Reed switch
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Image:Reedrelay.jpg Image:Reed switch (aka).jpg
A reed switch is an electric switch that consists of a pair of ferrous metal contacts on metal reeds in a hermetically sealed glass envelope (A hermetic seal is an airtight seal). A magnetic field from a permanent magnet or an electromagnet in close proximity to the switch will cause the contacts to pull together, thus completing an electrical circuit. A more complicated configuration contains a nonferrous normally closed contact that opens when the ferrous normally open contact closes.
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 an electromagnet is an electrical relay called a reed relay. In a reed relay, the electromagnet consists of a coil with the reed switch inside the coil.
A reed relay has two or more, usually normally open, contacts inside a vacuum or inert gas filled glass tube. This protects the contacts against atmospheric corrosion. The two contacts are closed by magnetism from a coil around the glass tube, or a permanent magnet moved towards it.
The hermetic sealing of a reed switch make them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard. The leads of the switch must be handled carefully to prevent breaking the glass envelope.
There are also versions of reed switches with Mercury wetted contacts.