A reed switch is a passive device consisting of two Reed Blades sealed inside a glass tube with an Inert Gas, which operates when brought near a magnetic field.
The reeds are hermetically sealed in cantilever form so that their free ends overlap and are separated by a small air gap. The contact area of each blade can be coated with one of many types of contact materials such as Ruthenium, Rhodium, Tungsten, Silver, Irridium, Molybdenum etc.
Due to the low inertia of the Reed Blades and the small gap, fast operation is achieved. The inert gas inside the sealed Reed Switch not only prevents oxidation of the contact material but also helps in making it one of the few devices that can be used in explosive environments.
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The reed switch is an electromechanical switch operated by an applied magnetic field. It was invented in 1922 by professor Valentin Kovalenkov at the Petrograd Electrotechnical University, and later evolved at Bell Telephone Laboratories in 1936 by Walter B. Ellwood into the reed relay. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open when a magnetic field is applied. The switch may be actuated by an electromagnetic coil, making a reed relay, or by bringing a permanent magnet near it. When the magnetic field is removed, the contacts in the reed switch return to their original position. The "reed" is the metal part inside the reed switch envelope that is relatively thin and wide to make it flexible. It somewhat resembles part of some reed plants. The term "reed" may also include the external wire lead as well as the internal part.
A common example of a reed switch application is to detect the opening of a door or windows, for a security alarm.