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Refining Avogadro constant could lead to a more reliable kilogram


The kilogram may finally get a break from its yo-yo diet. An international team of scientists is closer to redefining the unit of mass based on fundamental constants, instead of a piece of metal in France that loses weight only to put it back on again.

Since 1889, the international standard for the kilogram has been a cylinder of platinum, tucked under a glass jar inside another glass jar, stored in a vault outside Paris. But despite exceedingly stringent storage conditions, the cylinder (and six exact copies of it) gains weight from dust in the atmosphere, necessitating regular steam baths to remove the crud. On top of that, the seven cylinders change mass relative to each other by micrograms per century, for reasons no one can fully explain. So scientists want to redefine the basic metric unit of mass based on something that’s truly constant, just as the meter is defined as the distance light travels in one three-hundred-millionth of a second.

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