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. Efforts are under way to check and fine-tune measurements of fundamental natural quantities - including Avogadro's number - that scientists will use to give the kilogram a new mathematical definition.
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