheath should be securely
bonded by a heavy conductor to the water or gas pipe or to
other metallic structures that are in the vicinity. These bonds
serve to allow the current to flow from the cable sheath to the
other conductors, instead of forcing it to find circuit through
the ground or through the walls of the conduit. In some cases
the only remedy has been to run separate return circuits from
the maximum danger points on a cable directly to the power-
house from which the troublesome current emanates. All of
the cable sheaths entering a man-hole should be bonded to-
gether, the usual method of doing this being to brighten the
surface of the lead sheaths and to bend a No. 10 B. & S. cop-
per wire around each sheath, afterwards soldering the connec-
tion. This assures the fact that all of the cable sheaths will
be at an equal potential and that whatever bonds are run for
the protection of one sheath will afford protection for all. The
method of bonding to a gas pipe usually adopted is as follows :
The surface of the pipe is brightened for a space of about three
by eight inches with a coarse file. This surface is then heated
by a torch and tinned with ordinary solder. A copper plate
about three by seven inches previously tinned is then soldered
to the gas pipe, after which the bond wire leading from the
cable is wound into a flat coil and soldered to a copper plate.
UNDERGROUND CABLE CONSTRUCTION. 423
In bonding to a water pipe it is impossible to heat the pipe
sufficiently to make it take solder on account of the water flow-
ing within. The method to be followed is to provide a heavy
wrought-iron U-shaped band adapted to fit snugly around the
pipe. The ends of this band are screw-threaded and pass through
a yoke-piece bent to fit the Upper portion of the pipe. This
yoke-piece is then firmly screwed in place by nuts, the surface of
the pipe and the interior of the iron clamp having previously been
thoroughly brightened. The bond wire may then be soldered to
this yoke-piece and the whole device smeared with asphalt paint.
CHAPTER XXXIII.
TESTING.
TESTS of telephone lines, whether of bare wire on poles or of
overhead or underground cables, maybe divided into two general
classes :
First: Those which are for the determination of the existence
of certain conditions, without the necessity of measuring quanti-
tatively the extent to which those conditions exist ; in other
words, rough tests for the determination of grounds, crosses, or
breaks, usually made with instruments such as the magneto-bell,
telephone receiver and battery, and a few other such simple but
often in the hands of an experienced person most effective
instruments.
Second : Those for not only determining the existence of cer-
tain conditions, but also for their quantitative measurements.
These require the use of different and mor intricate instruments,
and in many cases the operator must be possessed of a fair
degree of mathematical training combined with an ingenuity for
meeting and mastering unusual problems that arise under differ-
ent conditions.
The magneto testing set is the most important instrument in
making tests under the first class. Such an instrument usually
-consists of a powerful magneto-generator so wound as to enable
it to ring its own bell through a resistance of from 25,000 to
75,000 ohms. A powerful magneto-telephone is carried on the
outside of the case in suitable clips, and may be switched in
circuit alternately with the generator by a small hand switch.
This magneto-telephone serves as both transmitter and receiver,
and enables the lineman or other party to communicate from a
pole top or man-hole with any other party on the circuit. Fre-
quently these sets are made to include microphone transmitter
and battery ; but, inasmuch as the instrument is seldom if ever
used to talk over very long circuits, the extra weight of these is
considered in most cases undesirable. A small, inexpensive gal-
vanoscope or current detector will also prove very convenient.
In testing for a ground on a wire, whether it be in a cable or
bare, and on poles, make sure that the far end of the line is open
7 ' STING.
425
and then connect one terminal of the magneto-bell to the near
end of the line and ground the other terminal. The ringing of
the bell would seem to indicate that the circuit was complete
and the line grounded in this case, but this is not always true,
and this test must therefore be relied on only with caution. The
static capacity of a long line or of a comparatively short length
of cable will often allow enough current to pass to and from the
line in charging and discharging to ring the magneto-bell.
For testing out local work where there is no room for this
capacity effect, the magneto-bell is invaluable.
A more reliable means of making tests for grounds or crosses
is to connect the current detector in series with several cells of
battery and to ground one terminal. Then with the other ter-
Fig. 340. Receiver Test for Crosses and Grounds.
minal make contact with the near end of the line. A kick of the
needle will take place in any event on closing the circuit, due to
the current flowing to charge the line, but a permanent deflection
will indicate a ground.
In testing for a cross, as for instance with some other wire in
the line or cable, one terminal of the magneto-bell or the gal-
vanoscope and batteries should be connected to the wire under
test and the other to all the other wires in the same lead, for
which purpose they are bunched. In case it is not convenient
to bunch them, however, the test may be made between the sus-
pected line and each of the others in succession.
Another and perhaps still more simple method for determining
a cross or ground is one described in Roebling's pamphlet on
Telephone Cables, and illustrated in Fig. 340, as applied to the
testing of a cable before it has been unreeled.
426 AMERICAN TELEPHONE PRACTICE.
N represents the near end, and F the far end of the wire
being tested. B is a battery, of about three cells. T is an
ordinary telephone receiver. The wire, N F, is carefully sepa-
rated from all the others at each end.
At the near end all the wires are stripped of insulation and,
except the one under test, are connected together and also with
the sheath. The wire, C, connects the sheath to one side of bat-
tery, B, and the other side of battery is connected to one side of
telephone receiver, T. The testing man rapidly taps with the
wire, N F, the unoccupied binding post of the receiver, T. The
first tap will produce in the receiver a distinct click, and if
the cable is long there may possibly occur a second faint click,
but if the wire, N F, is perfectly insulated, no more sound in the
telephone will follow the tapping. If, however, the wire, N F t is
crossed with any wire in the cable, or with the sheath, every tap
will be followed by a distinct click, and if there is moisture in the
paper, making a partial connection, clicking sounds will occur,
which are loud or faint, according to the amount of moisture
present.
The philosophy of this method of testing is very simple, and
serves to make the operation more readily understood.
When the wire, N F, is first connected to the battery, it be-
comes charged. During the process of charging a current flows
into the wire and passes through the coil of the receiver and
causes the click. If the wire is well insulated, the second tap,
immediately following, finds it charged, or nearly so, and there
is, therefore, no click, or a very faint one. If, on the contrary,
the wire under test is crossed with any of the other wires, or
imperfectly insulated from them, or from the sheath, the wire
will immediately discharge itself through the cross to the other
wires and the sheath, and there will be a flow of current at every
tap, and consequently a continuous clicking. If a conductor in
a perfectly insulated cable is very long, two or three taps or a
long first contact may be necessary to charge it completely.
If the cable is in place or if it is a bare aerial line that is being
tested this same method may be used. In case of a new cable it
is well to test every wire in this manner, and therefore the wire,
N F, should be put aside and another slipped out of the bunch
and tested in the same way, and so on until all have been gone over.
If any of them are found to be in trouble, it is well to care-
fully inspect the exposed ends to be sure they are properly
cleared from each other and from the sheath. If it is still found
to be defective, it should be plainly tagged.
TESTING.
427
In the manner just described, twenty-five minutes with two
men should be ample time for testing one hundred wires, the
testing operator listening and his helper attending to the connec-
tion of the different wires at N.
For this test as well as many others it is very convenient to
use a regular operator's receiver and head band, as it will save the
tester a very tired arm at the end of a long test. As a matter of
fact, the receiver is little appreciated as a testing instrument. A
very convenient set is formed by a watch-case receiver and head
band, and two small-sized cells of dry battery, strapped together
so as to be carried in the coat pocket. The receiver and battery
are connected in series, the free terminals of the circuit being
formed by flexible cords about four feet long. These cords
should terminate in convenient clips, or contact points adapted
Fig. 341. Continuity Test.
to make contact with the wires to be tested. This arrangement
leaves both hands free at all times, and is wonderfully sensitive.
