Fri. Apr 19th, 2019

Scientists to change definition of kilogram

Scientists are set to change the way the kilogramme is defined.

Currently, it is defined by the weight of a platinum-based ingot called “Le Grand K” which is locked away in a safe in Paris.

Researchers are expected to vote to get rid of it in favour of defining a kilogram in terms of an electric current.

The decision is to be made at the General Conference on Weights and Measures, in Paris.

But some scientists such as Purdey Williams at the National Physical Laboratory have mixed feelings about the change.

“I haven’t been on this project for too long but I feel a weird attachment to the kilogram,” he said.

“I think it is such an exciting thing and this is a really big moment. So I’m a little bit sad about (the change). But it is an important step forward and so the new system is going to work a lot better. So it is also a really exciting time. And I can’t wait for it to happen.”

Le Grand K has been at the forefront of the international system of measuring weights since 1889. There are also several close replicas.


But the master kilogram’s days are numbered. Its weight has changed over the years because it has deteriorated. The kilogram, like the pope, is infallible, so other weights have to be adjusted accordingly.

In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering, those responsible for maintaining the system plan to overturn Le Grand K’s increasingly flawed rule.

The fluctuation is about 50 parts in a billion, less than the weight of a single eyelash. But although it is tiny, the change can have important consequences. Coming in is an electrical measurement which Dr Stuart Davidson, head of mass spectrometry at NPL, says is more stable, more accurate and more egalitarian.

“We know from comparing the kilogram in Paris with all the copies of the kilogram that are all around the world that there are discrepancies between them and Le Grand K itself,” he said.

“This is not acceptable from a scientific point of view. So even though Le Grand K is fit for purpose at the moment, it won’t be in 100 years’ time.”