Concept:A potentiometer measures the accurate e.m.f. of a cell by using a null-deflection method, in which no current is drawn from the cell during the measurement.
Explanation:When a voltmeter is connected across a cell, it draws a small current
I through the cell's internal resistance
r.
As a result, the voltmeter reads the terminal potential difference
V=E−Ir, which is slightly lower than the true electromotive force
E of the cell.
This happens because some energy is lost inside the cell due to its internal resistance.
A potentiometer, however, works by balancing the unknown e.m.f. against a known potential gradient along a uniform wire.
At the balance point, the galvanometer connected to the cell shows zero deflection.
This zero deflection indicates that no current is being drawn from the cell at the moment of measurement.
Since no current flows through the cell, there is no potential drop across its internal resistance.
Therefore, the value obtained is the true e.m.f. of the cell, making the potentiometer the most accurate instrument for this purpose.
Answer:D. potentiometer