Abstract: A coin testing apparatus comprises transmitting and receiving coils on opposite sides of a coin passageway. The transmitting coil is connected to high and low frequency oscillators. In the high frequency channel the signal is amplitude controlled by a voltage controlled amplifier rectified by a rectifier and smoothed by a long time-constant circuit. The initial rise in level caused by a coin entering between the coils is detected by an instantaneous level change comparator which responds to the rate of change of signal level at the output of the long time-constant circuit becoming equal to a preset threshold and causes a normally closed switch to be opened. When the switch is closed a comparator compares the signal with a reference value from a source and adjusts the gain of the amplifier until the signal corresponds to the reference value.