The continuity test, or test for broken wires, may be made
with the same simple instruments. The wires to be tested should
all be grounded or connected to a return wire at the far end. At
the near end, one pole of a magneto-bell, or of the battery and
galvanoscope, or of the receiver, should be connected to ground or
the return wire and the other terminal connected successively to
the terminals of the line, which, of course, should all be separated.
A ring in the case of the magneto, or a permanent deflection
of the needle in the case of the galvanoscope, or a continuous
clicking in the receiver, will indicate that the wire is continuous.
The same precaution as previously pointed out must, however, be
428 A AMERICAN TELEPHONE PRACTICE.
observed with the magneto-bell. This same test for continuity
is well illustrated in Fig. 341, in which case a vibrating bell in-
stead of the receiver or galvanoscope is used.
In testing a cable all defective wires should be marked
"crossed," "grounded," or " broken " at the end at which they
are tested. The corresponding ends of the tagged wires at the
other end of the cable should then be found and similarly marked.
If there are not the requisite number of good wires in a new
cable it should be rejected.
Lead-covered cables, manufactured by reliable firms, are always
subjected to a much severer test than it is possible for the pur-
chaser to give them before they leave the factory. It is there-
fore considered by many as unnecessary to make a test on new
cable on reels, purchased from reputable firms, unless some injury
in shipment is suspected.
It is often desirable to be able to pick out a certain wire at
some intermediate point in an open cable, or in a large bunch of
insulated wires, in order to establish a branch connection. This
is easily done by the foregoing methods if the cable is to be cut,
but frequently this is not the case. It may be done without
cutting by the following simple method : Ground the wire or
wires desired at the distant end, being sure that these wires are
free from all the others at both ends. Then having loosened the
bunch of wires at the point at which the branch is to be taken
off, test each by means of a needle-pointed instrument, con-
nected to ground through a bell or receiver and battery. The
needle-point can readily pierce the insulation and make good
contact with the conductor within. A knowledge of this very
simple test will often save an immense amount of trouble.
In the second class of tests that is, those requiring quantitative
measurements there are three distinct subdivisions, which are as
follows : Tests for resistance or conductivity, tests for capacity,
and tests for insulation. Tests for the location of faults in lines
always depend on the application of one or more of these.
There are three principal methods of making resistance tests:
First, by the use of a Wheatstone bridge, which is accurate for all
resistances except those very large or those very small. Second,
the fall of potential method, which is of value in measuring very
small resistances, as of a large conductor, such as a trolley-wire or
heavy feeder. This method has little use, therefore, in telephone
work where all conductors are comparatively small. Third, by
the use of a sensitive galvanometer in series with a battery.
This method is the most accurate for the determination of
TESTING.
429
extremely high resistances and is, therefore, of great use in meas-
urements of insulation resistance.
For general resistance measurements the Wheatstone bridge
is the most suitable, being very accurate and exceedingly simple
in manipulation. In order to appreciate the possibilities of this
instrument its underlying principles should be understood. In
Fig. 342,^,^,^, and X represent resistances. G is a galvanometer
or instrument for detecting ^the flow of current. The four
Fig. 342. Diagram of Wheatstone Bridge.
resistances are connected together as shown, the galvanom-
eter being connected in the "bridge" between the junctures of
A and R, and B and of X. A battery, B ', is connected between the
junctures of A and B, and of R and X. Each resistance, A, B, R,
and X, forms, what is termed an arm of the bridge.
The two fundamental laws upon which the action of the bridge
is based may be stated as follows :
1. No current will flo^v -between points of equal potential; and
2. The drop in potential along the various parts of a conductor
is proportional respectively to the resistances of those parts.
Referring again to the diagram, it is evident that a current
from the battery flows to the point, ^, where it divides, part flow-
ing through A R and part through B X, after which they unite
and pass to the negative pole of the battery. But what of the
galvanometer? Evidently by Rule I the only time at which no
current will pass through it will be at the time when the points,
f and h, are at the same potential. By Rule 2 these points
will be at the same potential only when A bears the same re-
lation to R as J3 does to X.
430 AMERICAN TELEPHONE PRACTICE.
That is
A : R :: B : X, or, by alternation,
A R
B ~~ X '
A little algebra will render the above evident if not so already.
Call a the drop of potential between the points e and z, /
that between e and/, and c that between e and h.
Then
b : a :: A : A + R by Rule 2.
Similarly
B
'- a '
For a condition of equal potentials at /and h so that no cur-
rent will flow through the galvanometer, b must = c.
Then
A B
A + R B + X
whence : AB -f ^A r = ^ + BR,
and ^A" = BR.
Dividing by BX, we have
A R
B JT
which is the equation of the ratios between the resistances of the
arms of the bridge, to insure no flow of current through the
galvanometer.
The resistance to be measured forms the arm X of the bridge,
and in order to determine its value the resistances in the various
arms are adjusted till no current flows through the galvanom-
eter. Then the equation just derived holds good and may be
solved for X,
thus X = J* R.
A
The arms A and B are best termed the " ratio arms " of the
bridge and arm R the rheostat arm.
In commercial forms of the Wheatstone bridge, A and B are
usually' so arranged that each may be given the values, 10, 100,
and 1000 ohms, and in some cases I ohm and 10,000 ohms also.
TESTING. 43 J
The ratio arms, A and B, may therefore be adjusted to bear any
convenient ratio to each other from - - to IQQQ , or, in
1000 10
some instances, from - to - L2^2_ . The rheostat arm
10,000 I
is in reality a rheostat capable of being adjusted to any value
from I to about 11,000 ohms.
In some bridges a sealed battery is furnished with and forms
a part of the instrument. In those having no battery, suitable
binding posts are provided, usually marked BB, between which
the battery may be connected. Other binding posts, usually
marked XX, are furnished for connecting the terminals of the
unknown resistance to be measured.
Two keys are usually furnished, one in the battery circuit and
the other in the galvanometer circuit. Each keeps its circuit
normally open.
The operation of the bridge is very simple. First some ratio
between the arms A and B is determined upon. The battery is
then connected between the proper binding posts, and likewise
the resistance to be measured is connected between its binding
posts.
The battery key is first depressed and then the galvanometer
key. A deflection of the galvanometer needle will take place
which by its direction will after a few trials show whether the
resistance in the rheostat arm is too great or too small. The
rheostat is adjusted accordingly until the galvanometer needle
shows no deflection upon the operation of the keys. We then
know that our equation
4- = 4- h lds
> A.
and consequently
X = * *'
That is, the unknown resistance is equal to the ratio between B
and A multiplied by the resistance in the adjustable arm.
Considerable judgment may be exercised in the choosing of
the appropriate ratio in the ratio arm to obtain the greatest
accuracy. Obviously if a very high resistance is to be measured
the ratio should be large, and vice versa.
In bridges having resistances of 10, 100, and 1000 ohms in the
432 AMERICAN TELEPHONE PRACTICE.
ratio arms, the following values in arms A and B will give the
best results :
Resistance to be measured. A arm. B arm.
