Re: Vortex gap loss measurements

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

hi Jim,

On 4 Sep 00, at 12:36, Tesla list wrote:

> Original poster: "Jim Lux" <jimlux-at-jpl.nasa.gov> 
> 
> > > > I have metered a Jacob's ladder to watch the effects of arc 
> > > > length and it showed voltage climbing as the spark rose. The 
> > > > current hardly varied. The changes in arc length were pretty 
> > > > significant and the voltage would have roughly doubled for a 
> > > > change of around 10 in arc length. Agree with all your other 
> > > > conclusions.
> > > 
> > > Of course, you were driving the arc from a current limited source (i.e.
> a
> > > ballasted transformer of some sort)?
> > 
> > It was a rectified and capacitor filtered high frequency 
> > flyback (real flyback) supply so it was current limited. I 
> > would expect a non-limited supply to blow fuses and/or melt 
> > something.  Output voltage is metered so the voltage would 
> > have been averaged by the inertia of the meter movement but 
> > the arc climbed quite slowly enough to show a definite trend.
> > I used that same supply to power a desktop mini-twin at one 
> > stage. The arc voltage was not at all proportional to the arc 
> > length. 
> 
> Essentially a constant current source, then.  You'd expect the voltage to
> rise a bit (the arc isn't a perfect conductor), especially because as it
> stretches, you have to increase the power (i.e. the voltage, given the
> constant source impedance) to keep the larger plasma hot enough to conduct.
> 
> 
> Obviously, there is power going other places in the system, because the
> thermal losses, to a first order, are going to be proportional to length. 
> It would be VERY interesting to figure out a way to measure the arc column
> diameter in this experiment.  

Interesting conclusion. It is actually throughput limited 
rather than loss limited. Let us say chunks of energy/time 
integrated over time with a maximum imposed. The losses 
elsewhere are not at all great. True flyback. If one examines 
on a small timescale, there will be individual discharges so 
arc current isn't constant when examined closely enough. So 
average voltage reflecting average resistance is the 
situation. It would be interesting to measure arc temperature 
at various points on the ladder. I agree, something seems at 
odds here. Probably the supply itself is not as constant as 
one would wish since output regulation is achieved by 
monitoring primary current. No load makes the core operate 
well up the B-H curve probably.

> I wonder if I were to take a video camera, put a suitable ND filter so that
> the exposure is good, and collect images simultaneously with voltage
> readings (a very bright bar graph, or a video titler driven from a dvm, or
> some sort of synchronized capture (storage scope, strip chart recorder).
> You could use a fairly high zoom, so the arc occupies a good fraction of
> the screen, and manually pan up as the arc moves.
> 
> Even better, I should find a copy of Meek, et.al., who probably studied
> this stuff in gory detail and made detailed measurements.  They certainly
> spent a lot of time characterizing long air sparks. However, these days, we
> have all sorts of tools that they didn't have in the 30's and 40's to do
> this kind of measurement.

I suppose this is all academically interesting. Regarding 
application to the TC primary gap, it seemed obvious to me a 
long time ago that a lot of energy was being lost as reflected 
in the heat and light in the arc and various attacks on the 
problem seem to bear this out so when it comes to TC 
efficiency (getting as much Ep as possible to the output 
load/discharges), anything done to reduce those losses given 
the K's we have to operate at seems like a good idea.

Regards,
Malcolm