The Relay

A method of switching on a large current from a small one. Relays like this were used to detect the weak current in telegraph cables (10 - 20mA). The relay translates the signal on the telegraph line into an equivalent signal in a local circuit.

How it worked A current passes through the electromagnets generating a magnetic field, which moves the small magnetized armature between them. This action brings two contacts together, allowing current to flow through the local circuit. Click here to see how the relay was used in a simple circuit.