Re: Sparks

[brian whatcott <inet@intellisys.net>]

At 22:47 9/4/98 +0100, you wrote:
> I am a little surprised that dry air is a better insulator \not 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.
> What do you mean by "soft" vacuum?
>
> Ludwik Kowalski

I quite deliberately included this air insulator allusion because, it
appears, the datum does not occur in the usual physics curriculum.

 The topic has however been discussed (as all physical topics must have
been) not too long ago on this phys-l list.

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
atmospheric pressure was lower.
 The practical solution was simply to pressurize 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.

 For the purposes of this story one can take a soft vacuum to be available
below 80 000 ft up in the atmosphere.

Brian
brian whatcott <inet@intellisys.net>
Altus OK