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 circuits 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 investigations 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 and sheathed with lead. This type of cable was a great improvement 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.
Fig. 5.
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 of 4-wire cores made up of two 2-wire cores twinned together. The Manufacture of this type of cable has been greatly improved in 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 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.
Fig. 6.
The conception of a repeater which could be inserted in a telephone 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 solved. Since that time progress has been so rapid as to cause almost a 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 communication 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 communications of a country. For instance, in order to provide telephonic communication between, say London and Manchester, Leeds, Newcastle and Glasgow, it has hitherto been necessary to erect line conductors weighing on the average 600 lb. per circuit mile. A London Newcastle line thus requires about 180,000 lb. of copper. It is now possible by using four telephone repeaters at intermediate points between those two cities to provide equally good communication over conductors weighing only 80 lb. 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 interruption by storms.
Fig. 7.
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 construction 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
