A reed sensor is comprised of a small hermetically sealed glass cylinder usually sealed and packaged in plastic or metal having two leads that directly provide a switching function. Reed sensors come in hundreds of different sizes and shapes suitable for a variety of applications. The sensitivity to closure is measured in milli-tesla (mT) or ampere turns (AT). The reed sensor typically uses an external magnet to close the contacts. When the magnet is brought close enough to the reed sensor, the contacts will close or pull-in (PI). Withdrawing the magnet, the contacts will open or drop-out (DO). Hysteresis is another useful parameter. Hysteresis is just the ratio of the pull-in divided by drop-out (PI/DO).