the principle of a metel detector is quite simple . inside the machine are wire coils . a transmitter sends an electronic current through the coils . the current goes from clockwise to counterclockwise and back to clockwise again . this current creates an electromagnetic fied . the direction of the current flow is reversed several thousand times every second . after a few second,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.