the principle of a metal detectors is quit simple. inside the machine are wire coils. a transmitter sends an electronic current through the coils. the current flow goes from clockwise to counterclockwise and back to clockwise again. 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 milloseconds longer for the magnetic pulse to go away. this delay sets off an alarm.