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Definition of kilogram set to change

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The master kilogram, sealed in a jar and stored in a safe in Paris, no longer weighs the same as this “exact” copy – which researchers say is “not scientifically acceptable”

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.”

Why kill off the kilogram?

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.

How wrong is Le Grand K?

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.”

How does the new system work?

Electromagnets generate a force. Scrap yards use them on cranes to lift and move large metal objects, such as cars. The pull of the electromagnet, the force it exerts, is directly related to the amount of electrical current going through its coils. There is, therefore, a direct relationship between electricity and weight.

So, in principle, scientists can define a kilogram, or any other weight, in terms of the amount of electricity needed to counteract its force.

Here’s the tricky part

There is a quantity that relates weight to electrical current, called Planck’s constant – named after the German physicist Max Planck and denoted by the symbol h.

But h is an incredibly small number and, to measure it, the research scientist Dr Bryan Kibble built a super-accurate set of scales. The Kibble balance, as it has become known, has an electromagnet that pulls down on one side of the scales and a weight – say, a kilogram – on the other.

The electrical current going through the electromagnet is increased until the two sides are perfectly balanced.

By measuring the current running through the electromagnet to incredible precision, the researchers were able to calculate h to an accuracy of 0.000001%.

This breakthrough has paved the way for Le Grand K to be deposed by “Die Kleine h“.

What are the advantages of the new system?

Every few decades, all the replica kilograms in the world have to be checked against Le Grand K. The new system, if it is adopted, will allow anyone with a Kibble balance to check their weights anytime and anywhere, according to NPL’s Dr Ian Robinson.

“It feels really good to be at this point. I feel it is the right decision. Once we’ve done this it will be stable for the foreseeable future,” he said.

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