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I'm aware about ionisation but even to ionise a gas you need an electron. So how in a discharge tube, which has a low pressure gas and a high voltage, does current flow? How is the first electron initially obtained?

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  • $\begingroup$ U can also search Paschen's law $\endgroup$ – user6760 Jan 19 at 8:22
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the first electrons that initiate the rest of the current flow can be plucked free of the electrode surface in two ways: 1) field emission, where the strength of the electric field at the surface is big enough to pull electrons out of it, or 2) the photoelectric effect, where a stray ultraviolet photon happens to strike the electrode surface and eject an electron from it.

Once you have a "loose" electron like that in that strong electric field, it gets accelerated until it smacks into a gas molecule, knocking an electron off it, and now you have two loose electrons and one positive ion that will get accelerated.

Two things then occur: 1) a growing avalanche of fast electrons sweeping their way through the gas in the tube towards the positive electrode, and 2) a slower drift of heavy ions moving toward the negative electrode. When one of these heavy ions strikes the negative electrode it can knock loose several electrons which then join the others in the growing cascade, speeding off towards the positive electrode.

In a short time you have ionized enough of the gas molecules in the gap between the charged electrodes in this way that its electrical resistance drops to a low value and a large flow of current then develops between the electrodes. If that current is not limited by an external resistance, you'll get a power arc between the electrodes, which will get burned. If there is limiting resistance in the high voltage circuit driving the tube, then you will obtain a glow discharge, whose characteristics can be varied by varying the amount of allowed current.

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  • $\begingroup$ I think it's fair to mention also (3) thermionic cathodes, probably the most common type used in lighting, where arc is started by preheating the electrodes. All T12/T8 lamps with 2 pins on each end are of this kind (opposed to CCFL). Also, CFL do operate in arc mode (dominated by thermal emission and ionisation, negative impedance), not glow mode (Townsend multiplication and secondary emission on non-thermal cathode, positive impedance). This is why the CFL use constant-current sources. See the graph in physics.stackexchange.com/q/116907/115253; point I is the glow-arc transition. $\endgroup$ – kkm Jan 20 at 21:45
  • $\begingroup$ I omitted the thermionics, even though I am a vacuum tube enthusiast, because I did not consider it relevant in the context of a discharge tube- which I took to be of the physics class demonstration variety. $\endgroup$ – niels nielsen Jan 20 at 22:02
  • $\begingroup$ Ah, that's fair. Perhaps I read the question to be more general. $\endgroup$ – kkm Jan 20 at 22:14

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