Spark Gaps

From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Wednesday, April 08, 1998 10:55 AM
To:  Tesla List
Subject:  Re: Spark Gaps

> ----------
> From:  L.Robertson [SMTP:LWRobertson-at-email.msn-dot-com]
> Sent:  Tuesday, April 07, 1998 2:06 AM
> To:  Tesla Builders
> Subject:  Spark Gaps
> Hi Folks ...
> I am slowly but surely coming to the conclusion that
> the reason rotary spark gaps don't work all that well
> for low power coils is that they are mostly not very 
> good spark gaps.
> If a few bolt ends don't work at all well for a static gap,
> why should they work any better as a rotary?
> Think for instance about a rotary gap made of 1/4 "
> tungsten - is this a good  gap? - with only two gaps,
> one on each end of the rotary piece I don't think so.
> How does this differ from two 1/4 " bolts facing each
> other?
In theory (note this disclaimer), here are some reasons why rotary gaps
might be better:

1) you could make the gap at closest approach much smaller than the static
gap. Normally, the electrodes are much farther apart. Smaller gap means
less loss because of shorter arc. Less loss means less heat which means
less electrode heating, etc.

2) Better electrode cooling, particularly if you use multiple electrodes.
They are whirling around stirring up the air. In fact, ask Greg Leyh about
windage losses in a BIG rotary gap.

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.

4) Easier control of output power. With a rotary gap, one can adjust the
number of breaks per second, which in turn affects the total output power.
Particularly with a DC power supply, this is quite handy. With an AC supply
(typical), you do have interactions with the line frequency, hence the
numerous posts on making a synchronous RSG.