In a screw gauge, the main scale has divisions in millimetre and circular scale has `50` divisions. The least count of screw gauge is

Options

A. `2` microns
B. `5` microns
C. `20` microns
D. `50` microns

Explanation

To find the least count of a screw gauge, we can use the formula:

\[ \text{Least Count} = \frac{\text{Pitch}}{\text{Number of divisions on the circular scale}} \]

### Step-by-Step Solution:

1. **Identify the Pitch**: The pitch of a screw gauge is the distance moved by the screw when it is rotated by one complete turn (360 degrees). In this case, we need to know the pitch value. However, since it is not provided in the question, we will assume a common pitch value for a screw gauge, which is typically 1 mm.

2. **Identify the Number of Divisions**: The circular scale has 50 divisions as given in the problem.

3. **Calculate the Least Count**:
Using the formula for least count:
\[ \text{Least Count} = \frac{\text{Pitch}}{\text{Number of divisions}} \]
Substituting the values:
\[ \text{Least Count} = \frac{1 \text{ mm}}{50} = 0.02 \text{ mm} \]

4. **Convert to Micrometers**: Since 1 mm = 1000 micrometers, we convert the least count to micrometers:
\[ 0.02 \text{ mm} = 0.02 \times 1000 \text{ micrometers} = 20 \text{ micrometers} \]

5. **Final Result**: Therefore, the least count of the screw gauge is:
\[ \text{Least Count} = 20 \text{ micrometers} \]

### Summary:
The least count of the screw gauge is 20 micrometers.