This rather fragile switch is normally open (NO). If a magnet it positioned near the metal contacts, the magnetic field causes the contacts to attract each other and the switch turns on.
This device is often found in door and window alarms. When the door is opened the magnet (in the door) moves away from the switch (in the door frame). This breaks the circuit and the alarm sounds.
The glass tube is filled with an inert gas so corrosion is minimised. This makes reed switches more reliable.
