What are neon signs? How do they work?

All of the inert gases—helium, neon, argon, krypton, xenon, and radon—can be made to emit light once they pass their ionization potential by inducing a threshold voltage across them through two terminals. This is how fluorescent lamps work. A starter and ballast kick the voltage up from 115 to around 7,000 or 8,000, with very low current, to overcome this threshold, at which point the voltage will drop but the lamp will still remain lit until it’s switched off.

Xenon flash lamps work a little differently due to the electrical structure of the xenon atom. These devices rely on a special-purpose trigger transformer to produce the high voltage needed to make the xenon tube glow, but it will only do so with a rapid, intense flash of light. This is why xenon strobe lamps have a regulator circuit, to energize the trigger transformer so the lamp will actuate at the desired intervals.

High voltage rather than high current is used to power fluorescent lamps because too much current will wear out the terminals, making them spongy and useless. This is especially true of mercury vapor lamps with tungsten terminals.

Inert gases have been used to fill glass tubes similar to vacuum tubes, and these were used as voltage regulators before solid state devices took over.

Incidentally, neon and argon are the only two inert gases which glow with colored light (orange-red and bluish lilac respectively). All the others glow white, so when you see yellow, green, or pink fluorescent signs, their glass tubes are either made of stained glass, or coated with colored varnish.

Radon is radioactive and doesn’t need high voltage to produce a glow. It will emit light when brought into contact with zinc sulfide.