A relay is an electrical switch controlled by the signal from another circuit. An electromagnetic or electromechanical relay is based on an electromagnet (coil of wire surrounding a soft iron core). When the signal current is flowing, a hinged iron armature is attracted to the core and moves the contacts of the switch. Shown here are two Schrack RP 710024 electromechanical relays made by Tyco Electronics. One of the relays has its enclosure removed so that the internal construction can be seen more clearly. Photo July 2009.