Is the kilogram shrinking?

The kilogram is one of the few units of measure based on an actual phyisical object: a cylinder made of platinum and iridium, kept under triple-locked protection in Sevres, France. But the reference kilogram now weighs 50 micrograms less than copies that were made at the same time and are stored elsewhere. Is the kilogram shrinking -- or are the copies somehow becoming more massive? (cnews)


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