Which is used in high temperature thermometry ?

Explanation

To solve the question "Which is used in high temperature thermometry?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Thermometry**:
- Thermometry is the science of measuring temperature. High-temperature thermometry specifically refers to measuring temperatures that are significantly above room temperature.

2. **Identifying Suitable Materials**:
- For high-temperature thermometry, we need materials that can remain in a liquid state over a wide temperature range and have low vapor pressure to ensure accurate readings.

3. **Considering Liquid Metals**:
- Liquid metals are often used in thermometers because they have a high thermal conductivity and can expand uniformly with temperature changes.

4. **Evaluating Gallium**:
- Gallium (Ga) is a liquid metal that remains in a liquid state at room temperature (approximately 302 K).
- It has a boiling point of 2673 K, which provides a wide range for temperature measurement (from 302 K to 2673 K).

5. **Analyzing Properties**:
- Gallium has low vapor pressure, which is advantageous for high-temperature measurements as it minimizes the risk of evaporation affecting the readings.
- It is also relatively non-toxic compared to other liquid metals like mercury.

6. **Conclusion**:
- Based on these properties, gallium is identified as a suitable material for high-temperature thermometry.

### Final Answer:
Gallium is used in high-temperature thermometry.

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