In The History of Landmines, Mike Croll says
The Polish detector had two coils, one of which was connected to an oscillator which generated an oscillating current of an acoustic frequency. The other coil was connected to an amplifier and a telephone. When the coils came into proximity to a metallic object the balance between the coils was upset and the telephone reported a signal. The equipment weighed just under 30 pounds and could be operated by one man. The Polish detector saw service throughout the war and the Mark 4c version was still used by the British Army until 1995.