Monday, June 16, 2014

The Reed Switch

 The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by Walter B. Ellwood. The patent was granted in 1938 (US2289830 ). It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass tube filled with inert gas. When the unit is exposed to a magnetic field, the two ferrous "reeds" or leaf springs inside the switch pull together and the switch closes. When the magnetic field is removed or reversed, the reeds separate and the switch opens. It's a type of non-contact switch with numerous applications including a few in radio.

In 1967 Electrical Review called it "revolutionary" Telecom Journal was equally amazed.  In his patent Ellwood explained the advantages:
"One of the chief advantages of this invention is that it eliminates the complicated mechanical and magnetic structures of the usual electromagnetic relays and enables these to be replaced with a simple pair of magnetic contacts which perform both the magnetic and electrical functions essential to an electromagnetic circuit closing device."
He didn't even mention that it was protected from oxidation by the inert gas barrier in the sealed tube.
Ellwood was from Missouri, but got his PhD at Columbia University in New York in 1933.  He had actually been hired in 1932 by Bell labs... but they let him finish that pending thesis. He was 30 years old.

The two most common places I see reed switches in radio are actually not in the broadcasting booth. It's in the transmitter shack. Against the door jamb is usually a reed switch, and attached to the door is a magnet. (or vice versa) when the door swings open an alarm sounds. It's a simple mechanism but it spooks those copper thieves. Ellwood also invented ferrite bead dissipators to reduce contact damage from reflections of electric waves in transmission lines.