Spark Gaps

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Friday, April 10, 1998 10:25 PM
To:  Tesla List
Subject:  Re: Spark Gaps

Tesla List wrote:
> ----------
> From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
> Sent:  Friday, April 10, 1998 10:26 AM
> To:  Tesla List
> Subject:  Re: Spark Gaps
> >
> > ----------
> > From:  Antonio C. M. de Queiroz [SMTP:acmq-at-compuland-dot-com.br]
> > Sent:  Thursday, April 09, 1998 1:11 PM
> > To:  Tesla List
> > Subject:  Re: Spark Gaps
> >
> > Jim Lux wrote:
> >
> > > 3) Faster interruption of the spark. As opposed to just waiting for the
> > > current to go through a zero, the gap is physically separated. The
> sudden
> > > circuit opening causes the voltage across the primary inductor to rise,
> > > inducing a similar rise in the secondary. The faster the interruption
> > > (di/dt) the more the rise.
> >
> > Do this really work? This would be like operating a Tesla coil as an
> > induction coil. The theory saying that there is a great increase in the
> > secondary voltage or a two-coil system after the opening of the spark
> > gap (Corums) may be due to this effect (otherwise I don't see how).
> I have no theoretical basis for my statement, above. It was proposed as a
> possible reason why one might want a rotary gap. The other, related, theory
> I have come across has to do with the energy transfer back and forth
> between the primary and secondary. The idea being that if one removes the
> resonant primary at a time when the energy is mostly in the secondary, then
> that optimizes the performance: as opposed to letting it dissipate as heat
> in the primary.
> Interestingly, with a static gap, I wouldn't expect the gap to "open" at a
> current maximum, the contrary, in fact, resulting in essentially ZERO
> di/dt. With a rotary gap, one could conceivably interrupt the arc (by
> stretching it long enough) while significant current is still flowing
> through the arc. Now, the di/dt is pretty significant.
> I'm too lazy to go back through everyone's posts on modelling the coil, so
> I'll ask a redundant question: Have you looked at this aspect of the
> gap/primary circuit? Say you modeled the gap as a perfect switch that opens
> and closes in some sort of synchronism with the primary waveform, what sort
> of waveforms are created? in a theoretical sense, at least.

Jim and all,

All measurements that I've made, and AFAIK, all measurements that others
have on this list have made on actual operating coils seem to confirm
that, for air-gaps of all types, the primary arc is only broken at a
point where the primary current is zero, AND at a point where the
primary tank energy is ALSO zero (i.e., secondary energy is at a maximum
point). Based on all empirical evidence thus far, I'm now convinced that
it's virtually impossible to force quenching with an air-insulated spark
gap of any type if there's any substantial energy remaining in the
primary tank circuit, and certainly if there's any primary current
flowing at the time.

Thratron or transistor switches were used by Richard Hull and Malcolm
Watts respectively to switch off primary current at earlier current
"zeros" (of the coil's fundamental operating frequency) during a number
of very interesting quenching experiments last year. Malcolm's attempts
to turn off a low-power transistor-switched primary circuit at points
other than primary current zeros did result in the expected high voltage
spikes stemming from rapid di/dt, but it's not clear that the an arc's
characteristics will permit this to actually occur under any reasonable
circumstance in higher power air-gap systems.
The GOOD news is that nature has apparently conspired to turn the gap
off when most of the energy is where we want it - in the secondary,
thereby achieving a natural synchonism of sorts.  :^)

The BAD news is that this usually doesn't happen at the first primary
current notch (where the first primary-to-secondary energy transfer has
completed). Instead, the arc reignites, letting the secondary's energy
cycle back into the primary and back again to the secondary. In fact,
most operating coils seem to actually quench at the 3rd-5th current
notch (or worse!).  :^(

However, EVEN BETTER news is that, as heavier streamer losses begin to
remove increasingly more energy from the secondary, we begin to achieve
1st or 2nd notch quenching just as the sparks really begin a flyin'. 
Hmmm... Mother Nature must really love coilers after all!  :^)

Safe coilin' to you, Jim. BTW, GREAT web page at your site!

-- Bert --