This circuit provides an accurate measure of the internal resistance of an analog instrument. Just activate the circuit and adjust the potentiometer from 5k (4k7) to half scale of the instrument. When this occurs, the adjusted resistance is equal to that of the instrument. The circuit is powered by a single common battery. It will be convenient to connect a 1k resistor in series with the circuit to avoid a short-circuit current in the instrument, causing it to burn if its resistance is too low.
