That is a magnetic switch (the smaller section is usually the magnet) and it will sound when the two pieces are separated from each other. Not of any use as a driveway alarm. Used on doors and windows mostly, as the door or window is opened (one section mounted on the moveable door or window-- and the other affixed to the frame-- the two sections separate and the alarm sounds.
This is a definition from Wik:
"The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied. The switch may be actuated by a coil, making a reed relay,[1] or by bringing a magnet near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back to its original position.
An example of a reed switch's application is to detect the opening of a door, when used as a proximity switch for a burglar alarm. "
You can do a search for driveway alarms if that is what you need.
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