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: "Greg Leyh" <lod-at-pacbell-dot-net>
> > 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.
>
>
>Hi Greg,
>
>Didn't you also construct a toroid for Electrum using stainless tubing or 
>was that for your
>previous coil?
>
>Ed W.


Hi Ed,

About a month after Electrum was finished I built a new, larger toroid
for my old coil at the shipyard.

The presence of fast (400ns) dart-leader bursts in Electrum's sphere-to-arc
current waveforms showed the importance of a low-impedance top capacitance.
It can be seen here that the leader currents are significant, and originate
almost exclusively from the local sphere capacitance:

http://www.lod-dot-org/electrum/sphere05us.jpg

Rather than a larger toroid though, I first tried an actual capacitor stack,
placed through the center of the secondary.  After re-tuning, this produced
notably longer arcs, although the primary energy was increased in the tuning
process.  The stack flashed over a few times in testing, although I couldn't
see exactly where.  In the end I opted for larger C in the form of a toroid,
as this would be more reliable, and offer additional shielding to the top
of the coilform itself.

The new 26" x 108" toroid fabrication is shown here:

http://www.lod-dot-org/teslacoils/maketoroid.html

This toroid uses 2" Al tubing, and is actually 23" larger in OD than the
Electrum sphere.  With the new toroid the coil performance improved
dramatically -- the arcs now typically strike the ground.