French Kilogram doesn't measure up

A Cylinder from 1889 that has been the international prototype for a kilogram and is stored under lock and key near Paris is mysteriously losing its weight - if ever so slightly. It appears to have lost 50 micrograms, compared with the average of dozens of copies, according to the International Bureau of Weights and Measures in Sevres, where it is kept.

The bureau's Richard Davis said: "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."

Only the one in Sevres really counts. It is kept in a triple-locked safe at a chateau and rarely sees the light of day - and then mostly for comparison with other cylinders.

"It's not clear whether the original has become lighter, or the national prototypes have become heavier," said Michael Borys, of Germany's national measures institute. "But by definition, only the original represents exactly a kilogram."