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WORLD | 13-11-2018 11:34

This is heavy: The kilogramme is getting an update

The gold-ball sized metal cylinder at the heart of the world's system for measuring mass is heading into retirement.

No, your bathroom scales won't suddenly become kinder and a kilo of fruit will still weigh a kilo. But the way scientists define the exact mass of a kilogramme is about to change.

Until now, its mass has been defined by the granddaddy of all kilos: a golf ball-sized metal cylinder locked in a vault in France. For more than a century, it has been the one true kilogramme upon which all others were based.

Now, the kilogramme is getting an update. Gathering in Versailles, west of Paris, governments are expected on Friday to approve plans to instead use a scientific formulation to define the exact mass of a kilo. The change will have practical applications in industries and sciences that require ultra-precise measurements of mass.

The 'Grand K'

Made of a corrosion-resistant alloy of 90 percent platinum and 10 percent iridium, the international prototype kilo, the so-called 'Grand K,' has rarely seen the light of day. Yet its role has been crucial, as the foundation for the globally accepted system for measuring mass upon which things like international trade depend.

Three different keys, kept in separate locations, are required to unlock the vault where the Grand K and six official copies — collectively known as "the heir and the spares" — are entombed together under glass bell-jars at the International Bureau of Weights and Measures, in Sevres on the western outskirts of Paris.

Founded by 17 nations in 1875 and known by its French initials, the BIPM is the guardian of the seven main units humanity uses to measure its world: the metre for length, the kilogramme for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance and the candela for luminous intensity.

Of the seven, the kilo is the last still based on a physical artefact.

"Everything else has been recycled and replaced and improved. This is the last improvement that dates back to the original conception in 1875. So that's a tribute to what was done in 1875, that it's lasted this long," Martin Milton, the BIPM director, said in an Associated Press interview.

Milton says the change will have applications in computing, manufacturing, pharmaceuticals, the study of climate change and other sciences where precise measurements are required.

"The system will be intrinsically correct by reference to the laws of science, the laws of nature," he said. "We won't have to depend on just assuming that one particular object never changes."

- TIMES/AP

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