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[Illustration: THOMAS A. EDISON Pioneer Electrical Investigator and
Inventor of Numerous Telegraph, Telephone, Lighting, and Other
Electrical Devices.]
Cyclopedia
of
Telephony and Telegraphy
_A General Reference Work on_
TELEPHONY, SUBSTATIONS, PARTY LINE SYSTEMS, PROTECTION, MANUAL
SWITCHBOARDS, AUTOMATIC SYSTEMS, POWER PLANTS, SPECIAL
SERVICE FEATURES, CONSTRUCTION, ENGINEERING,
OPERATION, MAINTENANCE, TELEGRAPHY, WIRELESS
TELEGRAPHY AND TELEPHONY, ETC.
_Prepared by a Corps of_
TELEPHONE AND TELEGRAPH EXPERTS, AND ELECTRICAL ENGINEERS OF
THE HIGHEST PROFESSIONAL STANDING
_Illustrated with over Two Thousand Engravings_
FOUR VOLUMES
CHICAGO
AMERICAN SCHOOL OF CORRESPONDENCE
1919
COPYRIGHT, 1911, 1912,
BY
AMERICAN SCHOOL OF CORRESPONDENCE
COPYRIGHT, 1911, 1912
BY
AMERICAN TECHNICAL SOCIETY
Entered at Stationers' Hall, London
All Rights Reserved
Authors and Collaborators
* * * * *
KEMPSTER B. MILLER, M.E.
Consulting Engineer and Telephone Expert Of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers
* * * * *
GEORGE W. PATTERSON, S.B., Ph.D.
Head, Department of Electrical Engineering, University of Michigan
* * * * *
CHARLES THOM
Chief of Quadruplex Department, Western Union Main Office, New York City
* * * * *
ROBERT ANDREWS MILLIKAN, Ph.D.
Associate Professor of Physics, University of Chicago
Member, Executive Council, American Physical Society
* * * * *
SAMUEL G. McMEEN
Consulting Engineer and Telephone Expert Of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers
* * * * *
LAWRENCE K. SAGER, S.B., M.P.L.
Patent Attorney and Electrical Expert
Formerly Assistant Examiner, U.S. Patent Office
* * * * *
GLENN M. HOBBS, Ph.D.
Secretary, American School of Correspondence
Formerly Instructor in Physics, University of Chicago
American Physical Society
* * * * *
CHARLES G. ASHLEY
Electrical Engineer and Expert in Wireless Telegraphy and Telephony
* * * * *
A. FREDERICK COLLINS
Editor, _Collins Wireless Bulletin_
Author of "Wireless Telegraphy, Its History, Theory, and Practice"
* * * * *
FRANCIS B. CROCKER, E.M., Ph.D.
Head, Department of Electrical Engineering, Columbia University
Past-President, American Institute of Electrical Engineers
* * * * *
MORTON ARENDT, E.E.
Instructor in Electrical Engineering, Columbia University, New York
* * * * *
EDWARD B. WAITE
Head, Instruction Department, American School of Correspondence
American Society of Mechanical Engineers
Western Society of Engineers
* * * * *
DAVID P. MORETON, B.S., E.E.
Associate Professor of Electrical Engineering, Armour Institute of
Technology
American Institute of Electrical Engineers
* * * * *
LEIGH S. KEITH, B.S.
Managing Engineer with McMeen and Miller, Electrical Engineers and
Patent Experts Chicago
Associate Member, American Institute of Electrical Engineers
* * * * *
JESSIE M. SHEPHERD, A.B.
Associate Editor, Textbook Department, American School of Correspondence
* * * * *
ERNEST L. WALLACE, B.S.
Assistant Examiner, United States Patent Office, Washington, D. C.
* * * * *
GEORGE R. METCALFE, M.E.
Editor, _American Institute of Electrical Engineers_
Formerly Head of Publication Department, Westinghouse Elec. & Mfg. Co.
* * * * *
J. P. SCHROETER
Graduate, Munich Technical School
Instructor in Electrical Engineering, American School of Correspondence
* * * * *
JAMES DIXON, E.E.
American Institute of Electrical Engineers
* * * * *
HARRIS C. TROW, S.B., _Managing Editor_
Editor-in-Chief, Textbook Department, American School of Correspondence
Authorities Consulted
The editors have freely consulted the standard technical literature of
America and Europe in the preparation of these volumes. They desire to
express their indebtedness particularly to the following eminent
authorities, whose well-known works should be in the library of every
telephone and telegraph engineer.
Grateful acknowledgment is here made also for the invaluable
co-operation of the foremost engineering firms and manufacturers in
making these volumes thoroughly representative of the very best and
latest practice in the transmission of intelligence, also for the
valuable drawings, data, suggestions, criticisms, and other courtesies.
* * * * *
ARTHUR E. KENNELY, D.Sc.
Professor of Electrical Engineering, Harvard University.
Joint Author of "The Electric Telephone," "The Electric Telegraph,"
"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
Motors," "Electric Railways," "Incandescent Lighting," etc.
* * * * *
HENRY SMITH CARHART, A.M., LL.D.
Professor of Physics and Director of the Physical Laboratory, University
of Michigan.
Author of "Primary Batteries," "Elements of Physics," "University
Physics," "Electrical Measurements," "High School Physics," etc.
* * * * *
FRANCIS B. CROCKER, M.E., Ph.D.
Head of Department of Electrical Engineering, Columbia University, New
York; Past-President, American Institute of Electrical Engineers.
Author of "Electric Lighting;" Joint Author of "Management of Electrical
Machinery."
* * * * *
HORATIO A. FOSTER
Consulting Engineer; Member of American Institute of Electrical
Engineers; Member of American Society of Mechanical Engineers.
Author of "Electrical Engineer's Pocket-Book."
* * * * *
WILLIAM S. FRANKLIN, M.S., D.Sc.
Professor of Physics, Lehigh University.
Joint Author of "The Elements of Electrical Engineering," "The Elements
of Alternating Currents."
* * * * *
LAMAR LYNDON, B.E., M.E.
Consulting Electrical Engineer; Associate Member of American Institute
of Electrical Engineers; Member, American Electro-Chemical Society.
Author of "Storage Battery Engineering."
* * * * *
ROBERT ANDREWS MILLIKAN, Ph.D.
Professor of Physics, University of Chicago.
Joint Author of "A First Course in Physics," "Electricity, Sound and
Light," etc.
