Metal Detectors

Metal detectors use electromagnetic induction to detect metal. Uses include demining (the detection of land mines), the detection of weapons such as knives and guns, especially at airports, geophysical prospecting, archeology and 'treasure hunting'. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect steel reinforcing bars in concrete and pipes and wires buried in walls and floors.

In it's simplest form, a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. If a piece of metal, which is electrically conductive, is close to the coil eddy currents will be induced in the metal, and this produces an alternating magnetic field of its own. If another coil is used to measure the magnetic field (acting as a magnetometer) the change in the magnetic field due to the metallic object can be detected.

(Text courtesy Wikipedia.org)