# [TCML] JAVATC - Question

bartb bartb at classictesla.com
Tue Mar 17 19:02:51 MST 2009

```Hi Jim,

jimlux wrote:
>> One of the main points that Paschen showed was a curve identifying
>> breakdown voltage with gap distance or pressure. It's a curve because
>> of the molecular density of air and the probability of electrons
>> getting from point A to B.
>
> Ahem... Paschen actually didn't take any data down low enough to get
> to the minimum.  His data fits pretty well on a straight line, and
> says only that p*d = constant for a given voltage.  I just got a copy
> of the graphs from his paper, and I'll put them up in a day or two.
> (the paper's already out there.. send me an email and I'll send you

Yes, your correct. I was speaking of Pachens Curve as shown in the North
report. I already have copies of Pachen's paper in German and a
transcript in English, but the data tables are still in German. I would
like to see the actual data graphs when you get those up. I know there's
not enough there to show everything and it would tend to be linear in
the narrow window he made actual measurements in air.

> In the "straight line" region (where Paschen actually worked), you're
> looking at avalanche breakdown.  But when the gap gets small or the
> density gets low, (i.e. the mean free path starts to be comparable to
> the gap), other factors come into play. There's some well known data
> for small gaps where the breakdown does NOT occur at the shortest
> distance.

I agree. This is the area in Norths report shown on a graph (Ch-7,
pg-55). And it makes sense. As /d /reduces to the point where free
electrons become far a few between, density is reduced and thus it
becomes more difficult to cause breakdown.

>> In general, as the pressure is increased, the breakdown voltage
>> should increase and mainly due to the product of d x p (distance and
>> pressure). So if we double p, we should halve d for the same arc
>> voltage. This is one of my main points about hyperbaric gaps I've
>> tried to mention in the past regarding "cause of performance". Say we
>> double the pressure and leave the gap distance unchanged. Did we just
>> double our arc voltage?
> Probably not.  You may have doubled the breakdown voltage, but not the
> arc voltage, especially for mm length arcs, where the cathode drop is
> the bulk of the voltage.
Maybe terminology is a problem here. For me, breakdown voltage is the
arc voltage (that is what I meant as arc voltage).
Do you have another definition for arc voltage?

I'm speaking of a typical sphere style gap with normal TC spacing. For
example, 2 spheres of 1"D with a gap distance of 0.25". In this example,
should you double the pressure leaving everything else the same, I think
you have almost doubled the arc voltage as well.

I typically use Norths equations for this stuff, but their not that
For North, the field strength is 34,822V and the breakdown voltage is
18,812V. Your equations show a field strength of 31,836V and breakdown
voltage of 20,216V. This is with pressure at 1 atmosphere. If we double
the pressure to 1520 Torr (mm Hg), then field strength 59,203V and
breakdown voltage is 37,594V. Almost double, meaning that if hyperbaric
gaps truly do increase pressure, then the breakdown voltage must be
increasing as a result. And what would result in a better performing gap.

I'm sure in reality, a hyperbaric gap is not doubling the pressure,
simply increasing it by some margin. But, that margin is increasing the
breakdown voltage. If you set a standard atmosphere blown gap
(increasing the distance to accommodate the equivalent hyperbaric gap
breakdown voltage), only then would you have a true comparison. The
hyperbaric gap may "win" at the end of the day, but at least it would be
an apples to apples comparison. From a mechanical build standpoint, I
think the hyperbaric gap has already won.

Take care,
Bart

```