Chemistry / Chem 0092 · Atom · ~20 seconds
The Kilogram
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The kilogram, SI's unit of mass, is defined since 2019 by a fixed value of the Planck constant, not an object.
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Before 2019 the kilogram was tied to a platinum-iridium cylinder in France; today a lab with the right apparatus realizes it directly.
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Some references still call it "a metal cylinder's mass" — accurate before 2019, now outdated. (NIST/BIPM, Aug 20, 2026.)
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