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History of the Celsius temperature scale

A temperature scale must be based on one or two standard temperatures,called fixed points. For those it was natural to choose temperatures within thetemperature domain of practical interest, i.e. from about plus forty to minustwenty in modern Celsius degrees. Thermometers were simply used in meteorology, in horticulture, and sometimes for indoor use. As fixed points one could use the humanbody temperature or temperatures of local origin such as the observatory cellarin Paris or the highest temperature in sunshine in London. Of course also the freezingand boiling points of water were used, but it was not self-evident that they really were universal and e.g. independent of the geographic latitude.

Anders Celsius should be recognized as the first to perform and publish carefulexperiments aiming at the definition of an international temperature scale onscientific grounds. In his Swedish paper "Observations of two persistentdegrees on a thermometer" he reports on experiments to check that thefreezing point is independent of latitude (and also of atmospheric pressure!).He determined the dependence of the boiling of water with atmospheric pressure (in excellent agreement with modern data). He further gave a rule for thedetermination of the boiling point if the barometric pressure deviates from a certain standard pressure.

The position of the degree zero on a temperature scale has created a lot ofdiscussions. The French Réaumur scale had zero at the freezing point but as regardsother scales one placed zero outside the ordinary temperature region, thus avoidingthe mixture of positive and negative numbers. The zero could be placed at a low temperature, a method used by the Danish astronomer Ole Rømer and then adopted byFahrenheit. Celsius was accustomed to the Réaumur thermometer, but he also used athermometer made by the French astronomer Joseph-Nicolas Delisle with zero at theboiling point, thus creating a reversed scale with increasing numbers for decreasingtemperatures, but avoiding negative numbers.

This reversed scale was not as awkward as we think today and it served its purpose.The change to our modern direct scale was inevitable in the long run, however,but there is no sense in trying to give the credit to any single person. Linné has often been named as the inventor with reference to his famous work "Hortus Cliffortianus" of 1737, five years before Celsius' publication.However, in studying Linné's work it is obvious that the few temperatures he mentionesare impossible as modern Celsius' degrees. Linné met Fahrenheit in Holland and itis clear that he used Fahrenheit temperatures when he returned home to Sweden. Most probably Linné abandoned the Fahrenheit scale when he learned about Celsius'research. After the death of Celsius in 1744 thermometers with a direct scaleappeared in the meteorology reports under different names, such as 'CelsiusNovum', 'Ekström' and 'Strömer'. Linné had a great interest inthermometers, and he ordered a thermometer with the modern scale from theinstrument maker Daniel Ekström around 1744. The scale is mentioned in adissertation by Samuel Naucler in 1745, but this does not indicate Linné asthe single inventor of the modern scale. Certainly the change of scalebecame obvious to the Uppsala scientists in general. In an account of thehistory of the thermometer in the Proceedings of the Royal Swedish Academyof Sciences 1749, Pehr Wargentin, Secretary of the Academy of Sciences,mentiones Celsius, his successor Strömer and the instrument maker Ekströmin connection with the direct scale. Linné is not mentioned at all. Nosingle person can be given the credit.

From the scientific point of view the most important contribution to the moderntemperature scale is due to Celsius because of his careful experiments on the fixedpoints. The direction of the scale comes in second place, and was hardly a great intellectual achievement.