How does a metal detector work?

The search head contains a coil of wire carrying an alternating or pulsed electric current, which produces a rapidly changing magnetic field. When this field passes through a metal object, it induces small swirling electric currents in the metal, called eddy currents. These currents generate their own weak magnetic field, which is picked up by a receiver coil or by the same coil between pulses. The detector's electronics turn this into a sound or reading, and many models can estimate what type of metal has been found. The same principle is used in airport security gates, landmine clearance and treasure hunting.