At a basic level, a Reed Switch is an electrical switch that is operated by a magnetic field. A Reed Switch consists of two ferromagnetic metal blades, or “reeds,” that are hermetically sealed within a glass envelope. The glass envelope typically contains nitrogen gas to eliminate the presence of oxygen and ensure that the electrical contacts do not oxide. The presence of a magnetic field of sufficient strength will either close or open the electrical contacts depending on whether the Reed Switch is of the normally-open or normally-closed type. The unit of measurement used to describe the magnetic sensitivity of a Reed Switch is the Ampere-turn (AT). For a normally-open switch, the “Pull-In” value is the AT value at which the electrical contacts close, and the “Drop-Out” value is the AT value at which the contacts open.