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, that 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.
Humankind is about to sever one of the links between its present and its past.
The so-called “Grand K” kilogram, a cylinder of polished platinum-iridium alloy that has been the world’s sole true kilo since 1889, is to be retired.
Nations gathered in Versailles, west of Paris, are expected on Friday to instead approve the use of a scientific formula to define the exact weight of a kilogram.
The change will have no discernable impact for most people. Their bathroom scales won’t get kinder and kilos and grams won’t change in supermarkets.
But scientists are hailing the vote as a mini-revolution in the field of weights and measures, which underpins vital human activities like international trade.
And it will mean redundancy for the Grand K and its six official copies.