A coil, mounted vertically on a base, with 10 turns of insulated copper wire and a diameter of 15 cm. Electrical connection to the coil is via 4 mm sockets, which are mounted on the base. When AC is passed through the coil, an alternating magnetic field is induced, which can be detected using a search coil.
Requires an AC power supply which can deliver up to 5 Amps at low voltage (e.g. a Westminster power supply). The current should not exceed 5A.