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Electromagnetism

Relay.

The relay consists of two circuits.

Circuit 1 is a simple electromagnet which requires only a small current.
When the switch is closed, current flows
and the iron rocker arm is attracted to the electromagnet.
The arm rotates about the central pivot and pushes the contacts together.
Circuit 2 is now switched on.

Circuit 2 may have a large current flowing through it,
to operate a powerful motor or very bright lights.

When the switch is opened the electromagnet releases the rocker arm
and the spring moves the contacts apart.
Circuit 2 is now switched off.

The advantage of using a relay is that a small current (circuit 1)
can be used to switch on and off a circuit with a large current (circuit 2).
This is useful for two reasons:
(i) circuit 1 may contain a component such as an LDR,
which only uses small currents,
(ii) only the high current circuit
needs to be made from thick wire.

A relay is used to operate the starter motor in cars
and the heating circuit in diesel engines.

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