of the switching mechanism is controlled by the falling back of a
relay armature at the exchange.
The mechanism at the exchange is so designed that by the
provision of a sluggish relay or equivalent device certain con-
trolling connexions remain unchanged throughout the whole of
the train of impulses, but the long current corresponding to the
period between two trains is utilized for effecting a change-over
so that each train operates a separate switching device. Briefly
the effect is to produce successive selection as already indicated.
The operation of the digit 7 in a typical case would operate the
mechanism so that an idle outgoing connecting line of the 7th
thousand is found. The following digit, i, will pick out an idle
outgoing connecting line to the ist hundred in the 7th thousand
being the 7ist hundred. At this point it is common practice to
give the subscriber connexion with a switch that can select any
one of the 100 lines in the group, so that the next two digits 4 and
6 will call line 46 in the 7ist hundred, viz. line 7146.
Automatic systems may in the main be roughly placed in two
classes: (a) direct impulse systems; (b) stored impulse systems.
In the direct impulse system may be placed the systems of The
Automatic Electric Co., Siemens Bros. & Co., The North Electric
Co., and The Relay Automatic Telephone Co. (including licensees of
the firms concerned).
The Automatic Electric Co. and Messrs. Siemens Bros. & Co.
manufacture what is known as the " Strowger " system, the switches
of which in operating utilize two motions, first, a vertical action in
which the brush is not making contact followed by a rotary action in
which the brush is either hunting for an idle connecting line in the
case of a preliminary switch or moving towards the contact of the
wanted subscriber's line in the case of a final switch. Two magnets
are involved in this operation. There is a third magnet used for
effecting the release at the end of the conversation, the operation of
release consisting in the restoration of the moving element to normal,
first in a rotary direction by means of a main spring and, secondly, in
a vertical direction under the influence of gravity.
Fig. 4 shows the elementary circuit connexions for one of these
systems. The calling subscriber on lifting his receiver operates relay
" LR " which in turn operates " RR." The impulses are received on
" LR " whose armature falls back for each impulse placing an earth
connexion intermittently on the circuit of " S " and " VM." Both
" RR " and " S " are sluggish relays that release slowly so that the
succession of " breaks " and " makes " from the impulses do not
affect them. When the long current between two trains of impulses
occurs the circuit of " S " is disconnected for a period which is long
enough for its operation, and " S " electromagnetically operates the
switch " SS " so that " VM " is thrown out of circuit and " RM " is
substituted. " VM " is the magnet which operates the vertical
stepping by means of a ratchet and pawl device. " RM " similarly
effects the rotary stepping. The next train of impulses will clearly
operate " RM " after which the change-over devices depending on
" S " are made to cut out the accessory mechanical apparatus, leav-
ing only the connexions suitable for giving the engaged signal,
ringing the subscriber, and talking. At the end of the conversation
the caller hangs up his receiver, the armature of " LR " falls back
permanently, " RR " is deenergized and the release magnet " REL "
is operated thus restoring the switch to normal : " REL " disengages
a detent which permits a coiled spring to restore the wipers or
brushes in a rotary direction after which gravity carries them
vertically to their home positions at which stage the " REL " circuit
is opened at " ON." The transmission system consists of two relays
" LR " and " LC " associated with two condensers as shown at the
top of fig. 4. This will be recognized as the " Stone " C. B. system
1 _ *
The North Electric Co. manufactures a switch operated by direct
impulses in which the motions are the reverse of the systems just
described. Rotary action, with the brush away from the contacts,
is followed by " trunk " hunting in a vertical direction. This per-
mits of vertical contacts which are less liable to the adverse influence
of dust than horizontal contacts, and would also permit of a jarger
number of trunks than ip, being placed in one group without seriously
interfering with the design of the equipment.
Both the Relay Automatic Telephone Co. and the North Electric
Co. manufacture automatic systems which do not use mechanism as
usually understood. The systems consist of aggregations of relays,
combined so as to provide a number of connecting or " trunking "
paths through the equipment. In the case of the Relay Automatic
Telephone Co.'s system the calling subscriber operates his dial in the
usual way and at the same time finds an " outgoing " trunk. His
impulses operate relay devices known as the " recorder " and the
" marker," which latter places an electrical condition on the called
subscriber's line so that it immediately operates somewhat like a
called line, and finds an idle " incoming " trunk which is placed in
communication with the outgoing trunk already seized by the calling
party. These two trunks are automatically placed in contact and
together form the connecting link for the conversation.
