How to Spot a Psychopath

A reader writes:

What are those little LED-like, but flickery, orange lights, seen in nightlights, electric-blanket power lights, etc? I've seen them in antique radios and as indicator lights in other ancient gear, so I presume they're not actually LEDs.

Minnie

They're neon bulbs. One giveaway is the colour; a plain neon tube, just low-pressure neon in a glass envelope, glows naturally with that orange-red colour when you put enough volts across it.

("Neon lights" that aren't orange-red may still contain neon, but have a phosphor coating on the inside of the glass that turns the light another colour. White fluorescent lights are all actually mercury-vapour tubes with a phosphor coating. The amount of mercury in even a large fluorescent lamp is very small.)

For a large neon tube, the voltage from end to end has to be up in the kilovolts. But if you make a little teeny neon bulb with electrodes only a few millimetres apart, you only need a bit more than a hundred volts to get it to glow.

This makes teeny neon bulbs a natural fit for indicator-light duty in countries with 115V-ish mains power. You still need to use a current-limiting resistor in series to discourage the lamp from zipping up past the C on this graph and burning up, but that's all you need. In countries with 230V-ish mains, you just need a larger resistor value.

If you run a neon lamp directly from 50 or 60Hz AC mains power like this, the bulb flickers at twice the mains frequency, because the two electrodes light up in turn, but only when the mains waveform is giving the bulb enough voltage to light. (From DC, the lamp won't flicker, but only one electrode will light up.) The older the lamp, the more flickery it will become, until eventually it doesn't light up at all. Little neon lamps ought to last 20,000 hours or more, but many modern ones seem to be of lousier quality.

(Incandescent bulbs don't visibly flicker when run from AC, because a tungsten filament has enough thermal inertia to keep glowing at very close to full brightness even when the mains waveform is crossing the zero-volts mark. A fluorescent tube driven almost-as-directly as a neon bulb from mains power will also flicker, which a lot of people hate. Modern high-frequency electronic ballasts solve this problem, for fluorescent tubes and compact-fluorescent lamps.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.