* * * * *
KEMPSTER B. MILLER, M.E.
Consulting Engineer and Telephone Expert; of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago.
Author of "American Telephone Practice."
* * * * *
WILLIAM H. PREECE
Chief of the British Postal Telegraph.
Joint Author of "Telegraphy," "A Manual of Telephony," etc.
* * * * *
LOUIS BELL, Ph.D.
Consulting Electrical Engineer; Lecturer on Power Transmission,
Massachusetts Institute of Technology.
Author of "Electric Power Transmission," "Power Distribution for
Electric Railways," "The Art of Illumination," "Wireless Telephony,"
etc.
* * * * *
OLIVER HEAVISIDE, F.R.S.
Author of "Electro-Magnetic Theory," "Electrical Papers," etc.
* * * * *
SILVANUS P. THOMPSON, D.Sc., B.A., F.R.S., F.R.A.S.
Principal and Professor of Physics in the City and Guilds of London
Technical College.
Author of "Electricity and Magnetism," "Dynamo-Electric Machinery,"
"Polyphase Electric Currents and Alternate-Current Motors," "The
Electromagnet," etc.
* * * * *
ANDREW GRAY, M.A., F.R.S.E.
Author of "Absolute Measurements in Electricity and Magnetism."
* * * * *
ALBERT CUSHING CREHORE, A.B., Ph.D.
Electrical Engineer; Assistant Professor of Physics, Dartmouth College;
Formerly Instructor in Physics, Cornell University.
Author of "Synchronous and Other Multiple Telegraphs;" Joint Author of
"Alternating Currents."
* * * * *
J. J. THOMSON, D.Sc., LL.D., Ph.D., F.R.S.
Fellow of Trinity College, Cambridge University; Cavendish Professor of
Experimental Physics, Cambridge University.
Author of "The Conduction of Electricity through Gases," "Electricity
and Matter."
* * * * *
FREDERICK BEDELL, Ph.D.
Professor of Applied Electricity, Cornell University.
Author of "The Principles of the Transformer;" Joint Author of
"Alternating Currents."
* * * * *
DUGALD C. JACKSON, C.E.
Head of Department of Electrical Engineering, Massachusetts Institute of
Technology; Member, American Institute of Electrical Engineers, etc.
Author of "A Textbook on Electromagnetism and the Construction of
Dynamos;" Joint Author of "Alternating Currents and Alternating-Current
Machinery."
* * * * *
MICHAEL IDVORSKY PUPIN, A.B., Sc.D., Ph.D.
Professor of Electro-Mechanics, Columbia University, New York.
Author of "Propagation of Long Electric Waves," and "Wave-Transmission
over Non-Uniform Cables and Long-Distance Air Lines."
* * * * *
FRANK BALDWIN JEWETT, A.B., Ph.D.
Transmission and Protection Engineer, with American Telephone &
Telegraph Co.
Author of "Modern Telephone Cable," "Effect of Pressure on Insulation
Resistance."
* * * * *
ARTHUR CROTCH
Formerly Lecturer on Telegraphy and Telephony at the Municipal Technical
Schools, Norwich, Eng.
Author of "Telegraphy and Telephony."
* * * * *
JAMES ERSKINE-MURRAY, D.Sc.
Fellow of the Royal Society of Edinburgh; Member of the Institution of
Electrical Engineers.
Author of "A Handbook of Wireless Telegraphy."
* * * * *
A. H. McMILLAN, A.B., LL.B.
Author of "Telephone Law, A Manual on the Organization and Operation of
Telephone Companies."
* * * * *
WILLIAM ESTY, S.B., M.A.
Head of Department of Electrical Engineering, Lehigh University.
Joint Author of "The Elements of Electrical Engineering."
* * * * *
GEORGE W. WILDER, Ph.D.
Formerly Professor of Telephone Engineering, Armour Institute of
Technology.
Author of "Telephone Principles and Practice," "Simultaneous Telegraphy
and Telephony," etc.
* * * * *
WILLIAM L. HOOPER, Ph.D.
Head of Department of Electrical Engineering, Tufts College.
Joint Author of "Electrical Problems for Engineering Students."
* * * * *
DAVID S. HULFISH
Technical Editor, _The Nickelodeon_; Telephone and Motion-Picture
Expert; Solicitor of Patents.
Author of "How to Read Telephone Circuit Diagrams."
* * * * *
J. A. FLEMING, M.A., D.Sc. (Lond.), F.R.S.
Professor of Electrical Engineering in University College, London; Late
Fellow and Scholar of St. John's College, Cambridge; Fellow of
University College, London.
Author of "The Alternate-Current Transformer," "Radiotelegraphy and
Radiotelephony," "Principles of Electric Wave Telegraphy," "Cantor
Lectures on Electrical Oscillations and Electric Waves," "Hertzian Wave
Wireless Telegraphy," etc.
* * * * *
F. A. C. PERRINE, A.M., D.Sc.
Consulting Engineer; Formerly President, Stanley Electric Manufacturing
Company; Formerly Professor of Electrical Engineering, Leland Stanford,
Jr. University.
Author of "Conductors for Electrical Distribution."
* * * * *
A. FREDERICK COLLINS
Editor, _College Wireless Bulletin_.
Author of "Wireless Telegraphy, Its History, Theory and Practice,"
"Manual of Wireless Telegraphy," "Design and Construction of Induction
Coils," etc.
* * * * *
SCHUYLER S. WHEELER, D.Sc.
President, Crocker-Wheeler Co.; Past-President, American Institute of
Electrical Engineers.
Joint Author of "Management of Electrical Machinery."
* * * * *
CHARLES PROTEUS STEINMETZ
Consulting Engineer, with the General Electric Co.; Professor of
Electrical Engineering, Union College.
Author of "The Theory and Calculation of Alternating-Current Phenomena,"
"Theoretical Elements of Electrical Engineering," etc.
* * * * *
GEORGE W. PATTERSON, S.B., Ph.D.
Head of Department of Electrical Engineering, University of Michigan.
Joint Author of "Electrical Measurements."
* * * * *
WILLIAM MAVER, Jr.
Ex-Electrician Baltimore and Ohio Telegraph Company; Member of the
American Institute of Electrical Engineers.
Author of "American Telegraphy and Encyclopedia of the Telegraph,"
"Wireless Telegraphy."
* * * * *
JOHN PRICE JACKSON, M.E.
Professor of Electrical Engineering, Pennsylvania State College.
