A Metal Detector is an electronic instrument which detects thepresence of metal nearby. Metal detectors are useful for finding metalinclusions hidden within objects, or metal objects buried underground. Theyoften consist of a handheld unit with a sensor probe which can be swept overthe ground or other objects. If the sensor comes near a piece of metal this isindicated by a changing tone in earphones, or a needle moving on an indicator.Usually the device gives some indication of distance; the closer the metal is,the higher the tone in the earphone or the higher the needle goes. Anothercommon type are stationary "walk through" metal detectors used forsecurity screening at access points in prisons, courthouses, and airports todetect concealed metal weapons on a person's body.
The simplest form of a metal detector consists of an oscillatorproducing an alternating current that passes through a coil producing analternating magnetic field. If a piece of electrically conductive metal isclose to the coil, eddy currents will be induced in the metal, and thisproduces a magnetic field of its own. If another coil is used to measure themagnetic field (acting as a magnetometer), the change in the magnetic field dueto the metallic object can be detected.