  Temperature scale and the Boltzmann constant: The newest system of units is
based on a compatible set of natural constants with fixed values. An example is
the Boltzmann constant k which defines the thermal energy content kT. To
express the base unit T, the absolute temperature in kelvin, an international
agreement for the temperature scale is needed. The scale is defined using fixed
points, which are temperatures of various phase transitions. Especially
important has been the triple point of water at 273.1600 K. These fixed point
temperatures determine the international temperature scale ITS within the Si
system. Temperature measurement itself is based on physical laws and on the
properties of appropriate thermometric materials selected to determine the
temperature scale. For determining the Boltzmann constant, new precision
techniques have been developed during the last two decades. Examples are
different types of gas thermometry, which ultimately are based on the ideal gas
law, and thermal noise of electric charge carriers in conductors. With these
means it has become possible to fix the value of the Boltzmann constant with a
relative uncertainty of < 1 ppm. As of 2019, the value of k has been agreed to
be fixed at 1.380649x10^(-23) J/K. This agreement replaces the earlier
definition of a Kelvin degree.
