At a meeting in Versailles, France, countries voted to approve wide-ranging changes that underpin vital human activities like global trade and scientific innovation. The international system of measurements has been overhauled with new definitions for the kilogramme and other key units. 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 secured 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.
The change will have no discernible impact for most people. Their bathroom scales won’t get kinder and kilos and grams won’t change in supermarkets.
A mini revolution
But scientists are hailing the vote as a mini revolution in the field of weights and measures. And it will mean redundancy of 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 that has set the standard for more than a century.
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, very small or very, very large masses.
Even in retirement, the Grand K and its six official copies collectively known as “the heir and the spares” will still be kept in the high-security vault on the outskirts of Paris where they are stored. That’s because scientists want to keep on 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 kilograms that nations around the world use 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. Scientists instead should be able to accurately calculate an exact kilo without having to measure one lump of metal against another.