The kilogram is a relic in the International System of Units (SI). It is the only base unit defined by an artifact—Le Grand K, a platinum and iridium cylinder in Sèvres, France. A refined SI, scheduled to debut in 2018, will be artifact free (see the article by David Newell, Physics Today, July 2014, page 35). In it, the kilogram will be empirically realized in terms of Planck’s constant h, whose value will be defined once and for all. In preparation for the new SI’s debut, several research groups are working to measure h as precisely as possible before its value is set. Now Stephan Schlamminger and a team of NIST scientists report one of the sharpest determinations yet, a result precise to 34 parts per billion. The value h = 6.62606983 × 10−34 J·s is consistent with other state-of-the-art determinations, including the currently most precise...

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