The Kelvin temperature of a system can be measured by a constant-volume gas thermometer.


A constant-volume gas thermometer is a device used to measure temperature based on the principles of ideal gas behavior. It operates on the principle that the pressure of a gas is directly proportional to its temperature when the volume is kept constant.


The constant-volume gas thermometer consists of a bulb filled with a gas (usually helium or hydrogen) connected to a manometer. The bulb is immersed in the substance whose temperature needs to be measured. As the temperature changes, the gas inside the bulb also expands or contracts, resulting in a change in pressure.


To measure the temperature using a constant-volume gas thermometer, the following steps are followed:

1. Calibration: The thermometer is first calibrated by determining the relationship between pressure and temperature. This is done by immersing the thermometer in a series of known temperature baths and recording the corresponding pressure readings.

2. Measurement: Once calibrated, the thermometer is immersed in the substance whose temperature needs to be measured. The gas inside the bulb expands or contracts, causing a change in pressure. This change in pressure is then measured using the manometer.

3. Calculation: The change in pressure is used to calculate the temperature using the ideal gas law equation:

PV = nRT

where P is the pressure, V is the volume, n is the number of moles of gas, R is the gas constant, and T is the temperature in Kelvin.

Since the volume is kept constant in a constant-volume gas thermometer, the equation simplifies to:

P = nRT

The temperature can be calculated by rearranging the equation as:

T = P / (nR)

The pressure and gas constant values are known, allowing the temperature to be determined.


- High accuracy: Constant-volume gas thermometers can provide highly accurate temperature measurements.
- Wide temperature range: These thermometers can measure a wide range of temperatures, from cryogenic temperatures to high temperatures.
- Simple design: The design of constant-volume gas thermometers is relatively simple, making them easy to use and maintain.
- Standardization: The use of constant-volume gas thermometers is standardized, ensuring consistent and comparable temperature measurements.


A constant-volume gas thermometer is capable of measuring temperature in Kelvin. Its working principle is based on the relationship between pressure and temperature according to the ideal gas law. The use of a constant-volume gas thermometer provides accurate and reliable temperature measurements, making it a valuable tool in various scientific and industrial applications.