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1 .This is Scientific American 60-Second Science. I'm Sophie Bushwick. Got a minute?
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2 .How do you define one kilogram?
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3 .Easy: it's the exact mass of a metal cylinder called the International Prototype Kilogram, IPK for short, kept in controlled conditions in France.
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4 .But the standard kilogram has gained weight since its creation in 1875.
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5 .To trim it and its many replicas down to size, we need to clean them.
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6 .The report is in the journal Metrologia.
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7 .When scientists analyzed the surface of an IPK replica, they found it carried tens of additional micrograms.
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8 .These changes are due to contaminants building up on the kilograms, despite their careful storage under glass.
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9 .The additional mass is equal to just a few grains of sand.
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10 .But the kilogram is such an essential unit that tiny changes to different replicas are important.
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11 .To remove the contaminants, we need a standard and reproducible cleaning method.
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12 .Researchers exposed one kilogram measure to ozone and ultraviolet light,
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13 .which removed the carbon-based contamination without damaging the metal.
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14 .But to permanently standardize the kilogram, we'll need to define it in terms of fundamental constants, not a lump of metal in a jar.
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15 .Thanks for the minute, for Scientific American 60-Second Science. I'm Sophie Bushwick.
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