How it began
Anders Celsius was a Swedish astronomer at Uppsala. As a student he had helped Erik Burman, professor of astronomy at Uppsala University, with meteorological observations at a time when there was no accepted standard for thermometers. Through a series of careful experiments he showed that the melting point of snow or ice and the boiling point of water, once adjusted for atmospheric pressure, were constant everywhere. In 1742 he published the results in the annals of the Royal Swedish Academy of Sciences in a paper on two persistent degrees on a thermometer.
Before it existed
Daniel Gabriel Fahrenheit had built precise alcohol and mercury thermometers from 1709 and 1714, but there was no simple way to make instruments built in different places agree. Many fixed points were in use, including human body temperature and even the melting point of butter, and no one knew whether they were truly fixed. Ole Rømer's scale of 1702 used the boiling point of water and the temperature at which snow begins to form. Readings from two places could be compared only by carrying the same thermometer between them or calibrating two against each other.
How it works
Celsius proposed three steps for calibrating a thermometer. It was placed in thawing snow or ice made from pure water and that point was marked. It was then placed in boiling water and that point was marked, corrected for the atmospheric pressure read from a barometer. The distance between the two marks was divided into one hundred equal units. Celsius numbered his scale the other way around from the modern one: freezing was 100 degrees and boiling was 0.
How it spread
The scale was soon reversed so that freezing was 0 and boiling 100. Carl Linnaeus may have been the first to do so, but his thermometers, like many of those Celsius used, were built by Daniel Ekström, and it is not known which of them made the change. The Frenchman Jean Pierre Christin has also been suggested. The modified scale became popular in Sweden and France in particular and was called the centigrade scale.
What changed first
Thermometers built in different laboratories could now be calibrated to the same scale, so results could be compared from one laboratory to another. Scientists were able to work out how the physical properties of materials, such as the volume of gases, liquids and solids, vary with temperature.
What lasted
Celsius's experiments established two universal fixed points, which is why his contribution is regarded as the most important one to the modern temperature scale. A common scale also let weather records from different locations be compared, which led in time to an understanding of weather patterns and to weather forecasting. The centigrade scale is now called the Celsius scale in honor of its inventor.
