The principle of a metal detector is quite simple. Inside the machine are wire coils. A transmitter sends an eletronic current through the coils. The current flow goes from clockwise to counterclockwise and back to clockwise again. This current creates an electromagnetic field. The dorection of the current flow is reversed several thousand times every second. After a few seconds, this current or pulsr slowd 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 objec, it will take a few milliseconds longer for magnetic pulse to go away. This delay sets off an alarm