Research articleAdapting the International System of Units to the twenty-first century
Abstract
We review the proposal of the International Committee for Weights and Measures (Comité International des Poids et Mesures, CIPM), currently being considered by the General Conference on Weights and Measures (Conférences Générales des Poids et Mesures, CGPM), to revise the International System of Units (Le Système International d'Unitès, SI). The proposal includes new definitions for four of the seven base units of the SI, and a new form of words to present the definitions of all the units. The objective of the proposed changes is to adopt definitions referenced to constants of nature, taken in the widest sense, so that the definitions may be based on what are believed to be true invariants. In particular, whereas in the current SI the kilogram, ampere, kelvin and mole are linked to exact numerical values of the mass of the international prototype of the kilogram, the magnetic constant (permeability of vacuum), the triple-point temperature of water and the molar mass of carbon-12, respectively, in the new SI these units are linked to exact numerical values of the Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant, respectively. The new wording used expresses the definitions in a simple and unambiguous manner without the need for the distinction between base and derived units. The importance of relations among the fundamental constants to the definitions, and the importance of establishing a mise en pratique for the realization of each definition, are also discussed.
Footnotes
References
- 1BIPM. 2006The International System of Units8th edn.Sèvres, FranceBureau International des Poids et MesuresSee http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf. Google Scholar
- 2BIPM. 2010Resolution A for the 24th Meeting of the CGPM. On the possible future revision of the International System of Units, the SI. See http://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf. Google Scholar
- 3BIPM. 2010Draft Chapter 2 for SI Brochure, following redefinitions of the base units. . See http://www.bipm.org/cc/CCU/Allowed/20/Watermark_SI_Brochure_Draft_Ch2_29Sep10.pdf. Google Scholar
- 4
Mills I. M., Mohr P. J., Quinn T. J., Taylor B. N.& Williams E. R . 2005Redefinition of the kilogram: a decision whose time has come. Metrologia 42, 71-80doi:10.1088/0026-1394/42/2/001 (doi:10.1088/0026-1394/42/2/001). Crossref, ISI, Google Scholar - 5
Mills I. M., Mohr P. J., Quinn T. J., Taylor B. N.& Williams E. R. . 2006Redefinition of the kilogram, ampere, kelvin and mole: a proposed approach to implementing CIPM recommendation 1 (CI-2005). Metrologia 43, 227-246doi:10.1088/0026-1394/43/3/006 (doi:10.1088/0026-1394/43/3/006). Crossref, ISI, Google Scholar - 6
Taylor B. N.& Mohr P. J. . 1999On the redefinition of the kilogram. Metrologia 36, 63-64doi:10.1088/0026-1394/36/1/11 (doi:10.1088/0026-1394/36/1/11). Crossref, ISI, Google Scholar - 7
Mohr P. J. . 2008Defining units in the quantum based SI. Metrologia 45, 129-133doi:10.1088/0026-1394/45/2/001 (doi:10.1088/0026-1394/45/2/001). Crossref, ISI, Google Scholar - 8
Cabiati F.& Bich W. . 2009Thoughts on a changing SI. Metrologia 46, 457-466doi:10.1088/0026-1394/46/5/009 (doi:10.1088/0026-1394/46/5/009). Crossref, ISI, Google Scholar - 9
Gläser M., Borys M., Ratschko D.& Schwartz R. . 2010Redefinition of the kilogram and its impact on its future dissemination. Metrologia 47, 419-428doi:10.1088/0026-1394/47/4/007 (doi:10.1088/0026-1394/47/4/007). Crossref, ISI, Google Scholar - 10
Taylor B. N. . 2009Molar mass and related quantities in the new SI. Metrologia 46, L16-L19doi:10.1088/0026-1394/46/3/L01 (doi:10.1088/0026-1394/46/3/L01). Crossref, ISI, Google Scholar - 11
Milton M. J. T., Williams J. M.& Bennett S. J. . 2007Modernizing the SI: towards an improved, accessible and enduring system. Metrologia 44, 356-364doi:10.1088/0026-1394/44/5/012 (doi:10.1088/0026-1394/44/5/012). Crossref, ISI, Google Scholar - 12
Mohr P. J., Taylor B. N.& Newell D. B. . 2008CODATA recommendedvalues of the fundamental physical constants: 2006. Rev. Mod. Phys. 80, 633-730doi:10.1103/RevModPhys.80.633 (doi:10.1103/RevModPhys.80.633). Crossref, ISI, Google Scholar - 13
Bordé C. J. . 2005Base units of the SI, fundamental constants and modern quantum physics. Phil. Trans. R. Soc. A 363, 2177-2201doi:10.1098/rsta.2005.1635 (doi:10.1098/rsta.2005.1635). Link, ISI, Google Scholar - 14
Becker P.& Bettin H. . 2011The Avogadro constant: determining the number of atoms in a single-crystal 28Si sphere. Phil. Trans. R. Soc. A 369, 3925-3935doi:10.1098/rsta.2011.0222 (doi:10.1098/rsta.2011.0222). Link, ISI, Google Scholar - 15
Stock M. . 2011The watt balance: determination of the Planck constant and redefinition of the kilogram. Phil. Trans. R. Soc. A 369, 3936-3953doi:10.1098/rsta.2011.0184 (doi:10.1098/rsta.2011.0184). Link, ISI, Google Scholar - 16
Davis R. S. . 2011The role of the international prototype of the kilogram after redefinition of the International System of Units. Phil. Trans. R. Soc. A 369, 3975-3992doi:10.1098/rsta.2011.0181 (doi:10.1098/rsta.2011.0181). Link, ISI, Google Scholar - 17
Pitre L., Sparasci F., Truong D., Guillou A., Risegari L.& Himbert M. E. . 2011Determination of the Boltzmann constant using a quasi-spherical acoustic resonator. Phil. Trans. R. Soc. A 369, 4014-4027doi:10.1098/rsta.2011.0197 (doi:10.1098/rsta.2011.0197). Link, ISI, Google Scholar - 18
Milton M. J. T. . 2011A new definition for the mole based on the Avogadro constant: a journey from physics to chemistry. Phil. Trans. R. Soc. A 369, 3993-4003doi:10.1098/rsta.2011.0176 (doi:10.1098/rsta.2011.0176). Link, ISI, Google Scholar - 19
Milton M. J. T.& Mills I. M. . 2009Amount of substance and the proposed redefinition of the mole. Metrologia 46, 332-338doi:10.1088/0026-1394/46/3/022 (doi:10.1088/0026-1394/46/3/022). Crossref, ISI, Google Scholar
