```Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi Danny:

It's possible to achieve arcs several times the secondary winding length, by
using the e-field shaping inherent in a toroid top load to encourage
horizontal discharges.  An appropriately placed breakout point on the toroid
will also help send the streamers where you want them.

Insulating the primary is likely to be futile unless heroic efforts are
made.  Streamers have a way of going where they want and going around
deliberate barriers.  Placing a grounded target sounds reasonable, but in
practice, again, streamers will go where they want.

Two toroids are used for two reasons.  To reduce primary strikes, you want
to move the top load away from the primary.  But you'd also be moving the
toroid away from the top of the secondary.   One of the main reasons we use
toroids is that it prevents arcing off the top of the secondary windings.
If the toroid is too far away, it can't protect the secondary.  Adding a
smaller toroid just above the secondary serves to protect it, and the larger
main toroid can then be placed higher up to increase the strike distance to
the primary.

And sometimes folks want more top capacitance than a single toroid has.
They have one, so they build another one and just stack them.

There's no simple formula to calculate the final capacitance of two stacked
toroids.  The answer is not a simple sum or percentage of the sum, but
depends on many factors.  An extremely accurate program called E-Tesla6 may
be found on Terry's web site,
http://hot-streamer-dot-com/TeslaCoils/Programs/Programs.htm.  This is the only
way short of building and testing the two toroids of determining their
effective capacitance and resultant resonant frequency.

Regards, Gary Lau
MA, USA

=====================================================

Original poster: "Danny Stone by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<dansto-at-pacbell-dot-net>

I am in the process of designing a coil, using the wonderful software
available online.
The coil is 6 inches in diameter, and the estimated spark length is about
40 inches. Using an aspect ratio of 1:4, the coil length is 24 inches.
Naturally, I'm adding some space at the top and bottom of the coil form,
for a total form length of about 30 inches.
You probably can guess my question: With the topload (a toroid) sitting on
top of this form, how can I avoid arcs to the primary, since the distance
is less than the estimated spark length? The obvious solutions are
insulation (of the primary) or providing a grounded point to attract arcs.
Neither are highly desirable for my application.
I've wondered why some of the coils I've seen on the web have two toroids,
and I recall reading somewhere on the list that this configuration can
prevent (or alleviate) arcing to the primary. Is this correct? If so, how
does this work? Is this primarily why the stacked toroids are used?
Finally, how do I calculate the total capacitance of the stacked toroids?
Do I treat them as two separate toroids, calculating their free-space
capacitance (from the equation) and summing them as two parallel capacitors?

Thanks,
Danny

```