Re: [ATM] A perfect sphere

[Mark Suchting <masuch@tpg.com.au>]

At 03:21 AM 6/25/2007, Richard F.L.R. Snashall wrote:
>Mitchell R wrote:
> >
> > I would wonder why they would not simply make a square or some other shape.
> > If you measure all the sides accurately and know it is "plum and square" it
> > would be easier to make I would think. Just a thought,
> >
>
>Don't corners and edges wear more quickly?

Richard ,  You got it in one. Even a knife edge has some sort of 
roughness that is hard to measure. A sphere has no corners that can 
be knocked off, and silicon is very homogenous so a pretty fair 
estimate of the the # of molecules contained for a mass tandard if 
the 3D shape and surface roughness are known.

I worked at this facility for 10 years and interesting to note that 
the manufacture and sale of these spheres to Europe as mass standards 
has actually been going on for nearly 20 years. The spheres are 
ground and polished by hand in cups just like the Gypsies made 
crystal balls. Apart from a lot of patience and stamina  in the 
fingers the grinding and polishing did not use sophisticated 
equipment. . What has changed over the years is the development of 
computerised methods of contact profilometry to measure the 3D shape 
and prove the sphericity . That's where this  expertise in 
a  unique  Australian industry  comes in.  The Gypsies probably had 
no idea how incredibly round their crystal balls were !

Mark 

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