Jeff Liebermann wrote in message . ..
On Sun, 21 Mar 2004 08:47:34 +0000, John Woodgate
wrote:

I read in sci.electronics.design that Jeff Liebermann
wrote (in mppp50ho4dr08ahkb3dlbqkcfkp0ih
) about 'CB Radios, Cellphones and Gasoline Vapor
Ignition', on Sun, 21 Mar 2004:
The gap necessary to create an arc with 22 volts is:
22V / 20,000V/in = 0.001 inches Kinda small, but given a microscope,
a 1 mil spark gap will arc.

But it takes about 350 V to do so. The relationship between voltage and
gap length is very non-linear below about 500 V.

I didn't know that it wasn't linear. I just assumed that it takes the
same amount of energy to peel electrons off of a single atom (ionize)
regardless of gap seperation. A wider gap requires more voltage to
ionize more atoms to create a longer conduction path, but the energy
per atom is the same. I also couldn't find (Google) any useful
references that showed this non-linearity. Unless the heat generated
by the ionization contributes to assisting furthur ionization, my
seat-o-de-pants physics says it should be linear (for DC).

You need to read up on the physics involved. The critical point is
that a free electron in the gas has to have a long enough mean free
path to pick up enough energy by falling down the electric field to be
able to ionise a molecule when it does hit one, generating one more
electron in an inelastic collision.

If it hits a molecule before it acquires enough energy, in an elastic
collision, it will end up travelling in a different direction with the
same energy, but with a good chance of losing the energy that it had
accumulated. Think "drunkards walk".

The minimum in the Paschen curve corresponds to the point where the
mean free path is longer than the gap.

There's also the minor detail of RF excitation versus DC. As I
vaguely remember from my 35 years ago college welding classes, TIG
welding uses RF to strike the arc because it takes less
power/energy/whatever to start the arc. We're allegedly talking about
striking an arc across 0.001" with a 5 watt, 27MHz transmitter
terminated with a 50 ohm load. If it's non-linear in the opposite
direction, the calcs are gonna be no fun.

RF excitation works better than DC becasue it doesn't sweep the
electrons out of the gap as they are created (by cosmic rays or local
radioactivity) in the way that a DC field does. Like I said earlier,
the physics was worked out about a hundred years ago, and the
calculations shouldn't be too difficult now that we can use computers
for the tedious bits.

I have everything it takes to test this. Microscope slide, with two
sewing pins glued with hotmelt goo and seperated by 0.001". Apply RF
and watch through the microscope. I'll see if I can throw something
together and post photos (time permitting).

Everything except a sound undertanding of the theory. I've got a copy
of a reprint of volume 2 of "Conduction of Electricity Through Gases"
- Ionisation byCollision and the Gaseous Discharge - by J.J. Thompson
and G.P. Thompson.
My copy was published by Dover Press in 1969, and reprints the 1933
third edition. The first - singe volume - edition was published in
1903. I bought it when I was fiddling around building a starter for a
xenon arc lamp, back in 1972. It proved quite useful.

-------
Bill Sloman, Nijmegen