The Kibble balance: weighing mass against the Planck constant

When the kilogram was redefined in 2019, the artifact in Sèvres gave way to a fixed numerical value of the Planck constant. The instrument that made that possible — and that now realizes the kilogram — is the Kibble balance, named for the late Bryan Kibble of NPL who conceived it in 1975.

The idea is an elegant two-mode experiment. In weighing mode, the gravitational force on a mass is balanced by the electromagnetic force on a current-carrying coil in a magnetic field. In velocity mode, the same coil is moved through the same field and the induced voltage is measured. Combining the two modes cancels the hard-to-characterize geometry of the coil and field, leaving mass expressed through electrical quantities — which, via the Josephson and quantum Hall effects, trace directly to the Planck constant.

The deeper significance is architectural: mass realization is no longer the monopoly of a single artifact. Any suitably equipped laboratory can, in principle, realize the kilogram independently. National institutes including NIST, NRC, PTB, LNE and BIPM operate full-scale instruments, and NIST has demonstrated smaller tabletop Kibble balances aimed at direct dissemination at lower accuracy — pointing at a future where the traceability chain for mass gets shorter for everyone.