A gas thermometer is a

thermometer A thermometer is a device that measures temperature (the hotness or coldness of an object) or temperature gradient (the rates of change of temperature in space). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb ...

that measures the

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

of a gas by variation in the volume or pressure of the gas.

Constant-pressure thermometer

According to

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 ...

, the volume of gas is directly proportional to the temperature of that gas, when its pressure and mass are kept constant; that is, :

V \propto T\,

In other words, :

\frac=k \Rightarrow\frac=\frac \Rightarrow\frac=\frac

where

V

is the volume,

T

is the

thermodynamic temperature Thermodynamic temperature, also known as absolute temperature, is a physical quantity which measures temperature starting from absolute zero, the point at which particles have minimal thermal motion. Thermodynamic temperature is typically expres ...

and

k

is the constant for the system. The constant

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. This works on the same principle as mercury thermometers. If a change in volume can be measured with one standard known temperature (such as the melting point of water), the second unknown temperature can be determined.

Constant-volume thermometer and absolute zero

The constant-volume gas thermometer works on the basis of the pressure-temperature law: when the volume of a gas is kept constant, its pressure is directly proportional its temperature. :

P\propto T

That is,

\frac=\frac

The constant volume gas thermometer plays a crucial role in understanding how

absolute zero Absolute zero is the lowest possible temperature, a state at which a system's internal energy, and in ideal cases entropy, reach their minimum values. The absolute zero is defined as 0 K on the Kelvin scale, equivalent to −273.15 ° ...

could be discovered long before the advent of

cryogenics In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a universa ...

. Consider a graph of pressure versus temperature made around standard conditions (well above absolute zero) for three different samples of any ideal gas ''(a, b, c)''. To the extent that the gas is ideal, the pressure depends linearly on temperature, and the extrapolation to zero pressure occurs at absolute zero. Note that data could have been collected with three different amounts of the same gas, which would have rendered this experiment easy to do in the eighteenth century.

History

References

Thermometers Gases fr:Thermomètre#Thermomètre à gaz {{thermodynamics-stub

Constant-pressure thermometer

Constant-volume thermometer and absolute zero

History

See also

References