Scientists prepare to say au revoir to current definition of a kilogram

The definition of a kilogram is set to change – but this doesn’t mean we’ll all be losing weight.

Currently, a kilo is defined by the weight of Le Grand K, a mix of 90 per cent platinum and 10 per cent iridium hailed as the “perfect” kilogram since the late 1800s.

Since then, the ingot – which is sealed under bell jars in a vault in Sevres, near Paris – has deteriorated, causing its weight to change.

And today, the international General Conference on Weights and Measures is meeting in Versailles to vote on whether the kilogram will be redefined.

Its expected that the vote will be unanimous, and that a scientific formulation will instead be used to define the exact mass of the kilo.

Martin Milton, the Bureau International des Poids et Mesures (BIPM) director, said: “This, if you like, is a moment of celebration because it’s like the last standard remaining from 1875 that will finally be replaced by new innovation.”

“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.”

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The metal kilo is set to be replaced by a definition based on Planck’s constant, one of the most celebrated equations in physics.

This would mean enable scientists to accurately calculate an exact kilo without having to measure replica kilograms against Le Grand K.

This is good news for many areas of science where precise measurements are required, according to Milton: “The system will be intrinsically correct by reference to the laws of science, the laws of nature. We won’t have to depend on just assuming that one particular object never changes.”