What is the principle of potentiometer?

Explanation

### Step-by-Step Solution

1. **Understanding the Potentiometer**: The potentiometer is an instrument used to measure the electromotive force (emf) of a battery or to find an unknown voltage in a circuit. It operates on the principle of comparing potential differences.

2. **Components of a Potentiometer**: A typical potentiometer consists of a long wire (or multiple wires) of uniform cross-section. The wire is usually made of a material with a known resistivity, and it is connected in a circuit with a constant current flowing through it.

3. **Principle of the Potentiometer**: The key principle of the potentiometer is that the potential drop (voltage) across a segment of the wire is directly proportional to the length of that segment. This can be expressed mathematically as:
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