Under 100 ohms, 1000 10
100 to 1000 ohms, ..... 1000 100
1000 to 10,000 ohms, .... 1000 1000
10,000 to 100,000 ohms, . . . . 100 1000
100,000 to 1,000,000 ohms, ... 10 1000
As to the accuracy of measurements attainable by the use of
the Wheatstone bridge, the following table represents the claim
of one reliable manufacturer:
.01 of an ohm to an accuracy of I per cent.
r it t< T/
I ohm " " " ^ " "
10 ohms " " " \ " "
100 " " " " ?/8 "
1000 " " " " */8 "
10,000 " " " " I " "
100,000 " " " " # " "
1,000,000 " " " " 5 " "
If using the no volt lighting circuit as battery power I meg-
ohm may be measured accurate to j per cent.
There is no doubt but that with a well-made bridge with a
sensitive galvanometer, these results may be equaled if not sur-
passed. Great care must be taken in using a voltage as high as
no, as there is danger of burning out the coils. Such high volt-
age should be used only in measuring very high resistances, and
the ratio arms should be adjusted to give as high a multiplying
ratio as possible.
A particular form of bridge which has come into extensive use
in this country and which possesses several unique features is
shown complete in Fig. 343 and in plan view in Fig. 344.
The various adjustments of the arms are accomplished by
placing plugs in the various holes between the brass blocks
arranged in rows as shown in the latter figure. Between each
successive pair of blocks are arranged resistance coils having the
resistances in ohms designated on the plan. Placing a plug in a
hole between two blocks short-circuits the resistance connected
between those two blocks. The rheostat arm of this bridge is
represented by the top and bottom row of blocks, and if all plugs
TESTING.
433
are removed the resistance in this arm will amount to 11,110
ohms. The ratio arms A and B are represented by the left- and
right-hand halves respectively of the center row. A galva-
Fig. 343. Portable Testing Set.
nometer and suitable battery, together with battery and galva-
nometer keys, are all mounted in a carrying case as shown.
The connections of this instrument are indicated in Fig. 344,
Fig. 344. Plan of Portable Testing Set.
and are 3& follows : The top row of blocks is connected to the
bottom r*>w by a heavy copper bar joining the right-hand blocks.
This connection is made very heavy so as to interpose no extra
resistance in the rheostat. On the rheostat formed by the
434
AMERICAN TELEPHONE PRACTICE.
upper and lower rows of blocks any resistance from i to
1 1, no ohms may be obtained, the resistance being added by
leaving out plugs. The lower left-hand block of the rheostat is
connected to the lower binding post, D, forming one terminal of
the unknown resistance. The upper post, C, forming the other
terminal of the unknown resistance, is connected to block, X,
which block is also joined to the galvanometer key. The block,
R, is connected to the upper left-hand block of the rheostat.
The end blocks of the middle row are connected together and
to the -+- terminal of the battery. The terminal of the bat-
tery is connected through the battery key to the lower left-hand
end of the rheostat. One galvanometer terminal is connected
directly to the block, R, the left-hand block of the rheostat,
and to the back contact of the galvanometer key. The other
galvanometer terminal is connected through the key to the
block, X.
By carefully following out these connections it will be apparent
that the parts as connected form three arms of a Wheatstone
bridge, the fourth, of course, being the unknown resistance
& 345- Circuits of Portable Testing Set.
joined to the line posts. This is shown diagrammatically in Fig.
345, where the corresponding parts are similarly lettered.
It will be noticed that this latter figure is practically the same
as Fig. 342, with the addition of the center blocks, A, B, X, R,
forming a sort of commutator. The object of this arrangement
is to make it possible to reverse the connections of arms, A and
B, with R and X. Thus with the plugs in the position shown by
the black dots, the connection is precisely as shown in Fig. 342,
A
and the equation - = JiL holds true.
B X
If, however, the plugs are
inserted in the holes on the other diagonal, arm, A, will be con-
nected to arm, X, and arm, B, to arm, R, and the equation of
the bridge will be - = -^L
TESTING. 435
The bridge arms, A and B, have not the same range of resist-
ances in this bridge, A having only i, 10, and 100 ohm coils,
while the resistances of ^are 10, 100, and 1000 ohms. Therefore,
if a ratio of looo to I for measuring large resistances is desired,
the plugs are inserted in the commutator along the arrow H
(Fig. 344) ; while an opposite'arrangement of the plugs along the
arrow L will give a ratio of I to 1000 for very small resistances.
In this bridge the galvanometer key is so arranged as to short-
circuit the galvanometer while the key is up.
The galvanometers usually furnished with the complete
bridges consist of a needle so mounted as to swing freely in a
horizontal plane. This needle is given a tendency to point in
one direction sometimes by the action of the earth's magnetic
field and sometimes by the field of a powerful permanent mag-
net. By causing the current through the bridge wire to flow
through a coil, either stationary and surrounding the needle, or
movable and carried on the needle, the needle is caused to
swerve from its normal position and to place itself at right-angles
to the lines of force due to the permanent field. The deflection
of the needle is great or small according to the strength of the
current, and to the right or left according to the direction of the
current.
In many of the tests to be described later a galvanometer of
greater sensitiveness is required, and some form of reflecting in-
strument is used. In these the needle carries a small circular
mirror, which reflects a spot of light from a lamp or some other
source against a scale. In this arrangement every movement of
the needle causes the spot of light to move along the scale, and
a little consideration will show that the angle through which the
reflected ray of light moves is double that through which the
needle travels. Thus this reflected ray of light serves as a
needle of any desired length, and has the advantages of magnify-
ing the angular movement of the needle to twice its real amount,
and of possessing no mass, and therefore no inertia.
The two galvanometers used to the greatest extent for quanti-
tative measurements in practical work are the Thomson and the
D'Arsonval.
The Thomson galvanometer is made in a great variety of
forms. The needle consists of several very light bar-magnets
arranged side by side and with opposing poles together, so that
the directive influence of the earth's field shall be very slight.
The needle is directly attached to a small silvered glass mirror,
and is suspended within the coil or coils by means of a silk or
43 6
AMERICAN TELEPHONE PRACTICE.
quartz* fiber. The current to be measured is passed through the
coils, and the magnetic field set up thereby causes the needle to
swerve from its normal position. The Thomson galvanometer is
used in the most delicate tests, and is essentially a laboratory in-
strument. It has the disadvantage of being affected to such an
extent by external magnetic fields as to render its use impos-
sible in many cases. A passing street car or variations in the
Fig. 346. D'Arsonval Galvanometer.
current flowing in a neighboring circuit will cause the needle to
swing violently, thus making accurate work out of the question.
These disadvantages may be overcome to some extent by in-
closing the galvanometer in a heavy iron case such as an old
safe but they tend to make it a very undesirable instrument
for portable work. Where the instrument can be permanently
set up and properly guarded, it is unequaled for delicacy and
accuracy.
For nearly all practical engineering work, the D'Arsonval gal-
vanometer is sensitive enough, and has the advantage of being
much more convenient for general work. In this the needle is a
coil instead of a permanent magnet, and is suspended within the
TESTING.
437
field of a powerful permanent magnet instead of in a coil. The
needle carries a mirror, as in the Thomson instrument. The
current to be measured is passed through the coil, and as this
coil lies in the field of the permanent magnet, a rotation of the
coil ensues, the action being identical with that which causes
the armature of an electric motor to revolve.
In Fig. 346 is shown a much-used form of D'Arsonval gal-
vanometer made by Queen & Co., Philadelphia. The field is built
up of a number of horizontal permanent magnets, between the
Fig. 347. Suspension of D'Arsonval Galvanometer.
poles of which is suspended the needle. The needle system
is shown in detail in Fig. 347. It consists of a coil of wire, W,
wound on a boxwood frame, D, and supported by means of the
flat phosphor-bronze filament, A, from the torsion pin, E. The
current is led in by means of the torsion pin, E, and suspension
wire to the coil ; thence to the spiral spring, B, and by means of
the bottom contact out to the external circuit. A ring, F, is
438 AMERICAN TELEPHONE PRACTICE.
joined above the coil frame, and another, G, below the coil frame.
