Kibble balance

Kibble balance. An electromechanical instrument of exceptional precision used to measure mass by relating it to fundamental constants of nature, most notably the Planck constant. It operates by comparing mechanical power to electrical power, a principle first proposed by Bryan Kibble of the National Physical Laboratory in the 1970s. The device played the definitive role in the 2019 redefinition of the SI base unit of mass, the kilogram, liberating it from dependence on a physical artifact.

Principle of operation

The core principle relies on the equivalence between mechanical and electrical power, realized in two distinct operational modes. In the first, or "weighing mode," an electrical current is passed through a coil suspended in a permanent Magnetic field, generating a Lorentz force that balances the gravitational force on a test mass. In the second, or "velocity mode," the same coil is moved at a known velocity through the same magnetic field, inducing a Voltage across its terminals. By precisely measuring the current, voltage, velocity, and local gravitational acceleration, the instrument relates mass to the Planck constant via the Josephson effect and the Quantum Hall effect, which provide exact quantum standards for voltage and electrical resistance.

Historical development

The concept originated from the work of Bryan Kibble at the National Physical Laboratory in Teddington during the 1970s, evolving from earlier Watt balance designs. Key theoretical and experimental advancements were made throughout the late 20th century by metrology institutes worldwide, including the National Institute of Standards and Technology in the United States and the International Bureau of Weights and Measures near Paris. A major international effort, coordinated by the Consultative Committee for Mass and Related Quantities, culminated in the successful use of Kibble balances to fix the Planck constant's value, enabling the 2019 redefinition of the SI base units.

Design and components

A typical apparatus features a rigid balance beam, often suspended by a flexure pivot to minimize friction, with a test mass pan on one arm and a moving coil assembly on the other. The coil resides in a high-stability, radially symmetric magnetic field generated by a system of permanent magnets and yokes, a design critical for ensuring field uniformity. Sophisticated laser interferometers, such as those based on the Michelson interferometer, measure the coil's velocity with nanometer precision. The entire instrument is housed in an elaborate Vacuum chamber to eliminate effects of air buoyancy and convective currents, and is often installed on stable foundations isolated from seismic vibrations.

Role in redefining the kilogram

Prior to 2019, the kilogram was defined by the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder stored at the International Bureau of Weights and Measures. The Kibble balance provided an invariant, quantum-based alternative by enabling an exact measurement of mass in terms of the Planck constant. In the years leading to the redefinition, several national metrology institutes, including those in the United States, Canada, France, and China, operated Kibble balances to determine the constant with extraordinary uncertainty, on the order of a few parts in 100 million. This collective data was essential for the General Conference on Weights and Measures to unanimously adopt the new definition, effectively retiring the artifact known as "Le Grand K".

Comparison with other methods

The primary alternative approach for realizing the new kilogram definition is the Avogadro project, which uses a highly polished sphere of pure Silicon-28 to count atoms and determine the Avogadro constant. While both methods anchor mass to fundamental constants, the Kibble balance is a real-time, electromechanical instrument, whereas the Avogadro project is a meticulous material-based counting experiment. Other historical methods, such as traditional mechanical balances or Hydrostatic weighing, are far less precise and traceable to artifacts rather than constants. The Kibble balance's direct traceability to quantum electrical standards gives it a unique role in disseminating the kilogram unit to national measurement laboratories.

Category:Measuring instruments Category:Metrology Category:Weighing instruments