A relay's an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch's operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay's able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier. Basic design and operation A simple electromagnetic relay, such as the… (More on Relay)