In 1744, coincident with the death of Anders Celsius, the Swedish botanist Carl Linnaeus (1707–1778) reversed Celsius's scale. On he published the design of a mercury thermometer, the "Thermometer of Lyon" built by the craftsman Pierre Casati that used this scale. Some credit Christin for independently inventing the reverse of Celsius' original scale, while others believe Christin merely reversed Celsius' scale. In 1743, the Lyonnais physicist Jean-Pierre Christin, permanent secretary of the Academy of Lyon, inverted the Celsius scale so that 0 represented the freezing point of water and 100 represented the boiling point of water. The BIPM's 10th General Conference on Weights and Measures (CGPM) in 1954 defined one standard atmosphere to equal precisely 1,013,250 dynes per square centimeter (101.325 kPa). This pressure is known as one standard atmosphere. He proposed that the zero point of his temperature scale, being the boiling point, would be calibrated at the mean barometric pressure at mean sea level. He also determined with remarkable precision how the boiling point of water varied as a function of atmospheric pressure. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that the melting point of ice is essentially unaffected by pressure. In 1742, Swedish astronomer Anders Celsius (1701–1744) created a temperature scale that was the reverse of the scale now known as "Celsius": 0 represented the boiling point of water, while 100 represented the freezing point of water. Note the reversed scale, where 100 is the freezing point of water and 0 is its boiling point. The degree Celsius remains exactly equal to the kelvin, and 0 K remains exactly −273.15 ☌.Īn illustration of Anders Celsius's original thermometer. The newly-defined exact value of the Boltzmann constant was selected so that the measured value of the VSMOW triple point is exactly the same as the older defined value to within the limits of accuracy of contemporary metrology. This means that the triple point is now a measured value, not a defined value. On, the kelvin was redefined so that its value is now determined by the definition of the Boltzmann constant rather than being defined by the triple point of VSMOW. This means that a temperature difference of one degree Celsius and that of one kelvin are exactly the same.
Until, the temperature of the triple point of water was defined as exactly 273.16 K (0.01 ☌). Absolute zero, the lowest temperature possible, is defined as being exactly 0 K and −273.15 ☌.
This definition also precisely related the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. The 1743 scale reversal was proposed by Jean-Pierre Christin.īy international agreement, between 19 the unit degree Celsius and the Celsius scale were defined by absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW), a precisely defined water standard. the boiling point was 0 degrees and the freezing point was 100 degrees). Prior to 1743 the values were reversed (i.e.
Since 1743 the Celsius scale has been based on 0 ☌ for the freezing point of water and 100 ☌ for the boiling point of water at 1 atm pressure. Before being renamed to honor Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (symbol: ☌) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference or range between two temperatures. The degree Celsius is a unit of temperature on the Celsius scale, a temperature scale originally known as the centigrade scale. A thermometer calibrated in degrees Celsius
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