Weighty mystery: kilo standard sheds 50 micrograms

Updated September 16, 2007 00:50:00

A riddle: When is a kilogram that is no longer a kilogram still a kilogram?

Answer: When the chunk of metal that serves as the official international standard for the kilogram, under triple lock-and-key in France since 1889, inexplicably sheds a little weight.

Very little, though. The cylinder of platinum and iridium, sitting under three concentric glass bells like a metallic round of goat's cheese, is 50 micrograms lighter than the average of several dozen copies.

That may not seem like much, but in an era when measures of all kinds - distance, time, space - have reached levels of precision unimaginable even a generation ago, a few millionths of a kilogram is a big deal.

Scientists at the International Bureau of Weights and Measures in Sevres, near Paris, where the standard is kept, are baffled.

"We do not know how this phenomenon occurred since the prototype and the copies are preserved in the same environment. They were also made at the same time and in the same manner," the bureau's director, Richard Davis, said.

But if Dr Davis and his colleagues do not have an explanation for the missing micrograms, they do have a solution.

Sometime before 2010, he said, the mysteriously shrinking cylinder will be replaced with a new-and-improved model, a perfect sphere measuring 93 millimetres in diameter and made of pure silicon-28.

"The advantage of silicon-28 is that it is stable. The mass does not change over time," said Dr Davis.

"That will permit us, then, to have an invariable mass for the kilogram standard."


Topics: physics, human-interest, offbeat, science-and-technology, france

First posted September 16, 2007 00:44:00