State the principle of potentiometer. With the help of circuit diagram, describe a method to find the internal resistance of a primary cell.

Explanation

### Step-by-Step Solution

**Step 1: State the Principle of Potentiometer**
The principle of a potentiometer states that the potential drop across a segment of a wire of uniform cross-section carrying a constant current is directly proportional to the length of that segment. Mathematically, this can be expressed as:
\[ V \propto L \]
where \( V \) is the potential drop and \( L \) is the length of the wire segment.

**Step 2: Draw the Circuit Diagram**
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