For 130 years the world’s official unit of mass has been defined using a one kilogramme block of metal stored in a Parisian vault at the International Bureau of Weights and Measures.

The International Prototype Kilogram was certified in 1889 by the first General Conference on Weights and Measures. From its home in Paris this unassuming paperweight size cylinder of dense metal alloy made from platinum and iridium sets the standard against which all other measures of mass are calibrated.

But this is set to change next May, when the kilogramme will be redefined and the metal block jettisoned in favour of a new standard based on a measurement method developed at the UK National Physical Laboratory (NPL). This will enable more accurate, reliable and accessible measures of mass at scales never possible before.

Mass is the only remaining fundamental SI metric unit whose measurement relies on a physical object rather than a natural constant — the prototype metre bar was replaced as the standard length measure in 1960 by a measure based on speed of light in a vacuum.

The international prototype kilogram stored at the International Bureau of Weights and Measures in Paris

In a talk at last week’s British Festival of Science in Hull, Stuart Davidson, a metrologist from the National Physical Laboratory, outlined why the measurement of mass needs to change.

“Relying on a single physical object as a standard is unsatisfactory because it is intrinsically unstable,” he said, adding that processes of oxidation and hydrocarbon contamination add mass to the cylinder and even with careful cleaning it is impossible to keep the standard mass constant.

“Innovations by UK physicists and engineers at NPL underpin the new standard which is about to be introduced across the globe.”

This new standard will be maintained by the Kibble balance, a metrological instrument, originally developed by British physicist Bryan Kibble in the 1970s, which balances electrical and gravitational forces. It measures the force produced by an electric current flowing through a coil in a magnetic field required to support the weight of a standard kilogramme mass.

This force can be measured very accurately using “quantum electrical standards”, which are related to fundamental constants of nature, such as the charge on an electron, which will not vary over time.

The technology has been honed to meet the exacting requirements of a 20 microgram error per kilogramme needed for a standard measure. Two Kibble balance systems have now reached this level — one at the US National Institute for Standards and Technology and the other at the Canadian National Research Centre.

NPL is currently working on the next generation of the Kibble balance to deliver a low cost national standard measurement system. Scientists at the lab are working to miniaturise the balance so that it can be used more widely.

This will be of particular value to the pharmaceutical, biotechnology and microfabrication industries where accurate measurements of small forces and masses are crucial.



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