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 knagel-at-cnct-dot-com (Keith Nagel) writes; 
> Joe Duszynski asks "Whats the inductance of a spherical inductor" 

> L = (2*PI)/9  * mu0 * a * n,2 

> where mu0 is the free space permeability, a is the radius of sphere, and n 
> is turns. 

That's interesting to know. I've seen some of these in antique radio equipment 
(ticker coils, perhaps?) If some of the antique radio buffs here know
for sure, 
it would be fun to find out. 

> Now, perhaps you can tell me why you need this? These coils have some 
> interesting 
> properties; for one, the H field inside has !no! curl. Weird, huh? And 
> before the 
> flames start, nothing in Maxwells equation prevents this. The field is still 
> a dipole.   

I don't think anyone here need flame you on that; curl-free fields aren't 
impossible, they're just unusual; particularly where a magnetic field is 
concerned. One quick example of a curl-free field would be in an area where 
the field lines were parallel and all had the same magnitude. Wether or not 
that's the case in the spherical inductor would take some work, so I'll  
just take your word for it. Thanks for an interesting bit of info. 

Wes B.