These are normally a sufficient distance apart to enable the sys-
tem to swing freely, but when packing for transportation the
torsion head may be pressed down until the rings above men,
tioned firmly clamp the coil. In this condition it will withstand
shipment satisfactorily. To the right is shown the coil more
clearly. The two points, U and /,, have soldered to them the
ends of the coil, W. The mirror is shown at C.
The great advantage of the D'Arsonval galvanometer is that
It is unaffected by variations in the external magnetic field. It
may even be used close to dynamo machinery without being
sensibly affected.
In order to read the deflection produced by a current, in any
Fig. 348. Scale and Telescope.
form of reflecting galvanometers, two methods may be em-
ployed. One is to cause the needle to reflect a spot of light
from a stationary source, upon a horizontal scale, and by watch-
ing the movement of the spot the number of scale divisions de-
flection may be accurately determined. Another and better
way is to focus a telescope on the mirror, in such manner that
the horizontal scale will be visible in the telescope. The mirror
in its movements will reflect different portions of the scale into
the telescope, and the deflection may thus be observed with
great precision. When this method is used the numbers on the
scale should be reversed, in order to appear normal in the tele-
scope., Fig. 348 shows a telescope and scale as arranged for
this purpose.
Complete testing sets, containing reflecting galvanometers,
TESTING. 439
bridges, batteries, keys, and other accessories, are frequently
mounted in one case, and so arranged as to fold within small
compass when not in use. This arrangement is very convenient,
but has one disadvantage the manipulation of the keys and
plugs jar the box to such an extent as to make the readings
on the galvanometer unreliable. The separately mounted gal-
vanometer is therefore in general to be preferred. Of course
this applies only to reflecting galvanometers.
It is frequently found that a current that it is desired to
measure is so large that it sends the spot of light completely off
the scale, thus rendering the measurement of the deflection im-
possible. In order to increase the range of the galvanometer so
as to make it available for measuring both large and small cur-
rents, certain resistances called shunts may be placed in parallel
with the galvanometer coil as in Fig. 349. The resistance of
S
Fig. 349. Galvanometer and Shunt.
the shunt being known, it is easy to calculate the amounts of
the currents that pass through the galvanometer coil and the
shunt.
Calling R s the resistance of the galvanometer, R* that of the
shunt, I g the current through the galvanometer, 7 S that through
the shunt, and / the total current through both, then
7=4+ /..
Also when E is the difference of potential between the com-
mon terminals of the galvanometer and shunt,
E , E
7 g = _ and 7 8 = = _.
E = LR.-.--I. R s . Hence /. = ^-
Substituting this value of 7 S , in the first equation, we have
~ R* e \ ~RJ X*
7? i T2
The quantity - is called the multiplying power of the
440 AMERICAN 7^ELEPHONE PRACTICE.
shunt, because it represents the number by which the current
through the galvanometer must be multiplied, in order to give
the value of the current being measured.
Shunt boxes are usually provided for a given galvanometer
with a number of coils specially arranged to give such con-
venient values of the multiplying powers, as 10, 100, and 1000.
For this purpose the various coils of the shunt box have resist-
ances of |, -jfo, and -rrj-g-of the resistance of the galvanometer.
To better show this relation, assume that a multiplying power
of 1000 is desired, then
I000 =
- R K - R..
iooo i 999
A commercial form of shunt box is shown in Fig. 350, the
various multiplying values of the shunt being obtained by plug-
ging the block corresponding to the multiplying power desired.
Fig. 350. Shunt Box.
For moderate deflections, the current traversing the coils of a
reflecting galvanometer may, without sensible error, be taken as
proportional to the deflection of the spot of light on the scale,
or to the deflection read through the telescope. The current is
of course inversely proportional to the total resistance of the
circuit, and from this it follows that the deflections are inversely
proportional to the resistance. This fact enables the galva-
nometer to be used for measuring unknown resistances by com-
paring the deflection obtained when a given E. M. F. acts
through a known resistance with that obtained when the same
E. M. F. acts through an unknown resistance.
The general method of measuring resistances by the use of a
TESTING. 44i
galvanometer is to note the deflection obtained with a given
battery and a known resistance in the circuit, and from this to
compute what is called the working constant. This working
constant may be defined as the number of scale divisions deflection
that ivould be obtained by causing the current from the given bat-
tery to pass through the galvanometer and a resistance of one
megohm. Of course such a deflection as this can exist in our
imagination only, but it serves, nevertheless, as a convenient
standard upon which to base our calculations. Having obtained
the working constant, a reading is taken of the deflection pro-
duced by passing the battery current through the galvanometer
in series with the unknown resistance. As the deflections are
inversely proportional to the resistances, the unknown resist-
ance is then readily computed.
If measurements of comparatively low resistance are to be
made, then the resistance of the battery and of the galva-
nometer must be taken into consideration as well as that of the
resistance placed in circuit with them, but as the measurements
here considered will be those of very high resistances only, the
resistance of the battery and of the galvanometer may be neg-
lected. For the purpose of taking the constant, connections
are made as shown in Fig. 351, where B is the battery, G the
Fig. 351. Circuits for Galvanometer Constant.
galvanometer, S the shunt, and R the known resistance.
Usually the value of R is ^ of a megohm, or 100,000 ohms.
With the g-fg- shunt a certain deflection will be obtained when
the circuit is closed. Obviously, if the shunt were not present
the deflection would be 1000 times as great, because only -j^
of the current passes through the galvanometer. Therefore the
total deflection, if it could be measured, that would be pro-
duced through the galvanometer and the 100,000 ohms resist-
ance, would be the deflection noted multiplied by IOOO. If,
now, the resistance, R, had a value of I megohm instead of T V
megohm, the deflection would have been only yV as great as
this. Therefore to find the number of scale divisions deflec-
442 AMERICAN TELEPHONE PRACJ^ICE.
tions which the galvanometer alone would give with I megohm
in circuit, we multiply the deflection noted by 1000 and by -^.
In general we may say : to find the working constant, multiply
tJie deflection obtained by the multiplying power of the shunt, and
by the value of the known resistance in megohms.
As a numerical example let us assume that with the -y-J-j- shunt
and the -fa megohm resistance, we obtain a deflection of 200
scale divisions, then the working constant is
200 x 1000 X = 20,000.
10
In other words, 20,000 would be the number of scale divisions
obtained were the entire current from the battery allowed to
pass through the galvanometer with one megohm in series.
With 50 cells of battery (45 or 50 volts), the constant under
ordinary working conditions with a good D'Arsonval galva-
nometer, will be from 10,000 to 25,000. With a Thomson instru-
ment a much higher constant may be obtained. Mr. George
D. Hale of the Western Electric Company's cable-testing de-
partment uses a large four-coil Thomson instrument with 600
volts obtained from a motor generator. With this he obtains a
constant of 528,000, and by adjusting the suspension for greater
delicacy can obtain as high as 2,000,000. Of course this is
entirely impracticable for portable instruments, and is, in fact,
unnecessary, as good work may be done with a constant of
2O,OOO. In ordinary testing a battery of 50 cells is sufficient.
Of course a higher working constant may be obtained with a
larger battery, and frequently 100 cells are used.
INSULATION TESTS.
