Redefinition of the kilogram, ampere, kelvin and mole: a proposed approach to implementing CIPM recommendation 1 (CI-2005)

Abstract

The International System of Units (SI) is founded on seven base units, the metre, kilogram, second, ampere, kelvin, mole and candela corresponding to the seven base quantities of 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 a recommendation on preparative steps towards redefining the kilogram, ampere, kelvin and mole so that these units are linked to exactly known values of fundamental constants. We propose here that these four base units should be given new definitions linking them to exactly defined values of the Planck constant h, elementary charge e, Boltzmann constant k and Avogadro constant N A , respectively. This would mean that six of the seven base units of the SI would be defined in terms of true invariants of nature. In addition, not only would these four fundamental constants have exactly defined values but also the uncertainties of many of the other fundamental constants of physics would be either eliminated or appreciably reduced. In this paper we present the background and discuss the merits of these proposed changes, and we also present possible wordings for the four new definitions. We also suggest a novel way to define the entire SI explicitly using such definitions without making any distinction between base units and derived units. We list a number of key points that should be addressed when the new definitions are adopted by the General Conference on Weights and Measures (CGPM), possibly by the 24th CGPM in 2011, and we discuss the implications of these changes for other aspects of metrology.

A more fundamental International System of Units

“…14 In 2007 the 23rd CGPM recommended continuing the pursuit of relevant experiments to provide data for the fixed values of the fundamental constants, developing experimental procedures for practical realizations (mises en pratique) of the units within the new framework, and initiating awareness campaigns of a new SI based on fundamental constants.…”

Adapting the International System of Units to the twenty-first century

“…(The idea of defining the entire SI by a single scaling statement was first put forward in the latter paper [5] and is due to P. J. Mohr.)…”

Field representation of a watt balance magnet by partial profile measurements

“…The watt balance, which was originally proposed by Dr. B P Kibble at the National Physical Laboratory (NPL, UK) in 1975 [1], is an experiment currently for precision measurement of the Planck constant h, and in future for maintaining one of the seven base units, the kilogram [2]. The operation of a watt balance is divided into two independent measurement modes, respectively known as the weighing mode and the velocity mode.…”