Scientists are baffled by the weight loss of the international prototype for the kilogram.
The reference kilogram, a 118-year-old cylinder which is kept in a triple-locked safe at the International Bureau of Weights and Measures (IBWM) in Sevres, has mysteriously shed 50 micrograms.
Physicist Richard Davis of the IBWM told the Associated Press that scientists do not know why the standard for all kilograms around the world has got lighter.
He 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."
A changing kilogram could affect electrical engineers and other scientists and could ultimately affect even some non-metric measurement systems, though Mr Davis said that it would not affect people's everyday lives.
"For the lay person, it won't mean anything," he said, adding that 50 micrograms is about the weight of a fingerprint. "The kilogram will stay the kilogram, and the weights you have in a weight set will all still be correct."
