Redefining the SI Base Units

November 2, 2011
Contact: Peter Mohr
(301) 975-3217

The latest official brochure for the international system of units.

Metrology is poised to undergo a profound change that will benefit scientists, engineers, industry and commerce – but which almost no one will notice in daily life.
The international General Conference on Weights and Measures (CGPM) has approved a plan to redefine four of the seven base units of the International System of Units (SI) in terms of fixed values of natural constants. The initiative would make possible new worldwide levels of consistency and accuracy, simplify and normalize the unit definitions, and liberate the system from dependence on the prototype kilogram, an artifact adopted in 1889 and still used as the world’s physical standard for mass.
On Oct. 21, 2011, CGPM, the diplomatic body that has the authority under the Meter Convention to enact such a sweeping change, passed a resolution declaring that the kilogram, the ampere, the kelvin and the mole, “will be redefined in terms of invariants of nature; the new definitions will be based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (N_A), respectively.”
That action follows – and results directly from – decades of pioneering metrology research around the globe, some of it accomplished by various groups at NIST and its antecedent, the National Bureau of Standards (NBS), that are now part of PML. And it echoes the recommendations made by three PML scientists and two European colleagues in an influential 2006 paper in Metrologia.
....more