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Shrinking kilogram bewilders physicists
Associated Press | Sep. 12, 2007 | JAMEY KEATEN

Posted on 09/12/2007 2:47:48 PM PDT by decimon

By JAMEY KEATEN, Associated Press Writer 6 minutes ago

PARIS - A kilogram just isn't what it used to be. 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.

"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.

"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."

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.

"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."

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.

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.

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.

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.


TOPICS: Miscellaneous; News/Current Events; Technical
KEYWORDS: kilogram; stringtheory
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To: spunkets

Why?


41 posted on 09/12/2007 3:06:42 PM PDT by Recovering_Democrat ((I am SO glad to no longer be associated with the party of Dependence on Government!))
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To: decimon

Global warming causing it to expand and become lighter.

Just a guess ;)


42 posted on 09/12/2007 3:07:08 PM PDT by grjr21
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To: bajabaja

Yes, but water (with its various isotopes of hydrogen and oxygen) varies slightly in weight from sample to sample (as well as with temperature) and thus they wanted something more stable. Thus, the cylinder reference.

They could define it using the cubic decimeter of water again, using and H-1 as the references, 101325 Pa pressure (assuming said atmosphere is saturated with water) and 277.14 K (the point at which water is most dense) and I think you’d probably be able to get a stable enough standard for it. Of course, that’s just my speculation.


43 posted on 09/12/2007 3:07:08 PM PDT by jmyrlefuller ("The Price is Right has given away more money than anyone except welfare"-- Bob Barker)
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To: Red Badger
Local gravity may have changed slightly..............

I'm not sure if your comment is serious or not, but that's not possible. The scale measured the relative weight of two cylinders and that should stay constant regardless of any hypothetical change in gravitational pull.

44 posted on 09/12/2007 3:07:31 PM PDT by Alter Kaker (Gravitation is a theory, not a fact. It should be approached with an open mind...)
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To: decimon

Global warming is causing it to evaporate.


45 posted on 09/12/2007 3:07:34 PM PDT by puroresu
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O-16 and H-1... wonder why the O-16 got dropped?


46 posted on 09/12/2007 3:08:18 PM PDT by jmyrlefuller ("The Price is Right has given away more money than anyone except welfare"-- Bob Barker)
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To: Red Badger
Local gravity may have changed slightly..............

Hm. True. Never thought of that. :)

47 posted on 09/12/2007 3:09:01 PM PDT by Recovering_Democrat ((I am SO glad to no longer be associated with the party of Dependence on Government!))
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To: Captain Rhino
Slow release of gases entrained during the original casting process?

Ah, another plausible idea. Good, Rhino. (Why did you ever choose THAT name?) :)

48 posted on 09/12/2007 3:09:47 PM PDT by Recovering_Democrat ((I am SO glad to no longer be associated with the party of Dependence on Government!))
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To: decimon

Radioactive impurities decaying over time?


49 posted on 09/12/2007 3:10:03 PM PDT by lesser_satan (FRED THOMPSON '08)
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To: decimon

That’s what you get when you base all of the world’s measurements on the French Enlightenment.


50 posted on 09/12/2007 3:10:12 PM PDT by Cicero (Marcus Tullius)
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To: Red Badger
Local gravity may have changed slightly..............

I'd go for that explanation.

51 posted on 09/12/2007 3:10:37 PM PDT by Tallguy (Climate is what you plan for, weather is what you get.)
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To: Captain Rhino

“Slow release of gases entrained during the original casting process?”

Best explanation yet, IMO. Some of the noble metals can absorb or dissolve astounding amount of hydrogen. Palladium, IIRC, can dissolve 500 times its volume of hydrogen. Platinum, similar. Where would the H come from in a crude furnace of over 110 years ago? Reducing atmosphere in the furnace..?


52 posted on 09/12/2007 3:11:40 PM PDT by Attention Surplus Disorder (When Bubba lies, the finger flies!)
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To: jmyrlefuller

The range of expertise on this board, again, impresses. Thanks for that response.


53 posted on 09/12/2007 3:12:04 PM PDT by bajabaja
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To: Red Badger
Local gravity may have changed slightly..............

I doubt they're being 'weighed'. More likely placed on a balance beam and compared to the copy.

54 posted on 09/12/2007 3:13:26 PM PDT by TC Rider (The United States Constitution ? 1791. All Rights Reserved.)
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To: StevieJ

Nice hookah you got there, Stevie.


55 posted on 09/12/2007 3:18:01 PM PDT by decimon
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To: Petronski
Doesn’t everybody’s Gram get smaller as she ages? It’s usually osteoporosis.

That makes it all the more heinous to kilogram.

56 posted on 09/12/2007 3:18:16 PM PDT by Sloth (You being wrong & me being closed-minded are not mutually exclusive.)
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To: Sloth
Grand Matricide?


As opposed to Petty Matricide?

57 posted on 09/12/2007 3:25:27 PM PDT by Petronski (Tribe still -11 . . . Jake Westbrook is a bum.)
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To: decimon

maybe there’s a stray fingerprint on some of the copies?


58 posted on 09/12/2007 3:28:14 PM PDT by stefanbatory
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To: decimon

Atomic decay, perhaps.


59 posted on 09/12/2007 3:28:29 PM PDT by Brilliant
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To: Recovering_Democrat
You asked if the mass change could be due to a change in entropy. The Pt-Ir alloy has a stable crystal structure. 50 µg of mass is worth 4.5*109J of energy. An entropy change is a configurational energy change at some temp T. See the problem?
60 posted on 09/12/2007 3:28:52 PM PDT by spunkets ("Freedom is about authority", Rudy Giuliani, gun grabber)
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