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Potentiometer preferred over voltmeter for measuring emf because?

RankGuru · Accepted Answer

1. **Voltmeter Limitation**: A voltmeter requires some current to show a deflection. This current causes a voltage drop ($Ir$) across the internal resistance of the cell being measured. Thus, it measures terminal voltage ($E - Ir$), not true EMF ($E$).
2. **Potentiometer Mechanism**: A potentiometer uses a 'null method'. At the balance point, the potential provided by the potentiometer wire exactly opposes the EMF of the cell.
3. **Result**: At balance, **zero current** is drawn from the cell.
4. **Highest Precision**: Because $I=0$, the measured potential difference is exactly equal to the EMF ($E$). It effectively acts like an infinite resistance voltmeter.

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