A : parctically a voltmeter will measure the voltage across the battery but not its EMF.
R : EMF of a cell is measured with the help of a potentiometer .

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion , then mark
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion then mark.
C. If Assertion is ture statement but Reason is false , then mark
D. If both Assertion and Reason are false statements then mark.

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided:

**Assertion (A):** Practically, a voltmeter will measure the voltage across the battery but not its EMF.

**Reason (R):** EMF of a cell is measured with the help of a potentiometer.

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- A voltmeter is designed to measure the potential difference (voltage) across two points in a circuit. When connected across a battery, it measures the voltage drop across the battery terminals.
- However, due to the internal resistance of the battery, the voltage measured by the voltmeter (V) is given by the formula:
\[ V = E - I \cdot r \]
where \(E\) is the EMF of the battery, \(I\) is the current flowing through the circuit, and \(r\) is the internal resistance of the battery.
- Therefore, the voltmeter measures the voltage drop (V) across the battery, which is less than the EMF (E) when current flows. Thus, the assertion is correct.

2. **Understanding the Reason:**
- A potentiometer is an instrument used to measure the EMF of a cell without drawing any current from it. It works on the principle of null deflection, where the potential difference across the potentiometer wire is equal to the EMF of the cell when no current flows through the galvanometer.
- When the jockey touches the wire at a point where the potential difference matches the EMF, the current through the galvanometer becomes zero, indicating that the EMF is being measured accurately.
- Therefore, the reason is also correct.

3. **Conclusion:**
- Both the assertion (A) and the reason (R) are true.
- However, the reason (R) does not explain the assertion (A) directly. The assertion discusses the limitations of a voltmeter in measuring EMF, while the reason explains a method to measure EMF using a potentiometer.

### Final Answer:
Both assertion and reason are true, but the reason is not the correct explanation of the assertion. Therefore, the correct option is **B**.

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