A reed switch is a small electromechanical device generally having two ferromagnetic leads that are separated internally in a hermetically sealed glass envelope. When the reed switch, which comes in various sizes, is brought into a magnetic field, the reed leads will close creating a switching function. Closing the reed contacts takes a certain magnetic strength measured in milli-Tesla (mT) or ampere turns (AT). This is called the pull-in point (PI). When removing the magnet the contacts will open at some point. This is called the drop-out point (DO). Hysteresis is another useful parameter, it is the ratio of the pull-in divided by drop-out (PI/DO). 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.