A magnetic contact is an automatic alarm for monitoring doors, windows or other movable parts. The magnetic contact consists of a reed contact and a permanent magnet. The reed contact includes a switch, that is located in a small dust and waterproof glass tube.
The reed contact is actuated without physical contact by a permanent magent. The reed contact is installed e.g. into the door frame, the magnet into or onto the door leaf. When the door is closed, the magnet effects the reed contact (contact closed). If the door or window is opened wider than the given sensing distance, the contact is interrupted with closer type contacts (contact opern), or a switchover happens with changeover type contacts.

When the door or window is opened, the reed contact is opened as the magnet is taken away, the magnetic field altered and the signal line interrupted.
Magnet contacts are also available with waterproof design (e.g. IP 67). They are then sealed in a impact-proof synthetic casing.