Field trial of 50-cycle signalling on toll lines
14 October 1936
The rapid progress in recent years of subscriber-to-subscriber dialling over toll lines has necessitated a departure from d.c. signalling in use on the shorter junctions between urban exchanges. D.c. signalling can only be applied economically to lines not exceeding 1400 ohms resistance. Also, d.c. signalling is unsuitable for lines which are phantomed or equipped with repeaters. Fifty-cycle a.c. has been in use for signalling and dialling on toll lines, but the sending voltage required was too high, thereby causing interference, and the impulses were distorted. The new type of 50-cycle a.c. signalling system operates with low sending voltage and current thus preventing excessive distortion while retaining a circuit of simple form and low transmission loss. The paper describes by means of photographs and circuit diagrams the principles of the new system. The applications to a tandem toll line and a line repeater are explained by means of circuit diagrams.