Why is a potentiometer universally preferred over an ordinary moving-coil voltmeter for measuring the true electromotive force (EMF) of a chemical cell?

Answer and explanation

Correct answer: C. At null balance, it draws zero current and measures true open-circuit EMF accurately

  1. Step 1: An ordinary voltmeter has a finite internal resistance (), so it must draw a small current () from the cell to produce a pointer deflection.
  2. Step 2: Because of this current draw, the voltmeter measures terminal voltage , which is always less than the true EMF .
  3. Step 3: A potentiometer operates on the null deflection principle: at the balance point, the opposing potential drop across the wire exactly balances the cell EMF, so current drawn from the test cell is strictly zero ().
  4. Step 4: Therefore, the potentiometer behaves as an ideal voltmeter with infinite input resistance (), measuring the exact open-circuit EMF.
Answer
At the null balance point, the potentiometer draws zero current from the cell under test, measuring its true open-circuit EMF
Why not the others: 'Potentiometer wire has zero resistance' is false; the wire must have uniform resistance to establish a potential gradient; 'Can only measure AC' is false; it measures DC potentials; 'Voltmeter increases cell EMF...' is fabricated.

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