Telephonic repeaters and long distance telephony

The paper reviews the development of long-distance telephony and shows the great improvements effected by the introduction of telephonic repeaters employing thermionic valves. Improvements of greater importance introduced up to April, 1921, when the paper was written, were made by the use of the microphonic transmitter, hard drawn copper wire, loading, cable balancing, phantom circuits and the triode repeater. The repeater is defined and is shown to have the following advantages: it increases the telephonic range and makes possible the use of finer wire or underground cable. From a consideration of the theory of the triode it is shown that hard valves are preferred because of their longer life and greater stability. Post Office tests for magnification, speech, stability, life and characteristics are described, and results of some experiments are given. An account of the Edison, single-relay and double-relay systems is given, and the difficulties encountered in their designs are indicated. The double relay is shown to be preferable on the grounds of adaptability and simplicity. Results are given, indicating that if repeaters were inserted at intervals equivalent to 15 miles of standard cable or telephone lines an improvement of 15 to 20 m.s.c. would be brought about. With regard to cost it is stated that a single repeater costs £50 a year to maintain but leads to economy, e.g. a saving of £15 000 a year on London´s main trunk lines alone. Some indication is given as to the probable development of long-distance telephone systems. A forecast of 50-lb. trunk lines, and transcontinental and transoceanic telephoney is made. References are given in the Appendix.