Plot to redefine the kilogram nears climax

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Science  19 May 2017:
Vol. 356, Issue 6339, pp. 670-671
DOI: 10.1126/science.356.6339.670

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Le Grand K could be facing retirement at last. For more than a decade, metrologists have sought to replace the legendary standard for the kilogram—a 128-year-old slug of platinum iridium alloy in a Paris vault—with one based on an immutable constant of nature. This week in Paris, teams from several countries presented (almost) final measurements of the Planck constant, the key to the new standard. If those numbers agree, the General Conference on Weights and Measures (CGPM) could redefine the kilogram next year. Metrologists have already redefined in a similar way another key unit in the International System of Units, the meter. In 1983, CGPM defined the speed of light as exactly 299,792,458 meters per second, allowing the meter to be redefined as the distance light travels in 1/299,792,458 of a second. Scientists aim to redefine the kilogram with an electrical device called a Kibble balance, in which a weight is balanced by a magnetic force on a coil carrying electric current. Do everything just right and you can define the kilogram in terms of a current and a voltage, with the Planck constant, the fundamental constant of quantum mechanics, entering through the techniques used to make the electrical measurements. Knowing the Planck constant, researchers can then use a Kibble balance to manufacture a standard kilogram. Before the new standard can be adopted, however, scientists must measure the constant as precisely as possible so its value can be fixed. Most researchers now expect at least three measurements to meet the criteria for fixing the constant and moving forward with redefining the kilogram.