reed switch

Switch containing two or three thin iron strips, or reeds, inside a sealed glass tube. When a magnet is brought near the switch, it induces magnetism in the reeds, which then attract or repel each other according to the positions of their induced magnetic poles. Electric current is switched on or off as the reeds make or break contact.

A simple burglar alarm can be constructed by inserting a magnet into the closing edge of a door, and a reed switch into the part of the doorframe facing the magnet: when the door is closed the magnet remains close to the switch, keeping a circuit open; when the door is opened the magnet moves away, closing the switch and setting off an alarm.