I'm not privy to any of the thinking here, but my guess is that
internal stresses are more evenly distributed in spheres than other
shapes.  Changes in volume due to temperature are more 'correct' since
a cylinder, cube, etc may not change in volume as evenly due to those
stresses.

It sounds like they may be working on more accurately defining the kg.
 In other words, they have already determined it to x decimal points,
but there is a need for more decimal points of accuracy and precision.
 These new spheres will become the new standard, and due to a variety
of differences from the old standard will be easier to quantify to a
higher degree than previously, and should last longer.  By measuring
the molar mass, they can define a kg as x number of silicon atoms,
which will forever remain the same definition, and future generations
can measure it to whatever precision is needed, much like the second
is defined as x vibrations of the cesium atom, and now any
improvements are merely in our measuring accuracy.

-Adam

On 6/15/07, Michael Rigby-Jones <Michael.Rigby-Jones@bookham.com> wrote:
>
>
> >-----Original Message-----
> >From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]
> >On Behalf Of Jinx
> >Sent: 15 June 2007 14:06
> >To: pic microcontroller discussion list
> >Subject: [EE] The grind's almost over to forge two perfect spheres
> >
> >
> >http://www.nzherald.co.nz/section/story.cfm?c_id=82&objectid=10445968
> >
> >"Using a single crystal of silicon-28 grown by Russian and
> >German scientists over three years, a team of Sydney
> >scientists and engineers will grind and polish two silvery
> >balls, each weighing precisely one kilogram, with
> >imperfections of less than 35 millionths of a millimeter"
> >
> >All very clever and nice but I don't understand why if -
> >
> >"The aim is not to change the value of the kilogram, but to
> >ensure its stability for all future times," Giardini said. "It
> >will no longer depend on an actual physical object and this is
> >going to allow us to relate the mass to the individual atoms."
> >
> >and
> >
> >"On completion, the spheres will be measured for volume in
> >Australia, Germany and Japan, then measured for mass. Belgian
> >scientists will look at the molar mass of the crystal used to
> >calculate the number of molecules in each sphere."
> >
> >they can't work out how many molecules without building them.
> >And how are they weighing them ? Why isn't *that* the standard ?
>
> What a load of old balls.
>
> It does say the current 118 year old standard held ina French valut is degrading.  Garlic errosion perhaps?
>
> Now I can understand wanting to make a nice standard that is impervious to oxidisation etc. but why does it need to be a sphere?  Surely that is just making things difficult for themselves.  Not only would it be easier to make something with flat sides, you could put it on a table without it rolling off and smashing itself it bits.
>
> Mike
>
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