Kelvin scale

The Kelvin scale is the SI absolute temperature scale. It uses the same degree size as Celsius (T_K = T_C + 273.15) but places its zero at absolute zero, the point at which thermal motion is minimal. Because its zero is physical rather than conventional, the kelvin is a true ratio scale: 600 K really does correspond to twice the mean thermal energy per particle of 300 K, a statement meaningless on the Celsius or Fahrenheit scales.

Every fundamental equation of thermodynamics and statistical mechanics — the ideal gas law, the Boltzmann factor e^(−E/kT), the efficiency of a Carnot engine, the definition of entropy — is written in kelvin, because each depends on temperature measured from absolute zero rather than from an arbitrary reference.

Introduced by William Thomson (Lord Kelvin) in 1848 on thermodynamic grounds, the scale was for decades fixed by the triple point of water (exactly 273.16 K). Since the 2019 SI redefinition the kelvin is defined by fixing the Boltzmann constant, tying temperature directly to energy.

Proposed by William Thomson in 1848 from the temperature-independence of Carnot efficiency; realised through the triple point of water until 2019, then redefined via the Boltzmann constant.