Metallic Polar-Duplex Telegraph System for Long Small-Gage Cables

In connection with carrying out the toll-cable program of the Bell System, a metallic-circuit polar-duplex telegraph system was developed. The metallic-return type of circuit lends itself readily to the cable conditions, its freedom from interference allowing the use of low potentials and currents so that the telegraph may be superposed on telephone circuits. The new system represents an unusual refinement in d-c. telegraph circuits, the operating current being of the same order of magnitude as that of the telephone circuits on which the telegraph is superposed. The following are some of the outstanding features of the present system. Sensitive relays with closely balanced windings are employed in the metallic circuit, and ``vibrating circuits'' are provided for minimizing distortion of signals. Repeaters are usually spaced about 100 miles apart. Thirty-four-volt line batteries are used and the line current is four or five milli-amperes on representative circuits. Superposition is accomplished by the compositing method which depends upon frequency discrimination; the telegraph occupying the frequency range below that of the telephone. New local-circuit arrangements have been designed, employing polar relays for repetition of the signals; these arrangements are suitable for use in making up circuits in combination with carriercurrent and ground-return polar-duplex telegraph sections. New forms of mounting are employed in which a repeater is either built as a compact unit or is made up of several units which are mounted on I-beams, and subsequently interconnected.