Gas thermometer

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Two variants of a gas thermometer

A gas thermometer measures temperature by the variation in volume or pressure of a gas.[1] One common apparatus is a constant volume thermometer. It consists of a bulb connected by a capillary tube to a manometer. The bulb is filled with a gas such that the volume of the gas in the bulb remains constant. The volume is related to temperature by k, known as Charles's Law. The pressure of the gas in the bulb can be obtained by measuring the level difference in the two arms of the manometer.[2] Gas thermometers are often used to calibrate other thermometers.[3]

Plots of pressure vs temperature for three different gas samples extrapolate to absolute zero.

The constant volume gas thermometer plays a crucial role in understanding how absolute zero could be discovered long before the advent of cryogenics. Consider a graph of pressure versus temperature made not far from 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.[4] 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.


See also[edit]


  1. ^ "AMS Glossary (gas thermometer)". 
  2. ^ "Gas properties". 
  3. ^ "The Ideal-Gas Temperature Scale". 
  4. ^ Tipler, Paul; Mosca, G. (2008). "17.2: Gas thermometers and the absolute temperature scale". Physics for Scientists and Engineers (6th ed.). Freeman. ISBN 9781429201322.