Non-Polarised Telegraph Relay
This consists of an Electro-magnet wound with rather fine covered wire to 160 ohms.
Opposite its poles is fixed, on a pivoted stem, an iron armature. The distance between the armature and the cores can be regulated by a screw which slides the electromagnet.
The armature when not attracted by the cores is drawn back by a spring which is adjustable. When a current passes through the coils they magnetise the iron cores (which are united by an iron yoke at the back end) and these attract the armature as long as the current passes. When it ceases the armature is drawn back by the spring. The starting and stopping of the current are controlled by a Key in the telegraph circuit. The stem carrying the armature then acts as a Key in a local battery circuit and so reproduces the signals on a telegraph 'Sounder'. These relays are used on 'open' and 'closed' circuit telegraph lines, but cannot be used when reversals are employed.