A new era of measurement-Kilogram set to be redefined

Scientists from around the world are gathering in France today to decide the fate of the kilogram.

For nearly 130 years, the kilogram has been based on a lump of metal called the Big K, locked in a vault near Paris.

A replica of the international prototype kilogram in Paris, France.

Key points

  • More than 60 countries will take part in a historic vote in Versailles to redefine the kilogram
  • Our unit for mass will no longer be based on the weight of the Big K, but on fixed value from nature: Planck’s constant
  • The conference will also formalise new definitions for three other units: the ampere for current, Kelvin for temperature and mole for molecules

But at 11:00am Paris time (9:00pm AEDT), representatives from 60 nations, including Australia, will vote on a proposal to define the humble unit using pure, unadulterated physics.

It’s not a change that you’ll notice when you go shopping, but it is necessary if we want to keep pace with technology now and into the future, said Australia’s chief metrologist Bruce Warrington.

The International Bureau of Weights and Measures, which governs changes to measurements, has already updated several of its units.

“We have atomic clocks that tell a second, and we use lasers using the speed of light to measure our metre, but the kilogram is still this 19th-century artefact,” said Dr Warrington, CEO of the National Measurement Institute.

History of the kilogram

Back in the 18th century a kilogram was equivalent to a certain volume of water, Dr Warrington explained.

But in 1889, it was replaced with a cylinder of platinum and iridium known as the international prototype kilogram (IKP) — or the Big K.

Today the original Big K is held under lock and key in the Pavillon de Breteuil, Saint-Cloud, near Paris.

A Kibble balance uses quantum mechanics to precisely measure an object’s weight.

So what’s going to change?

Instead of using Big K as a yardstick, the scientists are proposing to use one of the fundamental laws of nature known as the Planck constant to measure a kilogram.

The Planck constant is the amount of energy released in light when atoms jump around, explained physicist Tim Bedding of Sydney University.

“The committee is going to define the value of the Planck constant to be a particular number,” Dr Bedding said.

That number will now be exactly 6.62607015 x 10^-34 Joule seconds.

“Therefore, we’ll use that to define the kilogram — in the same way that the speed of light (299,792,458 metres per second) is fixed and we use that to define the metre,” Dr Bedding said.

To make their measurements, scientists will use a sensitive electromagnetic instrument known as a Kibble balance.


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