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{{Short description|Scale of temperature}}
{{About|the temperature scale|other uses|Réaumur (disambiguation)}}
{{distinguish|Rømer scale}}
[[File:Apotheke Weisser Storch 6.JPG|thumb|upright|Old thermometer in a pharmacy in [[Vienna]], showing [[room temperature]] by Reaumur scale.]]
[[File:Réaumur - Celsius thermometer.jpg|thumb|Réaumur and Celsius scale on thermometer. Private collection, central Europe.]]
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The '''Réaumur scale''' ({{IPA|fr|ʁeomy(ː)ʁ}}; '''°Ré''', '''°Re''', '''°r'''), also known as the "octogesimal division",<ref>''Division octogesimale'' in French.</ref> is a [[temperature]] scale for which the [[melting point|melting]] and [[boiling point]]s of [[water]] are defined as 0 and 80 degrees respectively.  The scale is named for [[René Antoine Ferchault de Réaumur]], who first proposed a similar scale in 1730.<ref name="Times07">{{cite news | url=https://www.thetimes.co.uk/article/how-reaumur-fell-off-the-temperature-scale-gg862jbms8b | title=How Reaumur fell off the temperature scale | access-date=2022-12-14 | date=17 October 2007 | newspaper=[[The Times]] | first=Paul | last=Simons }}</ref>

== Change in scale ==
Réaumur's [[thermometer]] contained diluted [[ethanol|alcohol]] (ethanol) and was constructed on the principle of using 0° for the melting temperature of water, and graduating the tube into degrees, each of which was one-thousandth of the volume contained by the bulb and tube up to the zero mark. He suggested that the concentration of alcohol employed be such that it began boiling at 80 °Ré&nbsp;– that is, when it had expanded in volume by 8%. He chose alcohol instead of mercury because it expands more visibly, but this posed problems: his original thermometers were very bulky, and the low boiling point of alcohol made them unsuitable for many applications. Instrument-makers generally chose different liquids, and then used 80 °Ré to signify the boiling point of water, causing much confusion. 

During 1772 [[Jean-André Deluc]] studied the several substances then used in thermometers and concluded that [[mercury (element)|mercury]] thermometers were the best for practical use, on the grounds that among these substances its volume varies the most linearly with the method of mixtures. In detail, if two portions of water of equal masses A, B were mixed, and let the resulting water be C, and if we immerse a thermometer in A, B, C, we obtain lengths <math>l_A, l_B, l_C</math>. Deluc expected that <math>l_C = \frac 12 (l_A + l_B)</math>, and similarly for other ratios of mixtures. He found that thermometers made using mercury allowed the closest fit to his expectation of linearity.<ref>{{Cite book |last=Chang |first=Hasok |url=https://www.worldcat.org/oclc/457147642 |title=Inventing temperature : measurement and scientific progress |date=2007|page=64 |isbn=978-0-19-533738-9 |location=Oxford |oclc=457147642}}</ref>

From the late 18th century mercury was used almost without exception.<ref>Herbert Dingle. ''The scientific adventure: essays in the history and philosophy of science.'' Sir Isaac Pitman and Sons, London, 1952. Page 131.</ref>

== Use ==
The Réaumur scale was used widely in Europe, particularly in France, Germany and Russia, and was referenced in the works of [[Thomas Jefferson]], [[Thomas Mann]], [[Fyodor Dostoevsky]], [[Gustave Flaubert]], [[James Joyce]], [[Leo Tolstoy]], and [[Vladimir Nabokov]].<ref>Thomas Mann, [[The Magic Mountain]], p. 93, Random House, [https://books.google.com/books?id=nm5YDwAAQBAJ&dq=reaumur+the+magic+mountain+thomas+mann&pg=PA93 Chapter IV on] [[Google Books]].</ref><ref>Fyodor Dostoyevsky, [[Crime and Punishment]], p. 268 Vintage Books. [http://www.gutenberg.org/files/2554/2554-h/2554-h.htm#2HCH0020 Part 3, Chapter VI on] [[Project Gutenberg]].</ref><ref>Leo Tolstoy, [[Anna Karenina]], p. 685, Wordsworth Editions Limited</ref><ref>Vladimir Nabokov, The Real Life of Sebastian Knight, p. 1. Penguin Books.</ref> For example, in Query VII of ''[[Notes on the State of Virginia]]'', Jefferson converts readings from the Fahrenheit thermometer at Williamsburg to the Réaumur scale. At the beginning of Book X of ''[[The Brothers Karamazov]]'', the narrator says, "We had eleven degrees of frost", i.e. −11&nbsp;°Ré, equivalent to {{degC|−14}} or {{degF|7}}.<ref>[[The Brothers Karamazov]], trans. Pevear and Volokhovsky (Vintage, 1991), p. 515.</ref>  By the 1790s, France had chosen the [[Celsius]] scale as part of the [[metric system]], rather than the Réaumur measurement,<ref name="Times07"/> but the Réaumur scale continued to be used in some parts of Europe until at least the mid-19th century,<ref>{{cite book|last=Draper|first=John William|title=Textbook on chemistry|year=1852|publisher=Harper and Brothers|location=New York|page=[https://archive.org/details/bub_gb_HKwS7QDh5eMC/page/n34 20]|url=https://archive.org/details/bub_gb_HKwS7QDh5eMC|quote=draper, john william.}}</ref> and in parts of Russia until the early 20th. While living in Paris, George Orwell mentions "translating Réaumur into Fahrenheit" even in 1929. Its main modern uses are in some Italian and Swiss factories for measuring milk temperature during cheese production,<ref>[http://secondversion.blogspot.com/2007/07/hard-to-kill.html Reaumur thermometer for Parmigiano-Reggiano cheese)]</ref> and in the Netherlands for measuring temperature when cooking sugar syrup {{citation needed|date=October 2021}} for desserts and sweets.

Uses outside of Europe include a Mexican colonel in [[Antonio_López_de_Santa_Anna|Santa Anna]]'s army in San Antonio, Texas in 1836 recording the night's temperature as {{degF|34}} and 1&nbsp;°Ré.<ref>{{cite book|last=Hansen|first=Todd E.|title=The Alamo Reader|year=2003|publisher=Stackpole Books|page=364}}</ref>

== Conversion between different temperature units ==
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{{Timeline Temperature Conversion}} 313.15 K = 40 °C = 104 °F = 563.67 °Ra = 28.5 °Ro = 13.2 °N = 90 °D = 32 ºRé

== See also ==
* [[Outline of metrology and measurement]]

== Notes and references==
{{Reflist|30em}}

{{Scales of temperature}}
{{Portal bar|Energy}}

{{DEFAULTSORT:Reaumur Scale}}
[[Category:Obsolete units of measurement]]
[[Category:Scales of temperature]]
[[Category:Science and technology in France]]
[[Category:Scales in meteorology]]