Which of the following types of thermometer is used for the calibration of other thermometers?

Step 1: Analyze the question The question asks which thermometer serves as the standard for calibrating other thermometers.

Step 2: Explain the concept A standard thermometer must give very accurate, reproducible readings over a wide temperature range. It should also be largely independent of the substance used.

The constant volume gas thermometer meets these requirements:

• Gases (especially hydrogen, helium, or nitrogen at low pressure) behave almost like an ideal gas, so pressure is directly proportional to absolute temperature at constant volume (P ∝ T).
• It covers a very wide temperature range.
• It is highly accurate and sensitive.
• Different gases give nearly the same readings, so it does not depend strongly on the particular substance.

Because of this, it is used as a standard to calibrate other thermometers.

Step 3: Evaluate the options

• A. Liquid-in-glass thermometer: Convenient, but it has a limited range, and the liquid's expansion is not perfectly uniform. It is a thermometer that gets calibrated, not a standard.
• B. Constant volume gas thermometer: Highly accurate with a wide range, so it is the standard. ✔
• C. Optical pyrometer: Used to measure very high temperatures (such as furnaces) without contact, and it is less precise.
• D. Thermocouple: Good for rapidly changing or remote temperatures, but it needs calibration itself.

Step 4: Answer The correct option is B. constant volume gas thermometer.

Exam tip: Remember "Gas = Gold standard." When a question asks about a standard or calibrating thermometer, think of the constant volume gas thermometer. Pyrometer means very hot objects, and thermocouple means fast or remote readings.