World poised to adopt new metric units

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Science  09 Nov 2018:
Vol. 362, Issue 6415, pp. 625-626
DOI: 10.1126/science.362.6415.625

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For 130 years, Le Grand K, a gleaming cylinder of platinum-iridium alloy at the International Bureau of Weights and Measures in Sèvres, France, has served as the world's standard for mass. Now, in a revolution far less bloody than the one that cost King Louis XVI his head, it will cede its throne as the one, true kilogram. When the 26th General Conference on Weights and Measures convenes next week in Versailles, France, representatives of the 60 member nations are expected to vote to redefine the International System of Units so that four of its base units—the kilogram, ampere, kelvin, and mole—are defined indirectly, in terms of physical constants that will be fixed by fiat. They'll join the other three base units—the second, meter, and candela (a measure of a light's perceived brightness)—that are already defined that way. To take effect on 20 May 2019, the rewrite aims to make the units more stable and allow investigators to develop ever more precise and flexible techniques to mete them out. But even proponents of the arcane changes acknowledge they may bewilder nonexperts.