Joint Author of "Alternating Currents and Alternating-Current
Machinery."
* * * * *
AUGUSTUS TREADWELL, Jr., E.E.
Associate Member, American Institute of Electrical Engineers.
Author of "The Storage Battery, A Practical Treatise on Secondary
Batteries."
* * * * *
EDWIN J. HOUSTON, Ph.D.
Professor of Physics, Franklin Institute, Pennsylvania; Joint Inventor
of Thomson-Houston System of Arc Lighting; Electrical Expert and
Consulting Engineer.
Joint Author of "The Electric Telephone," "The Electric Telegraph,"
"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
Motors," "Electric Railways," "Incandescent Lighting," etc.
* * * * *
WILLIAM J. HOPKINS
Professor of Physics in the Drexel Institute of Art, Science, and
Industry, Philadelphia.
Author of "Telephone Lines and their Properties."
[Illustration: GROSSE POINT EXCHANGE RACK Detroit Home Telephone
Company, Detroit, Mich. _The Dean Electric Co._]
[Illustration: LINE SIDE OF LARGE MAIN DISTRIBUTING FRAME]
Foreword
The present day development of the "talking wire" has annihilated both
time and space, and has enabled men thousands of miles apart to get into
almost instant communication. The user of the telephone and the
telegraph forgets the tremendousness of the feat in the simplicity of
its accomplishment; but the man who has made the feat possible knows
that its very simplicity is due to the complexity of the principles and
appliances involved; and he realizes his need of a practical, working
understanding of each principle and its application. The Cyclopedia of
Telephony and Telegraphy presents a comprehensive and authoritative
treatment of the whole art of the electrical transmission of
intelligence.
The communication engineer--if so he may be called--requires a knowledge
both of the mechanism of his instruments and of the vagaries of the
current that makes them talk. He requires as well a knowledge of plants
and buildings, of office equipment, of poles and wires and conduits, of
office system and time-saving methods, for the transmission of
intelligence is a business as well as an art. And to each of these
subjects, and to all others pertinent, the Cyclopedia gives proper space
and treatment.
The sections on Telephony cover the installation, maintenance, and
operation of all standard types of telephone systems; they present
without prejudice the respective merits of manual and automatic
exchanges; and they give special attention to the prevention and
handling of operating "troubles." The sections on Telegraphy cover both
commercial service and train dispatching. Practical methods of wireless
communication--both by telephone and by telegraph--are thoroughly
treated.
The drawings, diagrams, and photographs incorporated into the Cyclopedia
have been prepared especially for this work; and their instructive value
is as great as that of the text itself. They have been used to
illustrate and illuminate the text, and not as a medium around which to
build the text. Both drawings and diagrams have been simplified so far
as is compatible with their correctness, with the result that they tell
their own story and always in the same language.
The Cyclopedia is a compilation of many of the most valuable Instruction
Papers of the American School of Correspondence, and the method adopted
in its preparation is that which this School has developed and employed
so successfully for many years. This method is not an experiment, but
has stood the severest of all tests--that of practical use--which has
demonstrated it to be the best yet devised for the education of the
busy, practical man.
In conclusion, grateful acknowledgment is due to the staff of authors
and collaborators, without whose hearty co-operation this work would
have been impossible.
Table of Contents
VOLUME II
MANUAL SWITCHBOARDS _By K. B. Miller and S. G. McMeen_[A] Page[B] 11
Common-Battery Switchboards--Line Signals--Cord
Circuit--Lamps--Mechanical Signals--Relays--Jacks--Switchboard
Assembly--Transfer Switchboard--Transfer Lines--Handling
Transfers--Multiple Switchboard--Busy Test--Influence of
Traffic--Magneto-Multiple Switchboard--Multiple Boards: Series,
Branch-Terminal, Modern Magneto, Common-Battery--Western Electric No. 1
Relay Board--Western Electric No. 10 Board--Types of Multiple
Boards--Apparatus--Trunking--Western Electric and Kellogg Trunk Circuits
AUTOMATIC SYSTEMS _By K. B. Miller and S. G. McMeen_ Page 135
Automatic vs. Manual--Operation--Selecting Switch--Line Switch--Trunking
Systems--Two- and Three-Wire Systems--Subscriber's Station
Apparatus--First and Second Selector Operation--Connector--Release after
Conversation--Multi-Office System--Automatic Sub-Offices--Rotary
Connector--Party Lines--Two-Wire Automatic System--Lorimer
System--Central-Office Apparatus--Operation--Automanual
System--Operation--Subscriber's Apparatus--Operator's
Equipment--Switching Equipment--Distribution of Calls--Connection--Speed
POWER PLANTS AND BUILDINGS _By K. B. Miller and S. G. McMeen_ Page 227
Currents Employed--Types--Operator's Transmitter Supply--Ringing-Current
Supply--Auxiliary Signaling Current--Primary Sources--Duplicate
Apparatus--Storage Batteries--Power
Switchboards--Circuits--Central-Office Building--Arrangement of
Apparatus--Manual Offices--Automatic Offices
SPECIAL SERVICE FEATURES _By K. B. Miller and S. G. McMeen_ Page 271
Private-Branch Exchanges--Switchboards--Supervision--With Automatic
Offices--Battery Supply--Ringing Current--Inter-Communicating
Systems--Magneto System--Common-Battery Systems--Types--Long-Distance
Switching--Operator's Orders--Trunking--Way Stations--Traffic--Measured
Service--Charging--Rates--Toll Service--Local Service
TELEGRAPH AND RAILWAY WORK _By K. B. Miller and S. G. McMeen_ Page 321
Phantom, Simplex, and Composite Circuits--Ringing--Railway
Composite--Telephone Train Dispatching--Railroad
Conditions--Transmitting Orders--Apparatus--Telephone Equipment--Types
of Circuits--Test Boards--Blocking Sets--Dispatching on Electric
Railways
REVIEW QUESTIONS Page 359
INDEX Page 373
[Footnote A: For professional standing of authors, see list of Authors
and Collaborators at front of volume.]
[Footnote B: For page numbers, see foot of pages.]
[Illustration: PORTION OF TERMINAL ROOM OF LARGE COMMON-BATTERY OFFICE
Prospect Office, New York Telephone Co.]
