Observations on demo coil

From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Friday, April 17, 1998 5:04 PM
To:  tesla-at-pupman-dot-com
Subject:  Observations on demo coil

Using the demo coil described in my post of few days ago, I played around a

First, I lengthed the primary spark gap out from the current total length
of about .13 inches to a bit farther apart, although still close enough
that the NST can fire it with no primary circuit connected. As expected,
the spark length increased somewhat, from 8-9" to a 1/4" copper tube end,
to about 12".

Then, I had a spare 2700 pF doorknob, so I thought I would throw that in
parallel with the existing 2. I figured that the new tuning would be no
farther in than 4 turns on the pancake inductor instead of the existing 6
(completely neglecting a variety # of turns squared terms in the Wheeler
formula). The idea is that this would increase the stored energy by 50%,
which should be a good thing...

I did it, retuned, and the coil works a lot worse. Now just thin wispy
sparks of about the same length as before. Interestingly, the tuning is
really non critical, too. You get this same mediocre performance anywhere
from 4 turns to 5.5 turns, without much difference. Pretty low Q?

Furthermore, my NST seems to have become intermittent (internal
breakdown?). Guess a trip to the scrap yard is in order.

Before, the stored energy (5.4 nF) was .39 Joules, at 120 bps = 47W. Now,
with 8.1 nF, the stored energy is .58 J, at 120 bps about 70 W. This, being
driven from a 270 VA NST.

One idea is that with the bigger cap, the primary is physically smaller, so
less well coupled to the secondary.

Another is that I partially killed the NST earlier, so the comparison
really isn't valid; although, returning to the 5.4 nF case and retuning did
return to the previous performance.

So much for easy optimization of the demo coil. Maybe I should get busy on
that new secondary to replace the current rickety one. Time to fire up the
lathe (which sure does make winding a small secondary a whole lot easier).
A few hundred turns at 100 RPM goes by pretty fast.