By Anke Henrich
Since 1889, this small metal cylinder has lain in Paris, protected under three glass domes. Just 39 millimeters tall and 39 millimeters in diameter, it is made from 90 percent platinum and 10 percent iridium. The first General Conference on Weights and Measures in Versailles chose it as the base unit for weight. In the 130 years that followed, every scale in the world was calibrated according to this metal cylinder.
But scientists have now found - and no-one knows why - that the most historically significant piece of metal in the world is losing weight: 50 millionths of a gram every hundred years, to be precise. That might not sound a lot, but it causes problems for scientists and high-tech producers. Since May 2019, the kilo has now been redefined as an unchanging physical constant on the basis of 25 quadrillion atoms of a high-purity silicon ball and equations from quantum mechanics. The method was developed by physicists from the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany’s national metrology institute. It has the unbeatable advantage of allowing researchers all over the world to reconstruct a kilo in their own experiments at any time.
The ampere, mole and kelvin will all also be redefined for a final time based on physical constants.