Relay

A relay is an electro-mechanical device that, depending on the presence or absence of a voltage (and thus current) across a pair of contacts, opens and closes circuits between other pairs of contacts. How many pairs, and whether they are open or closed when incoming voltage is applied, will depend on the design of the specific relay.

Generally, the incoming current is fed into a coil, which functions as an electromagnet. That either attracts (or not) a hinged, spring-loaded, piece of metal, which has contacts which can make or break the circuits with the output contacts, depending on its position.

Early computing devices used relays for logic; a few of them did not use any electronics at all, just relays.