The earliest practical stored impulse system is the " Lorimer "
system as used at Hereford (England). In this case the dial as
already described is not used, but a lever device is associated with
each telephone. The levers are set in definite positions corresponding
to the number to be called, and the switches at the exchange are
set in motion by the subscriber operating a subsidiary crank that
forms part of the calling device. The operation of this crank also
winds up the mechanism of the calling device so that the operation
of the switches on the exchange can electromagnetically release the
calling device and run it down. The running down of the calling
device in conjunction with the operation of the exchange switches
controls the action of the latter by means of an electrical circuit
established through the setting of the levers so that the exchange
switches are made to trunk, hunt and find lines in a manner corre-
sponding to the lever setting. The mechanism at the exchange is
driven by a motor and can therefore be provided with robust con-
tacts. A subsidiary device known as a pilot switch can be made to
operate and alter the connexions between digits or at any other stage
of the call so as to control the sequence of switching operations.
The Western Electric Co.'s rotary and panel type systems possess
the obvious advantages to be derived from storing connexions and
the interpolation of controlling operations at any stage of a call.
In both these systems the subscriber's telephone is equipped with a
standard dial, and the impulses are taken up by the sender storing
device at the exchange. This device is set into position by the
impulses and subsequently controls the action of the selective
switches in a manner somewhat similar to that indicated for the
Lorimer system above mentioned. In the Western Electric Co.'s
systems banks of contacts for 200 and 500 lines respectively ar
employed, as against loo-line banks for direct-impulse systems, s<
that the number-storing device is also required to perform thi
functions of a numerical transformer changing the call record fron
the decimal system as dialled into whatever system is necessary fo
the correct operation of the switches. An additional feature asso
ciated with the panel system of the Western Electric Co. is th<
provision of a translator which consists of a cross-connecting device
so arranged that any number dialled can be converted from time t(
time into some other number. This is particularly desirable in th<
case of large cities in which the selection of exchanges is effected bj
means of a code. It will be seen on reference to fig. 2 that eight of thi
finger holes have in addition to the digits a group of three letters
These are arranged in alphabetical order from A to Y omitting Q
The use of these letters is to facilitate the calling of subscribers ir
areas where manual telephones coexist, and where in the ordinarj
course very cumbersome numbers would otherwise be involved. The
arrangement provides a means of facilitating the conversion of ar
existing manual area to automatic working. The number " Mayfaii
2148 " is printed in the directory " MAYfair 2148," and so long a;
any manual exchanges in the area exist the numbers would be passed
in the ordinary way, but as automatic exchanges are introduced the
subscriber will obtain connexion by dialling " MAY 2148." It will ol
course be recognized that to dial MAY is really to dial " 629," and
the switching equipment must be such that the dialling of this code
will give the subscriber connexion to an idle junction circuit outgoing
to the Mayfair exchange whether that exchange be an automatic or
a manual one. Owing to variations in traffic the size of the junction
groups to Mayfair will vary from time to time, and redistribution of
junction lines in the automatic equipment will be essential. The
translator mentioned provides the means whereby this can !><
effected, because the transformation by means of the " impulse
cross connexion field "will make it practicable for "MAY" to lir
reconverted into any combination of the 10 digits when taken three
at a time.
The traffic problem involved in the provision of connecting cir-
cuits or trunks at automatic exchanges is one of considerable interest
and importance, as is illustrated by the extent to which it figures in
the bibliography appended.
Wire Plant. If the distribution to the subscribers is under-
ground throughout, the main cables are now subdivided into
smaller units, bifurcated or multiple branching joints being made
between the main and subsidiary cables. The latter cables are
accessible in footway boxes, and are terminated in such a way
that one or more pairs of conductors can be led direct into any
adjacent premises by a small lead-covered paper-core cable.!
The end of the small cable in the subscriber's premises is ter-|
minated in such a manner as to prevent the ingress of moisture. |
If, however, the distribution is by means of aerial wires from
a pole which is erected to serve a small zone, the cable is continued '
to a point about 2 ft. below the lowest arm of the pole, and is
terminated in a solid or sealed joint from which separate lead- i
covered leads extend the pairs of conductors to insulators.
For long-distance service up to about 1910 the wires were
erected on pole lines along roads, railways and canals. The
hard-drawn copper wire of high conductivity (invented by T. B.
Doolittle in 1877) is invariably used; and wires weighing from
150 to 800 Ib. per mile have been employed. It was necessary
to carry the long-distance lines through underground cables in
the approaches to large English cities, but owing to the inef-
ficiency of cable wires as compared with aerial wires for speech
transmission, the length of underground cable sections was
kept down to a minimum. As the long distance service ex-
panded and the number of lines increased it became increasingly
difficult to find routes for new pole lines, especially near large
cities, and the need for improvement in the efficiency of cable
wires became a very pressing matter.
