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Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. Relays were first used in long-distance [[Electrical telegraph|telegraph]] circuits as signal repeaters: they refresh the signal coming in from one circuit by transmitting it on another circuit. Relays were used extensively in [[telephone exchange]]s and early computers to perform logical operations.
The traditional [[electromechanical]] form of a relay uses an [[electromagnet]] to close or open the contacts, but relays using other operating principles have also been invented, such as in [[solid-state relay]]s which use [[semiconductor]] properties for control without relying on [[moving parts]]. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called ''[[protective relay]]s'' or ''[[safety relay]]s''.
Latching relays require only a single pulse of control power to operate the switch persistently. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effects. Magnetic latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling.
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