(来源：英语麦当劳－英语快餐EnglishCN.com)

By JAMEY KEATEN, Associated Press Writer
PARIS - A kilogram just isn't what it used to be. (来源：英语聊天室 http://chat.EnglishCN.com)

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. (来源：英语麦当劳－英语杂志 EnglishCN.com)

"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."

(来源：英语麦当劳－英语学习门户 EnglishCN.com)

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. (来源：英语学习门户网站EnglishCN.com)

"They depend on a mass measurement and it's inconvenient for them to have a definition of the kilogram which is 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. (来源：EnglishCN英语问答中心[e问e答])

"For the lay person, it won't mean anything," said Davis. "The kilogram will stay the kilogram, and the weights you have in a weight set will all still be correct." (来源：英语聊天室 http://chat.EnglishCN.com)

Of all the world's kilograms, 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 — mostly for comparison with other cylinders shipped in periodically from around the world.

(来源：最老牌的英语学习网站 EnglishCN.com)

"It's not clear whether the original has become lighter, or the national prototypes have become heavier," said Michael Borys, a senior researcher with Germany's national measures institute in Braunschweig. "But by definition, only the original represents exactly a kilogram." (来源：英语分类信息 http://fl.englishcn.com)

The kilogram's fluctuation shows how technological progress is leaving science's most basic measurements in its dust. The cylinder was high-tech for its day in 1889 when cast from a platinum and iridium alloy, measuring 1.54 inches in diameter and height. (来源：最老牌的英语学习网站 EnglishCN.com)

At a November meeting of scientists in Paris, an advisory panel on measurements will present possible steps toward basing the kilogram and other measures — like Kelvin for temperature, and the mole for amount — on more precise calculations. Ultimately, policy makers from around the world would have to agree to any change. (来源：EnglishCN英语问答中心[e问e答])

Many measurements have undergone makeovers over the years. The meter was once defined as roughly the distance between scratches on a bar, a far cry from today's high-tech standard involving the distance that light travels in a vacuum.

(来源：英语分类信息 http://fl.englishcn.com)

One of the leading alternatives for a 21st-century kilogram is a sphere made out of a Silicon-28 isotope crystal, which would involve a single type of atom and have a fixed mass.

"We could obviously use a better definition," Davis said. (来源：英语麦当劳－英语杂志 EnglishCN.com)