This circuit was found in a 1963 magazine. The magazine no longer exists, but the circuit can be mounted using BC558 transistors, a 6 V x 50 mA lamp and for the diode we can use the 1N4002 or 1N4004. The circuit allows to compare impedances of high value coils (above 100 mH, and even to measure them if the potentiometer is calibrated.) The transformer has 6 + 6 V secondary with current from 250 mA.
