Re: Vortex gap loss measurements

["Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

> > > 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.  

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.