One of the principal uses of the galvanometer in line testing
is in the measurement of insulation resistance. The insulation
resistance of any line or conductor is the joint resistance of all
the leaks from the line to the ground or to other conductors.
On a pole line every insulator forms a leak to earth, and on a
line having 40 poles to the mile there would be 40 such leaks in
parallel. The insulation resistance of a line as a whole varies
inversely as its length, if the insulation is uniform. Evidently,
a line two miles long would have one-half as great an insulation
resistance as a similar line one mile long, because on the latter
there would be only half as many leaks as on the former. In
TESTING. 443
general it may be stated that a line n miles long will have only
as great an insulation resistance as a similar line one mile in
length. In order to obtain a standard of insulation resistance
independent of the length of the line, it is convenient to express
the insulation resistance as so ( many megohms per mile. The
insulation resistance per mile is found by multiplying the insula-
tion of the line as a whole by the length of the line in miles.
In order to measure the insulation resistance of a line the
constant of the galvanometer is first taken and then the known
resistance is cut out of circuit and the line insulation resistance
substituted for it. Assuming that the insulation resistance to
be measured is that of a wire in a cable, the terminals of the
circuit which were connected with resistance,^, in Fig. 351, will
be connected one with the wire and the other with the sheath
of the cable as shown in Fig. 352. Care must be taken that the
Fig- 352. Insulation Resistance of Cable.
wire being measured is carefully insulated from the sheath at
the other end of the cable. The shunt, S, is then cut out of
circuit in order that the full current may pass through the gal-
vanometer. Before completing the circuit with the cable con-
ductor and sheath, however, the key, K, should be closed in
parallel with the galvanometer, in order to prevent the rush of
current that will take place in charging the cable, from causing
the needle to give too violent a kick. After a short time the
key is opened and all of the current diverted through the
galvanometer. The galvanometer then receives only that cur-
rent which leaks from the core of the cable to the sheath
through the insulation. Under these circumstances a certain
deflection will be noted, and by comparing this deflection with
the constant already obtained the value of the insulation resist-
ance in megohms is readily determined.
To illustrate, suppose that a deflection of 75 scale divisions is
obtained with the apparatus connected as in Fig. 352. If the
constant is 20,000, as already determined, we know that the
444
AMERICAN- TELEPHONE PRACTICE.
insulation resistance must be 20,000 divided by 75, or 266 meg-
ohms, thus indicating that the total insulation resistance of the
cable is 266 megohms. That this is true is evident from the
fact that the constant, 20,000, represents the number of scale
divisions deflection that would be obtained were only one meg-
ohm in the circuit. The deflections are inversely proportional
to the resistance in the circuit, and therefore the total insulation
resistance is equal to the deflection through one megohm
divided by the deflection through the insulation resistance, or
20,000 divided by 75. To sum up these operations :
1st. Obtain the galvanometer constant or deflection obtained
when the galvanometer in series with one megohm resistance is
Fig. 353. Connections for Insulation Test.
subjected to the potential of the battery. 2d. Find the deflec-
tion obtained when the galvanometer and insulation resistance
in series are subjected to the potential of the battery. 3d.
Divide the constant by the deflection obtained through the
insulation resistance, the result being the insulation resistance
of the cable expressed in megohms. 4th. To find the insulation
resistance per mile, multiply the total insulation resistance by
the length of the cable in miles.
If the insulation of the cable is low, a shunt must be used in
obtaining the deflection through the insulation resistance. If the
insulation resistance is high, the deflection will be small and no
shunt will be required. The purpose of the shunt is merely to
keep the deflections on the scale so that they may be read.
In Fig. 353 is shown a convenient arrangement of connections
for making insulation tests. In this, B is the battery of say
50 cells, R the -^ megohm box, 5 the shunt box, G the galva-
nometer, and V a convenient switch for throwing either the -^
TESTING. 445
megohm box or the line insulation into circuit with the galva-
nometer and battery. When the levers of the switch, F, are in
the position represented by the dotted line, the circuits are
those for taking constant of the galvanometer, and when in the
position shown by full lines, the circuits are those for obtaining
the deflection through the insulation of the cable. Various
forms of keys for changing the direction of the battery current
through the galvanometer, and for performing other switching
operations with the greatest possible convenience, are obtainable,
and form an important part of all testing outfits. The scope of
this work will not permit of their detailed description.
In making insulation tests the resistance of the lead wires to
the cable or line need not be taken into account. It is a matter
of the greatest importance, however, that these wires are perfectly
insulated from each other. It is a very easy matter in making
tests of this nature to measure the wrong quantity.
One very important matter in connection with insulation tests
has not yet been spoken of. When the reading is being taken,
with the cable or line insulation in circuit, it will be noticed that
a maximum deflection is obtained at first, and that this gradu-
ally diminishes, as though the insulation resistance were increas-
ing. This is due to what is called electrification, a phenomenon
that is not very thoroughly understood. When the electromo-
tive force of the battery is first applied to the cable or line,
there is a sudden rush of current, due to the charging of the
conductors. The charges, however, apparently soak in to the
insulation to a slight extent, thus allowing more current to flow
to the conductors. After the first rush due to the first charging
of the conductors, there is still a flow of current, due in part to
this soaking 1 in, and in part to the actual leakage through the
insulation. It is the current . due to the latter that we are
concerned with in insulation measurements, and therefore we
must wait till the soaking in process ceases, when the flow of
current will be practically constant, being that through the
insulation. In nearly all telephone-testing work, one minute is
allowed for electrification, after which the reading is taken of
the deflection. When one is thoroughly familiar with his instru-
ments he may often, where great accuracy is not required,
estimate what the deflection at the end of one minute will be,
by watching the deflection for 30 or 40 seconds. This method
saves time, but must be used with extreme caution.
With a constant of 20,000 a reading taken on a wire in a piece
of good new telephone cable, one-quarter mile long, would
446 AMERICAN TELEPHONE PRACTICE,
probably show a deflection of 8 or 10 scale divisions upon the
closure of the key. This would decrease to about 6 scale
divisions in 2 seconds, and to about 2 scale divisions in 30 sec-
onds, after which it would remain constant. The reading of 2
divisions at the end of the minute would indicate an insulation
r 20,000
resistance of = IO,OOO megohms, or 2500 megohms per
mile.
As examples of deflections on the different wires in various
cables the following are given :
Dry paper cable, f mile long, two years old. Galvanometer
constant 22,000: Readings, 12-15-15-10-10-15-15-15-13 scale
divisions.
Another dry paper cable, 2750 feet long, one year old. Gal-
vanometer constant 19000 : Readings, 5-6-5-4-5-5-5-5-6, etc.
A piece of jute and ozite cable five years old, 6000 feet long,
gave the following with a constant of 20,000: 7500-2500-1000-
1000-600-800-900-800-1000. It was necessary to use the T V
shunt in taking these readings.
Another piece of the same kind of cable, 800 feet long, with
a constant of 20,000, gave 175-200-250-270-160-110-120-160-
110-125.
CAPACITY TESTS.
A very important measurement, especially in telephone cables,
is the determination of the capacity of the line conductors with
respect to all neighboring conductors. The usual method of
making capacity tests is to note the deflection produced when a
B
r >- 354- Capacity Test.
condenser of known capacity, after having been charged to a
known potential, is discharged suddenly through the galvanom-
eter, and to compare this with the deflection obtained when
the cable or conductor being measured, after being charged to
the same potential, is discharged through the galvanometer. The
TESTING. 447
deflections produced under these circumstances are proportional
to the charges, and therefore to the capacities of the standard
condenser and the line or cable. The circuits for obtaining the
deflection produced by the discharge of the condenser are
shown in Fig. 354, where C is the standard condenser, B the
battery, and G the galvanometer. When the key is depressed
the condenser is charged to the full potential of the battery, B.
