A relay is a
device that is used when we want one circuit to control another. This is especially
useful when one circuit may have to carry a larger current than the control
devices can take. A good example of this is the use of a computer (small current
output) to control a stage lighting circuit (large current).
To switch on the
relay a small current must flow in the circuit RS. When this happens the coil and
soft iron core become an electromagnet. The iron armature is attracted to it - the
contacts are closed and so the circuit PQ (large current) is made.