R Mech Machines LLP manufactured  a metal detector in its most basic form consists of an oscillator that generates an alternating current that travels through a coil that generates an alternating magnetic field. If a piece of electrically conductive metal is placed close to the coil, eddy currents  are created in the metal, which produces its own magnetic field. The change in the magnetic field due to the metallic object can be noticed if another coil is used to measure the magnetic field .