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On May 20th, the kilogram will no longer be defined by a lump in France
On May 20th, World Metrology Day, the scientific community will officially change the definition of the kilogram. For 130 years, the kilo has been defined by a physical cylinder of platinum-iridium alloy, known as Le Grand K and stored in a vault outside of Paris. But every time scientists handled it, the cylinder lost atoms -- an estimated 50 micrograms over its lifetime. So, beginning Monday, the kilogram will officially be measured by a physical constant known as the Planck constant.
