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 value, obtained by the National Research Council of Canada. The NIST result was achieved with the institute’s new watt balance.

The operating principle of the watt balance is simple: It matches an electrical force produced by a current-carrying coil in a magnetic field with the gravitational force on a mass in Earth’s gravitational field. The electrical quantities can be precisely measured via two quantum phenomena, the Josephson and quantum Hall effects. Thus, the watt balancing act relates mass—in the NIST experiment, a copy of Le Grand K—and h. Although the balance idea is straightforward, the device itself is an elaborate, sophisticated contraption, as the figure shows.

After h is fixed by NIST’s and others’ measurements, the watt balance will be used to determine the kilogram. Since NIST used a Grand K replica in its measurement of h, the process may appear circular. The point is that artifacts change with time due to chemical and physical degradation but the value of h will endure. In essence, the kilogram defined by Le Grand K will live on in the eternal Planck’s constant. (D. Haddad et al., Rev. Sci. Instrum. 87, 061301, 2016.)