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Re: Answer to your question




Chris,

	The advantage of a rotary spark gap is that it is a good "quencher".  At
high frequencies, it is not only important that the gap arcs, but also
important that the arc goes out.  The ability of a spark gap to
extinguish is called "quenching".

	Recall that in the tesla coil circuit, the capacitor charges off of the
60 cycle source voltage.  When the capacitor reaches a certain maximum
voltage, there is enough voltage across the spark gap to cause an arc. 
During arcing, the capacitor has both ends connected directly to the
primary coil.  This forms the resonant circuit.

	If the spark gap did not ever quench, the resonant circuit would only
oscillate a few times, meaning only one or two streamers out of your
secondary, and then nothing.  The spark gap would still be arcing, if it
never quenched, and the transformer would be shorted out, meaning that
the capacitor would not recharge.

	The rotary spark gap achieves better quenching that a static gap in the
following manner.  As the metal "probes", be they bolts, nails, tungsten
rods, or whatever, come close to each other, an arc between them becomes
more and more imminent.  Finally, they rotate close together enough for
an arc to occur.  However, as the gap continues to rotate, the probes
move directly past each other, and then the distance between them begins
to increase. (I'm talking about this like it happens slowly, really it
happens hundreds of times per second).  As the distance increases, the
arc becomes more and more difficult to sustain.  Finally, the distance
between the probes is large enough that the arc goes out.  Thus we have
quenching.

	Sometimes, in a static gap the air between the nails, or probes, or
whatever, gets hot enough to keep the air ionized, meaning that the arc
will sustain itself indefinitely.  This is bad, as explained earlier.

	If you have any more questions, I'm full of answers, so don't be shy.

						Brent
						"StretchMonster"

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