Since 1948, the unit has been called "Celsius." "Centigrade" was the name of the unit before the change, with "centi" meaning a hundred and "grade" being a scale.
The other main measurement of temperature is theFahrenheit scale, but it is less used. The Celsius scale, based on multiples of ten, is used with SI, ormetric, measurements.
In 1742,Anders Celsius made a "reversed" version of the modern Celsius temperature scale in which 0 was the freezing point of water and 100 the melting point of ice. In his paperObservations of Two persistent Degrees on a thermometer, he wrote about his experiments. He showed that the melting point of ice was basically unaffected byair pressure. Ice would turn into water at the same temperature whether it was at sea level or on a mountain.
That was not the case for the boiling point of water, which is lower with less pressure, such as on a mountain. He decided that zero on his temperature scale, the boiling point of water, would be set at the standardbarometric pressure at sea level, which is now known as oneatmosphere. In 1954, Resolution 4 of the 10th CGPM[1] (theGeneral Conference on Weights and Measures) set what exactly is one standard atmosphere (101.325kPa or 14.6959psi).
In 1744, the year that Celsius died, the famous Swedish botanistCarolus Linnaeus (1707–1778) used a reversed version[2] of Celsius's scale when he bought his firstthermometer. Its scale had zero represent the melting point of ice and 100 represent the boiling point of water, as what is used today. His custom-made "Linnaeus thermometer" to be used in his greenhouses, was made byDaniel Ekström, who was Sweden's leading maker of scientific instruments. Ekstöm's workshop was in the basement of the StockholmObservatory. As then often happened before modern communications, many physicists, scientists, and instrument makers are given credit with independently making the same measurement scale;[3] among them werePehr Elvius, the secretary of the Royal Swedish Academy of Sciences (which had an instrument workshop), who Linnaeus had also talked to;Christin of Lyons; Ekström, the instrument-maker; andMårten Strömer (1707–1770) who had studied astronomy under Anders Celsius.
The first known document[4] reporting temperatures in this modern "forward" Celsius scale is the paperHortus Upsaliensis, dated 16 December 1745, by Linnaeus, who wrote to one of his students, Samuel Nauclér. In it, Linnaeus reported the temperatures inside the orangery at the Botanical Garden ofUppsala University:
...since the caldarium (the hot part of the greenhouse) by the angle of the windows, merely from the rays of the sun, obtains such heat that the thermometer often reaches 30 degrees, although the keen gardener usually takes care not to let it rise to more than 20 to 25 degrees, and in winter not under 15 degrees...
For the next 204 years, the scientific and thermometry communities worldwide called this scale the "centigrade scale." Temperatures on the centigrade scale were often reported as "degrees" or "degrees centigrade." The symbol for temperature values on thiescale was °C (in several formats over the years).
There were three reasons for the decision to use the wordCelsius:
All common temperature scales would have their units named after someone closely associated with them: Kelvin, Celsius, Fahrenheit, Réaumur and Rankine.
The symbol °C that for centuries had been used in association with the namecentigrade could continue to be used but now meantCelsius. (Linnaeus was an important influence on the modern scale, but Celsius had first developed it.)
The new name meant thatcentigrade could again mean only the French name for the unit of angular measurement.
It would take nearly two decades, however, for school textbooks to change fromcentigrade toCelsius, and many people today still use the old name.
To turn a temperature in degrees Celsius intokelvins, 273.15 is added: K = C + 273.15. For example, 0°C, the temperature at which water freezes, is 273.15 K.
To turn a temperature in kelvins into degrees Celsius, 273.15 is subtracted: C = K - 273.15. For example, 310K is the same temperature as 36.85°C, about the temperature of ahuman body.
To turn a temperature in degrees Celsius into degreesFahrenheit, the former is multiplied by 9/5 and then 32°F is added: F = (9/5) * C + 32.
To turn a temperature in degreesFahrenheit into degrees Celsius, 32 is subtracted and the result is multiplied by 5/9: C = (F - 32) * (5/9).
↑According toThe Oxford English Dictionary (OED), the term "Celsius' thermometer" had been used at least as early as 1797. The term "The Celsius or Centigrade thermometer" was again used as the name of a particular type of thermometer at least as early as 1850. However, this was probably used by the general public, not by scientists. As an example, the twelve-volume 1933 edition of OED did not even have a listing for the wordCelsius (but did have listings for bothcentigrade andcentesimal in the context of temperature measurement).
