Redefinition of the kilogram

doi:10.1088/0026-1394/42/2/001

Redefinition of the kilogram: a decision whose time has come

Ian M Mills et al 2005 Metrologia 42 71-80 doi: 10.1088/0026-1394/42/2/001

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Ian M Mills¹, Peter J Mohr², Terry J Quinn³, Barry N Taylor² and Edwin R Williams²
¹ Department of Chemistry, University of Reading, Reading, RG6 6AD, UK
² National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA
³ Emeritus Director, Bureau International des Poids et Mesures, Pavillon de Breteuil, F-92312 Sèvres, Cedex, France
E-mail: i.m.mills@reading.ac.uk, mohr@nist.gov, terry.quinn@physics.org, barry.taylor@nist.gov and edwin.williams@nist.gov

Abstract. The kilogram, the base unit of mass in the International System of Units (SI), is defined as the mass $m({\cal K})$ of the international prototype of the kilogram. Clearly, this definition has the effect of fixing the value of $m({\cal K})$ to be one kilogram exactly. In this paper, we review the benefits that would accrue if the kilogram were redefined so as to fix the value of either the Planck constant h or the Avogadro constant N_A instead of $m({\cal K})$ , without waiting for the experiments to determine h or N_A currently underway to reach their desired relative standard uncertainty of about 10⁻⁸. A significant reduction in the uncertainties of the SI values of many other fundamental constants would result from either of these new definitions, at the expense of making the mass $m({\cal K})$ of the international prototype a quantity whose value would have to be determined by experiment. However, by assigning a conventional value to $m({\cal K})$ , the present highly precise worldwide uniformity of mass standards could still be retained. The advantages of redefining the kilogram immediately outweigh any apparent disadvantages, and we review the alternative forms that a new definition might take.

Print publication: Issue 2 (April 2005)
Received 23 December 2004
Published 28 February 2005

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Redefinition of the kilogram: a decision whose time has come

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