A gas thermometer is a

thermometer A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermomete ...

that measures

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

by the variation in volume or pressure of a gas.

Volume Thermometer

This thermometer functions by

Charles's Law Charles's law (also known as the law of volumes) is an experimental gas law that describes how gases tend to expand when heated. A modern statement of Charles's law is: When the pressure on a sample of a dry gas is held constant, the Kelvin ...

states that when the temperature of a gas increases, so does the volume. Using

, the temperature can be measured by knowing the volume of gas at a certain temperature by using the formula, written below. Translating it to the correct levels of the device that is holding the gas. This works on the same principle as mercury thermometers. :

V \propto T\,

or :

\frac=k

V

is the volume,

T

is the

thermodynamic temperature Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics. Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...

k

is the constant for the system.

k

is not a fixed constant across all systems and therefore needs to be found experimentally for a given system through testing with known temperature values.

Pressure Thermometer and Absolute Zero