Potentiometer

NCERT Class 12, Chapter 3 explains that a potentiometer measures potential difference by a null method, in which the potential drop along a length of uniform wire is balanced against the cell's emf so that at the balance point no current flows through the cell being tested. Because no current is drawn at balance, there is no potential drop across the cell's internal resistance, and the instrument records the true emf rather than the terminal voltage. A voltmeter, by contrast, always draws some current and therefore reads a slightly lower terminal voltage. The option about larger internal resistance is wrong because it is the zero-current null condition, not merely high resistance, that guarantees the true emf. The option about amplification is wrong because a potentiometer does not amplify; it compares potentials directly. The option restricting it to emfs above ten volts is wrong because potentiometers are especially suited to small emfs. The instrument works by maintaining a steady current through a long uniform wire from a driver cell, creating a uniform potential gradient along its length; the cell under test is balanced against a portion of this gradient. Because the sensitivity improves with a smaller potential gradient, a longer wire or lower driver current lets the potentiometer resolve very small differences in emf. A consistency check confirms that only a zero-current measurement avoids the internal-resistance drop, which is precisely why the potentiometer gives the accurate emf rather than the terminal voltage.