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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.
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.