Kibble balance

A Kibble balance uses a coil in a magnetic field to compare a test mass's weight with an electromagnetic force. In weighing mode, current through the coil produces an upward force that balances the downward force on the mass. In a second, velocity mode, the coil moves through the same field and induces a voltage. The force and induced-voltage relations involve the same magnetic field and coil geometry; combining them cancels those difficult-to-measure quantities. The resulting relation equates electrical power with mechanical power, conventionally written IV = mgv, where I and V are current and voltage, m is mass, g is local gravitational acceleration, and v is coil velocity.

The balance connects its electrical measurements to quantum electrical effects: the Josephson effect provides a precise voltage standard, while the quantum Hall effect underpins a precise resistance standard. Together with measurements of local gravity and coil velocity, these allow mass to be related to the Planck constant. Since the SI redefinition that took effect in 2019 fixes the numerical value of the Planck constant, a Kibble balance can realize the kilogram without depending on a particular physical prototype. Earlier experiments also used a known mass to determine the Planck constant.

The instrument was conceived by British physicist Bryan Kibble at the National Physical Laboratory in the 1970s and was originally called a watt balance, reflecting its comparison of power. The international metrology community later adopted the name Kibble balance in recognition of its inventor. National metrology institutes have built different designs; for example, NIST's fourth-generation instrument, NIST-4, became the United States' standard for realizing mass. The BIPM also operates a balance with design choices including a one-mode measurement scheme alongside the conventional two-mode approach.

A Kibble balance is a complex laboratory instrument: high-accuracy measurements require carefully controlled coil motion, a well-characterized magnetic field, precise electrical standards, and an accurate local measurement of gravity. Vacuum operation can reduce effects such as air buoyancy. Kibble balances are used primarily in national metrology laboratories to realize and disseminate the kilogram and support high-precision mass standards. Research groups are also developing smaller designs that could bring SI-traceable mass measurement to more settings, though these applications remain at different stages of development.