The reed switch is an electromechanical switch operated by an applied magnetic field. It was invented in 1922 by professor Valentin Kovalenkov at Leningrad 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. Description The most common type of reed switch contains a pair of magnetizable, flexible, metal reeds whose end portions are separated by a small gap when the switch is open. The reeds are hermetically sealed within a tubular glass envelope. Another type of reed switch contains one flexible reed that moves between a fixed normally-open contact and a fixed normally-closed contact. The normally-closed contact is non-ferromagnetic and is closed by the flexible reed's spring force. Although reed switches with multiple poles are possible, more often an assembly of single-pole reed switches is used for multi-pole applications. A magnetic field from an electromagnet or a permanent magnet will cause the reeds to attract each other, thus completing an electrical circuit. The spring force of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. Another configuration contains a non-ferromagnetic normally-closed contact that opens when the ferromagnetic normally-open contact closes. A thin layer of non-ferromagnetic material is applied to the reed switch contact area to serve as an electrical contact switching (wear) surface and, for normally-open contacts, as a magnetic spacer whose thickness is important in controlling the magnetic field level at which the contact opens (the drop-out). Reed