Paris – A kilogram just isn’t what it used to be. The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, seems to be mysteriously losing weight – if ever so slightly.
Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, said the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.
“The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart,” he said.
The kilogram’s uncertainty could affect even countries that don’t use the metric system – it is the ultimate weight standard for the U.S. customary system, where it equals 2.2 pounds. For scientists, the inconstant metric constant is a nuisance, threatening calculation of things.
“They depend on a mass measurement, and it’s inconvenient for them to have a definition of the kilogram … based on some artifact,” said Davis, who is American.
But don’t expect the slimmed-down kilo to have any effect, other than possibly envy, on wary waistline-watchers: 50 micrograms is roughly equivalent to the weight of a fingerprint.
“For the lay person, it won’t mean anything,” Davis said. “… The weights you have in a weight set will all still be correct.”
Of all the world’s kilograms, only the one in Sevres really counts. It is kept in a triple-locked safe at a chateau.
“It’s not clear whether the original has become lighter, or the national prototypes have become heavier,” said Michael Borys, a researcher with Germany’s national measures institute. “But by definition, only the original represents exactly a kilogram.”
