Metals when present near a magnetic field create disturbance in the field. This is similar to the waves created by a stone thrown into a stream of water. Metal detectors are equipped with sensitive electronic circuitry to detect disturbances created by metal objects in the magnetic field. The hand held metal detectors have two coils which create magnetic fields of similar strengths. Whenever a metal object comes near any of the coils it disturbs the magnetic field there. This upsets the balance between the two fields and alerts an electronic circuitry. Some detectors do not have any coils, but instead sense the disturbances created by metal objects in the natural field present in the surroundings. The door-frame type of metal detectors works this way. They have a series of coils in the frame which continuously monitor the surrounding magnetic field. Any disturbance caused in the field is sensed and an alarm set off.