the International Prototype of the Kilogram (IMAGE)
Caption
From 1889 to 2019, the international prototype of the kilogram was the measure of all masses. By definition, 1 kg was the mass of this piece of metal. The international prototype of the kilogram ('Le grand k'), together with its official copies (the so-called témoins -- witnesses), is kept in a safe at the Bureau International des Poids et Mesures (BIPM) near Paris. In addition, there are further national copies worldwide. On the occasion of mass comparisons which took place every couple of years, the copies appeared to be growing heavier in comparison to the international prototype of the kilogram. No one really knows why. It was therefore clear that the metal kilogram needed to be retired.
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BIPM
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