

Kilogram's Future Hangs In The Balance - vibhavs
http://www.npr.org/templates/story/story.php?storyId=112003322

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aerique
In one sentence they say the kilogram's been taken out only three times and
then later they suggest it might have gotten lighter because of periodic
washings. Looking at the picture of how the kg is kept I assume the latter
sentence is nonsense.

Can someone explain this to me?

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martey
From <http://en.wikipedia.org/wiki/Kilogram> :

 _Since the IPK and its replicas are stored in air (albeit under two or more
nested bell jars), they gain mass through adsorption of atmospheric
contamination onto their surfaces. Accordingly, they are cleaned in a process
the BIPM developed between 1939 and 1946 known as “the BIPM cleaning method”
that comprises lightly rubbing with a chamois soaked in equal parts ether and
ethanol, steam cleaning with bi-distilled water, and allowing the prototypes
to settle for 7–10 days before verification._

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mseebach
I thought that a kilogram was the mass of 1000 cubic centimeters of water at
melting temperature, and that a centimeter is 1/100 of a meter which is the
distance that light travels in a vacuum in the time it takes a caesium-133
atom to emit a little more than 30 periods of radiation (9,192,631,770 p/s /
299,792,458 m/s).

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tokenadult
That was the historical basic idea, but that doesn't provide the kind of
precision desired for a very exact definition of the kilogram. The article
submitted here reports an attempt at a very precise, ascertainable definition
of the kilogram.

From the New York Times article kindly linked by eru in another comment:

"The kilogram was conceived to be the mass of a liter of water, but accurately
measuring a liter of water proved to be very difficult. Instead, an English
goldsmith was hired to make a platinum-iridium cylinder that would be used to
define the kilogram."

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mseebach
Yeah, but that's 140 years ago. I'd like to think that we've progress since
then. Also, it's pretty important to get the definition right. If my
definition is right, then sneezing on the kilogram won't make every weight
wrong, if will make the kilogram not quite have the mass of one kilogram.

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tokenadult
_I'd like to think that we've progress since then._

The submitted article reports the direction progress has taken since then.

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mseebach
Yes. I'm merely expressing my surprise that the definition is, in fact, a
physical object, and not at this point expressed in terms of other, discreetly
measurable quantities.

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gloob
I'm sure you could measure the physical object either discreetly or
discretely, or even both at once if the situation was dire.

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abrahamsen
What is the problem with defining the kg in terms of the already existing
atomic mass unit?

See <http://en.wikipedia.org/wiki/Atomic_mass_unit>

The atomic mass unit is already associated with the mole, which is in SI.

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sethg
I think the trick is to define the kg in terms of something that is not only a
universal constant but can also be practically measured.

If I redefine the gram as the mass of 6e23 atoms of carbon-12, and you give me
a big pile of carbon-12 and say "what's the mass?" and I can't count exactly
how many atoms are in the pile, then my definition isn't very useful.

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sophacles
From the article: "... around 50 micrograms (billionths of a kilogram)". Oh
NPR -- I thought you were better than that :(

Edit: njm below is correct, i misread that as "billionths of a gram)". I
rescind my above frowny face and apply it to myself :(

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njm
Am I missing something here? If a microgram is 10^-6 gram and a kilogram is
10^3 grams...

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sophacles
No, i missed the that they were talking about kilograms not grams. Once again
my "no posting on the internet before coffee" rule shows its wisdom... and me
my foolishness for ignoring it. Good spot.

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Hexstream
And I thought the kilogram was defined in some platonic, mathematical way
relative to some well-known constant in the laws of physics all along...

Also, how did they decide "how much" the initial kilogram should weigh?

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RyanMcGreal
How do the Imperialists do it for the pound (lb)?

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tokenadult
In the United States, customary units are officially defined in terms of
metric units.

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mildweed
Good witty article titling, vibhavs. Its like Fark, but with substance.

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vibhavs
That was actually the original name of the article when I submitted it. Looks
like it has now been changed to "This Kilogram Has A Weight-Loss Problem."

