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In 1741 Anders Celsius, professor of astronomy at the University of Uppsala, Sweden, introduced a temperature scale with 0 the temperature at which water boiled and 100 the temperature at which water froze. Shortly after his death, the scale became known as the centigrade scale, but the fixed points were reversed, making 0 degrees the freezing point of water and 100 degrees its boiling point at atmospheric pressure.
In 1887 the International Commission on Weights and Measures adopted “as the standard thermometric scale for the international services of weights and measures the centigrade scale of the hydrogen thermometer, having as fixed points the temperature of melting ice (0°) and the vapor of distilled water boiling (100°) at standard atmospheric pressure, the hydrogen being taken at an initial manometric pressure of one meter of mercury.”
The Celsius scale is the centigrade scale with one change. Defined in 1954 at the 10th General Conference of Weights and Measures, temperature on the Celsius scale is the temperature on the Kelvin scale minus 273.15. This definition makes values on the Celsius and centigrade scale agree within less than 0.1 degree. For everyday purposes, the scales are identical. One reason for doing away with the word “centigrade,” was that it might be confused with one-hundredth of a grade, a unit of plane angle.
The problem was that the ice point, the “temperature of melting ice...at standard atmospheric pressure,” which was used to define zero degrees on the centigrade scale, cannot be measured with enough precision. Ideally one takes the temperature of a bath of pure, air-saturated water containing pure melting ice. But as ice melts it surrounds itself with a layer of insulating meltwater that is not air-saturated. The bath cannot be stirred because that would heat it.
In contrast, the Kelvin scale has a nearby set point, the triple point of water. The triple point is the temperature and pressure at which water can exist simultaneously as a solid, liquid and gas. Measurements of the temperature of the triple point are reproducible with an variation of 0.000 050 K or less. By the definition of the Kelvin scale the triple point of water is 273.16 kelvin. Replacing the hard-to-measure ice point with the triple point made possible more precise measurements. To indicate the change, the “zero is freezing, 100 is boiling” scale was given a new name.
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Last revised: 1 August 2009.