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A relay is an electrically operated or electromechanical switch composed of an electromagnet, an armature, a spring and a set of electrical contacts. The electromagnetic switch is operated by a small electric current that turns a larger current on or off by either releasing or retracting the armature contact, thereby cutting or completing the circuit. Relays are necessary when there must be electrical isolation between controlled and control circuits, or when multiple circuits need to be controlled by a single signal.
A relay is an electromagnetic switch; hence, its heart is the electromagnet, which is powered by a small current that acts as a lever or as the switch itself. This makes it possible to allow relatively small electric currents to leverage and control much larger electrical currents. Sensors are sensitive devices, and they only produce small amounts of electric currents, but in order for a sensor to drive larger pieces of equipment it needs something that would switch on this equipment by allowing larger currents to flow. In this way, the sensor can act as the control input for the relay so that when it is activated, larger currents flow to the equipment. For example, photo or light sensors can be assigned to control outdoor lights so that when it becomes dark outside the light sensors activate the relays, which act as the light switches.
The small control current is used to energize the electromagnet, which pulls the armature toward it. The armature makes contact with the other end of the circuit, which completes the circuit and allows current to flow. When the electromagnet is deactivated, the spring attached to the armature pulls it back, cutting off the flow of electricity.
Because of the feature of having separate control signals, relays were heavily used in earlier computers to form logic circuits such as with the Harvard Mark II, ARRA, Zuse Z2 and Zuse Z3. They were also widely used as a logical control for the complex switching found in early telephony systems such as the Strowger and Crossbar telephone exchange systems.