A '''relay''' is an [[electrically]] operated switch. Many relays use an [[electromagnet]] to operate a switching [[mechanism]] mechanically, but other operating principles are also used. Relays are used where it is [[necessary]] to control a [[circuit]] by a low-power [[signal]] (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the [[signal]] coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early [[computers]] to perform [[logical]] operations. | A '''relay''' is an [[electrically]] operated switch. Many relays use an [[electromagnet]] to operate a switching [[mechanism]] mechanically, but other operating principles are also used. Relays are used where it is [[necessary]] to control a [[circuit]] by a low-power [[signal]] (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the [[signal]] coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early [[computers]] to perform [[logical]] operations. |