CHAPTER XXII
THE SIMPLE COMMON-BATTERY SWITCHBOARD
=Advantages of Common-Battery Operation.= The advantages of the
common-battery system of operation, alluded to in Chapter XIII, may be
briefly summarized here. The main gain in the common-battery system of
supply is the simplification of the subscribers' instruments, doing away
with the local batteries and the magneto generators, and the
concentration of all these many sources of current into one single
source at the central office. A considerable saving is thus effected
from the standpoint of maintenance, since the simpler common-battery
instrument is not so likely to get out of order and, therefore, does not
have to be visited so often for repairs, and the absence of local
batteries, of course, makes the renewal of the battery parts by members
of the maintenance department, unnecessary. Another decided advantage in
the common-battery system is the fact that the centralized battery
stands ready always to send current over the line when the subscriber
completes the circuit of the line at his station by removing his
receiver from its hook. The common-battery system, therefore, lends
itself naturally to the purposes of automatic signaling, since it is
only necessary to place at the central office a device in the circuit of
each line that will be responsive to the current which flows from the
central battery when the subscriber removes his receiver from its hook.
It is thus that the subscriber is enabled automatically to signal the
central office when he desires a connection; and as will be shown, it is
by the same sort of means, associated with the cord circuits used in
connecting his line with some other line, that the operator is
automatically notified when a disconnection is desired, the cessation of
current through the subscriber's line when he hangs up his receiver
being made to actuate certain responsive devices which are associated
with the cord at that time connected with his line, and which convey the
proper disconnect signal to the operator.
Concentration of sources of energy into a single large unit, the
simplification of the subscriber's station equipment, and the ready
adaptability to automatic signaling from the subscriber to the central
office are, therefore, the reasons for the existence of the
common-battery system.
=Common Battery vs. Magneto.= It must not be supposed, however, that the
common-battery system always has advantages over the magneto system, and
that it is superior to the magneto or local-battery system for all
purposes. It is the outward attractiveness of the common-battery system
and the arguments in its favor, so readily made by over-zealous
salesmen, that has led, in many cases, to the adoption of this system
when the magneto system would better have served the purpose of utility
and economy.
To say the least, the telephone transmission to be had from
common-battery systems is no better than that to be had from
local-battery systems, and as a rule, assuming equality in other
respects, it is not as good. It is perhaps true, however, that under
average conditions common-battery transmission is somewhat better,
because whereas the local batteries at the subscribers' stations in the
local-battery system are not likely to be in uniformly first-class
condition, the battery in a common-battery system will be kept up to its
full voltage except under the grossest neglect.
The places in which the magneto, or local-battery, system is to be
preferred to the common-battery system, in the opinion of the writers,
are to be found in the small rural communities where the lines have a
rather great average length; where a good many subscribers are likely to
be found on some of the lines; where the sources of electrical power
available for charging storage batteries are likely either not to exist,
or to be of a very uncertain nature; and where it is not commercially
feasible to employ a high-grade class of attendants, or, in fact, any
attendant at all other than the operator at the central office.
In large or medium-sized exchanges it is always possible to procure
suitable current for charging the storage batteries required in
common-battery systems, and it is frequently economical, on account of
the considerable quantity of energy that is thus used, to establish a
generating plant in connection with the central office for developing
the necessary electrical energy. In very small rural places there are
frequently no available sources of electrical energy, and the expense of
establishing a power plant for the purpose cannot be justified. But even
if there is an electric light or railway system in the small town, so
that the problem of available current supply does not exist, the
establishment of a common-battery system with its storage battery and
the necessary charging machinery requires the daily attendance at the
central office of some one to watch and care for this battery, and this,
on account of the small gross revenue that may be derived from a small
telephone system, often involves a serious financial burden.
There is no royal road to a proper decision in the matter, and no sharp
line of demarcation may be drawn between the places where common-battery
systems are superior to magneto and _vice versa_. It may be said,
however, that in the building of all new telephone plants having over
about 500 local subscribers, the common-battery system is undoubtedly
superior to the magneto. If the plant is an old one, however, and is to
be re-equipped, the continuance of magneto apparatus might be justified
for considerably larger exchanges than those having 500 subscribers.
Telephone operating companies who have changed over the equipment of old
plants from magneto to common battery have sometimes been led into
rather serious difficulty, owing to the fact that their lines, while
serving tolerably well for magneto work, were found inadequate to meet
the more exacting demands of common-battery work. Again in an old plant
the change from magneto to common-battery equipment involves not only
the change of switchboards, but also the change of subscribers'
instruments that are otherwise good, and this consideration alone often,
in our opinion, justifies the replacing of an old magneto board with a
new magneto board, even if the exchange is of such size as to demand a
small multiple board.
Where the plant to be established is of such size as to leave doubt as
to whether a magneto or a common-battery switchboard should be employed,
the questions of availability of the proper kind of power for charging
the batteries, the proper kind of help for maintaining the batteries and
the more elaborate central-office equipment, the demands and previous
education of the public to be served, all are factors which must be
considered in reaching the decision.
It is not proper to say that anything like all exchanges having fewer
than 500 local lines, should be equipped with magneto service. Where all
the lines are short, where suitable power is available, and where a good
grade of attendants is available--as, for instance, in the case of
private telephone exchanges that serve some business establishment or
other institution located in one building or a group of buildings--the
common-battery system is to be recommended and is largely used, even
though it may have but a dozen or so subscribers' lines. It is for such
uses, and for use in those regular public-service exchange systems where
the conditions are such as to warrant the common-battery system, and yet
where the number of lines and the traffic are small enough to be handled
by such a small group of operators that any one of them may reach over
the entire face of the board, that the simple non-multiple
common-battery system finds its proper field of usefulness.
=Line Signals.= The principles and means by which the subscriber is
enabled to call the central-office operator in a common-battery system
have been referred to briefly in Chapter III. We will review these at
this point and also consider briefly the way in which the line signals
are associated with the connective devices in the subscribers' lines.
_Direct-Line Lamp._ The simplest possible way is to put the line signal
directly in the circuit of the line in series with the central-office
battery, and so to arrange the jack of the corresponding line that the
circuit through the line signal will be open when the operator inserts a
plug into that jack. This arrangement is shown in Fig. 307 where the
subscriber's station at the left is indicated in the simplest of its
forms. It is well to repeat here that in all common-battery manual
systems, the subscriber's station equipment, regardless of the
arrangement or type of its talking and signaling apparatus, must have
these features: First, that the line shall be normally open to direct
currents at the subscriber's station; second, that the line shall be
closed to direct currents when the subscriber removes his receiver from
its hook in making or in answering a call; third, that the line
normally, although open to direct currents, shall afford a proper path
for alternating or varying currents through the signal receiving device
at the sub-station. The subscriber's station arrangement shown in Fig.
