3 66 TELEPHONY 



the effect was practically the same as if the added inductance were uniformly distributed. 

 In subterranean lines the loading coils used are rings of iron wire wound with insulated 

 copper wire, and are placed a mile or two apart. The difficulties of introducing the 

 coils into a submarine telephone cable have only recently been overcome. A coil-loaded 

 cable about 9 miles long was laid in 1906 by Messrs. Siemens and Halske across Lake 

 Constance, and its improved telephonic qualities stimulated a larger experiment. 



In 1910 Messrs. Siemens Bros, made and laid for the British Post Office a coil-loaded 

 cable across the English Channel. This cable contains two complete telephone circuits, 

 each loaded with coils placed i nautical mile apart. This cable enables very excellent 

 telephonic speech to be maintained between London and Paris; and in a paper on " Sub- 

 marine Cables for Long Distance Telephone Circuits" (see Journal Inst. Elec. Eng., 

 1911, vol. 46, p. 309) Major O'Meara stated that with suitably heavy land lines of copper 

 conductors, 800 Ibs. to the mile, conversation through it between London and Astrakhan 

 on the Caspian Sea would be possible. A similar loaded cable has since been laid by 

 Messrs. Siemens Bros, between England and Belgium. In this last cable a very great 

 advance has been made in the quality of the gutta percha used for insulation. In con- 

 nection with the transmission of telephonic speech the ratio of the leakance (S) of the 

 dielectric to its capacity (C) has a great influence. The ratio of S/C for most gutta 

 percha is a number near 120, but Messrs. Siemens have reduced the value to 12 for the 

 Anglo-Belgian cable. In the Anglo-French the loading coils have an inductance of 

 100 millihenrys and are placed i nautical mile apart, and the value of S/C is 115. In 

 the Anglo-Belgian the coils have an inductance of 100 mh. and are also spaced 

 i nautical mile apart. The attenuation constant for the loaded cable is nearly 0.018. 



In addition to coil-loaded or Pupinised cables much work has been done with cables 

 uniformly loaded by winding them over with iron wire as carried out by Krarup in 

 Denmark. Many cables of Krarup type have been employed in Germany, and tele- 

 phonic engineers differ in opinion as to the relative advantages of cables of the Pupin and 

 Krarup type in respect to efficiency and cost, but evidence seems to show that for equal 

 efficiency the Pupin method is cheaper. In any case an improvement from 10 per cent 

 to 100 per cent or more can be obtained by uniform loading and more by coil loading in 

 the speech transmitting qualities. The problem of loading cables for short lengths 

 of 20 to 50 miles may be considered as solved, and the advantage established. 



Another important advance in practical telephony is the introduction of the Auto- 

 matic Exchange system. In the manual method an operator (generally a girl) is re- 

 quired at the central exchange to take from a caller his number and that of the desired 

 correspondent and connect the two wires by a flexible conductor with end plugs. The 

 delays and difficulties involved in this human element have led to the introduction of 

 automatic exchanges. Each customer has on his telephone a dial or indicator with 

 pointer which he sets successively to the integers, say four, composing the number of the 

 person he desires to call up. Each subscriber is connected to the exchange by a double 

 wire ending in a so-called ist selector switch, and the first movement of his index con- 

 nects him to a group of wires the numbers of which lie in that particular thousand. The 

 next movement of his index needle connects him to a 2d selector switch which picks out 

 a connection with a group of wires corresponding to the hundreds in the number called, 

 and this in turn operates a 3d selector switch which connects to the subscriber desired. 

 The power to work the switches is obtained from local accumulators and the whole opera- 

 tion takes only a few seconds. The advantages of the automatic system are that the 

 selector switches can be housed in basements or underground rooms unsuitable for man- 

 ual operators. The automatic system which has come into most generakise is the Strow- 

 ger system. Although new to England, it has been for some years in use in the United 

 States very successfully. In Los Angeles there are 47,000 subscribers in automatic 

 connection; in Chicago 30,000, and in San Francisco 25,000. The first exchange of this 

 type set up by the General Post Office in England was installed at Epsom, the second 

 at the General Post Office itself. Leeds is to have the third, and probably Portsmouth 

 and Brighton will soon be provided with automatic exchanges. The automatic system 



