Crystal Ball May Hold the Kilogram's Future

This article describes work directed at replacing the current standard for the kilogram (a platinum-iridium alloy cylinder) with something more permanent and less prone to aging. Though not the only candidate for the new kilogram standard, the approach described uses a high-purity sphere of polycrystalline silicon-28. This strategy requires precisely determining two important constants underpinning science - the Avogadro and Planck constants - and doing so in a complex manufacturing and analysis chain. The article describes the audacious plans to achieve the necessary precision and stability.

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