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ITS90 TextContents 
The International Temperature Scale of 1990 (ITS90)H. PrestonThomasPresident of the Comité Consultatif de Thermométrie and VicePresident of the Comité International des Poids et Mesures Division of Physics, National Research Council of Canada, Ottawa, K1A OS1 Canada Introductory NoteThe official French text of the ITS90 is published by the BIPM as part of the Procèsverbaux of the Comité International des Poids et Mesures (CIPM). However, the English version of the text reproduced here has been authorized by the Comité Consultatif de Thermométrie (CCT) and approved by the CIPM. The International Temperature Scale of 1990The International Temperature Scale of 1990 was adopted by the International Committee of Weights and Measures at its meeting in 1989, in accordance with the request embodied in Resolution 7 of the 18^{th} General Conference of Weights and Measures of 1987. This scale supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 Provisional 0.5 K to 30 K Temperature Scale. 1. Units of TemperatureThe unit of the fundamental physical quantity known as thermodynamic temperature, symbol T, is the kelvin, symbol K, defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water^{1}. Because of the way earlier temperature scales were defined, it remains common practice to express a temperature in terms of its difference from 273.15 K, the ice point. A thermodynamic temperature, T, expressed in this way is known as a Celsius temperature, symbol t, defined by: t / °C = T/K  273.15 . (1) The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference of temperature may be expressed in kelvins or degrees Celsius. The International Temperature Scale of 1990 (ITS90) defines both International Kelvin Temperatures, symbol T_{90}, and International Celsius Temperatures, symbol t_{90}. The relation between T_{90} and t_{90}, is the same as that between T and t, i.e.: t_{90} / °C = T_{90}/K  273.15 . (2) The unit of the physical quantity T_{90} is the kelvin, symbol K, and the unit of the physical quantity t_{90}, is the degree Celsius, symbol °C, as is the case for the thermodynamic temperature T and the Celsius temperature t.
^{1} Comptes Rendus des Séances de la Treizième Conférence Générale des Poids es Mesures (19671968), Resolutions 3 and 4, p. 104 Continues: 2 Principles of the International Temperature Scale of 1990 (ITS90)

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