f13.net

http://www.newsvine.com/_news/2007/09/12/957925-kilo-prototype-mysteriously-loses-weight

I thought they'd already gotten around to defining the gram as some number of carbon molecules, just like they did with the meter (distance light travels in a vaccuum in a certain amount of time, with time defined as oscillations of something or other -- another physical constant).

I had thought so, too, but apparently not considering the discussion of using a silicon sphere as a replacement.  In my view, if you have a fomulaic definition then you don't need a physical object at all.  However, I am not a scientist.

People like shit to hold?

According to wikipedia, it's the last of the SI units to be defined by an artifact and not a constant (and the only one to be fixed to a prefix definition -- defining kilogram, not gram).

The silicon sphere is actually an attempt to fix it to a constant (a fixed number of silicon atoms). According to wikipedia, looks like they're also looking at using Planck's constant (how they get mass from current is beyond me, but apparently it's a leading contender because they can almost get precise enough measurements standard off that method), fixed number of carbon atoms, electron mass and elementary charge.

Seems like they're moving towards it, they're just waiting for the development of a useful measurement system. (Apparently with meter, they use a helium laser and a fixed number of wavelengths -- easy enough to do in a lab to the requisite precision).

Laser for meter seems easier to do, as far as a "thing", so maybe that's it.  Still, if you know how many atoms of silicon then you don't need to actually constuct a sphere.  I think.

There was something in the wikipedia article about "why a sphere". I think because their tolerance for "Easily constructed measure of weight" is something like 25 picugrams. Whatever the funky lower-case u means in metric.

μ

It's a mu, it's for micro or 1 millionth i.e. E-6.

Damn Frenchies and their SI. :)

Perhaps the kilogram went swimming in cold water. 

Does this means everyone is now fatter?

Can't you already define a gram as being the mass of 1 mole (the Avogadro constant (http://en.wikipedia.org/wiki/Avogadro's_number)) of neutrons, or something like that?

Of course, the Avogadro constant is currently strictly defined in terms of the gram rather than an exact numerical value, but it seems better to just pick a number than to base the mass on a hunk of metal that is apparently evaporating as we speak.   :-P

As I said upthread -- they're working on it. :)

The problem isn't "definition" so much as it is "definition that can easily be created and measured in a given lab, and known to be precise within 25 micrograms" or something to that effect.

(http://www.mikeandjen.cc/images/shrinkage.jpg)

That was exactly the image I was thinking of WAP.  Exactly. 

Christ, first the wars, then the Euro cup 2008 qualifier to Scotland, and now bits of the kilo. The bloody French can lose anything.

 :-D