307, and those immediately following, is the simplest arrangement that
possesses these three necessary features for common-battery service.
[Illustration: Fig. 307. Direct-Line Lamp]
Considering the arrangement at the central office, Fig. 307, the two
limbs of the line are permanently connected to the tip and sleeve
contacts of the jack. These two main contacts of the jack normally
engage two anvils so connected that the tip of the jack is ordinarily
connected through its anvil to ground, while the sleeve of the jack is
normally connected through its anvil to a circuit leading through the
line signal--in this case a lamp--and the common battery, and thence to
ground. The operation is obvious. Normally no current may flow from the
common battery through the signal because the line is open at the
subscriber's station. The removal of the subscriber's receiver from its
hook closes the circuit of the line and allows the current to flow
through the lamp, causing it to glow. When the operator inserts the plug
into the jack, in response to the call, the circuit through the lamp is
cut off at the jack and the lamp goes out.
This arrangement, termed the direct-line lamp arrangement, is largely
used in small common-battery telephone systems where the lines are very
short, such as those found in factories or other places where the
confines of the exchange are those of a building or a group of
neighboring buildings. Many of the so-called private-branch exchanges,
which will be considered more in detail in a later chapter, employ this
direct-line lamp arrangement.
[Illustration: Fig. 308. Direct-Line Lamp with Ballast]
_Direct-Line Lamp with Ballast._ Obviously, however, this direct-line
lamp arrangement is not a good one where the lines vary widely in length
and resistance. An incandescent lamp, as is well known, must not be
subjected to too great a variation in current. If the current that is
just right in amount to bring it to its intended degree of illumination
is increased by a comparatively small amount, the life of the lamp will
be greatly shortened, and too great an increase will result in the
lamp's burning out immediately. On the other hand, a current that is too
small will not result in the proper illumination of the lamp, and a
current of one-half the proper normal value will just suffice to bring
the lamp to a dull red glow. With lines that are not approximately
uniform in length and resistance the shorter lines would afford too
great a flow of current to the lamps and the longer lines too little,
and there is always the danger present, unless means are taken to
prevent it, that if a line becomes short-circuited or grounded near the
central office, the lamp will be subjected to practically the full
battery potential and, therefore, to such a current as will burn it out.
One of the very ingenious and, we believe, promising methods that has
been proposed to overcome this difficulty is that of the iron-wire
ballast, alluded to in Chapter III. This, it will be remembered,
consists of an iron-wire resistance enclosed in a vacuum chamber and so
proportioned with respect to the flow of current that it will be
subjected to a considerable heating effect by the amount of current that
is proper to illuminate the lamp. As has already been pointed out,
carbon has a negative temperature coefficient, that is, its resistance
decreases when heated. Iron, on the other hand, has a positive
temperature coefficient, its resistance increasing when heated. When
such an iron-wire ballast is put in series with the incandescent lamp
forming the line signal, as shown in Fig. 308, it is seen that the
resistance of the carbon in the lamp filament and of the iron in the
ballast will act in opposite ways when the current increases or
decreases. An increase of current will tend to heat up the iron wire of
the ballast and, therefore, increase its resistance, and the ballast is
so proportioned that it will hold the current that may flow through the
lamp within the proper maximum and minimum limits, regardless of the
resistance of the line in which the lamp is used. This arrangement has
not gone into wide use up to the present time.
_Line Lamp with Relay._ By far the most common method of associating the
line lamp with the line is to employ a relay, of which the actuating
coil is in the line circuit, this relay serving to control a local
circuit containing the battery and the lamp. This arrangement and the
way in which these parts are associated with the jack are clearly
indicated in Fig. 309. Here the relay may receive any amount of current,
from the smallest which will cause it to pull up its armature, to the
largest which will not injure its winding by overheat. Relays may be
made which will attract their armatures at a certain minimum current and
which will not burn out when energized by currents about ten times as
large, and it is thus seen that a very large range of current through
the relay winding is permissible, and that, therefore, a very great
latitude as to line resistance is secured. On the other hand, it is
obvious that the lamp circuit, being entirely local, is of uniform
resistance, the lamp always being subjected, in the arrangement shown,
to practically the full battery potential, the lamp being selected to
operate on that potential.
[Illustration: Fig. 309. Line Lamp with Relay]
_Pilot Signals._ In the circuits of Figs. 307, 308, and 309, but a
single line and its associated apparatus is shown, and it may not be
altogether clear to the uninitiated how it is that the battery shown in
those figures may serve, without interference of any function, a larger
number of lines than one. It is to be remembered that this battery is
the one which serves not only to operate the line signals, but also to
supply talking current to the subscribers and to supply current for the
operation of the cord-circuit signals after the cord circuits are
connected with the lines.
In Fig. 310 this matter is made clear with respect to the association of
this common battery with the lines for operating the line signals, and
also another important feature of common-battery work is brought out,
viz, the pilot lamp and its association with a group of line lamps.
Three subscribers' lines only are shown, but this serves clearly to
illustrate the association of any larger number of lines with the common
battery. Ignoring at first the pilot relay and the pilot lamp, it will
be seen that each of the tip-spring anvils of the jacks is connected to
a common wire _1_ which is grounded. Each of the sleeve-contact anvils
is connected through the coil of the line relay to another common wire
_2_, which connects with the live side of the common battery. Obviously,
therefore, this arrangement corresponds with that of Fig. 309, since the
battery may furnish current to energize any one of the line relays upon
the closure of the circuit of the corresponding line. Each of the relay
armatures in Fig. 310 is connected to ground.
Here we wish to bring out an important thing about telephone circuit
diagrams which is sometimes confusing to the beginner, but which really,
when understood, tends to prevent confusion. The showing of a separate
ground for each of the line-relay armatures does not mean that literally
each one of these armatures is connected by a separate wire to earth,
and it is to be understood that the three separate grounds shown in
connection with these relay armatures is meant to indicate just such a
set of affairs as is shown in connection with the tip-spring anvils of
the jacks, all of which are connected to a common wire which, in turn,
is grounded. Obviously, the result is the same, but in the case of this
particular diagram it is seen that a great deal of crossing of lines is
prevented by showing a separate ground at each one of the relay
armatures. The same practice is followed in connection with the common
battery. Sometimes it is very inconvenient in a complicated diagram to
run all of the wires that are supposed to connect with one terminal of
the battery across the diagram to represent this connection. It is
permissible, therefore, and in fact desirable, that separate battery
symbols be shown wherever by so doing the diagram will be simplified,
the understanding being, in the absence of other information or of other
indications, that the same battery is referred to, just as the same
ground is referred to in connection with the relay armatures in the
figure under discussion.
