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Re: doorknob capacitor
I think Richard is being a little pessimistic here. I have
been running three of what sounds like the same capacitor here for
about a year (at most a minute at a time, to be honest) and they seem
to work OK. They do heat up, which will surely result in breakdown,
and the capacitance is temperature sensitive enough that the change
in coil tuning with (even room) temperature is quite appareciable.
If that's all I had I'd use it, while plotting to get something
better. I have a couple of the C.P. capacitors here which I have yet
to fire up, but I expect great things out of them.
By the way, I should have mentioned I'm using the doorknobs
with a 12 kV, 60 ma transformer running about 6 amps primary current.
Getting good (at least for me) results, 24" plus sparks from a 3" OD
by 16" high coil resonating around 270 kHz. By no means optimum, but
the results are good enough to satisfy visitors and allow for various
toroid experiments which completely verify the benefits reported by
the Richmond Richard.
My nephew, who teaches high school science down near San Diego,
also bought about 30 of the 4700 mmfd (most of them are nearer 3200,
by the way) capacitors from the same source I did, and his students
have been having a lot of fun making various coils, all powered with
the same 12 kV, 60 ma transformers of which George found an almost
unlimited, free (to high schools, at least) source.
By the way, having popped a few doorknobs at various times, I
can verify the loss of silver. However, for at least two of the capacitors
which failed, I believe it started with a break in the silver due to
the fact that the studs were not secure in the plastic potting, and probably
rotated a bit which broke the original continuity and provide a site
for arcing. As soon as that started, of course, the silver was indeed
"eaten away" by the very large surge currents.
For what it's worth,
Ed Phillips