The new SI, a unit system for the 21st century

The International System of Units (SI) is founded on the seven base units metre, kilogram, second, ampere, kelvin, mole and candela corresponding to the seven base quantities length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. At the 23rdGeneral Conference on Weights and Measures held in Paris in November 2007, Member States of the Metre Convention adopted Resolution 12 in which they recommended (among other things) that National Metrology Institutes and the BIPM • pursue the relevant experiments so that the International Committee can come to a view on whether it may be possible to redefine the kilogram, the ampere, the kelvin, and the mole using fixed values of the fundamental constants at the time of the 24th General Conference (2011), • initiate awareness campaigns to alert user communities to the possibility of redefinitions and that the technical and legislative implications of such redefinitions and their practical realizations be carefully discussed and considered. The proposal for such redefinitions, while long envisaged in general terms beginning with Maxwell in 1870, is based in detail on two recent papers by Mills, Mohr, Quinn Taylor and Williams (Metrologia 42 (2005) 227-246 and 43 (2006) 227-246). In these papers we proposed redefining the kilogram, ampere, kelvin and mole so that these units are explicitly linked to exactly known values of the Planck constant h, elementary charge e, Boltzmann constant k and Avogadro constant NA, respectively.