Is there anyway of measuring the potential difference between 2 points on a electrical circuit other than using a voltmeter? The reason I'm raising this question is because Voltmeters are developed purely using Ohm's Law and so certainly Ohm might have had his own way of measuring the p.d. for formulating the law. So.. Essentially the question is.. How did Ohm measure p.d. without a voltmeter..? Thats interesting isn't it?