The kilogram is still defined in terms of an artefact, namely, the same prototype sanctioned by the 1st CGPM in 1889. It has three main limitations: available in one place, not really invariant in terms of physical mass and not universal as based on an artefact. The definition of the mass unit could be revised as early as 2011 by basing it on exactly fixed value of a fundamental constant of nature. Thus, the possibility of redefining the kilogram in function of a true natural invariant has been discussed for about 30 years. In this context, the French metrology decided in 2002 to undertake the realisation of a watt balance experiment aiming at linking the kilogram to the Planck constant. The adoption of a new definition of the kilogram based on an atomic or a fundamental physical constant has to take its dissemination into account by avoiding great change for the accredited institutes and most of users. Of course, this will have consequences for the transfer mass standards used by the National Institutes of Metrology.

Key words

references and standards
units of measurement
watt balance
silicon single crystal
planck constant
avogadro constant