It is named for the German physicist Ludwig Boltzmann. Symbol, k. According to the 2006 CODATA recommended values, k = 1.380 6504 × 10−23 joule per kelvin, with a standard uncertainty of 0.000 0024 × 10−23 joule per kelvin.
Boltzmann himself (and his mentor Stefan) did not regard this number as a constant, but as a conversion factor. Apparently Max Planck was the first person to refer to it as a constant. By the 1920's the usage “Boltzmann constant” was thoroughly established.
Work on refining the value of the Boltzmann constant has continued at such centers as the Physikalisch-Technische Bundesanstalt.
Photograph courtesy Physikalisch-Technische Bundesanstalt
At its 24th meeting (Paris, October 2011), the CGPM decided to declare in advance its intention to make the value of the Boltzmann constant a matter of definition, rather than something to be determined experimentally. The new value will be exactly 1.380 6X × 10−23 joule per kelvin, where X stands for one or more yet to be determined digits. The new definition will not be adopted before 2014.
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Last revised: 1 January 2007.