Kibble balance

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A Kibble balance is a very precise instrument that measures mass using electric current and voltage. It lets scientists define the kilogram by fundamental constants instead of a physical object. It was originally called a watt balance because the mass it measures is linked to electrical power (current times voltage). In 2016, after Bryan Kibble’s passing, the device was renamed in his honor.

In 2019 the kilogram definition changed to rely on the fixed value of the Planck constant, not a physical kilogram artifact. The Kibble balance is an advanced version of an older device called the ampere balance. It uses a fixed Planck constant to relate electrical measurements to mass.

How it works in simple terms
- The balance has a coil in a magnetic field. In “weighing” mode, the current through the coil is adjusted so the electromagnetic force on the coil balances the weight of the test mass.
- In “moving” mode, the coil is moved through the magnetic field, which generates a voltage. By Faraday’s law, this voltage is related to the product of the magnetic field, coil length, and speed.
- By carefully measuring the current (I), voltage (U), and speed (v), and by knowing the local gravity (g) from a gravimeter, the unknown mass can be determined. The key idea is to cancel out the coil’s geometry so the result depends only on fundamental constants and the measurements, not on the exact coil shape.

Why gravity matters
The mass measurement depends on gravity because weight is mass times gravity. Gravity varies from place to place and over time, so g must be measured very precisely in the same lab. This is done with high-precision gravimeters.

What makes it reliable
- The device uses fixed electrical constants (through standard voltage and resistance references) tied to fundamental constants, so electrical measurements are connected to nature’s constants.
- The balance often operates in a vacuum to avoid air buoyancy, and it uses precise timing and interferometry to track movement and speed.
- There are large laboratory versions and smaller MEMS (micro-electromechanical) versions for different purposes, including calibration tasks in other instruments.

Context and history
The concept was proposed by Bryan Kibble in 1975. The first working version appeared in the late 1970s and improved over time. In the 2000s, national labs refined the design and measurements of the Planck constant became extremely precise, enabling the SI kilogram to be defined by fundamental constants rather than a physical object. Today, Kibble balances are used worldwide in national metrology institutes to realize the kilogram with extraordinary accuracy.