A method for testing coins which move along a runway, using a coil assembly which, on either side of the runway, has a transmission coil C, D adapted to be admitted by a transmission signal and a reception coil A, B on a common ferrite core wherein the reception coil smaller in diameter is closer to the runway than is the transmission coil and the diameter of the transmission coil is smaller than the diameter of the smallest coin to be assumed, and wherein the ferrite core has arranged thereon a secondary coil coupled to the transmission coil the signal of which is connected, as a negative-feedback signal, to the input of a differential amplifier in such a way that the signal of the transmission coil agrees with the transmitted signal provided to the other input of the differential amplifier.