Each line lamp in Fig. 310 is shown connected on one hand to its
corresponding line relay contact and on the other hand to a common wire
which leads through the winding of the pilot relay to the live side of
the battery. It is obvious here that whenever any one of the line relays
attracts its armature the local circuit containing the corresponding
lamp and the common battery will be closed and the lamp illuminated.
Whenever any line relay operates, the current, which is supplied to its
lamp, must come through the pilot-relay winding, and if a number of line
relays are energized, then the current flow of the corresponding lamps
must flow through this relay winding. Therefore, this relay winding must
be of low resistance, so that the drop through its winding may not be
sufficient to interfere with the proper burning of the lamps, even
though a large number of lamps be fed simultaneously through it. The
pilot relay must be so sensitive that the current, even through one
lamp, will cause it to attract its armature. When it does attract its
armature it causes illumination of the pilot lamp in the same way that
the line relays cause the illumination of the line lamps.
The pilot lamp, which is commonly associated with a group of line lamps
that are placed on any one operator's position of the switchboard, is
located in a conspicuous place in the switchboard cabinet and is
provided with a larger lens so as to make a more striking signal. As a
result, whenever any line lamp on a given position lights, the pilot
lamp does also and serves to attract the attention, even of those
located in distant portions of the room, to the fact that a call exists
on that position of the board, the line lamp itself, which is
simultaneously lighted, pointing out the particular line on which the
call exists.
Pilot lamps, in effect, perform similar service to the night alarm in
magneto boards, but, of course, they are silent and do not attract
attention unless within the range of vision of the operator. They are
used not only in connection with line lamps, but also in connection with
the cord-circuit lamps or signals, as will be pointed out.
[Illustration: Fig. 311. Battery Supply Through Impedance Coils]
[Illustration: Fig. 312. Battery Supply through Repeating Coils]
[Illustration: Fig. 313. Battery Supply with Impedance Coils and
Condensers]
=Cord Circuit.= _Battery Supply._ Were it not for the necessity of
providing for cord-circuit signals in common-battery switchboards, the
common-battery cord circuit would be scarcely more complex than that for
magneto working. Stripped of all details, such as signals, ringing and
listening keys, and operator's equipment, cord circuits of three
different types are shown in Figs. 311, 312, and 313. These merely
illustrate the way in which the battery is associated with the cord
circuits and through them with the line circuits for supplying current
for talking purposes to the subscribers. It is thought that this matter
will be clear in view of the discussion of the methods by which current
is supplied to the subscribers' transmitters in common-battery systems
as discussed in Chapter XIII. While the arrangements in this respect of
Figs. 311, 312, and 313 illustrate only three of the methods, these
three are the ones that have been most widely and successfully used.
_Supervisory Signals._ The signals that are associated with the cord
circuits are termed supervisory signals because of the fact that by
their means the operator is enabled to supervise the condition of the
lines during times when they are connected for conversation. The
operation of these supervisory signals may be best understood by
considering the complete circuits of a simple switchboard and must be
studied in conjunction with the circuits of the lines as well as those
of the cords.
[Illustration: Fig. 314. Simple Common-Battery Switchboard]
_Complete Circuit._ Such complete circuits are shown in Fig. 314. The
particular arrangement indicated is that employed by the Kellogg
Company, and except for minor details may be considered as typical of
other makes also. Two subscribers' lines are shown extending from
Station A and Station B, respectively, to the central office. The line
wires are shown terminating in jacks in the same manner as indicated in
Figs. 307, 308, and 309, and their circuits are normally continued from
these jacks to the ground on one side and to the line relay and battery
on the other. The jack in this case has three contacts adapted to
register with three corresponding contacts in each of the plugs. The
thimble of the jack in this case forms no part of the talking circuit
and is distinct from the two jack springs which form the line terminals.
It and the auxiliary contact _1_ in each of the plugs with which it
registers, are solely for the purpose of co-operating in the control of
the supervisory signals.
The tip and sleeve strands of the cord are continuous from one plug to
the other except for the condensers. The two batteries indicated in
connection with the cord circuit are separate batteries, a
characteristic of the Kellogg system. One of these batteries serves to
supply current to the tip and sleeve strand of the cord circuit through
the two windings _3_ and _4_, respectively, of the supervisory relay
connected with the answering side of the cord circuit, while the other
battery similarly supplies current through the windings _5_ and _6_ of
the supervisory relay associated with the calling side of the cord
circuit. The windings of these relays, therefore, act as impedance coils
and the arrangement by which battery current is supplied to the cord
circuits and, therefore, to the lines of the connected subscribers, is
seen to be the combined impedance coil and condenser arrangement
discussed in Chapter XIII.
As soon as a plug is inserted into the jack of a line, the line relay
will be removed from the control of the line, and since the two strands
of the cord circuit now form continuations of the two line conductors,
the supervisory relay will be substituted for the line relay and will be
under control of the line. Since all of the current which passes to the
line after a plug is inserted must pass through the cord-circuit
connection and through the relay windings, and since current can only
flow through the line when the subscriber's receiver is off its hook, it
follows that the supervisory relays will only be energized after the
corresponding plug has been inserted into a jack of the line and after
the subscriber has removed his receiver. Unlike the line relays, the
supervisory relays open their contacts to break the local circuits of
the supervisory lamps _7_ and _8_ when the relay coils are energized,
and to close them when de-energized; but the armatures of the
supervisory relays do alone control the circuits of the supervisory
lamps. These circuits are normally held open in another place, that is,
between the plug contacts _1_ and the jack thimbles. It is only,
therefore, when a plug is inserted into a jack and when the supervisory
relay is de-energized, that the supervisory lamp may be lighted. When a
plug is inserted into a jack and when the corresponding supervisory
relay is de-energized, the circuit may be traced from ground at the
cord-circuit batteries through the left-hand battery, for instance,
through lamp _7_, thence through the contacts of the supervisory relay
to the contact _1_ of the plug, thence through the thimble of the jack
to ground. When a plug is inserted into the jack, therefore, the
necessary arrangements are completed for the supervisory lamp to be
under the control of the subscriber. Under this condition, whenever the
subscriber's receiver is on its hook, the circuit of the line will be
broken, the supervisory relay will be de-energized, and the supervisory
lamp will be lighted. When, on the other hand, the subscriber's receiver
is off its hook, the circuit of the line will be complete, the
supervisory relay will be energized, and the supervisory lamp will be
extinguished.
