# Kelvin

Source: NIST
 Topics:

Lord William Thomson Kelvin (1824-1907)

The kelvin is the unit of thermodynamic temperature in the International System of Units (SI). It is one of the seven SI base units, and is named after William Thomson Kelvin (1824-1907), the Scottish mathematician and physicist who helped lay the foundation for modern physics. The definition of the unit of thermodynamic temperature was given in substance by the 10th General Conference on Weights and Measures (CGPM) (1954) which selected the triple point of water as the fundamental fixed point and assigned to it the temperature 273.16 °K, so defining the unit. The 13th CGPM (1967) adopted the name kelvin (symbol K) instead of "degree Kelvin" (symbol °K) and defined the unit of thermodynamic temperature as follows:

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Because of the way temperature scales used to be defined, it remains common practice to express thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T0 = 273.15 K, the ice point. This temperature difference is called a Celsius temperature, symbol t, and is defined by the quantity equation

t = T - T0

The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference or interval of temperature may be expressed in kelvins or in degrees Celsius. The numerical value of a Celsius temperature t expressed in degrees Celsius is given by

t/°C = T/K - 273.15

The kelvin and the degree Celsius are also the units of the International Temperature Scale of 1990 (ITS-90) adopted by the International Committee for Weights and Measures (CIPM) in 1989.