Kilogram's Future Hangs In The Balance | Hacker News Reader

Kilogram's Future Hangs In The Balance

npr.org

44 pointsvibhavs20 comments

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?

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.

This may be a better article http://www.nytimes.com/2003/05/27/science/27KILO.html.

And http://www.bipm.org/en/scientific/mass/pictures_mass/cleanin...

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).

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."

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.

I'd like to think that we've progress since then.

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

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.

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

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.

There have been attempts to redefine the mole.

http://www.iop.org/EJ/article/0026-1394/46/3/022/met9_3_022....

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.

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 :(

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

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.

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?

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

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

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

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."

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