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 24^{th} 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.