The key is then suddenly released, thus allowing the charge
from the condenser to pass through the galvanometer, thus
producing a certain throw of the needle. The connections are
then made as shown in Fig. 355, the same battery, , being used.
IL
will-
R
Fig- 355- Capacity Test.
When the key is depressed the cable is charged, and when
suddenly released this charge flows through the galvanometer
and produces another throw of the needle. By comparing the
throw produced by the charge of the condenser with that pro-
duced by the charge of the cable, a direct comparison may be
made between the capacity of the cable and that of the con
denser. Thus, if with the -g-j-g- shunt the discharge from the
condenser gave a deflection of 100 scale divisions, the capacity
of the condenser being T V microfarad, and if with the same shunt
the discharge of the cable produced a deflection twice as great, we
would know that the capacity of the cable was 2 X =
microfarad.
Convenient connections for making capacity tests are shown
in Fig. 356, where G is the galvanometer, 5 the shunt, C the
standard condenser, KK discharge keys, V the selecting switch,
and, B a battery of eight or ten cells. With the switch, V, at
the left and both discharge keys depressed, the current from the
battery will flow into the condenser, thus charging it. Upon
the sudden release of the discharge keys, the condenser will dis-
charge through the galvanometer and shunt, giving a deflection
which should be noted. With the switch, V, at the right, the
448
AMERICAN TELEPHONE PRACTICE.
cable may be charged and discharged in the same manner, and
the deflection produced by its discharge noted. About seven
cells of battery is usually sufficient for making capacity tests on
telephone cables. If a non-adjustable condenser only is avail-
able, one having a capacity of T V microfarad is probably most
desirable. For accurate work a subdivided condenser, having
its divisions so arranged as to be easily connected in multiple or
in series, or in combinations of the two, is very desirable. Then
Fig- 356. Circuits for Capacity Test.
the condenser capacity may be varied until the throw from the
condenser is nearly equal to that from the cable, thus greatly
minimizing the liability to error in the results. In making
capacity tests the wire under test should be carefully insulated
and all the other wires in the cable should be connected together
and to the sheath or ground. Fifteen seconds should always be
allowed for the charging of the cable.
If d is the throw due to the discharge of the condenser, d' that
to the discharge of the cable, K \\\z capacity 9f the condenser
in microfarads, and X the capacity of the wire being measured,
then
X : K :: d 1 : d
z-fx
If the throws of the galvanometer are too large to be measured,
the shunt must be used. In this case dor d' in the formula will
TESTING. 449
be the actual throws observed multiplied by the multiplying
power of the shunt.
THE LOCATION OF FAULTS.
When a break occurs in a wire in a line or cable, the ends
remaining insulated from other wires and the ground, the only
recourse is to capacity tests. The capacity of the two parts of
the wire will be proportional to their lengths, the wire being
uniform in size and in its relation to other wires, throughout its
length.
We may locate a break of this nature in several ways.
Measure the capacity of one end of the broken wire, then go
to the other end of the cable and do the same. Calling D the
length of the cable in feet, C the capacity of the first portion of
the wire, C that of the other, and X the distance in feet to the
break from the first end, then :
X : D :: C : C+ C
CD
When a good wire is available, and this is usually the case,
set up the instruments for capacity testing, and take a throw, d,
on the broken wire, another, d', on the good wire, and a third, d" ',
on the good wire with the broken wire connected to it at the
far end.
Evidently the throw on the whole broken wire would be
d" - d' + d.
Hence where D and X have the same significance as before
X : d :: d : d" - d' + d
r- dD
d" - d + d?
The location of breaks is much complicated by the presence
of poor insulation between ruptured portions, and between other
wires. The insulation resistances between these parts should
always be taken. If less than one megohm, the results obtained
by the capacity tests should not be relied on, and other methods
too complex for description here may be resorted to. It seldom
pays to open a lead-covered telephone cable for the purpose of
joining a few broken wires, the expense of making the splice
being usually in excess of the value of the wires.
45
AMERICAN TELEPHONE PRACTICE.
The location of crosses or grounds is rendered somewhat
difficult by the fact that there is nearly always some resistance
in the fault itself. If we know the resistance of the defective
wire and have no good wire running parallel with it, we may
proceed as follows, using a good Wheatstone bridge :
Measure the resistance of one end of the defective wire
through the fault to ground. Do the same at the other end.
Then calling R the total resistance of the wire (either known or
calculated from its size and length), R the measured resistance
from the first end, R" that from the other end, X the resistance
from the first end to the fault, Fthe resistance from the second
end to the fault, and Z the resistance of the fault, we have :
R = X + F.
R 1 = X + Z.
R " = Y + Z.
Solving these for X and F we have
R + R - R"
Y =
_ R - R + R"
j
2
which values are independent of the resistance of the fault.
Knowing the resistance to the fault, it is easy to compute the
distance to it, from the resistance per foot of the conductor.
When a good wire is available, the Varley loop test should be
used, as it is more accurate than the method just described.
FAULT
ig- 357. Varley Loop Test.
For this a Wheatstone bridge is used, and connected as in Fig.
357. The good and bad wires are joined at their distant ends,
and one terminal of the battery connected to the point, e, on
the bridge, while the other terminal is grounded. It is not
TESTING. 45 *
difficult to see that the partial ground or fault now bears the
same relation to the bridge as the point, t, in the diagram of
Fig. 342 ; the rheostat arm now includes the resistance, R, plus
the resistance of the bad wire to the fault, while the unknown
arm includes the resistance of the good wire, plus the resistance
of the bad wire on the other l side of the fault.
The equation of the bridge, when balanced, then becomes
A R + X
B ~~~- C + Y'
where R is the unplugged resistance of the rheostat, X the
resistance to the fault, Y the resistance beyond the fault, and C
that of the good wire.
Now calling L the resistance of the loop consisting of the
good and bad wires, we have
L = X + Y+ C,
or C + Y=L-X.
Substituting this in the second member of the equation of the
bridge, we have
A R + x
B~~ L- X"
A L- B R
whence^: ^ + ^
which is independent of the resistance of the fault. When the
two ratio arms of the bridge are given equal values we have
A = B, and the equation for X becomes :
Sometimes, in ordinary paper cables, a requirement is made
that a rubber-covered test wire shall be run through the center
of the cable, so that at least one good wire may always be avail-
able in testing. Where no good wire is available, a separate wire
may be strung to be used as the return in this test.
If the lead wires, from the instruments to the faulty wire, have
appreciable resistance, this should be measured, and deducted
from the value of X. After this the distance to the fault
may be readily obtained from the resistance per foot of the
conductor.
INDEX.
