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The Celsius scale is used for measuring temperature. Measurements in the Celsius scale are indicated by the suffix °C.

The scale is designed such that the freezing point of water is 0°C, and the boiling point of water is 100°C at a pressure of one atmosphere.

The Celsius thermometer (and hence the corresponding scale) was invented in 1742 by a Swedish astronomer, Anders Celsius, and published in a paper to the Annals of the Royal Swedish Academy of Science. It was already widely suggested that the freezing point of water was a good temperature calibration point, and Fahrenheit had determined that a relationship existed between barometric pressure and boiling point. Celsius' scientific contributions were:

Celsius' most important contribution was the following procedure for calibrating a mercury thermometer to this scale, allowing the production of standardized thermometers:

1. Put the cylinder AB of the thermometer (i.e., the bulb) in thawing snow and mark the freezing point of water C, which should be at such a height over the cylinder at A that the distance AC is half the distance between C and the water boiling point mark D.
2. Mark the boiling point of water D at a pressure of "25 tum 3 linier" (approximately 755 Hgmm).
3. Divide the distance in 100 equal parts or degrees; so that 0 degree corresponds to the boiling point of water D, and 100 to the freezing point of water C. When the same degrees have been continued below C all the way down to A the thermometer is ready.
(Translation from the original Swedish from www.santesson.com.)

You will notice from this description that the Celsius had the boiling and freezing points reversed from what we use today. In fact his original order never lasted much past his experimental efforts, as the instrument maker responsible for the manufacture of the thermometers reversed the order before many were made.

Originally the scale was known as the centigrade scale, owing to the one hundred graduations between freezing and boiling. In 1948 the name was officially changed by the 9th General Conference on Weights and Measures. This was partly in honor of its inventor, but mostly to remove what was by then over a hundred and fifty years of confusion caused by the metric use of the centi prefix for 1/100th and many European languages' existing words for "degree" being similar to "grade". (Note: centigrade predates metric by about 50 years.)

To convert from °F to °C and vice-versa use these formulas (F = temp in °F, C = temp in °C).

C = (5/9) * (F - 32)

F = ((9/5) * C) + 32
With the 1948 renaming of the degree centigrade to degree Celsius also came a redefinition in terms of the triple point of water, the temperature at which solid, liquid, and gas coexist. So it no longer used the melting point of water, now approximately 0,00°C instead of exactly. The triple point is at 0,01°C by definition.

In 1954 the subsequent Conférence Générale des Poids et Mesures redefined the absolute temperature scale to use the triple point, and the effect of this was to remove the exact value of the boiling point too. The degree Kelvin (renamed in 1967 as the kelvin) was defined to be the fraction 1 / 273,16 of the triple point of water. As the degree Celsius is identical in size to the kelvin, and as the triple point is 0,01°C exact by definition, this means that the Celsius scale now starts with its zero at 273,15 K, which is approximately the melting point of water, and with the boiling point at approximately 100,00°C or 373,15 K.

The next CGPM in 1960 created the SI, and the degree Celsius was not part of it: it was considered an everyday unit rather than a scientific one, the only SI unit for temperature being the degree Kelvin. The degree Celsius was relegated to a class with the hour, tonne, bar and a few others of important non-SI units still used with the SI. However in recent years this was changed and the degree Celsius was readmitted as an SI unit proper.

This means it can take all the standard SI prefixes, and this has an odd effect on the symbol. Unlike all the other SI symbols, the degree sign is attached to its number: 10°C, not 10 °C. But the prefixed ones take the space as normal: ten millidegrees Celsius is symbolized 10 m°C

The Celsius scale is also known as the practical scale. The Celsius temperature is symbolized t and is related to the thermodynamic temperature T by the equation t = T - T0, where T0 is the Celsius starting-point 273,15 K.

SI definitions and history at the BIPM official site: www1.bpm.org
In particular, degree Celsius included among derived units: http://www1.bipm.org/en/si/derived_units/2-2-2.html
The unit was readopted by the SI apparently by a 1989 decision of the CIPM, not the full CGPM: http://www1.bipm.org/en/si/base_units/kelvin.html

Cel"si*us (?), n.

The Celsius thermometer or scale, so called from Anders Celsius, a Swedish astronomer, who invented it. It is the same as the centigrade thermometer or scale.

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