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  • The old international prototype kilogram (IPK).
    Rights: Greg L, CC BY-SA 3.0 Published 17 August 2011, Updated 8 August 2019 Size: 590 KB Referencing Hub media

    The international prototype kilogram (IPK) made of a platinum/iridium alloy. Its height and diameter are the same at 39.17 mm. It is kept at the International Bureau of Weights and Measures in Sèvres on the outskirts of Paris.

    The IPK was retired on 20 May 2019, when the kilogram was formally redefined in terms of Planck's constant.

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