Kibblebalansz

Kibblebalansz, known in English as the Kibble balance, is a precision metrology instrument used to realize the unit of mass by relating mechanical power to electromagnetic power through the Planck constant. It was developed by Bryan Kibble in the 1970s at the National Physical Laboratory in the United Kingdom and has since been adopted by national metrology institutes around the world. The device plays a central role in modern mass measurements and in the realization of the kilogram.

The balance operates in two interrelated modes. In weighing mode, the gravitational force on a mass m is balanced by the magnetic force on a current-carrying coil in a strong magnetic field: m g ≈ B L I, where B is the magnetic flux density, L is the effective length of the coil in the field, and I is the current. In velocity (or moving) mode, the coil moves through the field, generating an electromotive force ε = B L v, with v the velocity. By combining these relations and comparing electrical quantities measured with quantum standards, the device ties mechanical power m g v to electrical power V I, which in turn is linked to the Planck constant h.

Relation to the Planck constant and the kilogram redefinition: Using Josephson and quantum Hall standards, electrical measurements are expressed in terms of h. The Kibble balance thus provides a practical realization of mass that is anchored to a fixed value of h. In 2019, the kilogram definition was redefined by fixing h to an exact constant, with Kibble balances serving as the primary tool for realizing the kilogram in national laboratories.

Practical considerations: A Kibble balance requires precise magnetic fields, careful mechanical design, temperature control, and vibration isolation. Measurements are time-consuming and sensitive to environmental conditions, but the device offers a robust, artifact-free path to mass that is linked to fundamental constants.