A student is recuired to measure emf of a cell the should use .

Options

A. Potentiometer
B. Voltmeter
C. ammeter
D. either (1) or (2)

Answer

To measure the electromotive force (EMF) of a cell, the student should follow these steps:

### Step-by-Step Solution:

1. **Understanding the Concept of EMF**:
- EMF is defined as the potential difference across the terminals of a cell when no current is flowing. This means the cell must be in an open circuit condition.

2. **Choosing the Right Instrument**:
- The student has three options: potentiometer, voltmeter, and ammeter.
- A voltmeter measures the potential difference across a component when it is connected in a circuit, which means it draws current from the cell. Therefore, it cannot measure EMF accurately.
- An ammeter measures the current flowing through a circuit, which is not relevant for measuring EMF directly.

3. **Using a Potentiometer**:
- A potentiometer is an instrument that can measure the EMF of a cell without drawing any current from it. It operates on the principle of null deflection, where the voltage across a known length of wire is balanced against the EMF of the cell being measured.

4. **Setting Up the Experiment**:
- Connect the potentiometer to the cell whose EMF is to be measured. Ensure that the circuit is open so that no current is drawn from the cell.
- Adjust the potentiometer until the galvanometer shows no deflection (null point). This indicates that the potential difference across the potentiometer wire equals the EMF of the cell.

5. **Reading the EMF**:
- At the null point, read the length of the potentiometer wire that corresponds to the EMF of the cell. This length can then be used to calculate the EMF using the known voltage per unit length of the potentiometer wire.

### Conclusion:
The correct instrument to measure the EMF of a cell is a **potentiometer** since it allows for measurement without drawing current from the cell.