Paving the way for the redefinition of the unit of temperature

At present, the kelvin is literally based on no more than water—on the triple point of water, to be more precise. The base unit of temperature is therefore dependent on a material whose properties may vary. But this is about to change: In the fall of 2018, the kelvin, as well as all the other units of the International System of Units (SI), will be redefined to rest on a solid and invariable foundation consisting of fundamental constants. The kelvin will be based on the Boltzmann constant, which scientists from the Physikalisch-Technische Bundesanstalt (PTB) have now determined by means of a dielectric-constant gas thermometer with such accuracy that all obstacles to the redefinition of the unit of temperature, the kelvin, have been removed. The results have been published in the current edition of the specialist journal Metrologia.