Réaumur scale

This article is about the temperature scale. For other uses, see Réaumur (disambiguation).

**Réaumur temperature conversion formulae**
	from Réaumur	to Réaumur
Celsius	[°C] = [°Ré] × ⁵⁄₄	[°Ré] = [°C] × ⁴⁄₅
Fahrenheit	[°F] = [°Ré] × ⁹⁄₄ + 32	[°Ré] = ([°F] − 32) × ⁴⁄₉
Kelvin	[K] = [°Ré] × ⁵⁄₄ + 273.15	[°Ré] = ([K] − 273.15) × ⁴⁄₅
Rankine	[°R] = [°Ré] × ⁹⁄₄ + 491.67	[°Ré] = ([°R] − 491.67) × ⁴⁄₉
For temperature intervals rather than specific temperatures, 1 °Ré = 1.25 °C = 2.25 °F Comparisons among various temperature scales

The Réaumur scale (°Ré, °Re, °R), also known as the "octogesimal division"^[1], is a temperature scale in which the freezing and boiling points of water are set to 0 and 80 degrees respectively. The scale is named after René Antoine Ferchault de Réaumur, who first proposed it in 1730.^[2]

Réaumur’s thermometer was constructed on the principle of taking the freezing point of water as 0°, 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. It was the dilatability of the particular quality of alcohol employed which made the boiling point of water 80°.^{[citation needed]} Mercurial thermometers, the stems of which are graduated into eighty equal parts between the freezing and boiling points of water, are not Réaumur thermometers in anything but name. Réaumur may have chosen the octogesimal division because the number 80 could be halved 4 times and still be an integer (40, 20, 10, 5);^{[citation needed]} the number 100, for instance, could only suffer this process twice (50, 25).

Use

The Réaumur scale saw widespread use in Europe, particularly in France and Germany as well as Russia, as referenced in works of Dostoyevsky, Tolstoy, and Nabokov.^[3]^{[citation needed]} But by the 1790s, France chose the Celsius scale for the metric system over the Réaumur measurements.^[2] Today it is only of historical significance except that it is still used in the measuring of milk temperature in cheese. It is used in some Italian dairies making Parmigiano-Reggiano and Grana Padano cheeses and in Swiss Alp cheeses.^[4]

Charles Minard's information graphic of the 1812 French invasion of Russia. The horizontal graph on the bottom, designed to be read from right to left, depicts the temperature on the army's return from Russia, in degrees below freezing on the Réaumur scale.

References

^ Division octogesimale in French.
^ ^a ^b Simons, Paul (17 October 2007). "How Reaumur fell off the temperature scale", The Times. Retrieved on 10 March 2008.
^ Vladimir Nabokov, The Real Life of Sebastian Knight, p. 1. Penguin Books.
^ Reaumur thermometer for Parmigiano-Reggiano cheese)