Re: Safety FAQ is here -- draft, asking for comments.
Tesla List wrote:
> >From rwstephens-at-ptbo.igs-dot-netThu Aug 15 21:17:47 1996
> Date: Thu, 15 Aug 1996 20:49:50 -0500
> From: "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Safety FAQ is here -- draft, asking for comments.
> > I have photographs of my 720va coil exhibiting just this phenomenon
> >when operating in a very light breeze. I have never been able to tie the
> >jagged sparks directly to the break rate bacause I don't know how to
> >make a measurement. I estimate the length of the "steps" in the spark
> >to be 2" to 3". One thought is that the spark is actually growing in
> >length from the bottom which is eminating from the toroid rather than
> >growing from the tip of the spark thru conduction of current in the
> >spark itself. I would be interested in other comments on this subject.
> Skip, All,
> I believe that the phenomenon which is being bandied about where some
> people believe that the discharge channel is growing on itself is
> completely illusory and false. I hereby offer my hypothesis.
> If we could, through high speed eyes, observe every single
> streamer that rushes out from our toroid, as an individual spark, and
> not confuse it with the one that flashed 2.5 milliseconds earlier, or
> the next one to flash 2.5 milliseconds later (based on a break rate
> of 400 PPS), then I think we would see a streamer that exists in a short space
> of time which has one fixed length during its entire duration!
> In a rotary break powered system, operating on 60 Hz, the system
> capacitor goes through a cycle where it is sometimes completely
> discharged (zero point in the 60 Hz waveform) to where it is half
> charged (45 degree points in the waveform) to where it is fully
> charged (90 degree points in the waveform), and at varying voltages
> at all points in between. Along come the contacts of our rotary
> break, commutating at random, usually half a dozen or so places along a 60
> hz sinewave. Some commutations will meet a fully charged capacitor
> and the result will be a single pulsed streamer for that commutation
> which is the longest that the system can produce. Another
> commutation will occur at less than maximum charge and the single
> output streamer driven by that commutation will be shorter. Since
> there is a beat frequency between the mains rate (or multiples of the
> mains rate) and the break rate, the train of ouput streamers will be formed in
> a repeating cycle of none, to short, to longer, to longest, to shorter, to shortest, to
> none, etc. This is repeated at the beat frequency. When this beat
> frequency is relatively fast compared to the response of the eye,
> like say 10 or 20 PPS or more, and given that the eye integrates what is
> seen (movies and television could not be possible without this
> persistence of the eye), an illusion is created where a short output streamer is
> seen to grow longer in steps, aparently building upon itself.
> Now, I'm not sure what the de-ionization time for the arc channel
> might be, and I suspect that the channel remains highly conductive
> after the light dies away (energy is dissipated) to make a good path for the next
> pulse. This is substantiated in the fact that a powerful streamer upon
> 'finding' a good solid target will remain attached to that target for
> an extended period of time with relatively little deviation of the
> arc channel path.
> I don't see this phenomenon one of streamers building on previous
> pulses, but rather a rapid succession of individual streamers,
> occuring in sequentially increasing and decreasing lengths, each
> with a differing length due solely and traceable to no other reason
> than to the power level that created each individual one of them.
> I had meant to post this earlier but have nor had time, I've noticed
> in the meantime that the idea of employing a high speed streak camera
> by Steve Roys has been suggested. If my hypothesis is indeed
> correct, a fast enough camera will certainly be able to verify it.
> Comments welcome as always.
> Regards, rwstephens
Excellent, and well thought out Robert!
If your hypothesis is correct, would the illusion go away or be reduced
if a synchronous rotary was used?
The "stepped leader" effect is probably behind the initial streamer
formation and sucessive growth along the channel formed by the previous
discharge. This effect almost certainly accounts for the long discharge
lengths we see versus versus output voltage. If, as you hypothesize, the
ion channel is not rapidly dissipated (the wind not a blowin', and a
recombination rate slow enough vs gap firing rate), each successive
"step" should increase by an appreciable fraction of the "single shot"
spark distance. The current discharge would follow the relatively
"easy" ionization trail left by its predecessor for most of the
distance, extending and breaking a new path only towards the end of the
channel. During its initial formation a discharge stream should "grow"
until the average energy output of the coil balances the energy lost in
reheating/ionizing the entire path, or until the discharge source moves
to a different point on the toroid and the growth process begins anew.
Once a fully formed channel is formed, it tend to stay in one place for
an extended period of time. Any observed "growth" that is seen during
this time may be attributable to the "beat" frequency mechanism you
describe. A crude streak camera might be made from a moving/rotating
mirror. The mirror wouldn't have to move very rapidly since we want to
see what happens over successive rotary firings, not the high-speed
propagation of a stepped leader within each discharge.
Another possibility might be to use a fan to create a constant "breeze",
and a small piece of foil to make the discharge start from the same
place on the toroid. One might be able to see the successive discharges
shrink/grow in real time...
Comments, flames, and brickbats welcomed as always!
-- Bert --