ScienceDaily (Mar. 1, 2005) — It’s time to replace the 115-year-old kilogram artifact as the world's official standard for mass, even though experiments generally thought necessary to achieve this goal have not yet reached their targeted level of precision. That the conclusion of an upcoming Metrologia journal article* authored by five eminent scientists from the United States, United Kingdom and France that was discussed at a scientific meeting of the Royal Society of London on Feb. 14-15.

The authors of this Metrologia paper suggest replacing the kilogram artifact—a cylinder of platinum-iridium alloy about the size of a plum—with a definition based on one of two unchanging natural phenomena, either a quantity of light or the mass of a fixed number of atoms.

The five authors, including three from the U.S. National Institute of Standards and Technology (NIST), one from the University of Reading in the United Kingdom, and a former director of the Bureau International des Poids et Mesures (BIPM) near Paris, conclude that redefining the kilogram now in terms of an invariable property of nature rather than a material object could immediately have many benefits. For instance, it would improve the precision of certain electrical measurements 50-fold and would enable physicists to make more precise calculations in studying the fundamental quantum properties of atoms and other basic particles. The paper outlines how this could be accomplished without impairing the current international system of mass measurements.

For further information see: http://www.nist.gov/public_affairs/newsfromnist_redef_kilogram.htm

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* I.M. Mills, P.J. Mohr, T.J. Quinn, B. Taylor, E. Williams, "Redefinition of the kilogram: A decision whose time has come," Metrologia, expected online publication, Feb. 2005.

Shown above is the U.S. National Prototype Kilogram, which currently serves as the nation's primary standard for measuring mass. It was assigned to the United States in 1889 and is periodically recertified and traceable to the primary international standard, "The Kilogram," held at the Bureau International des Poids et Mesures near Paris. (© Robert Rathe)

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