This current creates an electromagnetic field. The direction of the current flow is
reversed several thousand times every second. After a few seconds, this current or
pulse slows down and disperses. Any metallic object like keys, coins, a knife or
even a cell phone which happens to enter this electromagnetic field will influence
the speed of the current. If the pulse hits a metallic object, it will take a few
milliseconds longer for the magnetic pulse to go away. This delay sets off an
alarm.