Réaumur

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This article is about the temperature scale. For the commune in the Vendée département of France, see Réaumur, Vendée.

The Réaumur scale is a temperature scale named after René Antoine Ferchault de Réaumur, who first proposed it in 1731. The freezing point of water is 0 degrees Réaumur, the boiling point 80 degrees Réaumur. Hence, a Reaumur degree is 1.25 Celsius degrees or kelvins. The Réaumur temperature scale is also known as the octogesimal division (division octogesimale in French).

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°. 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); the number 100, for instance, could only suffer this process 2 times (50, 25).

The Réaumur scale saw widespread use in Europe, particularly in France and Germany, but was eventually replaced by the Celsius scale.^{[citation needed]} Today it is only of historical significance.