Reed proximity switch
Phoenix America

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Elwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied.

A magnetic field (such as from an energized coil around the glass tube or a permanent magnet moved towards it) will cause the contacts to pull together, thus completing an electrical circuit. The stiffness of the reeds causes them to separate, and open the circuit, upon removal of the magnetic field. A more complicated configuration contains a nonferrous normally closed contact that opens when the ferrous normally open contact closes. Good electrical contact is assured by plating a thin layer of precious metal over the flat contact portions of the reeds.