Potentiometer

A voltmeter is the everyday device used to measure the potential difference between two points in a circuit. An IDEAL voltmeter would not change the very potential difference it is trying to measure, and the only way to guarantee that is for it to draw absolutely no current -- which means it would need infinite resistance. In practice, no real voltmeter can be built with truly infinite resistance, so every ordinary voltmeter draws at least a small current and so disturbs the reading by a small, usually acceptable, amount. …