Final answer:
In a constant-volume gas thermometer, the property measured to indicate its temperature is the pressure of the gas. This relationship is based on the ideal gas law, and pressure serves as the key variable for temperature measurement in this device.
Step-by-step explanation:
A constant-volume gas thermometer contains a fixed amount of gas, and the property of the gas that is measured to indicate its temperature is the pressure of the gas. This is because, when the temperature of a gas increases at constant volume, its pressure also increases, and vice versa.
This relationship is described by Amontons' Law (part of the ideal gas law), which states that the pressure of a given amount of gas held at constant volume is directly proportional to its temperature.
To accurately measure temperature, metrologists use a constant-volume gas thermometer as a standard device. In this measurement, the pressure changes in proportion to temperature changes, making pressure the key measurable property when volume is held constant.
A low-density gas provides more accurate results in various gases, and extrapolating results to zero density gives a temperature directly proportionate to the pressure change, with zero pressure indicating absolute zero temperature.