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.
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