relay

relay electromechanical switch operated by a flow of electricity in one circuit and controlling the flow of electricity in another circuit. A relay consists basically of an electromagnet with a soft iron bar, called an armature, held close to it. A movable contact is connected to the armature in such a way that the contact is held in its normal position by a spring. When the electromagnet is energized, it exerts a force on the armature that overcomes the pull of the spring and moves the contact so as to either complete or break a circuit. When the electromagnet is de-energized, the contact returns to its original position. Variations on this mechanism are possible: some relays have multiple contacts; some are encapsulated; some have built-in circuits that delay contact closure after actuation; some, as in early telephone circuits, advance through a series of positions step by step as they are energized and de-energized.

relay set of fresh hounds, etc. posted to take up the chase from others XV; set of fresh horses at a stage XVII; relief gang XIX; (stage of) a relay race XX. — OF. relai (mod. relais), f. relayer (f. RE- + laier, ult. repr. L. laxāre; see LEASE), whence or from the sb. relay vb. †let go (fresh hounds) XV; provide with relays XVIII.

relay
1. In networking, a means of passing information between two or more networks, each offering a similar network function but each using a different protocol. In general a relay differs from a gateway or bridge in offering a store-and-forward service rather than a real-time service. As an example, a mail relay may be used to pass mail messages between networks using different mail protocols. See also cell relay, frame relay.

The term relay is used in some communities as synonymous with bridge or gateway. These three terms have meanings that vary between different communities at the same time, and within a given community at different times.

2. An electrically controlled switch enabling small signals to control much larger powers than would normally be possible as well as providing isolation between circuits. Various contact arrangements, e.g. normally open, normally closed, change-over and multiple poles, are available, as are bistable versions.