The neon bulb was a wonderful device for a young boy to play with - and for
those that do not entirely grow up. It energized at about 60 volts,
although that varied from tube to tube. ARC-5 receivers used them across
their input, to discharge static electricity. They were great for hams with
meager funds since you could place one near the final and determine if there
were parasitics, by the color of the glow.

Sensitivity could be increased by putting a dc voltage on the plates just
under what it took to fire. Then, a slight amount of rf would add to that
base and fire the bulb.

Little pocket testers would distinguish between ac or dc, by simply looling
to see whether one or two plates were lighted.

Light wuld affect the sensitivity, as well as heat. You could apply a
voltage just under the firing voltage, then put your finger near it and it
would light.

Although I never built one, it probably would have made a great lightning
indicator, with directional probes connected to indicate which quadrant the
lightning was coming from.

As I recall, we used to salvage neon bulbs from old flouroscent light
starters.

And, of course, they were good for neutralizing those old triode finals.

73, Colin K7FM