Metal detector at Bosworth

The museum at the Bosworth battlefield had this metal detector exhibit. You moved the detector horizontally along the top and noted that it buzzed when you could see a metal object in the tank below it. The detector consists of a coil with alternating current flowing in it. This sets up an oscillating magnetic field. When a metal object cuts this field, an eddy current is induced in the metal object in accordance with Faraday's Law. The eddy current is alternating because the field that creates it is oscillating. The oscillating alternating current in the metal object turns that into an electromagnet too. It produces its own magnetic field. This second magnetic field must always oppose the original magnetic field. If it added to it, energy would have been magicked up out of nowhere. Thus the second field weakens the first. If the metal detector contains a sensor coil for magnetic fields, then it can sense this weakening an trigger an audio alarm.