Abbott, A. V., 199, 367
Ader, Clement, 23, 36
Ahearn, T. F., 46
American transfer system, 334
Ampere, work of, i
Anchor for guy, 379
- pole, 377
Anders, George L., 236, 312, 318
Anders' step-by-step system, 312
Arago and Davy, work of, i
Arrester carbon, 273
combined carbon and heat
coil, 278
, Hayes, 275
, McBerty, 277
-, Rolfe, 280
Automatic shunt, 86
B
Barrett, Whittemore, and Craft, 330
Battery call instruments, circuits of,
101
charging, motor-generator, 116
Batteries, Fuller, 66
, gravity, 71
, Hayden, 64
, Le Clanche, 63
, primary, 62
, storage, 73
Bell, Alexander Graham, 6
Bell's early instruments, 7
magneto telephone, 7
Belt-driven magneto, 114
Berliner, Emile, n
Birmingham wire gauge, 350
Boiling out cable, 400
Bonding of cable sheaths, 422
Bourse ul, work of, 5
Braiding on cables, 393
Branch terminal multiple switch-
board, 204
Breaking strength, copper wire, 358
strength of wire, 347
Breaks in line, location of, 449
Bridging bell system, Carty, 297
telephone, circuits of, 99
Brown and Sharpe gauge, 350
Buell, Charles E., 243
Busy signal on party lines, 306
test on multiple switch-boards,
201, 203, 206, 260
Cable, boiling out of, 400
construction, underground, 409
hangers, 396
head, Cook, 403
head, interior, 408
head, Moon, 401
capacity of, 391
comparative cost, 389
drawing in, 419
dry-core, 392
lead covered, sizes of, 394
overhead, 389
rubber-covered, 389
rubber-covered, data concern-
ing, 39i
saturated-core, 392
sheaths of, 393
electrolysis on, 421
sizes of wire in, 393
splicing of, 399
stringing of, 397
terminals, pot-head, 405
testing with receiver, 425
Calling apparatus, 75
apparatus battery, 75
apparatus, commercial, 104
Cant-hook, 371
Capacity, 128
of telephone lines, 133
tests, 446
Carbon transmitter, 13
transmitter, action of, 32
Carry-hook, 369
Carty, J. J., 100, 136, 139, 241, 246,
297
bridging bell system, 297
Carty's experiments in line induction,
139
Cement arch conduit, 416
Charge, 128
Circuits of telephone, 90
Clamp for guy wire, 381
for messenger wire, 396
Clay conduits, 412
Clearing-out drop, 156
Climbers, Eastern, 384
, Western, 384
Colvin, F. R., 41
Come-along, 383
Common battery systems, 236
, Dean, 241, 249
, Hayes, 240, 254
453
454
INDEX,
Common battery systems, house, 267
multiple board, 257
repeating coil, 240
Scribner, 252
series, 237
Stone, 238
storage cell at subscriber's
station, 242
, Stromberg-Carlson, 263
Common return lines, induction
on,
return systems, 144
return wire, position of, 144
return, size of, 146
Concrete, 374
for conduit, 415
Condenser, capacity of, 129
Conduit, cement-arch, 416
, clay, 412
, concrete for, 415
, creosoted wood, 411
, Johnston distributing, 413
, mandrel for, 414
, mortar for, 416
, multiple duct, 412
, obstructions in laying, 418
, open box, 410
, pump log, 410
, requirements of, 409
, single du9t, clay, 414
, trench for, 414
, vitrified clay, 412
Constant of galvanometer, 441
Construction tools, 370
Continuity tests, 427
Cook, F. B., 88, 165
Cook-Beach transfer system, 232
Copper wire, 355
wire breaking strength of, 358
wire data concerning, 356
wire specifications, 357
Creosoted wood conduit, 411
Cresoting of poles, 364
Cross-arms, attachment to poles, 365
, sizes of, 364
, spacing of holes, 364
Cross- talk, 139
D
D'Arsonval galvanometer, 435
Dead-man, 372
Dean thermopile system, 245
Dean, W. W., 241, 243, 244, 249, 327
Desk set, 101
, circuits of, 102
Diaphragms, receiver, 29
Dickerson, E. N., 308
Dielectric, 129
Di gging bar, 370
Distributing boards, 281
, Ford & Lenfest, 286
, Hibbard, 283
Disturbances in telephone lines, 136
Drawing in of cables, 419
Drop, American, 162
and Jack combined, American,
179
and Jack combined, Western,
177
, Keystone, 162
, Warner, 160
, electrically self -restoring, 173
, mechanically self-restoring.
176
Dry-core cables, 392
Du Moncel, n
Dynamometer, 382
E
Edison, Thomas A., 13, 15, 119
Edison's transmitter, 13
Electrification, 445
Electrolysis. 421
Electromagnetic induction, 125
induction disturbances due to,
137
Electrom agnetism, 2
Electromotive force, active, 131
Electrolytic cell, 244
Electropoin fluid, 67
Electrostatic induction, 129
induction disturbances due to,
138
Erdman, A. W., 121
, Relay, 121
Ericsson, L. M., 42
Explosions in manholes, 421
Express system, 217
Faraday, 2
Faults, location of, 449
Fessenden, Professor R. A., 33
Field of force, 125
Ford and Lenfest, 286
distributing board, 286
Fuller cell, Standard, 66
Gaining template, 369
Gains in poles, 369
Galvanized steel strands, 395
Galvanizing of iron wire, 352
Galvanometer, 435
constant of, 441
shunt, 439
Gas in manholes, 421
Gauge, circular wire, 349
, micrometer, 349
Generator, Holtzer-Cabot, 105
, Western Telephone Construc-
tion Co., 104
, Williams, 108
, Williams- Abbott, 107
, constantly driven, 113
INDEX.
455
Gharky, W. D., 47
Gravity cell, 71
Gray, Elisha, 6, 339
Gray's transmitter, 10
Ground return systems, 144
Grounded to metallic circuits, connec-
tion of, 149
Grounds or crosses, location of, 450
Guy, anchor, 379 <
clamp, 381
Guying, head, 375
, side, 375
Guy, Y-, 378
H
Hampton, 217, 320-21
Hand barrow, 382
Hard rubber, imitation, 22
Harmonic signaling, 338
Hayes, H. V., 240, 255, 275
Head guys, 375
Henry, Joseph, i
Hibbard, Angus S., 281, 322
distributing board, 283
History of the telephone, i
Holtzer-Cabot house system, 269
Hook-switch, 90
, Warner, 94
House, Royal E., 8
House systems, 265
Hughes' microphone, 14
, David E., 13
Hunning, Henry, 15
I
Impedance, 127, 132
Incandescent lamps as signals, 192
Induction, electromagnetic, 125
Induction coil, action of, 16, 53
coil in base of transmitter arm
60
coil, commercial types, 58
coils, comparison of, 5
coils, selection of, 57
coils, Varley, 59
Iron wire, 352
wire, data concerning, 355
wire, grades of, 353
wire, specifications for, 354
Imitation hard rubber, 22
Insulation resistance, measurement
of, 442
Insulators, tests of, 367
J
Jack-strip, 210
Jacques, W. W., 50
Johnston, H. W., 413
Joint, Lillie, 386
, Mclntire, 386
, Western Union, 386
Jumper wires, 293
K
Kellogg, Milo G., 213
divided multiple switchboard,
214
Knudsen, A. M., 178
Lag screws, 365
Lamp signal controlled by relay, 195
directly in line circuit, 193
, life of, 199
switch-boards, 192
Leakage, disturbances due to, 137
Le Clanche batteries, 63
Lenfest and Ford, 286
Lightipe, J. A., 342
Lightning arrester, American, 273
arrester, ordinary form, 272
arrester, Western, 274
Lillie wire joint, 386
Line disturbances, remedy for, 140
Lines of force, 3
Listening and ringing apparatus, 163;
and ringing key, American,
167
and ringing key. Cook. 165
and ringing key, Western.
