The Kilogram redefined
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Since 1889, Le Grand K, a sleek cylinder of platinum-iridium metal, has ruled from its underground vault in Paris. An absolute monarch, it was the very definition of one kilogram of mass. Scientists from around the world made pilgrimages to it, bringing along their national kilogram standards to weigh in comparison.

“The mother ship is never wrong,” said Robert Vocke Jr., a chemist at the National Institute of Standards and Technology (N.I.S.T.) in Gaithersburg, MD.

New York Times reports that on Friday, in a small conference center located just steps from the Palace of Versailles, several dozen nations voted to overthrow Le Grand K and to redefine the kilogram and three other standard units of measure: the ampere, for electrical current; the kelvin, for temperature; and the mole, which describes the amount of a chemical substance.

The vote fulfills an 18th-century dream. Henceforth, all seven units in the International System of Units, otherwise known as the S.I., will no longer be defined by material objects and instead will be defined only by abstract constants of nature.

In 1990, metrologists discovered that Le Grand K had mysteriously become lighter than its six official copies by some 50 micrograms. The kilogram standard was in trouble.

The mission to redefine it began.

Over the years, two possibilities presented themselves: to measure the exact mass of one kilogram in terms of the electromagnetic force required to lift it, or in terms of the specific number of atoms in its mass. But like the meter before 1983, neither of those definitions linked the kilogram to fundamental constants.

Friday’s vote cements the values of the Planck and Avogadro constants, and releases the kilogram from its earthly form. The four new definitions — for the kilogram, ampere, kelvin and mole — will be officially take effect after World Metrology Day, next May 20. The transition will only be felt at the frontiers of science and technology; the everyday world will not notice.

“They mustn’t notice, because if they do, we haven’t done our jobs right,” said Dr. Robinson, of the N.P.L.

Le Grand K will join its counterpart, the meter bar, in the archives of metrology. Dr. Bettin will retire next year. Dr. Schlamminger has set his sights on big G, the universal gravitational constant.

“If we love to measure things,” he said. “There’s tons of stuff out there that can be measured.”


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