Réaumur (°Ré)
The Réaumur scale is a historical temperature scale in which water freezes at 0°Ré and boils at 80°Ré. Invented by French scientist René Antoine Ferchault de Réaumur in 1730, it was widely used across continental Europe, particularly in France, Germany, and Russia, until the late 19th and early 20th centuries when Celsius gradually replaced it.
Definition
On the Réaumur scale, water freezes at 0°Ré and boils at 80°Ré at standard atmospheric pressure. To convert Réaumur to Celsius: °C = °Ré × 5/4. To convert Celsius to Réaumur: °Ré = °C × 4/5. One Réaumur degree equals 1.25 Celsius degrees. Normal body temperature (37°C) is 29.6°Ré.
History
Réaumur introduced his scale in 1730, using diluted alcohol as the thermometric fluid. He set the freezing point of water at 0 and calibrated his thermometer so that the alcohol expanded by 80 units between freezing and boiling. The Réaumur scale became the dominant temperature scale in France, Germany, and Russia during the 18th century. In Germany, it remained in common use well into the 19th century. The shift to Celsius happened gradually: France adopted Celsius during metrication in the 1790s, Germany followed in the late 1800s, and Russia transitioned after the 1917 revolution. Today, the Réaumur scale survives only in the dairy and cheese-making industries of a few European countries.
Common Uses
The Réaumur scale has almost no modern applications. Its only surviving niche use is in some European cheese-making and dairy operations, where traditional recipes specify temperatures in Réaumur. Historical meteorological records from 18th- and 19th-century Europe, particularly Germany and Russia, use Réaumur temperatures. Some Italian pasta-making traditions also reference Réaumur.
Did You Know? Facts About Réaumur
- Réaumur used diluted alcohol instead of mercury, which limited his thermometer's upper range but made it more sensitive at lower temperatures.
- German meteorological records from the 18th and 19th centuries are in Réaumur, requiring conversion to Celsius for modern analysis.
- The 80-degree span was chosen because Réaumur's alcohol expanded by that factor between freezing and boiling - it was a measurement artifact, not a deliberate design choice.
- Some traditional Swiss and Italian cheese recipes still specify curdling temperatures in Réaumur.
- The Réaumur scale was used in Germany longer than in France, even though a Frenchman invented it.