Which of the following instruments can be used to measure e.m.f. most accurately.

1. Analyze the question You are asked which instrument measures e.m.f. (electromotive force) most accurately. The key word is "most accurately."

2. Explain the concept The e.m.f. of a cell is its potential difference when no current flows through it (open circuit). If current is drawn, some voltage is lost across the cell's internal resistance (V = E − Ir), so the reading is lower than the true e.m.f.

• Voltmeter: It has a high but finite resistance, so it draws a small current. The reading is slightly less than the true e.m.f.
• Ammeter: It measures current, not e.m.f., and has very low resistance.
• Galvanometer: It detects small currents and is used as a null detector. It does not give a direct e.m.f. value.
• Metre bridge: It compares or measures resistances (Wheatstone bridge principle).
• Potentiometer: It works by the null (balance) method. At the balance point, no current flows through the cell being tested, so the internal resistance has no effect. The potentiometer therefore measures the true e.m.f.

3. Solution For a potentiometer with a driver cell giving a uniform potential gradient along the wire:

E₁/E₂ = l₁/l₂

where l₁ and l₂ are the balance lengths for the two cells. Because the balance point is found with zero current drawn from the cell, the measurement is very accurate.

4. Answer The potentiometer is the most accurate instrument for measuring e.m.f.

Correct option: D. Potentiometer

5. Exam tip Remember: "Potentiometer = Potential, no current drawn." Whenever a question asks for the most accurate measurement of e.m.f. or potential difference, think of the null method, which draws zero current. A voltmeter is only an approximation because it always draws a little current.