These are normally used to secure an security\burglar\Access Control alarm circuit on a door, roller 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 (current flows), and open when apart. Normally CLOSED (NC) per pair.
We also have commercial grade and high security reed switches.