Quantum Balancing Act for the SI

Case study

Novel measurements support redefinition of the International System of Units.


The Challenge

Under protective bell-jars in a secure vault near Paris, there lies a measurement oddity – Le Grand K. This small cylinder of platinum-iridium is the International Prototype Kilogram (IPK) – the standard against which all other kilograms are measured world-wide. It is the only remaining material artefact used in the International System of Units (SI), but that’s set to change in 2019.

As part of a major revision of the SI that will also affect three other base units — the ampere, kelvin and mole – the kilogram will be redefined in terms of the Planck constant, h. These changes will shift the basis of the SI from imperfect recipes based on early 20th century physics to perfectly precise definitions that will endure in the 21st century and beyond, presenting metrologists with a range of new measurement opportunities and challenges.

The MSL Solution

The Planck constant offers a route to a new definition of the kilogram via other SI units, including the metre (length), second (time) and ampere (electric current). Each of these is already defined by, or realised in terms of a universal constant – the speed of light, electron transitions in a caesium atom, and the electronic charge, respectively. So, if we can measure the Planck constant with sufficient accuracy, we have everything we need to redefine the kilogram.

Linking the kilogram to the Planck constant is possible using an instrument called a Kibble balance. This uses electromagnetic forces, via a coil suspended in a strong magnetic field, to balance a kilogram mass. By carefully measuring local gravity, and coil parameters such as velocity, current and induced voltage over two operating modes, we can assign a mass value to a standard weight with an accuracy approaching 20 parts per billion.

MSL is home to the only Kibble Balance in the Southern Hemisphere, and it is unique in the world. Rather than using a horizontal beam to ‘weigh’ the mass, it uses a pair of pressure balances. Our magnet has been specifically designed to remain highly stable over time, and is carefully shielded to ensure that the magnetic field is consistent across both modes of operation.

The Impact

Across the globe, metrologists are working toward this common goal – to measure the Planck constant with sufficiently low uncertainty so that it can be used as the basis of a new definition of the kilogram. This will remove the need for primary material artefacts in the SI, and ensure that the kilogram can be realised in labs across the world.

The redefinition will allow for more accurate and reliable measurements of mass, as well as related quantities – e.g., density, volume, flow, force, torque and pressure. So, it will have an impact on many industries, from aviation and beverages to natural gas and pharmaceuticals. In the long term, it may even enable new technologies whilst maintaining continuity for practical users.


“Here at Buckley Systems, we are particularly drawn to innovative, novel projects, and are lucky enough to work with universities all over the world, as well as silicon-chip manufacturers and companies developing medical treatments that will save lives. Manufacturing components for MSL’s Kibble Balance, which aims to do something as fundamental as redefine the kilogram, was a lot of fun, and we look forward to collaborating again soon.”

Arron Sands – Chief Operations Officer, Buckley Systems