Permanent magnets produce invisible lines of flux that leave the north pole and re-enter the south pole. This magnetic flux interacts with the magnetic sensitivity lobes of the reed switch. These invisible lines of flux represent the hidden force behind a magnet, and it is these lines of flux that produce the force that closes the contacts. When the magnet is removed and the lines of flux are no longer influencing the reed switch, the magnetic forces are no longer present, and the contacts will open.