Why is the potentiometer preferred to a voltmeter for measuring emf of a cell ?

Explanation

### Step-by-Step Solution:

1. **Understanding EMF**:
- EMF (Electromotive Force) is defined as the potential difference across the terminals of a cell when no current is flowing through the circuit. This is the maximum potential difference that the cell can provide.

2. **Difference Between EMF and Terminal Voltage**:
- When a current flows through the cell, the voltage measured across its terminals is known as the terminal voltage. This voltage is always less than the EMF due to internal resistance within the cell.

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