The disadvantages of the earlier types of underground cables as
compared with aerial lines were: (a) much greater attenuation and
distortion of telephonic currents; (6) inability to superpose a third
circuit, known as a " phantom," on each pair of physical circuits.
Towards the end of the igth century Oliver Heaviside had proved
mathematically that uniformly distributed inductance in a tele-
phone line would diminish both attenuation and distortion, and that
if the inductance were great enough and the dielectric conductance
not too high the circuit would be distortionless, while currents ofaU
frequencies would be equally attenuated. Following up this idea
Prof. M. I. Pupin showed that by placing inductance coils in circuit
at distances apart less than half the length of the shortest component
wave to be transmitted, a non-uniform conductor could be made
approximately equal to a uniform conductor.
Pupin's system of " loading " telephone conductors has been
applied in England mainly to underground cables, and many iffl-
provements have been made in recent years, so that it is now possible
to obtain the same transmission efficiency from an underground
telephone circuit as from an aerial circuit of equivalent gauge.
The difficulties encountered in working underground cable cir-
cuits were exhaustively investigated by the British Post Office in
the first decade of this century, and it was proved that the inability to
obtain phantom circuits was due primarily to want of balance
between the electrostatic capacity of conductors in respect to (a)
other conductors and (b) to earth. As a result of the earlier investiga-
tions the method of laying up the conductors in pairs to form a
complete cable was radically changed. A type of cable known as
the " quadruple pair " was introduced. In this type the conductors
are lapped with insulating paper twinned together in pairs, and are
arranged in " cores " each containing four twisted pairs laid up
together around a centre, usually of yarn, forming a "quadruple
pair " core. The cores are laid up together to the number required
Physical ' Circuit X
; and sheathed with lead. This type of cable was a great improvement
i on the earlier " twin " cables, and permitted the formation of a
superposed "phantom" circuit on two physical circuits. Diagonal
pairs in the same core are selected for superposing.
In a later type of cable known as the " multiple twin cable " the
centre of yarn is dispensed with , and the cable consists of a number
1 of 4-wire cores made up of two 2-wire cores twinned together. The
i Manufacture of this type of cable has been greatly improved in
i recent years, and cables are now produced with very small out-of-
balance capacities between wire and wire, and between wire and
earth. It is still, however, necessary to balance the cables after
laying by a systematic method of jointing contiguous lengths,
whereby conductors are selected and jointed in such a manner as to
secure maximum uniformity of characteristics.
A method of loading the phantom circuit in telephone cables was
invented by G. A. Campbell and T. Shaw in the United States and
: patented in Great Britain in 1911. This method was applied to a
cable laid between London and Birmingham in 1914 and extended to
Liverpool in 1916.
The phantom circuit is obtained by means of specially wound
i transformers joined across the ends of the physical circuits. The
, cores of these transformers consist of a ring made up of very fine
soft iron wires. Fig. 5 illustrates the method of connecting.
Telephone Repeater. The art of long-distance telephony was
advanced a further and more important stage by the introduction
of a practicable type of telephone relay or repeater in 1913.
Balance I 4=
The conception of a repeater which could be inserted in a tele-
phone circuit and fulfil the same functions as a repeater in a telegraph
circuit is almost as old as the telephone itself. Early attempts at a
solution of the problem were invariably in the form of a sensitive
microphone attached to the reed or the diaphragm of a receiving
apparatus, but the fundamental defects of repeaters of this type,
due primarily to the inertia of moving mechanical parts, prevented
their successful application in commercial service. It was not
until the development of the 3-electrode thermionic tube had
reached the stage of commercial production for wireless telegraphy
purposes in 1913 that the problem of the telephone repeater could be
wived. Since that time progress has been so rapid as to cause almost
i complete revolution in long-distance telephony.
A modern telephone repeater for insertion at an intermediate point
in a long telephone line consists essentially of two thermionic tube
amplifiers, one for the up and one for the down side of the line
circuit, associated with apparatus for balancing the line circuits for
duplex working, the telephone circuit being necessarily a duplex
circuit. The general arrangement is shown in fig. 6.
The telephone repeater may be used to extend the range of speech
over existing lines, as for instance a London-Paris line may be
extended by a repeater at Paris to any distant city in direct com-
munication with Paris; a second repeater at the distant city may
relay the line again to a further point and so on. In fact it may be
said that telephonic speech is now possible over any length of wire
circuit. Speech through submarine cables is, however, still limited to
comparatively short distances.
