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    • IPK (international prototype kilogram)housed in nested bell jars
    Rights: Image reproduced with permission of the BIPM, which retains full internationally protected copyright (© BIPM) Published 17 August 2011 Size: 190 KB Referencing Hub media

    The IPK (international prototype kilogram) is housed under three nested glass bell jars with a carefully controlled atmosphere. The IPK was retired on 20 May 2019.

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      ARTICLE

      Changing the mass standard

      For more than a century, the base SI unit for mass was defined by a material artefact. Originally, 1 kilogram was defined as the mass of 1000 cm3 of water at 4C. Based on this, the international ...

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