These are used to secure an alarm circuit on a door or window. Sold as a pair, one has a magnet inside, the other a reed switch. They have powerful magnets, with self adhesive or screw mount, and screw terminal connections.

Reed Switch contacts: closed when both units are together (ie current flows), and open when apart.

Normally CLOSED (NC) per pair.

Contact operation: Contacts are closed cicuit when the magnet is alongside the switch. For alarm installation this is denoted as the "normal" state as the door/window is secure and the alarm is armed.