Synopsis.-In connection with carrying out the toll-cable pro-graph occupying the frequency range below that of the telephone. gram of the Bell System, a metallic-circuit polar-duplex telegraph New local-circuit arrangements have been designed, employing system was developed. The metallic-return type of circuit lends polar relays for repetition of the signals; these arrangenents are itself readily to the cable conditions, its freedom from interference suitable for use in making up circuits in combination with carrierallowing the use of low potentials and currents so that the telegraphl current and ground-return polar-duplex telegraph sections. New may be superposed on telephone circuits. The new system repre-forms of mounting are employed in which a repeater is either built sents an unusual refinement in d-c. telegraph circuits, the operating as a compact unit or is made up of several units which are mounted current being of the same order of magnitude as that of the telephone on I-beams, and subsequently interconnected. In the latter case circuits on which the telegraph is superposed.the usual arrangements for sending and receiving from the repeaterThe following are some of the outstanding features of the present are omitted, and a separate "monitoring" unit provided for consystem. Sensitive relays with closely balanced windings are em-nection to any one of a group of repeaters. ployed in the metallic circuit, and "vibrating circuits" are pro-The metallic system is suitable for providing circuits up to 1000 vided for minimizing distortion of signals. Repeaters are uxsually miles or more in length, the grade of service being better than that spaced about 100 miles apart. Thirty-Jour-volt line batteries are usually obtained from ground-return circuits on open-wire lines used and the line current is four or five mnilli-amperes on representa-for such distances. About 5.5,000 miles of this type of telegraph tive circuits. Superposition is accomplished by the compositing circuit are in service at present. method which depends upon frequency discrimiination; the tele-* * * * *
INTRODUCTIONto employ a number of repeaters with comparatively T HERE has been developed recently by the Bell large amplification and also to insert loading coils in the System a low-current metallic telegraph system, line at short intervals. As a result, the interference of the polar-duplex type, which is suitable for from superposed telegraph would be excessive unless superposition on telephone circuits in long small-gage the telegraph voltages and currents were kept far below cables. In certain sections where long-distance toll the values ordinarily employed for ground-return teletraffic is heavy, it becomes desirable, from the stand-graph. To allow the use of small currents and potenpoints of economy and continuity of service, to employ tials with ground-return telegraph would require the such cables to replace existing open-wire lines and to development of arrangements for neutralizing differprovide for future growth. The new telegraph system...
