Re: Sparks

["Ludwik Kowalski" <KowalskiL@Mail.Montclair.edu>]

THIS WAS POSTED EARLIER BUT WAS APPARENTLY LOST.

Yes, the breaking d.o.p. ("voltage"), for a given configuration of
electrodes, is lower in partial vacuum than in very good vacuum
or at a much higher pressure. But the term "better conductor"
usually refers to steady currents at d.o.p. which are very much
less than what is needed to produce a spark. In other words, I am
referring to conditions under which Ohm's law is applicable
while Brian is referring to "sparking conditions" under which
the value of resistivity depends on the electric field.

That is why I was "a little surprised that dry air is a better conductor

than the vacuum". Cosmic rays produce free charges in the air but not
in the vacuum. Less pressure --> less free charges per unit volume.


Ludwik Kowalski
Brian wrote:
> I mentioned the severely practical illustration that in WWII it was
soon
> found that waveguides for radar transmission - at that time almost all

> transmitters were magnetrons - which worked perfectly well on the
ground
> and at modest altitudes, began to break down at 20,000 ft plus, where
the
> the waveguides.
>
>  In Dr Bowman's splendid analysis which this casual observation
provoked,
> it was explained that the mean free path length of a charged particle
was
> crucial to the conductivity of the medium - so that when the vacuum
becomes
> really hard, the resistivity of the path rises again.