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 (sometimes also known as Herkon) is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferromagnetic 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. The switch may be actuated by a coil, making a reed relay, or by bringing a magnet near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back to its original position.