Re: Magnifier topload size?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Greg Leyh by way of Terry Fritz <teslalist-at-qwest-dot-net>" <lod-at-pacbell-dot-net>


>Original poster: "Edward Wingate" <ewing7-at-rochester.rr-dot-com>
>
> > Original poster: "Jeff W. Parisse" <jparisse-at-teslacoil-dot-com>
> >
> > A smooth metal toroid is the only pro way to go. Bumps or surface
> > perturbations lowers the breakout voltage by leaking off corona (or
> > streamers). A totally smooth toroid will develop the largest charge
> > before breakout and usually makes longer arcs. Many years ago (before
> > the founding of kVA) we compared many "homebuilt" alternatives to the
> > Smooth AL toroid and the results were pretty obvious that the smooth
> > toroids worked "better".
> >
> > Greg's method allowed visitors to climb into the sphere and look out. I
> > bet he'd have more pF with a smooth continuous surface.
>
>
>Jeff,
>
>When you say "better", did you A/B spark lengths at the same power level?
>
>I wasn't referring to the sphere. I think Greg had a tubing toroid in 
>addition to the sphere that was
>installed on Electrum in the final installation, but I don't recall what 
>the actual dimensions were.
>
>With identical sizes there is some difference in spark output between home 
>built and spun toroids,
>dependent on the care taken in construction. I have seen copper and 
>aluminum covered home built toroids
>constructed by Tim Davy in the UK that were as smooth as any spun toroid I 
>have ever seen seen.
>
>I was talking about a home built toroid at least twice the size of the 
>spun 8' or so that seems to be the
>practical limit in size for spun aluminum. There WOULD be an increase in 
>spark length with a toroid twice
>the size, homebuilt or not. How much increase TBD.
>
>The curve of the  toroid tends to better shield any surface imperfections 
>in larger sized toroids than in
>smaller diameter and cross section toroids.



The top electrode on Electrum consists of 30 1.5" stainless steel rings, 
sized to form a sphere of about 85" OD.  The gap between the rings is about 
1.75".  The sphere appears to breakout between 1.2 and 1.6 megavolts, 
calculated from secondary current waveforms.

Given a dielectric strength of air of 25kV/cm, an ideal sphere of this size 
would be able to hold off about 2.7 megavolts.  Of course if Electrum's 
sphere were ideal, the coil wouldn't fire at all... the secondary can't 
generate that much voltage.

I'd argue that the capacitance of Electrum's sphere is very close to that 
of an ideal sphere, since the electric field a short distance from the 
outer surface would be very nearly the same.  An isolated sphere 85" in 
diameter would have about 120pF, and the total Csec of Electrum (including 
secondary sheet capacitance and ground proximity effects) is around 135pF.