Rev. Sci. Instrum. 87, 061301 (2016)

Scientists' most authoritative reference — the International System of Units (SI) — is undergoing fundamental changes. In the new SI all seven base units will be defined via the values of seven defining constants. The redefinition of the kilogram, for example, relates to the value of Planck's constant h. So before the new definition of the unit of mass can be put into practice, h must be precisely determined.

Darine Haddad and colleagues have developed a new instrument for precise measurement of h. The apparatus is a state-of-the-art Watt balance, which compares a mechanical force (or weight) with an electromagnetic force generated by a moving conducting coil in a magnetic field. A temperature-stabilized underground room shielded from radio-frequency waves houses the instrument, sitting within a vacuum chamber on a concrete block.

After extensive alignment work, required to obtain the precise values of all of the quantities involved, data was collected over a few weeks. Proper analysis, taking uncertainty budgets into account, then led to a value of h = 6.62606983(22) × 10−34 J s. The authors plan to realize an even more precise value in a year's time.