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=== Charles's law ===
From 1787 to 1802, it was determined by [[Jacques Charles]] (unpublished), [[John Dalton]],<ref>J.{{Cite Dalton (1802), [book|url=https://books.google.com/books?id==3qdJAAAAYAAJ&pg=PA595|title=PA595 "Essay II. On the force of steam or vapour from water and various other liquids, both in vacuum and in air" and Essay IV. "On the expansion of elastic fluids by heat," ] ''Memoirs of the Literary and Philosophical Society of Manchester'',|date=1802|publisher=The vol. 8, pt. 2, pp. 550–74, 595–602.Society|language=en}}</ref> and [[Joseph Louis Gay-Lussac]]<ref>{{citation |author=Gay-Lussac, J. L. |author-link=Joseph Louis Gay-Lussac |year=1802 |title=Recherches sur la dilatation des gaz et des vapeurs |journal=Annales de Chimie |volume=XLIII |page=137}}. [http://web.lemoyne.edu/~giunta/gaygas.html English translation (extract).]</ref> that, at constant pressure, ideal gases expanded or contracted their volume linearly ([[Charles's law]]) by about 1/273 parts per degree Celsius of temperature's change up or down, between 0° and 100° C. This suggested that the volume of a gas cooled at about −273 °C would reach zero.
=== Lord Kelvin ===
=== Triple point standard ===
{{stack|[[Image:Phase-diag2.svg|thumb|upright=1.5|A typical [[phase diagram]]. The solid green line applies to most substances; the dashed green line gives the anomalous behavior of water. The boiling line (solid blue) runs from the triple point to the [[Critical point (thermodynamics)|critical point]], beyond which further increases in temperature and pressure produce a [[supercritical fluid]].]]}}
In 1873, William Thomson's older brother [[James Thomson (engineer)|James]] coined the term ''[[triple point]]''<ref>James{{Cite Thomsonjournal|last=Royal Society (1873Great Britain) [https://babel.hathitrust.org/cgi/pt?id|date=hvd.32044106377062;view1856-1905|title=1up;seq=47 "A quantitative investigation of certain relations between the gaseous, the liquid, and the solid states of water-substance"], ''Proceedings of the Royal Society'', '''22'''of London.|url=https: 27–36//catalog.hathitrust.org/Record/000505545|pages=69 From a footnote on page 28: " … the three curves would meet or cross each other in one point, which I have called the ''triple point''"v.|issn=0370-1662}}</ref> to describe the combination of temperature and [[pressure]] at which the solid, liquid, and gas [[Phase (matter)|phases]] of a substance were capable of coexisting in [[thermodynamic equilibrium]]. While any two phases could coexist along a range of temperature-pressure combinations (e.g. the [[boiling point]] of [[water]] can be affected quite dramatically by raising or lowering the pressure), the triple point condition for a given substance can occur only at a single pressure and only at a single temperature. By the 1940s, the triple point of water had been experimentally measured to be about 0.6% of [[standard atmosphere (unit)|standard atmospheric pressure]] and very close to 0.01 °C per the historical definition of Celsius then in use.
In 1948, the Celsius scale was recalibrated by assigning the triple point temperature of water the value of 0.01 °C exactly and allowing the [[melting point]] at standard atmospheric pressure to have an empirically determined value (and the actual melting point at ambient pressure to have a [[Low-pressure area|fluctuating]] value) close to 0 °C. This was justified on the grounds that the triple point was judged to give a more accurately reproducible reference temperature than the melting point.<ref name="CGPM 9 res 3 (1948)">{{cite web |title=Resolution 3 of the 9th CGPM (1948) |url=https://www.bipm.org/en/committees/cg/cgpm/9-1948/resolution-3 |website=bipm.org |publisher=BIPM |access-date=21 February 2022}}</ref>
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