_Salient Features of Supervisory Operation._ It will facilitate the
student's understanding of the requirements and mode of operation of
common-battery supervisory signals in manual systems, whether simple or
multiple, if he will firmly fix the following facts in his mind. In
order that the supervisory signal may become operative at all, some act
must be performed by the operator--this being usually the act of
plugging into a jack--and then, until the connection is taken down, the
supervisory signal is under the control of the subscriber, and it is
displayed only when the subscriber's receiver is placed on its hook.
_Cycle of Operations._ We may now trace through the complete cycle of
operations of the simple common-battery switchboard, the circuits of
which are shown in Fig. 314. Assume all apparatus in its normal
condition, and then assume that the subscriber at Station A removes his
receiver from its hook. This pulls up the line relay and lights the line
lamp, the pilot relay also pulling up and lighting the common pilot lamp
which is not shown. In response to this call, the operator inserts the
answering plug and throws her listening key _L.K._ The operator's
talking set is thus bridged across the cord circuit and she is enabled
to converse with the calling subscriber. The answering supervisory lamp
_7_ did not light when the operator inserted the answering plug into the
jack, because, although the contacts in the lamp circuit were closed by
the plug contact _1_ engaging the thimble of the jack, the lamp circuit
was held open by the attraction of the supervisory relay armature, the
subscriber's receiver being off its hook. Learning that the called-for
subscriber is the one at Station B, the operator inserts the calling
plug into the jack at that station and presses the ringing key _R.K._,
in order to ring the bell. The act of plugging in, it will be
remembered, cuts off the line-signaling apparatus from connection with
that line. As the subscriber at Station B was not at his telephone when
called and his receiver was, therefore, on its hook, the insertion of
the calling plug did not energize the supervisory relay coils _5_ and
_6_, and, therefore, that relay did not attract its armature. The
supervisory lamp _8_ was thus lighted, the circuit being from ground
through the right-hand cord-circuit battery, lamp _8_, back contacts of
the supervisory relay, third strand of the cord to contact _1_ of the
calling plug, and thence to ground through the thimble of the jack. The
lighting of this lamp is continued until the party at Station B responds
by removing his receiver from its hook, which completes the line
circuit, energizes relay windings _5_ and _6_, causes that relay to
attract its armature, and thus break the circuit of the lamp _8_. Both
supervisory lamps remain out as long as the two subscribers are
conversing, but when either one of them hangs up his receiver the
corresponding supervisory relay becomes de-energized and the
corresponding lamp lights. When both of the lamps become illuminated,
the operator knows that both subscribers are through talking and she
takes down the connection.
Countless variations have been worked in the arrangement of the line and
cord circuits, but the general mode of operation of this particular
circuit chosen for illustration is standard and should be thoroughly
mastered. The operation of other arrangements will be readily understood
from an inspection of the circuits, once the fundamental mode of
operation that is common to all of them is well in mind.
=Lamps.= The incandescent lamps used in connection with line and
supervisory signals are specially manufactured, but differ in no sense
from the larger lamps employed for general lighting purposes, save in
the details of size, form, and method of mounting. Usually these lamps
are rated at about one-third candle-power, although they have a somewhat
larger candle-power as a rule. They are manufactured to operate on
various voltages, the most usual operating pressures being 12, 24, and
48 volts. The 24-volt lamp consumes about one-tenth of an ampere when
fully illuminated, the lamp thus consuming about 2.4 watts. The 12- and
48-volt lamps consume about the same amount of energy and corresponding
amounts of current.
[Illustration: Fig. 315. Switchboard Lamp]
_Lamp Mounting._ The usual form of screw-threaded mounting employed in
lamps for commercial lighting was at first applied to the miniature
lamps used for switchboard work, but this was found unsatisfactory and
these lamps are now practically always provided with two contact strips,
one on each side of the glass bulb, these strips forming respectively
the terminals for the two ends of the filament within. Such a
construction of a common form of lamp is shown in Fig. 315, where these
terminals are indicated by the numerals _1_ and _2_, _3_ being a dry
wooden block arranged between the terminals at one end for securing
greater rigidity between them.
[Illustration: Fig. 316. Line Lamp Mounting]
The method of mounting these lamps is subject to a good deal of
variation in detail, but the arrangement is always such that the lamp is
slid in between two metallic contacts forming terminals of the circuit
in which the lamp is to operate. Such an arrangement of springs and the
co-operating mounting forming a sort of socket for the reception of
switchboard lamps is referred to as a _lamp jack_. These are sometimes
individually mounted and sometimes mounted in strips in much the same
way that jacks are mounted in strips. A strip of lamp jacks as
manufactured by the Kellogg Company is shown in Fig. 316. The
opalescent lens is adapted to be fitted in front of the lamp after it
has been inserted into the jack. Fig. 317 gives an excellent view of an
individually-mounted lamp jack with its lamp and lens, this also being
of Kellogg manufacture. This figure shows a section of the plug shelf
which is bored to receive a lamp. In order to protect the lamps and
lenses from breakage, due to the striking of the plugs against them, a
metal shield is placed over the lens, as shown in this figure, this
being so cut away as to allow sufficient openings for the light to shine
through. Sometimes instead of employing lenses in front of the lamps, a
flat piece of translucent material is used to cover the openings of the
lamp, this being protected by suitable perforated strips of metal. A
strip of lamp jacks employing this feature is shown in Fig. 318, this
being of Dean manufacture. An advantage of this for certain types of
work is that the flat translucent plate in front of the lamp may readily
carry designating marks, such as the number of the line or something to
indicate the character of the line, which marks may be readily changed
as required.
[Illustration: Fig. 317. Supervisory Lamp Mounting]
[Illustration: Fig. 318. Line Lamp Mounting]
[Illustration: Fig. 319. Individual Lamp Jacks]
In the types made by some manufacturers the only difference between the
pilot lamp and the line lamp is in the size of the lens in front of it,
the jack and the lamp itself being the same for each, while others use a
larger lamp for the pilot. In Fig. 319 are shown two individual lamp
jacks, the one at the top being for supervisory lamps and the one at the
bottom being provided with a large lens for serving as a pilot lamp.
