Which one of the following intruments is not used for the measurement of length?
Atomic clock
vernier callipers
Screw gauge
Spherometer

Options

A. Atomic clock
B. varnier callipers
C. Screw gauge
D. Spherometer

Explanation

To determine which instrument is not used for the measurement of length among the given options, we will analyze each instrument one by one.

### Step-by-Step Solution:

1. **Identify the Instruments**: We have four instruments listed:
- Atomic clock
- Vernier calipers
- Screw gauge
- Spherometer

2. **Analyze the Atomic Clock**:
- An atomic clock is a device that measures time based on the vibrations of atoms, specifically the electromagnetic radiation emitted by atoms when they change energy levels.
- It is primarily used for precise timekeeping and is essential in global positioning systems (GPS) and other applications that require accurate time measurement.
- **Conclusion**: The atomic clock is not used for measuring length.

3. **Analyze the Vernier Calipers**:
- Vernier calipers are precision instruments used to measure small lengths with high accuracy. They can measure internal and external dimensions as well as depths.
- They have a least count of around 0.01 cm (or 0.1 mm).
- **Conclusion**: Vernier calipers are used for measuring length.

4. **Analyze the Screw Gauge**:
- A screw gauge (or micrometer screw gauge) is another precision instrument used to measure very small lengths, typically with a least count of 0.001 cm (or 0.01 mm).
- **Conclusion**: Screw gauges are used for measuring length.

5. **Analyze the Spherometer**:
- A spherometer is used to measure the curvature of a surface and can also measure small distances with high precision, usually having a least count of around 0.0001 cm (or 0.001 mm).
- **Conclusion**: Spherometers are used for measuring length.

6. **Final Decision**:
- After analyzing all the instruments, we find that the only instrument that is not used for measuring length is the **atomic clock**.

### Answer:
The instrument that is not used for the measurement of length is the **Atomic Clock**.

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