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Réaumur temperature scale

In 1730 René Antoine Ferchault de Réaumur, a distinguished French scientist, proposed a scale that depended on only one endpoint, the freezing point of water, which was taken as zero. The thermometer was filled to the zero mark with a solution of alcohol and water at the temperature of the freezing point of water. The proportion of alcohol to water in the solution was chosen such that if its volume at the freezing point of water was 1000, its volume at the solution's boiling point was 1080. This temperature, the boiling point of the solution, was 80 degrees on Réaumur's scale.

As a practical matter, Réaumur's method of calibration was unworkable, and in practice by 1770 the instrument makers were making mercury thermometers using two end points (0 as the freezing point of water and 80 degrees the boiling point of water), dividing the space between them into 80 intervals, and calling them Réaumur degrees. Thus one Réaumur degree is 1¼th of a degree centigrade, and a temperature in degrees Réaumur is 80% of the temperature in degrees centigrade.

This scale was adopted in France and most of Europe, excluding only Britain and Scandinavia, prior to the metric system.

On 12 Germinal an II (1 April 1794) the revolutionary French government instituted the centigrade scale as part of the new, decimal, metric system, thus doing away with the Réaumur scale.

sources

Réaumur, ayant placé dans un matras de verre une certaine quantité d’alcool mélange d’eau, observa qu’il se dilatait de 80 milliémes de son volume lorsque, après l’avoir mis dans de l’eau commençant a se congeler, on le portait dans l’eau bouillante; il marqua zéro le premier point, et 80 le second : les degrés de Reaumur sont donc des millièmes de la dilatation de l’alcool hydraté, entre le point 0° et son point d'ébullition qu’il confondait avec celui de l’eau. Micheli Ducrest fit déjà observer que l’ébullition de l’eau se produisait au-dessus du point 100 du thermomètre de Réaumur, ce qu’il montra avec des thermomètres remplis d’air et fermés à la température ordinaire (¹). Lui-même adopta, comme 0, la température des caves profondes et marqua 100 à la température d’ébullilion de l’eau. La définition du Système de Réaumur n’en est pas moins demeurée telle qu’il l’avait donnee, la température vraie d’ebullition de l’eau étant plus tard marquée 80°. En réalité, les thermométres que Reaumur construisit donnaient à peu près des degrés centigrades.

(¹) La dilatation de l’alcool éthylique pur entre 0° et son point d’ébullition (78°,5), est de 0,0935; donc d’après la graduation de Réaumur, les thermomètres à alcool pur marquent 93°,5 à 78°,5 C.

Réaumur, having placed in a glass flask a certain amount of alcohol mixed with water, observed that it was expanding by 80 parts per thousand when, after putting in water starting to freeze, he it wore in boiling water. He called the first point zero, and the second 80. Réaumur degrees are therefore thousandths of expansion of hydrated alcohol, between 0° point and its boiling point mixed with that of water. Micheli Ducrest was already observed that the boil water occurred above paragraph 100 of Réaumur's thermometer, which he showed with thermometers filled with air and closed at room temperature (¹). He himself adopted, as 0, the temperature of the deep caves, and fixed 100 as the temperature of boiling water. The definition of Réaumur system does not remain as he had given it; the true temperature of boiling water is latterly marked 80°. In reality, the thermometers that Réaumur made read in nearly centigrade degrees.

1. The expansion of pure ethyl alcohol from 0° to its boiling point (78.5°), is 0.0935; therefore following the graduation of Réaumur, thermometers containing pure alcohol read 93.5° at 78.5° C.

Ch.-Ed. Guillame.
Unités et Étalons.
Paris: Gauthier-Villars et Fils, [1873].
Pages 130-131.

for further reading

W. E. Knowles Middleton.
A History of the Thermometer and its Use in Meteorology.
Baltimore, MD: Johns Hopkins Press, 1966.