File:EB1911 Telegraph - Single-current Relay Working.jpg

English: If long circuits were worked direct with ordinary instruments, high battery power would be required in order to send sufficient current to actuate the apparatus. In such cases it is usual to employ a local battery to produce the signals, and to close the local battery circuit by means of a circuit-closing apparatus called a relay, which is practically an electromagnetic key which has its lever attached to the armature of the magnet and which can be worked by a very weak current. The arrangement at a station worked by relay on the “single-current” system is shown, where L is the line wire, joined through the key K to one end of the coil of the relay magnet R, the other end of which is put to earth. When a current passes through R the armature A is attracted and the local circuit is closed through the armature at b. The local battery B₁ then sends a current through the instrument I and records the signal. In the form of relay indicated in the figure the armature is held against the stop a by a spring S.