Telephone Construction Co., 169
key, American, 169
key plug socket, 172
Location of breaks in line, 449
of crosses in grounds, 450*
of faults, 449
Lockout systems on party lines, 301
Lockwood, Thomas D., 312
Loop test, Varley, 450
Low, George P., 227
M
Magneto generator, 75
generator, voltage of, 83
testing set, 424
testing set, errors from use of,
425
Magnets, permanent, 81
Make-and-break telephone, 5
Manholes, 419
explosions, 421
McBerty, F. R., 243, 277, 325
McCluer system, 146
McDonnell, J. L., 315
Mclntire sleeve joint, 386
Measurement of capacity, 446
of insulation resistance, 442
of resistance with Wheatstona
bridge, 431
Meissner ringing device, 171
Messenger wire, 394
wire clamp, 396
Micrometer gauge, 349
Mile-ohm, 348
45 6
INDEX.
Morse telegraph, 4
Mortar for conduit, 416
Motor-generator, 115
for battery charging, 116
Multiple duct conduits, 412
switchboard, 200
switchboard, branch terminal,
204
switchboard, series, 201
transmitter circuits, 246
N
Ness automatic switch, 269
O
O'Connell, J. J., 163, 192
O'Connell key, 163
Oersted, work of, i
Ohm's law, 124
Overhead cable construction, 389
Packing, 45
Party line, Anders' strength and
polarity, 318
, Barrett, Whittemore, and
Craft, 330
, bridged grounded, 297
, busy signal on, 306
, classification of, 294
-, Currier and Rice, 339
, Dean, 327
, Dickerson, 308
, Gray and Pope, 339
, harmonic selective signaling,
338
-, Harter, 344
, Hibbard, 322
, Lightipe, 342
, Lockwood, 312
, McBerty, 325
, non-selective, 294
, Reid and McDonnell, 313
, Sabin and Hampton, 320
, secrecy on, 301
, series grounded, 295
, step-by-step selective signal-
ing, 308
, strength and polarity, 318
, Wood, F. B., 315
Payne, George F., 47
Permanent magnets, 81
Permeability, 80
Phelps, George M., 13
Phonograph, busy test, 227
Pike-poles, 371
Pilot lamp, 198
Plug for branch terminal system,
212
for metallic circuit system, 158
listening device, 170
Polarized bell, 78
, construction of, 84
, Holtzer-Cabot, 106
, Williams, in
, Williams- Abbott, 108
Pole, anchor, 377
brace, 381
buck, 369
guards, 362
holes, depths of, 371
line construction, 360
line, route of, 368
top terminal, 403
tops, grading of, 368
Poles, creosoting of, 364
heights of, 362
life of, 360
number to the carload, 363
numbers to the mile, 361
sizes of, 360
vulcanizing of, 364
weight of, 363
woods for, 360
Pope, Frank L., 339
Pot-head terminals, 405
Power circuit and telephone line,
388
Power generators, 113
Preece and Stubbs, 55
Protection of cable sheaths from
electrolysis, 44
Protective devices, 272
Pump-log conduits, 410
R
Raising poles, 372
Receiver, Ader, 23
, American Electric Telephone
Co., 27
, Bell double-pole, 25
, Bell single-pole, 19
cords, 30
diaphragm, 29
, efficiency of, 18
, Ericsson, 28
, faults in, 22
, Holtzer-Cabot Electric Co., 27
, Stromberg-Carlson, 25
, Western Telephone Construc-
tion Co., 24
Reid, R. T., 315
Reis, Philip, 5
telephone instruments, 5
Relay circuits, simple, 118
circuits, two-way, 113
, Erdman, 121
lamp system, 195
, Stone, 122
telephone, 118
Repeater telephone, 118
Repeating coils, 149
Resistance measurement with
Wheatstone bridge, 431
INDEX.
457
Ringer, 78
Rodding, 419
Rolfe, C. A., 280
Rough tests, 424
Route of pole line, 368
Sabin, John I., 217, 315, 320
Saturated core cables, 392
Scribner, Charles E., 195, 198, 243,
252, 257, 302
Secrecy on party lines, 301
Self-induction, 124
, effect of, 126
Self-restoring switch-board drops, 1 73
Series, multiple switch-board, 201
telephone, circuits of, 96
transmitter circuit, 248
Sewall cement-arch conduit, 416
Shovels, 370
Shunt, automatic, 86
, centrifugal, 87
, Cook, 88
for galvanometers, 439
, Holtzer-Cabot, 88
, Western Electric, 86
, Western Telephone Construc-
tion Co., 87
, Williams, 89
Side guys, 375
Sound waves, 6
Specifications for copper wire, 357
for iron wire, 354
Specific inductive capacity, 129
Splicing of cable, 399
Spring jack, 153
, American, 158
, Keystone, 158
, metallic circuit, 157
for multiple switch-boards, 210
Steam power for drawing in cables,
420
Steel strands, 395
Step-by-step mechanisms, 308
Sterling Company, cable terminals,
408
Electric Company's switch-
board, 187
St. Louis Bell exchange, 260
Stone, John S., 122, 238, 243
relay, 122
Storage batteries, care of, 74
Strains on pole lines, 375
Strength and polarity signaling on
party lines, 318
Stringing of wires, 382
Sturgeon, William, i
Supporting strand for aerial cable,
^ 395
Sutton transmitter, 42
Switch-board drop, 154
for grounded lines, circuits of,
Switch-board, metallic circuit, 159
for small exchanges, 153, 183
Tamping bar, 372
Telephone lines, 136
Telescope for galvanometer, 438
Tensile strength of wire, 347
Tension in wires, 383
Terminal pole, 376
t pole top, 403
Testing, 424
set, magneto, 424
Tests for capacity, 446
for continuity, 427
for crosses and grounds, 425
for electrolysis, 421
with receiver, 425
Thermal arrester, Hayes, 275
, McBerty, 277
Thermopile system, Dean, 245
Thomson galvanometer, 435
Tie, Kelvin, 384
, latest method, 385
, ordinary, 384
Tin in cable sheaths, 393
Transfer systems, 216
, Western Telephone Construc-
tion Co., 227
Transpositions, 143
, method of making, 387
Trench for conduits, 414
Transmitter, Ader, 36
, Ahearn, 46
, Berliner's, n
, Universal, 37
, Blake, 34
, Carbon, 13
, Clamond, 37
, Colvin, 41
, Crossley, 35
, D'Arsonval, 36
, Ericsson, 42
, Gower, 36
, Runnings', 15
, Jacques, 50
, Johnson, 36
, multiple-electrode, 36
, Payne and Gharky, 47
, " solid back," 38
, Sutton, 41
, Turnbull, 36
, variable resistance, 10
, Western Telephone Construc-
tion Co., 43
White,
, 38
Tubular drops, 160
U
Underground cable construction,
409
INDEX.
Variable resistance transmitter, 10
Varley coils, 112
loop test, 450
Viele, F. S., 398
Vitrified clay conduit, 412
Vulcanizing of poles, 364
W
Warner, 94, 160
drop, 1 60
hook-switch, 94
Waves of sound, 6
Weight per mile-ohm, 348
Western switch-board, 183
- Union wire joint, 386
Wheatstone bridge, 428
White, A. C., 38, 237
Williams, J. A., 89, 108, in
Wilmington, Del., switch-board, 232 Y-Guy, 378
Wire, breaking strength of, 347
, conductivity of, 347
, copper, 355
, copper, specifications, 357
for guying, 381
for telephone use, 347
gauges, 348
gauge, Birmingham, 350
\ S au S e Brown and Sharpe.
350
gauges, table of, 351
iron, 352
iron, grades of, 353
iron, specifications for, 354
resistance of, 348
tensile strength of, 347
tension in, 38 -
Wood, F. B., 315
THE END.
YC 19357
THE UNIVERSITY OF CALIFORNIA LIBRARY