Overview

The Réaumur scale is a historical temperature scale that sets the freezing point of water at 0 and the boiling point at 80 degrees, dividing the interval into 80 equal parts. It is named after the French naturalist and physicist René-Antoine Ferchault de Réaumur, who introduced the system in 1731. The unit is commonly written as degree Réaumur and abbreviated as °Ré or sometimes as Ré or r. For general context on temperature measurement see measurement.

Definition and basic properties

Réaumur originally anchored his scale to two fixed reference points: the melting point of ice and the boiling point of water at standard atmospheric pressure. He set the melting point at 0 °Ré and the boiling point at 80 °Ré, so one degree Réaumur equals one eightieth of the temperature difference between those two states. The basic physical references are the melting of ice and melting of water ice (melting point and ice) and the boiling of water (boiling point and water), although the exact boiling point depends on ambient pressure.

How the scale was realized

Réaumur's original thermometers used spirit (ethanol) as the working fluid. Ethanol was attractive because it expands markedly with heat and gives a visible response to small temperature changes; many early instrument makers therefore preferred it. The liquid Réaumur used is referenced in technical histories as ethanol. Instruments of that era are generally called thermometers. Over time instrument makers found that other liquids, in particular mercury, produced more reproducible and linear readings in the practical temperature range; mercury is discussed in later experiments and evaluations (mercury).

History and scientific assessment

After Réaumur's proposal the scale saw wide use across parts of Europe in the 18th and 19th centuries. Scientific debate on heat and thermal behavior prompted comparisons of thermometer fluids and scales. Contemporary theories of heat and caloric are discussed in period literature (heat theories). One influential investigation by Jean-André Deluc studied different thermometer fluids and their reliability. Deluc performed mixing experiments and other tests (experiments) that checked whether measured temperatures behaved additively: for example, when two equal masses of water at different temperatures are combined, the resulting temperature should be the arithmetical mean of the original temperatures. Deluc's work supported the view that mercury thermometers gave more consistent and linear readings for such tests than spirit-filled instruments.

Conversion and comparison with other scales

Because the Réaumur interval between freezing and boiling of water is 80 degrees, conversion to the Celsius scale (with 100 degrees between the same points) is simple: degrees Celsius = 1.25 × degrees Réaumur. Conversely, degrees Réaumur = 0.8 × degrees Celsius. To convert to Fahrenheit use the relation degrees Fahrenheit = 2.25 × degrees Réaumur + 32. These relations assume the same reference pressures are used for the fixed points.

Uses, decline and legacy

The Réaumur scale was widely used for scientific and practical purposes in its day, including meteorology, industrial settings and household thermometry. As precision instruments improved and international standards emerged, the metric-based Celsius scale and the absolute Kelvin scale became dominant. Mercury thermometers and decimal-based temperature scales offered practical advantages, and during the 19th and 20th centuries most countries gradually abandoned Réaumur in official use. Nevertheless, the scale survives in some historical documents, older recipes and technical literature, and it is sometimes encountered in studies of historical science and technology. For further reading on historical instruments and measurement practice consult general references on early thermometry (measurement) and instrument design (thermometers).

Notable distinctions and practical considerations

  • Fluid choice: Réaumur's original spirit thermometers respond quickly but show non-linear expansion at temperature extremes; mercury later became preferred for linearity (mercury).
  • Fixed points dependence: The scale depends on pressure-sensitive phenomena (boiling of water), so readings must specify the reference pressure to be precise (water, boiling point).
  • Naming: Many thermometers manufactured after Réaumur's time carried 80 divisions between the ice and steam points but used different liquids; such instruments share the name but not the original liquid design (ethanol, mercury).

Today the Réaumur scale is chiefly of historical interest and of practical relevance for interpreting older scientific texts and recipes. For introductions to temperature scales and how they developed consult general histories of thermometry and heat (heat theories, measurement). For specific discussions about Réaumur or historical instruments, see specialized museum catalogs and histories of scientific instruments (experiments, mean, ice).