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Where do the various colors in electrical discharge of air with a high voltage originate from? Are these transitions in ionized nitrogen gas or unionized gas? Are chemical reactions involved? I assume the violet color is corona discharge. What are the transitions involved?

When I connected 2 nails to a $15\,\rm kV$ AC and kept them close enough, there is yellow plasma visible. But, when separated a little more, no visible spectrum visible. I assume electrons get enough energy to cause yellow transition in $\mathrm{N_2}$ in the first case, but infrared occurs in second. I don't know if discharge fails to occur altogether in second case, and if only corona is occurring. Are these the electronic transitions in $\mathrm{N_2}$ or the ionised $\mathrm{N_2^+}$.

I also want to know, up to what distance would the plasma be formed, and after that, only corona discharge would occur?

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  • $\begingroup$ What was the setup under which you saw these colors? And, of course, what research have you done to try to answer your basic question? $\endgroup$ – Carl Witthoft Dec 3 '14 at 18:32
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    $\begingroup$ I just connected 15kv ac neon sign transformer o/p to the space between two nails in normal air. As I increase the distance a little, no colors appear, only violet glow at both ends. That must be the corona discharge. Does it ionise air, and electronic transition of nitrogen give the violet color? I could not find the transition that gives this color online. Also, when the nails are close, yellow photons appear, but at a little more distance, they do not(and only violet colors at ends appear.) Does the transition shift to infrared? $\endgroup$ – Harshfi6 Dec 4 '14 at 3:18
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You do not many details of the setup, but I assume it an electrical discharge in air inside a closed ball, like in some museums? If such is the case, different elements mixed with the air will get the atoms either ionized or some electrons exited to exited to a higher energy level. When these decay back to their original energy they emitting light of a specific frequency during the process, because each element has a specific energy structure for its orbitals. For instance sodium usually emits in yellow. Other elements in other colors, but I don't remember which ones, so you might need to do a search if you want more details.

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