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Another low-pressure-air discharge Instead of a jar this time I used a long tube to get a better look at the positive column. Interestingly it is red-colored which is characteristic of nitrogen, with very little of the characteristic white of oxygen shining through which is presumably what makes air discharges look purple. Looks [i]almost[/i] like neon, but not as saturated of a red. Does anyone know why this discharge looks more nitrogen-y than other low pressure air discharges? Other pictures seem to look different. Anyway it looks cool. Here it is running on AC hence why you can see the purple negative glow on both electrodes. Runs at a maximum of 8mA on a small neon sign transformer, and gets pretty hot after a bit. I definitely had a leak in this setup so the pressure didn't get as low as I had hoped, but I was too lazy to figure out where it was leaking from. The glow you see from the glass is probably just dirt (I didn't clean it at all), not fluorescence, but I am not certain as I noticed it in the picture only after I put it all away. Keywords: Lamps

Another low-pressure-air discharge

Instead of a jar this time I used a long tube to get a better look at the positive column. Interestingly it is red-colored which is characteristic of nitrogen, with very little of the characteristic white of oxygen shining through which is presumably what makes air discharges look purple. Looks almost like neon, but not as saturated of a red. Does anyone know why this discharge looks more nitrogen-y than other low pressure air discharges? Other pictures seem to look different. Anyway it looks cool.

Here it is running on AC hence why you can see the purple negative glow on both electrodes. Runs at a maximum of 8mA on a small neon sign transformer, and gets pretty hot after a bit. I definitely had a leak in this setup so the pressure didn't get as low as I had hoped, but I was too lazy to figure out where it was leaking from.

The glow you see from the glass is probably just dirt (I didn't clean it at all), not fluorescence, but I am not certain as I noticed it in the picture only after I put it all away.

Screenshot_2026-06-03_134449.png IMG_0474.jpeg poscolumn.jpg Screenshot_2026-05-17_180135.png Screenshot_2026-05-10_110023.png
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Tuopeek   [Sat 06 Jun 2026 at 11:47]
I’ve noticed discharges in air produce this colour where the low-pressure results in the maximum current flow or maximum voltage drop, hence the heating. This will be the minimum point on the Paschen curve. Your pump must be managing a reasonable low pressure. I haven’t tried looking at the spectrum but guess it’s the result of a transition in Nitrogen molecule.

Glass can sometimes glow with UV or electron bombardment, but more noticeable as lower pressures. I tend to expect x-ray generation to occur at about the same time as the glass glows more than the gas discharge. Don't worry, X-ray generation only occurs at high potential and won’t be occurring here.
Drew   [Sat 06 Jun 2026 at 23:52]
Interesting to know! The glass glowing more than the discharge is an interesting rule of thumb, I'll remember that though of course 3600VOC isn't really gonna be able to make significant x-rays.
Ria   [Sun 07 Jun 2026 at 01:26]
Another excellent upload..! You're proving a real asset to the site, thanks so much Love Cat
Max   [Wed 17 Jun 2026 at 07:54]
Nice experiment and good picture of the plasma! It's always fun to experiment with low-pressure discharges in air in long tubes. If you could have reduced pressure further you would have seen the Faraday dark spaces near the alternating cathodes. Here nitrogen dominates the light emission in the visible domain because this is the majority species (nearly 80 % of the gas mix) and the plasma conditions (electron energy, dissociation of oxygen, limited production of other molecules, etc.) results in the preferential excitation of N2 molecules. To be more precise, the red emission is from nitrogen's first positive system, i.e., the energetic transitions from the B to A states of the molecule which result in spectral emission bands which are most intense between 565 and 880 nm. This spread, especially towards shorter wavelengths, is the reason why the light color is is not as saturated as in neon discharges, where the spectral emission grouped in a narrower spectral band.

Note that oxygen's contribution to the discharge's spectral emission is present, but it is not obvious to the naked eye due to the dominance of nitrogen and the fact that most of oxygen's molecular emission lies in the UV. There are several spectral lines from atomic oxygen in the visible domain, but they are in the same spectral range as that of nitrogen's first positive system.

Tuopeek - The minimum in Paschen's curve, i.e., the Stoletow point, is a characteristic of the electrical breakdown of the gas only, not of the steady-state operation of the ensuing glow discharge. The latter is described by the maintaining voltage-current curve (or characteristic), not Paschen's.

Comment 1 to 4 of 4
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