The most important application of the telephone repeater, and one
in which the greatest economies are possible, is in the internal corar
munications of a country. For instance, in order to provide tele-
phonic communication between, say London and Manchester, Leeds,
Newcastle and Glasgow, it has hitherto been necessary to erect line
conductors weighing on the average 600 Ib. per circuit mile. A
London Newcastle line thus requires about 180,000 Ib. of copper. It
is now possible by using four telephone repeaters at intermediate
points between those two cities to provide equally good communica-
tion over conductors weighing only 80 Ib. per circuit mile, and these
conductors may be contained in an underground cable which will
carry 240 circuits. The combination of telephone repeaters with
underground cables affords a service of greater efficiency than can
be obtained from heavy aerial lines, and a service free from inter-
ruption by storms.
Fig. 7 is a plan illustrating a scheme for providing telephonic
communication between all the important towns of Great Britain
by means of underground cables and telephone repeaters. The con-
struction of this extensive system was well advanced in 1921 and was
due for completion in 1925.
A list of representative types of main underground telephone
cables in Great Britain is given in the table.
Submarine Telephone Cables. The problem of loading deep-sea
cables with inductance coils, and thus increasing the possible range
of speech transmission, was successfully solved in 1910, when Messrs.
Siemens Bros. & Co. manufactured and laid for the British Post Office
between Dover and Calais a 4-core submarine cable loaded with
inductance coils at intervals of one nautical mile. The transmission
efficiency of this cable was rather more than three times as good as
that of a similar cable without loading coils.
In 1911 Messrs. Siemens introduced a form of balata dielectric as
a substitute for gutta-percha in loaded submarine cables on account
of the greatly reduced leakance of the former as compared with the
Particulars of Representative Types of Loaded Main Underground British Cables.
Number of Pairs
D. C. Constants of
Cable per Mile
A. C. Constants of Loaded Cable per
Mile Loop at 10 = 5,000
1 50 phantom
1 50 phantom
1 40 phantom
[ 20 phantom
[ 20 phantom
D. C. Coi
897 \3 4'
905 /s 41'
891 /5 36'
864 /8 40'
860 /8 7'
563 /6 48'
610 /8 55'
I 062 \i 4'
i 069 \2 57'
i 031 \i 4'
I 088 \ 5 45'
I 074 /2 26'
565 /l 18'
615 \2 II'
I 298 \2 35'
857 \2 II'
I 550 \3 3i'
I 023 \2 58'
I 855 \2 33'
I 240 \2 8'
* loo-lb. conductors used for telegraphs. f Not phantomed. J In course of construction.
latter. The effect was to reduce materially the attenuation constant
and increase the range of speech in loaded cables.
The improved dielectric was used in a cable laid in Aug. IQI2
between St. Margaret's Bay, Dover, and La Panne, Belgium. This
cable contained four copper conductors, each weighing 160 Ib. per
nautical mile and insulated by a dielectric weighing 150 Ib. per mile
(as compared with 300 Ib. per mile in the 1910 Anglo-French cable).
The variation of attenuation with frequency is much less in the 1912
cable than in the earlier one.
The Anglo-Belgian cable had another special feature, namely, the
provision of loading coils for a third circuit superposed on the two
physical circuits. The loading coils for all three circuits were placed
together at intervals of one nautical mile.
A similar cable with some further improvements in dielectric and
loading coils was laid across the Irish Sea between Nevin, Carnarvon-
shire, and Howth, co. Dublin, in 1913.
A submarine telephone cable of the continuously loaded type
was laid across the English Channel by the French Government
in 1912, between the same points as the 1910 coil-loaded cable. The
weight per nautical mile of dielectric is the same in both cables, but
each copper conductor of the former weighs 300 Ib. per mile as com-
pared with 160 Ib. per mile in the latter. The transmission efficiency
of the cables is practically equal, but the continuously loaded cable
provides an additional circuit by superposing. Experiments con-
ducted on this cable in 1914 proved the possibility of obtaining four
circuits from a continuously loaded 4-wire submarine cable by intro-
ducing an improved method of balancing the electrostatic capacity of
the conductors. The fourth circuit has not yet been successful in a
Several additional coil-loaded telephone cables were laid across
the English Channel during the war period. Details of these cables
are given in the Table.
With equal weights of conductor and dielectric, the relative trans-
mission efficiencies of (a) coil-loaded and (6) continuously loaded
4-wire submarine cables are as loo to 75, but the latter may provide
four circuits as compared with three in the former. Experience has
hown that the maintenance and repairs of coil-loaded cables are
attended by difficulties which are not met with in continuously
The introduction of telephone relays has made it possible fre-
quently to use submarine cables of a less efficiency than the coil-
loaded cables previously required. Consequently, it is practicable
to increase the use of continuously loaded cables, and the modern
tendency is in that direction. (W. No.)
The more important improvements made in the United
States during 1910-21 are briefly described below.