Kibble Balance

A see-saw that weighs a kilogram by counting Planck's constant — turning mass itself into a quantum measurement.

Realises the kilogram from first principles by linking mechanical force to a precisely measurable electrical quantity. After the 2019 SI redefinition fixed Planck's constant h to an exact value, the Kibble balance became the primary way national metrology institutes generate traceable kilograms — replacing the international prototype kilogram (Le Grand K) that had served since 1889.

A coil of known geometry hangs in a strong, uniform magnetic field B. In 'weighing mode' a current I through the coil produces an upward force BLI that balances the gravitational force mg of the test mass; the ampere is measured as a Josephson voltage divided by a resistance calibrated against the quantum Hall effect. In 'velocity mode' the same coil is moved through the same field at velocity v and the induced voltage U = BLv is measured against another Josephson reference. Dividing eliminates the unknown geometric factor BL: m = UI / (gv). Because U and I are tied to fundamental constants (Josephson and von Klitzing), the recovered mass is expressed in joule-seconds — i.e., directly in Planck's constant. Modern instruments achieve relative uncertainties of a few parts in 10⁸ on a 1 kg artefact.

In plain terms

Imagine a see-saw where one side holds an apple and the other side holds an electromagnet. To know exactly how heavy the apple is, you don't compare it to a reference apple — you measure how much electricity it takes to lift it, then use the rules of nature itself to translate that electricity back into mass.

Also called: watt balance, electromechanical mass-realisation device.

Why it matters

Defines the kilogram from a fundamental constant of nature instead of a single hand-polished metal cylinder.
Closes the SI redefinition begun in 2019 — the second, metre, ampere, kelvin, mole and candela now all derive from invariants.
Each NMI (NIST, NRC, BIPM, NPL, METAS, KRISS) can produce its own primary kilogram, ending dependence on a single artefact in Sèvres.
Same Josephson and quantum-Hall standards used here propagate into voltage, resistance, and electrical-power calibration worldwide.

Where it lives

NIST-4 Kibble balance (Gaithersburg, USA)
BIPM Kibble balance (Sèvres, France)
NRC Kibble balance (Ottawa, Canada)
METAS Kibble balance (Bern, Switzerland)

Key numbers

Defined constant: h = 6.62607015 × 10⁻³⁴ J·s (Exact, since 20 May 2019)
Relative uncertainty (NIST-4): ~1 × 10⁻⁸ (On a 1 kg artefact)
Magnetic field: ~0.5 T (Permanent SmCo or NdFeB yoke)
Coil velocity (velocity mode): ~2 mm/s (Tracked by laser interferometer)

Surprising facts

Every kilogram on Earth is now defined by counting Planck's constant — the kilo is a quantum quantity.
The 2019 SI redefinition fixed h = 6.62607015 × 10⁻³⁴ J·s exactly, with no uncertainty.
NIST-4 has reached relative uncertainty of about 1 × 10⁻⁸ on a 1 kg artefact — equivalent to weighing a fully fuelled Boeing 737 to within the mass of a single grain of sand.
The same instrument doubles as the world's best demonstration that the Josephson and quantum-Hall constants are linked: it tests the consistency of e and h to better than parts in 10⁸.

Open questions

Disagreement before SI redefinition: In the years leading up to 2018, different national Kibble balances and X-ray-crystal-density determinations of h disagreed at the 10⁻⁷ level — large enough that the redefinition committee had to weight several methods. After fixing h, the residual disagreements persist as differences between national mass scales today, with no single error source identified.
Surface-adsorption mass shifts: Kibble-balance test masses gain and lose adsorbed water and hydrocarbons at the microgram level depending on cleaning protocol, ambient humidity, and elapsed time since polishing. The exact functional form remains a measurement art rather than a closed theory, and contributes a sizeable share of the residual budget at the 10⁻⁸ level.

A short history

Bryan Kibble of the National Physical Laboratory (UK) proposed the watt-balance concept in 1975 to break the dependence on Le Grand K. Decades of refinement at NIST, NPL, BIPM, METAS and NRC followed. After Kibble's death in 2016 the instrument was renamed in his honour. The 26th General Conference on Weights and Measures (CGPM) voted in November 2018 to redefine the kilogram in terms of h; the change took effect on World Metrology Day, 20 May 2019.

Sources

Kibble, B.P., 'A measurement of the gyromagnetic ratio of the proton by the strong field method' (1976) — original watt-balance proposal (https://link.springer.com/chapter/10.1007/978-1-4684-2682-3_47)
BIPM — The new SI and the redefinition of the kilogram (2019) (https://www.bipm.org/en/measurement-units/rev-si/)
Haddad et al., Metrologia (2017) — NIST-4 Kibble balance result (https://iopscience.iop.org/article/10.1088/1681-7575/aa70bf)
NIST Kibble balance program (https://www.nist.gov/programs-projects/kibble-balance)