# Kelvin

##### Article

May 25, 2022

The kelvin (symbol K, sometimes incorrectly indicated as ° K) is a unit of measurement of temperature that belongs to the seven basic units of the international system of units.

## Definition

In 2018, the 26th General Conference of Weights and Measures modified the definition of the unit of measurement of temperature by reducing it to the mechanical units of the international system (second, meter, kilogram), exploiting the theorem of equipartition of the energy of mechanics statistics. A kelvin is the thermodynamic temperature difference corresponding to a thermal energy difference equal to k B. T. {\ displaystyle k_ {B} T} , where is it k B. {\ displaystyle k_ {B}} is the value of the Boltzmann constant in joules / kelvin. The value of the constant, which until now had not yet been measured with sufficient precision and still varied slightly from one conference to another, is finally considered sufficiently stable: Boltzmann's constant now by definition corresponds to 1 , 380649 × 10 - 23 J / K {\ displaystyle 1,380649 \ times 10 ^ {- 23} J / K} .

## History

The immediately preceding (re) definition of kelvin was adopted in 1954 by the 10th Conférence générale des poids et mesures, and was by " 1 273 , 16 {\ displaystyle {\ frac {1} {273,16}}} absolute temperature of the triple point of water ". By" absolute temperature "we mean the difference between the indicated temperature (in this case that of the triple point of water: 0 , 01 or C. {\ displaystyle 0,01 \ mathrm {^ {o} C}} ) and that of absolute zero ( - 273 , 15 or C. ) {\ displaystyle (-273.15 \ mathrm {^ {o} C})} . The zero of the kelvin scale is in fact absolute zero: it is an absolute temperature scale, like the Rankine scale. To understand the level of precision required in defining a unit of measurement, the Comité International des Poids et Mesures added the following clarification to the above definition of kelvin at its 2005 meeting: "This definition refers to water having an isotopic composition defined exactly by the following substance ratios: 0 , 00015576 {\ displaystyle 0,00015576} moles of H. 2 {\ displaystyle {\ ce {^ {2} H}}} per mole of H. 1 {\ displaystyle {\ ce {^ {1} H}}} , 0 , 0003799 {\ displaystyle 0,0003799} moles of OR 17 {\ displaystyle {\ ce {^ {17} O}}} per mole of OR 16 {\ displaystyle {\ ce {^ {16} O}}} , And