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Redefinition of the Kilogram, Ampere, Kelvin and Mole: A Proposed Approach to implementing CIPM Recommendation 1 (C1-2005)



I M. Mills, Peter Mohr, T J. Quinn, Barry Taylor, Edwin R. Williams


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 its 94th meeting in october, 2005, the International Committee for Weights and Measures (CIPM) adopted Recommendation 1 (C1-2005) on preparative steps towards redefining the kilogram, ampere, kelvin and mole so that these units are linked to exactly known values of fundamental constants. In fact, we see many advantages to linking these four base units to the Planck constant h, elementary charge e, Boltzmann constant k and Avogradro constant NA, respectively. By directly connecting them to these four fundamental physical constants, which are true invariants of nature, the SI would be significantly advanced, as would our knowledge of the values of the fundamental constants themselves. Not only would h, e, k, and NA have exact values, the uncertainties of many other constants would be either eliminated or significantly reduced. In keeping with this recommendation, it is the purpose of this paper to (i) provide the background for the current unit-redefinition effort; (ii) discuss the merits of redefining these units as proposed, present possible workings for the four new definitions, and suggest a novel way to define the entire SI explicitly in terms of constants without associating a particular unit with a particular constant; (iii) present the elements of a General Conference on Weights and Measures (CGPM) resolution that would formally implement the new definitions, possibly at the 24th CGPM in October 2011; (iv) discuss the impact of the new definitions on both practical metrology and the fundamental physical constants; and (v) discuss some of the issues related to the preparation of mises-en-pratiques of the four new definitions.


ampere, Avogadro constant, Kelvin, kilogram, SI Units


Mills, I. , Mohr, P. , Quinn, T. , Taylor, B. and Williams, E. (2021), Redefinition of the Kilogram, Ampere, Kelvin and Mole: A Proposed Approach to implementing CIPM Recommendation 1 (C1-2005), Metrologia (Accessed July 24, 2024)


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Created October 12, 2021