Why does this neon flicker?

--- If you pay a tutor to come to your house and teach you something that's one thing, but if you want a free basic education that's quite another. In the case of the latter, you'll be expected to go to the school where the information you want is located instead of having the school come to you.

As far as the answer to your question goes, heat generated by the neon plasma rises, reducing the ionization potential in the 'hot patch', causing the plasma to follow the convection currents until it rises high enough above the peaks of the electrodes that the ionization potential gets to be too high to maintain the arc. When that happens the plasma will quench and a new one will start at the base of the electrodes, which are probably closer to each other than the tips, a la Jacob's Ladder.

-- John Fields

or horn gap.

--
Yes, except that a horn gap is designed to quench a surge and keep an
arc from being re-initiated, while the gap at the bottom of a Jacob's
ladder is designed to _assure_ the initiation of an arc when the
previous one gets quenched.

I thought the spark in a Jacob's ladder moved in much the same way as a projectile moves in a rail gun, ie the current in the spark is intersecting field lines created by the current in the rails (or wires inn the ladder), causing it to move.

Is this correct?

-Ed

--
(You can't go wrong with psycho-rats.)       (er258)(@)(eng.cam)(.ac.uk)

/d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m
{moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto}
for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage

--- It may be part of the reason, but since it's AC, I don't think so. AIUI the bulk of the effect is due to the lowering of the ionization potential in the plasma and the heat of the plasma causing it to rise. An easy experiment to find out what part (if any) the intersecting field lines played would be to place the electrodes parallel to each other and run the thing horizontally. I'll do it if I can find the time to dig the stuff out and try it.

-- John Fields

Hnuh? Nothing very basic about this at all, old man. The combined efforts of 4 electronics related newsgroups haven't come up with a good explanation, The thermal/convection one, which seems in part what you suggest, will be investigated tonight and I'll post the results. You seem to be under the impression that there is an arc in a neon tube - there isn't, it is a glow discharge, a very different thing indeed. Check out the updated web page and then see if your explanation still holds.

Thanks anyway for your reply

best

Andy

Me neither. However, if the current is reversed, then the fields reverse, but the current in the spark reverses also, so does it still go the same way?

Anyway, I don't have the kit to test this with at the moment.

-Ed

--
(You can't go wrong with psycho-rats.)       (er258)(@)(eng.cam)(.ac.uk)

/d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m
{moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto}
for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage

--
It does until you prove otherwise, and I'll be looking forward to
reading about the methodology of your investigation. :-)

BTW, since you asked about the behavior of the lamps under AC
conditions, your DC data is meaningless, since the continuous cycle of
voltage rising to a strike followed by settling into a "valley", then
later deionization followed by another strike...  doesn't happen with
DC excitation.  

If I meant 'arc' I'd have written "arc", and I believe a glow
discharge is, in effect, a plasma, which is why I wrote "plasma".

http://science-education.pppl.gov/SummerInst/SGershman/Structure_of_Glow_Discharge.pdf

A very simplistic methodology, I'm afraid. I operated the bulb in three mutually perpendicular orientations. The result was that it made no difference at all to the areas glowing or to the flicker rate. I conclude that convection in the gas is not the cause of the flicker.

I didn't ask about the behaviour of lamps under AC conditions, I asked why the neon flickered. See the thread title. I gave the conditions under which I had observed the flickering. As it turns out the neon flickers under DC or AC conditions. My DC data is not, IMHO, meaningless, as it shows that any explanation relying on AC conditions is incorrect. Check

where you will be able to see the tube flickering under various DC conditions.

You wrote :-

"the ionization potential gets to be too high to maintain the arc."

which is why I thought you were referring to an arc. An easy mistake to make. ;-)

best

Andy

--
Yup! You're right on all counts.  So what do _you_ think the flicker
can be attributed to, or is it still a mystery?

I think that, and I'm stumped as to an experimental method to verify this with what I have available, build up of charge on the inner surface of the envelope is it. The envelope is a relatively poor conductor at room temperature so I think the charge distribution might be non-uniform and sufficient to cause the glow to flip to a different position. Once it has flipped then the charge would slowly dissipate from that area while it built up in a new area until the reverse flip occurred. This is all speculation unless I can find a non-destructive way to measure the charge. Any ideas on that very welcome.

I did wonder about RF oscillations but none were detectable from 150 kHz up to 30 MHz nor at 50 and 144 MHz. I have had Nixies that self oscillated at RF when fed from high impedance sources - the inter electrode capacitance forming a simple relaxation oscillator. They were very squeggy (unstable) at various periods.

I will do a few more investigations 'in my copious free time' and post the results on the page given before.

Thanks for your contributions

best wishes

Andy