Thermal transpiration

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Thermal transpiration (or thermal diffusion) refers to the thermal force on a gas due to a temperature difference. Thermal transpiration causes a flow of gas in the absence of any other pressure difference, and is able to maintain a certain pressure difference called thermomolecular pressure difference in a steady state. The effect is strongest when the mean free path of the gas molecules is comparable to the dimensions of the gas container.

Thermal transpiration appears as an important correction in the readings of vapor pressure thermometers,{{Cite journal | last1 = Watkins | first1 = R. A. | title = Thermomolecular Pressure Difference Measurements for Precision Helium−3 and Helium−4 Vapor-Pressure Thermometry | doi = 10.1063/1.1840762 | journal = The Journal of Chemical Physics | volume = 46 | issue = 3 | pages = 1007–1018 | year = 1967 |bibcode = 1967JChPh..46.1007W }}{{cite book | isbn = 978-3-540-46356-6 | title = Matter and Methods at Low Temperatures | edition = 3rd | last1 = Pobell | first1 = F. | author-link1 = Frank Pobell | year = 2007 | publisher = Springer }} and the effect is historically famous as being an explanation for the rotation of the Crookes radiometer. {{Cite journal | last1 = Reynolds | first1 = Osbourne | title = On certain dimensional properties of matter in the gaseous state. - Part I. Experimental researches on thermal transpiration of gases through porous plates and on the laws of transpiration and impulsion, including an experimental proof that gas is not a continuous plenum. - Part II. On an extension of the dynamical theory of gas, which includes the stresses, tangential and normal, caused by a varying condition of gas, and affords an explanation of the phenomena of transpiration and impulsion| doi = 10.1098/rstl.1879.0078 | journal = Philosophical Transactions of the Royal Society of London| volume = 170 | issue = 170 | pages = 727–845 | year = 1879}} {{cite journal |last=Maxwell |first=James Clerk | doi=10.1098/rstl.1879.0067 | title=On stresses in rarified gases arising from inequalities of temperature | journal=Philosophical Transactions of the Royal Society of London | year=1879 | volume=170 | pages=231–256 | s2cid=110911902 }}