A moving coil voltmeter is generally used to measure the potential difference across a conductor of resistance `'r'` carrying a current `i`. The resistance of voltmeter is `R`. For more correct measurement of potential difference

Options

A. `R=r`
B. `Rgtgtr`
C. `Rltltr`
D. `R=0`

Answer

To solve the problem regarding the correct measurement of potential difference using a moving coil voltmeter, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Setup**:
- We have a conductor with resistance \( r \) carrying a current \( i \).
- A moving coil voltmeter with resistance \( R \) is connected in parallel to the conductor to measure the potential difference across it.

2. **Potential Difference in Parallel Connection**:
- Since the voltmeter is connected in parallel with the conductor, the potential difference across the voltmeter (\( V \)) is the same as the potential difference across the conductor.
- Therefore, we can write:
\[ V = I_1 \cdot R = I_2 \cdot r \]
- Here, \( I_1 \) is the current through the voltmeter and \( I_2 \) is the current through the conductor.

3. **Current Ratio**:
- From the above equation, we can express the ratio of the currents:
\[ \frac{I_2}{I_1} = \frac{R}{r} \]

4. **Condition for Accurate Measurement**:
- For the voltmeter to measure the potential difference accurately, we want most of the current \( i \) to flow through the conductor rather than the voltmeter. This means:
\[ I_2 \gg I_1 \]
- This implies:
\[ \frac{I_2}{I_1} \gg 1 \]
- Substituting from our earlier expression, we have:
\[ \frac{R}{r} \gg 1 \]
- This means that the resistance of the voltmeter \( R \) should be much greater than the resistance of the conductor \( r \):
\[ R \gg r \]

5. **Conclusion**:
- Therefore, for a more accurate measurement of potential difference, the resistance of the voltmeter \( R \) should be much greater than the resistance of the conductor \( r \).

### Final Answer:
The correct condition for more accurate measurement of potential difference is:
\[ R \gg r \]