The underground metal detector search coils emit oscillating magnetic fields to detect the metal which you want.

A underground metal detector search coil is the rounded coil of wire on the end of the metal detector. The coil is fed a signal by the electronics in the body of the detector and transmits to the area surrounding it via an oscillating electromagnetic field. When the field comes into contact with a metal object, its shape will have changed. This is detected by the coil, which sends a signal back to the electronics in the metal detector, causing a sound that notifies the user of the presence of metal.

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