Advantages of a Potentiometer Over a Voltmeter

Compared with an ordinary voltmeter, a potentiometer used at its balance point has three real merits. First, it is MORE SENSITIVE than a voltmeter -- a very small potential difference, of order $10^{-6}$ V, can still be detected, because the potentiometer's least count (its smallest reliably measurable change) is far better than a voltmeter's. Second, a potentiometer can be used to measure BOTH a potential difference AND the true emf of a cell, whereas an ordinary voltmeter, because it always draws some current, can only ever measure a cell's terminal potential difference (which is slightly less than the true emf once any current is drawn) and never the emf itself. Third, precisely because it draws no current at balance, measurement of potential difference or emf with a potentiometer is very ACCURATE. …