Screw gauge is said to have a negative error.

Options

A. (a)when circular scale zero coincides with base line of main scale,
B. (b)when circular scale zero is above the base line of main scale.
C. (c)when circular scale zero is below the base line of main scale.
D. (d)None of the above.

Answer

To determine when a screw gauge is said to have a negative error, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Screw Gauge**:
- A screw gauge consists of a main scale and a circular scale. The main scale has a baseline, and the circular scale is used to measure small dimensions with high precision.

2. **Identifying Zero Error**:
- Zero error occurs when the zero mark of the circular scale does not align with the baseline of the main scale. This misalignment can lead to inaccuracies in measurement.

3. **Types of Errors**:
- **Zero Error**: When the zero of the circular scale coincides with the baseline of the main scale, there is no error.
- **Positive Error**: If the zero of the circular scale is below the baseline of the main scale, it indicates a positive error.
- **Negative Error**: If the zero of the circular scale is above the baseline of the main scale, it indicates a negative error.

4. **Analyzing the Options**:
- **Option A**: When the circular scale zero coincides with the baseline of the main scale, this indicates zero error, not negative error.
- **Option B**: When the circular scale zero is above the baseline of the main scale, this is the correct condition for a negative error.
- **Option C**: When the circular scale zero is below the baseline of the main scale, this indicates a positive error.

5. **Conclusion**:
- The correct answer is **Option B**: The screw gauge is said to have a negative error when the zero of the circular scale is above the baseline of the main scale.