National Physical Laboratory

Proposed SI Changes

In order to continue to improve the robustness of the SI system by basing the units on fundamental constants a proposal is under consideration which would redefine the kilogram, ampere, kelvin and perhaps the mole in terms of physical constants. This would be done by giving assigned values to these constants as was done with the speed of light in 1983.

This proposal has come about following progress over the last few years, in enabling a replacement for the last of the SI base units that remains defined by and artefact, the kilogram. Progress has been made in the watt balance. This is an experiment that will allow the kilogram to be redefined in terms of the Plank constant. The link between the kilogram and the Plank constant is not an obvious one but comes about via the electrical units. Progress also continues with experiments aimed at redefining the kilogram in terms of the Avogadro constant.

The SI is governed by the Metre Convention, an international treaty that established the international system in 1875. Most developed countries are signatories to that treaty. Changes to the SI are made through a political process centred at the BIPM starting with recommendations of the consultative committee on units. The authority to make changes to the SI resides with the General conference on Weights and Measures and they are advised on technical matters by the International Committee for Weights and Measures the CGPM. A guide to the committees and structures is available.

Decision of the Consultative Committee on units 2005

At the CCU meeting of July 2005 a proposal to redefine the kilogram at the 2007 CGPM was rejected, principally because of the unresolved discrepancy between the Avogadro and watt balance results. Instead they are looking forward to 2011 when it is proposed to redefine the kilogram in terms of Planck constant and also the ampere in terms of the electron charge, and possibly also at the same time the kelvin in terms of the Boltzmann constant. A further proposal is the redefinition of the mole in terms of the Avogadro constant. The recommendation is available and the full report of the meeting.

Future of the SI

In order to encourage communication, awareness and debate around the future of the SI, in February 2011 the BIPM (Bureau International des Poids et Mesures) created some dedicated webpages about the future of the SI. The webpages are dedicated to explaining the 'New SI' and give access to Draft Resolution A as well as Draft Chapter 2 of the next edition of the SI Brochure.

BIPM's 'New SI' website also contains the presentations given at the Royal Society Discussion Meeting, held in London on 24 and 25 January 2011, on the subject: The New SI: Units of measurement based on fundamental constants


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