what is the principle of METAL DETECTORS ?
Metal detectors respond to the metal which is not be able to seen.
Metal detector consist of a oscillator which produce an alternating current which passes through a coil and a change in magnetic flux across it develops. If an electrically conducting metal comes near to the coil, the eddy currents induce into the metal and correspondingly magnetic flux linked with the metal changes. This change in magnetic flux linked with the metal is detected by another coil.