The Kibble Balance: A Quantum Measurement of Mass |…

What is the Kibble Balance?

The Kibble balance, also known as the watt balance, is an instrument that measures mass by balancing a test mass against the force exerted by a magnetic field on a current-carrying coil. This method directly relates to Planck's constant (h), providing a quantum-based approach to defining the kilogram.

Originally developed in 1975 by Bryan Kibble, this balance is pivotal for modern metrology as it offers a more precise and stable way of measuring mass compared to traditional physical prototypes.

How Does It Work?

In the weighing phase, a coil carrying an adjustable current is placed in a magnetic field. By adjusting the current, the magnetic force on the coil can be matched to the gravitational force of a known test mass, creating a balance point. The servo null mechanism automatically adjusts the current until this equilibrium is achieved.

During the velocity calibration phase, the same coil is moved through the magnetic field at a constant velocity. This induces an electromotive force (EMF) in the coil, which can be measured by numerically differentiating the flux trace of the magnetic field. The induced EMF is then used to calculate the mass based on Planck's constant.

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Why Is It Important?

The Kibble balance provides a direct measurement of mass that does not rely on physical prototypes, which can degrade or be lost over time. By linking mass to fundamental constants like Planck’s constant, it ensures a more stable and accurate definition of the kilogram.

This method is crucial for advancing precision measurements in fields such as quantum physics, materials science, and fundamental research, contributing to our understanding of the physical world.

Real-World Applications

The Kibble balance has been instrumental in redefining the kilogram. In 2019, it was used as part of a new definition for the kilogram based on Planck's constant, ensuring that the unit of mass is more stable and universally accessible.

Beyond metrology, the principles underlying the Kibble balance are also applied in precision measurements in various scientific disciplines, enhancing our ability to conduct accurate experiments and research.

Frequently asked questions

How does the Kibble balance ensure accuracy?

The Kibble balance achieves high accuracy by balancing a test mass against a magnetic force that is precisely controlled. This process involves both weighing and velocity calibration phases, ensuring that the measurement of mass is tied to fundamental constants like Planck's constant.

What are the challenges in using the Kibble balance?

One major challenge is maintaining a stable magnetic field and precise control over the coil current. Additionally, the process requires careful calibration and can be time-consuming due to its complex nature.

How does the Kibble balance differ from traditional mass measurements?

Unlike traditional physical prototypes that may degrade or change over time, the Kibble balance provides a direct measurement based on fundamental constants. This makes it more stable and less prone to errors associated with physical objects.

Why is redefining the kilogram important?

Redefining the kilogram ensures that the unit of mass remains consistent and accurate across different laboratories and countries, improving precision in scientific research and industrial applications.

Try it live

Everything above runs in your browser — open Kibble Balance: 2D Beam & Flux Trace and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

▶ Open Kibble Balance: 2D Beam & Flux Trace simulation