In article , Jon Elson wrote:

John Albers wrote:
SNIP
80V DC output. According to various sources the break down voltage of
air is around 20V per .001 inch. I have a block of wood (pine) with a
notch in it holding two machine screws facing each other to create a
spark gap. Using a piece of .001 inch brash shim stock, I adjusted the
gap to be .001 inch. I am thinking that if 80V is supposed to jump a
.004 inch gap, then 80V should have no problem jumping a .001 inch gap.

I am not getting a spark. Should I be getting a spark with this setup?

Probably not. My experience is you'll never get a fat spark from a TV
anode power supply over 1". More like 1/8" or so. You may get some
ionization, but not enough to create a full conduction channel in air.

That's a function of current. A milliamp or two of "average current"
will typically, in my experience, get the voltage drop down to
somewhere around 15-20 KV per inch. 30 milliamps will get the voltage
drop down to somewhere around 6-7 KV per inch. A few amps plus some fumes
of metal vapor will get the voltage drop to a few hundred volts per inch.

So, I think you need a LOT more voltage per inch, maybe about 70 KV/inch,
SNIP

Varies from approx. 1 to 3 KV per millimeter, or 25 to 75 KV per inch,
depending on how the electrode geometry affects evenness of the electric
field between the electrodes. Can be less volts/distance at higher
voltages over 40 or 50 KV or so.
I have heard 20 KV/inch before as some sort of a "1-size-fits-all".

You will usually need a couple hundred volts or so to spark through air
no matter how small the gap is because of the cathode fall of a "glow
discharge".
Voltage drop in a spark gap will be less once the temperature of the air
rises. That couple hundred volts "cathode fall" will "collapse" to a much
lower voltage once an arc process on the cathode forms.

- Don Klipstein )