After more than a century, the international prototype kilogram – a cylindrical chunk of metal stored in a French vault – doesn’t weigh the same as its 40 replicas, distributed worldwide and used to standardize mass measurements. Suspecting that gunk accumulating on the metallic surfaces is to blame, scientists at Newcastle University have developed a high-tech way to clean the standards.
If the washing protocol makes it into labs around the world, it could help with the problem of drifting masses – at least until a different standard kilogram is developed, one that relies on a fundamental physical constantrather than on an actual thing.
“It sounds good,” said Richard Davis, a physicist and former head of the Mass Section at the International Bureau of Weights and Measures in France. “The technique they’re proposing is something that is not that expensive and could be implemented in different places without too much trouble.”
Forged from platinum and iridium in London, the official international standard has, since 1889, been stored in a vault near Paris belonging to the International Bureau of Weights and Measures. In 1884, 40 replicas of the roughly 2.2-pound cylinder were made; in 1889, 34 of the replicas were sent to countries around the world. They were supposed to be the kilo’s twins.
But in the late 1980s, scientists noticed that the original kilogram was about 50 micrograms lighter than its brethren. Because mass measurements are relative, it’s tough to determine whether the replicas are getting heavier or the original is getting lighter. Peter Cumpson, a metrologist at Newcastle, suspects that a good cleaning could help restore balance to the masses. But the cleaning of a standard must, of course, be standard and reproducible. Though the metal cylinders are bathed fairly regularly, the process involves hand-washing, which introduces mechanical rubbing that’s hard to reproduce; what scientists need is “a much more repeatable, controllable, reproducible method of cleaning these kilograms,” Cumpson said.