

Kilogram prototypes have all changed to different weights and no-one knows why - ck2
http://www.physorg.com/news/2011-04-scientists-kilo-weight.html

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ElliotH
Calling the Planck constant 'esoteric' seems odd even if expressing it in
total layman's terms could be difficult.

I don't see why explaining that it's a constant relating to photons, light and
energy would be too difficult, or too far away from the level the article is
pitched at.

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sorbus
Huh. I always thought that metric units of mass were defined by the mass of a
mole of whatever atom you were interested in: a mole of carbon masses exactly
12 grams, a mole of oxygen masses 16 grams, a mole of hydrogen masses 1 gram.

EDIT: corrected atomic mass of carbon. Not sure what I was thinking there ...

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tassl
Unless they have changed the definition, back when I studied it the mole was
defined as the number of particles found in 12 grams of C (C-12), 6.022*10^23.

So the mole depended on the gram.

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spc476
And here I thought that the gram was defined by one cubic centimeter of pure
water at 4°C.

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kragen
That was the idea, but they immediately made a prototype kilogram mass in 1799
when they came up with that definition, and it turns out that it was off by
about 25 ppm, and we've been working from that prototype ever since — so water
only has that density to four significant figures.

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michaelcampbell
Interesting article, but they kept trying to build a strawman about how
Americans don't care about the metric weights since it doesn't affect their
price of butter.

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rorrr
It's an article on physorg.com that was copied from USA Today that doesn't
specify the source of that information.

My guess is that it will turn out to be a bullshit story or a measurement
error.

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kragen
No, this has been an issue for years.
[http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_inter...](http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_international_prototype_kilogram)

