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 ametallic object, it will take a few milliseconds longer for the magnetic pulse to go away
this delay sets off an alarm