WattBalance

A watt balance, also known as a Kibble balance, is a precision instrument designed to measure the Planck constant by linking mechanical power to electrical power. It operates in two distinct modes—weighing and velocity—to relate a mass under gravity to electrical signals generated in a magnetic field. The device plays a central role in realizing the kilogram from fundamental constants rather than a physical artifact.

In weighing mode, a coil placed in a magnet produces a force F = BLI I that supports the gravitational force on a mass, so that mg is balanced by the magnetic force. In velocity mode, the coil is moved through the same magnetic field, generating an electrical voltage E proportional to the velocity, E = BLv. By combining the two measurements, mechanical power mgv is related to electrical power VI. When electrical units are tied to fundamental constants via the Josephson and quantum Hall effects, the comparison yields the Planck constant.

The watt balance has been developed and operated by several national metrology institutes as part of efforts to redefine the SI kilogram. Through precise measurements and cross-checks, it contributed to fixing h exactly in 2019, enabling mass to be realized from fundamental constants. Today, watt balances continue to serve as important tools for realizing the kilogram and for advancing precision measurements in metrology.