The following schematic illustrates the Simple Relay Circuit Diagram.
When the switch S is closed in a relay circuit, current flows in the coil, causing a strong magnetic field around the coil. The soft iron bar in the lamp circuit is attracted toward the right end of the electromagnet and makes contact with the conductor at A. A path is thus completed for current in the lamp circuit. When the switch is opened, the current flow through the electromagnet ceases and the magnetic field collapses and disappears. Since the attraction for the soft iron bar by the electromagnet no longer exists, the iron bar is pulled away from the contact by the piece of spring steel to which it is attached. This opens the contacts at A and breaks the circuit for the lamp.