kelvin

Historical perspective: Unit of thermodynamic temperature, kelvin

 

The definition of the unit of thermodynamic temperature was given by the 10th CGPM which selected the triple point of water, TTPW, as a fundamental fixed point and assigned to it the temperature 273.16 K, thereby defining the kelvin. The 13th CGPM adopted the name kelvin, symbol K, instead of "degree kelvin", symbol °K, for the unit defined in this way. However, the practical difficulties in realizing this definition, requiring a sample of pure water of well-defined isotopic composition and the development of new primary methods of thermometry, led to the adoption of a new definition of the kelvin based on a fixed numerical value of the Boltzmann constant k. The present definition, which removed both of these constraints, was adopted in Resolution 1 of the 26th CGPM (2018).

Suggested edited text:

The unit of thermodynamic temperature, the kelvin, was first formally defined by the 10th CGPM, which selected the triple point of water, TTPW, as a fundamental fixed point and assigned to it the temperature 273.16 K. The 13th CGPM adopted the name kelvin, symbol K, instead of "degree kelvin", symbol °K, for the unit defined in this way. However, the practical difficulties in realizing this definition, requiring a sample of pure water of well-defined isotopic composition and the development of new primary methods of thermometry, led to the development of a new definition that removes both of these constraints. The present definition of the kelvin, adopted in Resolution 1 of the 26th CGPM (2018), is based on a fixed numerical value of the Boltzmann constant, k.

 

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Résolutions/Recommandations

CGPM - Résolution 3 (1954)

Définition de l'échelle thermodynamique de température

CGPM - Résolution 3 (1967)

Unité SI de température thermodynamique (kelvin)

CGPM - Résolution 1 (2018)

Sur la révision du Système international d'unités (SI)