Reed relay and reed switches

A reed switch is an electric switch that consists of a pair of ferrous metal contacts in a hermetically sealed glass envelope. A permanent magnet placed in close proximity to the switch will cause the contacts to pull together, thus completing an electrical circuit.

Reed switches are commonly used in mechanical systems as proximity switches as well as in door and window sensors in burglar alarm systems. A reed switch combined with a copper coil is an electrical relay called a reed relay.