[Illustration: TERMINAL ROOM APPARATUS IN PROCESS OF INSTALLATION
Installed by Dean Electric Company at Detroit, Mich.]
=Mechanical Signals.= As has been stated the so-called mechanical
signals are sometimes used in small common-battery switchboards instead
of lamps. Where this is done the coil of the signal, if it is a line
signal, is substituted in the line circuit in place of the relay coil.
If the signals are used in connection with cord circuits for supervisory
signals, their coils are put in the circuit in place of the supervisory
relay coils. (These signals are referred to in Chapter III in connection
with Fig. 23.) They are so arranged that the attraction of the armature
lifts a target on the end of a lever, and this causes a display of color
or form. The release of the armature allows this target to drop back,
thus obliterating the display. Such signals, often called _visual
signals_ and _electromagnet signals_, should be distinguished from the
drops considered in connection with magneto switchboards in which the
attraction of the armature causes the display of the signal by the
falling of a drop, the signal remaining displayed until restored by some
other means, the restoration depending in no wise on when the armature
is released.
_Western Electric._ The mechanical signal of the Western Electric
Company, shown in Fig. 320, has a target similar to that shown in Fig.
254 but without a latch. It is turned to show a different color by the
attraction of the armature and allowed to resume its normal position
when the armature is released.
[Illustration: Fig. 320. Mechanical Signal]
_Kellogg._ Fig. 321 gives a good idea of a strip of mechanical signals
as manufactured by the Kellogg Company. This is known as the _gridiron_
signal on account of the cross-bar striping of its target. The white
bars on the target normally lie just behind the cross-bars on the shield
in front, but a slight raising of the target--about one-eighth of an
inch--exposes these white bars to view, opposite the rectangular
openings in the front shield.
[Illustration: Fig. 321. Strip of Gridiron Signals]
_Monarch._ In Fig. 322 is shown the visual signal manufactured by the
Monarch Telephone Company.
[Illustration: Fig. 322. Mechanical Signal]
=Relays.= The line relays for common-battery switchboards likewise
assume a great variety of forms. The well-known type of relay employed
in telegraphy would answer the purpose well but for the amount of room
that it occupies, as it is sometimes necessary to group a large number
of relays in a very small space. Nearly all present-day relays are of
the single-coil type, and in nearly all cases the movement of the
armature causes the movement of one or more switching springs, which are
thus made to engage or disengage their associated spring or springs. One
of the most widely used forms of relays has an L-shaped armature hung
across the front of a forwardly projecting arm of iron, on the
knife-edge corner of which it rocks as moved by the attraction of the
magnet. The general form of this relay was illustrated in Fig. 95.
Sometimes this relay is made up in single units and frequently a large
number of such single units are mounted on a single mounting plate. This
matter will be dealt with more in detail in the discussion of
common-battery multiple switchboards. In other cases these relays are
built _en bloc_, a rectangular strip of soft iron long enough to afford
space for ten relays side by side being bored out with ten cylindrical
holes to receive the electromagnets. The iron of the block affords a
return path for the lines of force. The L-shaped armatures are hung over
the front edge of this block, so that their free ends lie opposite the
magnet cores within the block. This arrangement as employed by the
Kellogg Company is shown in two views in Figs. 323 and 324.
[Illustration: Fig. 323. Strip of Relays]
[Illustration: Fig. 324. Strip of Relays]
A bank of line relays especially adapted for small common-battery
switchboards as made by the Dean Company, is shown in Fig. 325.
[Illustration: Fig. 325. Bank of Relays]
=Jacks.= The jacks in common-battery switchboards are almost always
mounted in groups of ten or twenty, the arrangement being similar to
that discussed in connection with lamp strips. Ordinarily in
common-battery work the jack is provided with two inner contacts so as
to cut off both sides of the signaling circuit when the operator plugs
in. A strip of such jacks is shown in Fig. 326.
[Illustration: Fig. 326. Strip of Cut-Off Jacks]
Ringing and listening keys for simple common-battery switchboards differ
in no essential respect from those employed in magneto boards.
[Illustration: Fig. 327. Details of Lamp, Plug, and Key Mounting]
=Switchboard Assembly.= The general assembly of the parts of a simple
common-battery switchboard deserves some attention. The form of the
switchboard need not differ essentially from that employed in magneto
work, but ordinarily the cabinet is somewhat smaller on account of the
smaller amount of room required by its lamps and jacks. An excellent
idea of the line jacks and lamps, plugs, keys, and supervisory signals
may be obtained from Fig. 327, which is a detail view taken from a
Kellogg board. In the vertical panel of the board above the plug shelf
are arranged the line jacks and the lamps in rows of twenty each, each
lamp being immediately beneath its corresponding jack. Such jacks are
ordinarily mounted on 1/2-inch centers both vertically and horizontally,
so that a group of one hundred lamps and line jacks will occupy a space
only slightly over 10 by 5 inches. Such economy of space is not required
in the simple magneto board, because the space might easily be made
larger without in any way taxing the reach of the operator. The reason
for this comparatively close mounting is a result, not of the
requirements of the simple non-multiple common-battery board itself, but
of the fact that the jack strips and lamp strips, which are required in
very large numbers in multiple boards, have to be mounted extremely
close together, and as the same lamp strips and jack strips are often
available for simple switchboards, an economy in manufacture is effected
by adherence to the same general dimensions.
[Illustration: Fig. 328. Simple Common-Battery Switchboard with
Removable Relay Panel]
A rear view of a common form of switchboard cabinet, known as the
_upright type_ and manufactured by the Dean Company, is shown in Fig.
328. In this all the relays are mounted on a hinged rack, which, when
opened out as indicated, exposes the wiring to view for inspection or
repairs. Access to both sides of the relays is thus given to the
repairman who may do all his work from the rear of the board without
disturbing the operator.
Fig. 329 shows a three-position cabinet of Kellogg manufacture, this
being about the limit in size of boards that could properly be called
simple. Obviously, where a switchboard cabinet must be made of greater
length than this, _i. e._, than is required to accommodate three
operators, it becomes too long for the operators to reach all over it
without undue effort or without moving from their seats. The so-called
_transfer board_ and the _multiple board_ (to be considered in
subsequent chapters), constitute methods of relief from such a condition
in larger exchanges.
[Illustration: Fig. 329. Three-Position Lamp Board]