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
(see 26.552).
Fig. 4.
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 permits of vertical contacts which are less liable to the adverse influence of dust than horizontal contacts, and would also permit of a larger number of trunks than 10, 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 corresponding to the lever setting. The mechanism at the exchange is driven by a motor and can therefore be provided with robust contacts. 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 are employed, as against 100-line banks for direct-impulse systems, so that the number-storing device is also required to perform the functions of a numerical transformer changing the call record from the decimal system as dialled into whatever system is necessary for the correct operation of the switches. An additional feature associated with the panel system of the Western Electric Co. is the provision of a translator which consists of a cross-connecting device so arranged that any number dialled can be converted from time to time into some other number. This is particularly desirable in the case of large cities in which the selection of exchanges is effected by means of a code. It will be seen on reference to fig. 2 that eight of the 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 in areas where manual telephones coexist, and where in the ordinary course very cumbersome numbers would otherwise be involved. The arrangement provides a means of facilitating the conversion of an existing manual area to automatic working. The number “Mayfair 2148” is printed in the directory “MAYfair 2148,” and so long as 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 of 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 be effected, because the transformation by means of the “impulse cross connexion field” will make it practicable for “MAY” to be reconverted into any combination of the 10 digits when taken three at a time.
The traffic problem involved in the provision of connecting circuits 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 underground 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 terminated 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-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 lb. 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 inefficiency 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 expanded 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; (b) inability to superpose a third circuit, known as a “phantom,” on each pair of physical circuits.
Towards the end of the 19th century Oliver Heaviside had proved mathematically that uniformly distributed inductance in a telephone 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 of all 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 im-
