Official prototype of kilogram mysteriously losing weight

PARIS, France (AP) -- A kilogram just isn't what it used to be.

Physicist Richard Davis of the International Bureau of Weights and Measures with the reference kilogram.

The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is
mysteriously losing weight -- if ever so slightly. Physicist Richard
Davis of the International Bureau of Weights and Measures in Sevres,
southwest of Paris, says 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. "We don't really
have a good hypothesis for it."

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 like electricity generation.

http://www.cnn.com/2007/TECH/science/09/12/shrinking.kilogram.ap/index.html