Scientists redefine kilogram as original measure loses mass

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A quiet revolution could this week change the standard for weighing everything from drug doses to jumbo jet fuel.

Scientists from more than 60 countries will vote on Friday on whether a lump of metal held in a Parisian vault should continue to be the definition of a kilogram.

Le Grand K, a small cylinder of titanium alloy, has set the standard since 1889. All the scales in the world are ultimately calibrated against it, even those weighing in pounds and ounces.

It’s so important to the global economy that three key-holders are needed to unlock the vault. When the Nazis occupied Paris they left untouched the building that houses “Le Grand K”.

The problem, though, is that while the mother of all kilograms has only been taken out of its protective case four times in the last century, it has lost atoms and therefore mass.

It amounts to just 20 billionths of a gram, about the weight of an eyelash, but in a world that needs to weigh objects with ever greater accuracy, that’s a big deal.

Work on a definition of a kilogram is taking place at the National Physical Laboratory
Image:
Work on the definition of a kilogram is taking place at the National Physical Laboratory

Britain has a copy of Le Grand K called Kilo 18, which it won in a lottery in 1889, and is stored at the National Physical Laboratory (NPL) in West London.

Stuart Davidson, a metrologist or weight scientist at NPL, is one of the trusted guardians.

“Once you get up to a few tens of tonnes – things like filling an aircraft with fuel – everything needs to be traceable back to a standard,” he told Sky News.

“The same is true when you get down to very small masses like a milligram – for example the active ingredients of pharmaceuticals.

“You like to know you are getting the right dose of drugs when you are given a prescription.”




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Shoppers will not notice any difference as they buy groceries
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Shoppers will not notice any difference as they buy groceries

Scientists at the lab are now part of the global effort to devise a more accurate, immutable definition of a kilogram that is no longer dependent on a physical object.

They are using what is known as a Kibble Balance, named after the British physicist who first conceptualised, to express the mass of a kilogram in terms of the amount of upward electromagnetic force is needed to balance the downward drag of gravity.

Then with some heavy-duty maths, they relate that to a fundamental physical law of nature.

By taking the answer – a number called Planck’s Constant – they can reverse the process and calibrate scales with unprecedented accuracy.

Ian Robinson, a fellow at NPL, has been leading the work.

He says labs around the world will be able to have a kibble balance, liberating the definition of a kilogram from its physical and geographical ties.

“You are not reliant on any one object anymore,” he said.

“Effectively our mass scale is spread out and everyone can contribute. I see it as egalitarian – a form of democracy for mass.”

Other important standard units have already been updated.

The metre is no longer defined by a rod of metal, but by the distance light travels in a set, and very small, fraction of a second.

And a second is no longer defined by a fraction of the time it takes for the Earth to complete one rotation, which scientists now know varies, but by vibrations in a caesium atom.

Michael de Podesta, a principle research scientist at NPL, said the public will not notice any difference when grocery shopping.

“But it means people like me won’t worry about the kilogram losing weight,” he said.

“It will make it future-proof.

“Scientists will be able to measure things in ever more detail and engineers fabricate things with ever more precision.

“Improvements in measurement will lead to advances in science.”


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