The reed switch is a small electromechanical switching device that has two ferromagnetic leads encased in a hermetically sealed glass cylinder. Ferromagnetic materials are materials that become magnetized when they enter or are influenced by a magnetic field. Iron, nickel, and cobalt are the most common ferromagnetic materials. The leads are internally aligned and separated by a gap. The reed switch can be as small as 4mm and as large as 50mm long.