Scientists are redefining the kilogram

Scientists are updating the definition of the kilogram, after years of nursing a sometimes dusty cylinder of metal in a vault outside Paris as the global reference for modern mass.

Friday vote is expected to make 1 kg relative to the Planck constant instead of a hunk of metal

A replica of the prototype of the kilogram at the Cité des Sciences et de l’Industrie in Paris. (Japs88/Wikimedia Commons, licensed under CC BY-SA 3.0)

Scientists are updating the definition of the kilogram, after years of nursing a sometimes dusty cylinder of metal in a vault outside Paris as the global reference for modern mass.

Just as the redefinition of the second in 1967 helped to ease communication across the world via technologies like GPS and the internet, experts say the change in the kilogram will be better for technology, retail and health — though it probably won't change the price of fish much.

The kilogram has been defined since 1889 by a shiny piece of platinum-iridium held in Paris. All modern mass measurements are traceable back to it — from micrograms of pharmaceutical medicines to kilos of apples and pears and tonnes of steel or cement.

The problem is, the "international prototype kilogram" doesn't always weigh the same. Even inside its three glass bell jars, it gets dusty and dirty, and is affected by the atmosphere. Sometimes, it really needs a wash.

"We live in a modern world. There are pollutants in the atmosphere that can stick to the mass," said Ian Robinson, a specialist in the engineering, materials and electrical science department at Britain's National Physical Laboratory.

"So when you just get it out of the vault, it's slightly dirty. But the whole process of cleaning or handling or using the mass can change its mass. So it's not the best way, perhaps, of defining mass."

What's needed is something more constant.

'Electronic kilogram'

So, at the end of a week-long meeting in the Palace of Versailles, Paris, the world's leading measurement aficionados at the International Bureau of Weights and Measures will vote on Friday to make an "electronic kilogram" the new baseline measure of mass.

Just as the metre — once the length of a bar of platinum-iridium, also kept in Paris — is now defined by the constant speed of light in a vacuum, so a kilogram will be defined by a tiny but immutable fundamental value called the "Planck constant."

The NIST-4 Kibble balance, which began full operation in early 2015 at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., measured Planck's constant to within 13 parts per billion in 2017, which was accurate enough to assist with the redefinition of the kilogram planned for 2019. (Jennifer Lauren Lee, for the U.S. National Institute of Standards and Technology (NIST))

The new definition involves an apparatus called the Kibble balance, which makes use of the constant to measure the mass of an object using a precisely measured electromagnetic force.

"In the present system, you have to relate small masses to large masses by subdivision. That's very difficult — and the uncertainties build up very, very quickly," Robinson said.

"One of the things this [new] technique allows us to do is to actually measure mass directly at whatever scale we like, and that's a big step forward."

He said it had taken years of work to fine-tune the new definition to ensure the switchover will be smooth.

But while the extra accuracy will be a boon to scientists, Robinson said that, for the average consumer buying flour or bananas, "there will be absolutely no change whatsoever."