On sparks

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>

Here's something of interest about sparks.  My s.s. apparatus (12" x 36"
secondary, ~3 ft sparks) is set up for a "test" mode in which I establish
each spark duration as only 128 cycles of the secondary's Fr--about 400
us.  The spark duration otherwise is about 7 ms.

Just to see what would happen, I thought to extend the function of my
128-cycle counter so as to interrupt the primary's excitation in bursts
of 128 cycles over the entire 7 ms duration.  400 us on, 400 us off, etc.

The sparks appear identical in form--jagged & branched--but they are, of
course, less "fat" when interrupted.  But that they appear otherwise
identical tells me that each burst of sparks travels essentially the same
path during the entire series of 400 us bursts.  I suspect that it's the
heated air, along the path, that induces all the repeating sparks to
follow along. 

The sound of the sparks is quite different also: they "ring"; one can
definitely hear the on/off square-wave burst which, of course, is in the
audio range.

Another interesting thing is that the secondary's E-field amplitude, as
picked up by a nearby scope probe, has this characteristic:  Rising
initially, asymptotically, to about 10 cm p-p to first break-out, at ~200
us; falling with the same shape to ~2 cm p-p during the remainder of the
first 400 us; falling essentially to 0 during the next 400 us "off"
period; rising again to ~2 cm p-p, with perhaps 10% initial overshoot;
and so forth to the end of the 7 ms period.  All the amplitudes following
the initial rise to 1st break-out are the same.  It might appear that the
overshoot is caused by the air having cooled somewhat during each 400 us
off-time.

I checked the secondary's return-current at the same time using a 1 ohm
series resistor monitored with a pair of differentially-connected probes.
 It has just the same shape.

Ken Herrick
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