Potentiometer Principle

A potentiometer measures potential difference by balancing it against the voltage drop along a uniform wire, and at the balance point the cell being tested drives no current, a property emphasised in NCERT Class 12, Chapter 3 (Current Electricity). Because zero current flows from the test cell at balance, there is no drop across its internal resistance, so the potentiometer reads the true emf rather than a reduced terminal voltage. A voltmeter, by contrast, always draws some current and therefore measures a slightly lower terminal voltage. The option about larger internal resistance touches on why good voltmeters are designed with high resistance, but it is not the defining reason here; the potentiometer's real strength is drawing literally zero current at the balance point. The option about amplifying the emf is wrong because a potentiometer performs no amplification whatsoever; it simply compares one potential against a known drop. The option about measuring current is wrong because the potentiometer fundamentally measures potential difference by a null method, not current. A plausibility check confirms the logic: with $I=0$ flowing through the test cell at balance, its terminal voltage equals $\epsilon -Ir=\epsilon -0=\epsilon$, so the value measured is exactly the true emf with no internal-resistance error at all.