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RE: Spheres vs Toriods

At 09:07 AM 4/9/96 +0700, Mark Napier wrote:
>Last year when I was listening to RQ talk about increasing terminal
>capacitance I began to stack things on top of the toroid.
<cut>
>I was trying to increase the surface area to see
>for my self how it would affect the tuning, freq., etc.  Well it
>showed me what RQ was saying.
<cut>
>If you stacked a sphere on the toroid that was
>larger in surface area than the toroid, would the spark only come out
>the top?  What if it was half a toroid on the bottom and half a sphere
>on top?  What about half a sphere with a radiused edge?  It seems
>these shapes would combine the best of both the sphere and the toriod,
>except for easy of construction, I guess.

Being only on my third coil (-at-1.28 MHz bare and dismal Q), I probably don't
know what I am talking about, but here goes:

I have, to date, tried about 500 different metal objects for use as a top
discharger for my coils.  In general, it seems to me that any discharger
likes to throw off spark wherever the radius of curvature is the smallest.
The "half a sphere with a radiused edge" I have modelled with styrofoam and
conductive silver paint.  The entire terminal is about 4 inches larger in
diameter than the secondary (2.73 inches).  I tuned the coil for maximum
output, and was able to draw a "continuous" spark off of the "edge" on a
plane level to the physical ground at a distance about 31 millimeters
further than at the top of the discharger.  Without the nailboard to draw
off spark, the terminal produced a "skirt" of >55 millimeter sparks off of
the "edge," mostly very short-lived and more or less evenly spaced around
the lip.  

As for the sphere with larger surface area than a toriod stacked on top of
it, the sparks didn't like the sphere at all;  nothing would come out of the
sphere unless I came close with the nailboard.  At this point, I couldn't
draw more than 2 or 3 millimeters of spark off of the sphere;  perhaps >~400
square inches of discharge terminal is too much for a 720 VA system?  (I
don't understand why my coils are necessarily so crappy while others, with
almost the exact same setup as myself can easily produce several tens of
centimeters of spark.)

In any case, my conclusion is that the spark will always try to find the
closest thing to a toroid that it can, no matter what that object is
electrically connected to.  Anyone else?

>Mark R. Napier
>napier-at-cats.ucsc.edu

Lawrence Wang
lcwang-at-students.wisc.edu