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A relay is a switch worked by an electromagnet. It is useful if we want a small current in one circuit to control another circuit containing a device such as a lamp, electric bell or motor which requires a large current.

The structure of a relay is shown in the figure below. 

When the controlling current flows through the coil, the soft-iron core is magnetized and attracts the L-shaped soft-iron armature. This rocks on its pivot and closes the electrical contacts in the circuit being controlled.