In , Jeff Liebermann wrote:
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.

It gets messy. You can see how messy it gets when you see what happens
in the cathode area of a "glow discharge".

A "glow discharge" is one of two common processes where positive ions of
the gas/vapor are accelerated by the cathode-adjacent electric field into
the cathode material, and where positive ions bombarding the cathode
dislodge electrons from the cathode to maintain the supply of free
electrons in the "discharge" (conductive path of glowing gas/vapor).
(The other of the two common discharge mechanisms where cathode
bombardment by positive ions dislodges electrons is the "cold cathode
arc". There is still another cathode process for a discharge known as the
"thermionic arc".)

The glow discharge cathode process has 5 layers, 3 dim/dark and 2
bright. There is some sort of 'natural spacing' and 'natural thickness'
of these layers, which varies with gas/vapor type and pressure and the
cathode material. There is also a characteristic voltage drop of the
cathode process known as the "cathode fall", and that is normally a few
times or several times the ionization potential of the gas/vapor.

There is such a thing as "normal glow", where the cathode process occurs
at its natural current density (for the gas/vapor type and pressure and
cathode material), and the first two dark layers and the two bright layers
and some minimal portion of the third dark layer have a tendency to occupy
some 'natural distance' (a function of gas/vapor type and pressure
and cathode material) between cathode and anode.

Then there is "abnormal glow", where the cathode process is forced into
a smaller space between electrodes and/or is conducting a current density
higher than 'natural' (for the gas/vapor type/pressure and cathode
material) due to more current flowing than is "natural" for the available
cross section of cathode process. When that happens, the "cathode fall"
is even higher than that of "nowmal glow".

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.

I don't know about that, but I have heard of RF glow discharges maybe
having the cathode process eliminating one bright layer and one dark layer
(for "electrodeless discharge" that occurs where insulation exists over
the cathode for example), and that may reduce the cathode fall.

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.

at best!!!

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).

Please do!!!

- Don Klipstein )