The principle of a metal detector is quite 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
milliseconds longer for the magnetic pulse to go away. This delay sets off an
alarm.