The definition of a kilogram, a unit roughly equal to 2.2 pounds, has changed forever.
Since 1889, the kilogram has been defined by a platinum-iridium alloy cylinder nicknamed Le Grand K, which is located in a triple-locked vault underneath the Pavillon de Breteuil, a building near Paris first unveiled by King Louis XIV in 1672.
But on Monday—World Metrology Day—Le Grand K lost its special status as the international prototype kilogram (IPK) and it will no longer represent this base unit of mass to the world.
From now on, the kilogram—along with the ampere, kelvin, mole, and candela—will be defined by fundamental physical and atomic properties instead of tangible human-made objects.
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