# Re: Sparks - Bright in the middle, how to verify it.

```Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>

>
>         eg in Lightning, multiple stokes flow, in quick succession
>         thru one plasma tube, at least on occiasion.  Stokes grow
>         stepwise, each further step extending from the ionized tube
>         of a preceding impulse.  The human eye is a very limited
>         instument, for some things...

Typical lightning is 2-3 strokes in a "flash".. Steps are 50-100 meters
long.  The steppy behavior is fairly well understood (or at least,
described), but the reason why it goes the direction it does at each step
isn't.

>
> > but the channel cooling is much faster, and that probably has a larger
> > effect than straight ionic recombination.  If the channel were still
> > hot enough to be significantly conducting, then it would perturb the
> > field and "attract" the sparks.
>
>         I'd wonder even more so at rapid cooling.  (if, of course, there is
>         data, then I'm wrong....) Cooling by pure radiation is not real
>         fast....

There is a sort of threshold for conductivity around 7000K, below, the
thermal ionization isn't all that much, above, it's pretty good (The "Saha
equation" gives the details).  At 7000K the radiation cooling is pretty
fast (that T^4 thing...).  It is from thermal balance calculations that the
models and data on lightning stroke diameters (ca 1 cm at 20 kA) is
derived... At 1000V/meter voltage drop and 20 kA, you get 20MW/meter
dissipation.  The mass of air in a 1/2 cm tube 1 meter long is about .025
gram, so, even with the weird thermal behavior of hot gases, it's going to
cool pretty darn fast. Assuming specific heat of 1 degree/gram cal, 7000
degrees * .025g/20MW*4.184J/cal gives 37 microseconds.  You really only
need to cool about 1000 degrees to make a big difference in conductivity
(although, there might still be lots of ions floating around....)

The other thing is, of course, that for a tesla coil, we're not dissipating
anywhere near the 100 kJ/meter that lightning has (except in that secret
death ray I have in my garage with all those maxwell caps, Bwaaa ha ha
ha...), so the spark channel is going to be even smaller, and the surface
area to volume ratio is going to make it cool even faster.

All in all, a fascinating area for research, and interestingly, one in
which tesla coils can provide useful data.  I've just got a data sheet on a
cheap consumer surveillance camera for about \$200 that claims 10
microsecond exposure times (of course, you probably can't set it
explicitly...) All those digital video boxes (like TiVo, and the \$50 Rabbit
picture in a picture add on) have fast video a/d's.... Maybe you could rig
up some logic and a video frame grabber to measure fine structure of
electric fields around the developing spark.  I've been thinking about
using a bunch of Logitech/Connectix "eyeball" cameras as the sensors for a
spinning mirror framing camera.

It is starting to get where consumer gear (or modified consumer gear) can
make measurements that would have been the exclusive province of a big
research lab...

>
>         best
>         dwp
> > This is an area of much uncertainty in the literature.  Nobody is really
> > quite sure why sparks go where they do when they do: Why is lightning
jagged
> > and forked?  The shape of the field will determine the general direction,
> > but not the fine structure.

```