This electronics beginner asks you to imagine two batteries of marginally unequal voltage, their negative poles connected together with the slider terminal of a potentiometer. One potentiometer terminal is connected to one battery positive and the other potentiometer terminal is connected to the other battery positive. Also connected between the battery positives, in parallel with the potentiometer, is a centre zero reading moving coil meter. Given an appropriate value for the potentiometer, will operating the slider null the meter? And, if it will, and the potentiometer is calibrated, will it indicate the same null point for any two batteries with exactly the same proportional difference of voltage, regardless of total voltage in the circuit? Also, is this the simplest circuit for establishing such comparisons? Or have I got this all terribly wrong?