Revamping the metric measure of mass

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

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.

Robert Rathe/NIST/Flickr 

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