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Landmark change to measurement of kilogramme approved

At a meeting in Versailles, France, countries voted to approve wide-ranging changes that underpin vital human activities.


A replica of the Grand K (Christophe Ena/AP)

A replica of the Grand K (Christophe Ena/AP)

A replica of the Grand K (Christophe Ena/AP)

The international system of measurements has been overhauled with new definitions for the kilogramme and other key units.

At a meeting in Versailles, France, countries voted to approve the wide-ranging changes that underpin vital human activities like global trade and scientific innovation.

The most closely watched change was the revision to the kilo, the measurement of mass.

Until now, it has been defined as the mass of a platinum-iridium lump, the so-called Grand K, which is kept in a secure vault on the outskirts of Paris.

It has been the world’s one true kilo, against which all others were measured, since 1889.

It is now being retired and replaced by a new definition based on a scientific formula.

In their vote, countries also unanimously approved updates to three other key units: the kelvin for temperature, the ampere for electrical current and the mole for the amount of a substance.

The vote was greeted by sustained applause and cheers after the 50-plus countries in attendance said yes, or “oui”, when asked one by one for their decision.

  • Kilogramme for mass
  • Kelvin for temperature
  • Ampere for electrical current
  • Mole for the amount of a substance

Nobel prize winner William Phillips called it “the greatest revolution in measurement since the French revolution”, which ushered in the metric system of metres and kilogrammes.

Scientists for whom the update represented decades of work clapped, cheered and even wept as the 50-plus nations approved the update. Some even sported tattoos on their forearms to mark the moment.

Jon Pratt of the US National Institute of Standards and Technology said the vote left him “a basket case” and “extremely emotional”.

“Those units, those constants chosen now, include everything we know, everything we have always known and provide that springboard for us to go pursue those things that we don’t know,” he said. “That was just leaving me in a puddle of tears.”

The change will have no discernable impact for most people. Their bathroom scales will not get kinder and kilos and grammes will not change in supermarkets.

But it will mean redundancy for the Grand K and its six official copies. The new formula-based definition of the kilogramme will have multiple advantages over the precision-crafted metal lump.

Unlike a physical object, the formula cannot pick up particles of dust, decay with time or be dropped and damaged. It also is expected to be more accurate when measuring very small or very large masses.

Even in retirement, the Grand K and its official copies — collectively known as “the heir and the spares” — will still be kept in the high-security vault where they are stored, because scientists want to keep studying them to see whether their masses gradually change over time.

Only exceedingly rarely have they seen the light of day since 1889, when they were taken out on a very few occasions to check whether other master kilogrammes around the world were still accurately calibrated, give or take the mass of a dust particle or two.

The metal kilo is being replaced by a definition based on Planck’s constant, which is part of one of the most celebrated equations in physics but also devilishly difficult to explain.

Suffice to say that the updated definition will, in time, spare nations the need to occasionally send their kilos back to France for calibration against the Grand K.

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