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Revamping the metric measure of mass

Precise count of Avogadro’s number furthers efforts to redefine kilogram

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6:00am, July 27, 2015

MEASURING UP  This metal cylinder is the United States’ copy of the international kilogram kept in France. Scientists hope to replace this standard, the last physical object used to define a unit of the metric system, with a definition of mass based upon fundamental physical constants.

The metric system is on track for a mass makeover.  

In an effort to provide accurate measurements at all scales, scientists are preparing to redefine four basic units by the end of 2018. The shift will most notably affect the kilogram, the base measure of mass and the last member of the International System of Units still defined by a physical object. Current efforts are under way to check and fine-tune measurements of fundamental natural quantities — such as Avogadro’s number — for use in giving the kilogram a new mathematical definition.

Since 1889, the standard for mass has been a 1-kilogram cylinder of platinum and iridium metal at the Bureau International des Poids et Mesures in Sèvres, France. While this standard is handled carefully, it’s at risk of becoming dirty or damaged, says Michael Stock, a physicist at the French bureau. “Any material object can change over time,” he says.

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