Nations approve landmark change to definition of kilogram

Lester Mason
November 16, 2018

At a meeting in Versailles, France, countries voted to approve wide-ranging changes that underpin vital human activities like global trade and scientific innovation. It's a base unit of measurement, but one that's hard to define in definite terms without having something to base it off. Kilograms are now marked by a cylindrical lump of platinum and iridium known as the "Big KG" in Paris, but scientists are seeking to redefine it by far more, well. scientific means.

Accurate measurement is critical in many areas of the world today, such as in drug development, nanotechnology and engineering and is among reasons the Le Grand K rule is being changed. It has been the world's one true kilo, against which all others were measured, since 1889. But the platinum-iridium cylinder outside of Paris that has been the worldwide standard for the kilo since 1889 has been troubling physicists, engineers, and anyone else who relies on very precise units of mass to build or measure anything.

World scientists for whom the update represents decades of work clapped, cheered and even wept as the 50-plus nations one by one said "yes" or "oui" to the update. Such a small difference would not affect most people, but for industries that rely on very exact measurements, this difference would require constant adjustment to maintain accuracy. Bathroom scales won't suddenly get kinder and kilos and grams won't change in supermarkets.

Since 1889, the kilogram has been defined as the exact weight of an actual piece of metal dubbed "Le Grand K" sitting in the International Bureau of Weights and Measures in Sèvres, France. The new formula-based definition of the kilogram will have multiple advantages over the precision-crafted metal lump that has set the standard for more than a century.


But scientists are hailing the vote as a mini-revolution in the field of weights and measures, which underpins vital human activities like worldwide trade.

The Grand K and its six official copies, kept together in the same safe on the edge of Paris and collectively known as the "heir and the spares", will be retired but not forgotten.

Unlike a physical object, the formula can not pick up particles of dust, decay with time or be dropped and damaged. For one thing, you can't just ask the French to send you the reference article every time you want to run an experiment-scientists needed precise measuring units they could generate for themselves, wherever they were. And, since energy and mass are equivalent, it can also be used to measure the latter.

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