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.

Race between teams in which each team member successively covers a specified portion of the course. In track events, such as the 4 × 100-m and 4 × 400-m relays, the runner finishing one leg passes a baton to the next runner while both are running within a marked exchange zone. In swimming competitions, such as the 4 × 100-m and 4 × 200-m freestyle races and the 4 × 100-m medley, the swimmer completing one leg touches the edge of the pool to signal the start of the next teammate's leg.

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