how does an electrical mains-tester work?

That's exactly what it does

It's not an incandescent light bulb, it's a neon with a high impedance series resistor

It does go through you, but there is an enormous resistance built in to the screwdriver and the current is very low (the neon lamp needs only fractions of a milliamp), and you usually don't feel it.

The current flows both ways as both the neon lamp, and usually the circuit you're testing, are AC (although it will work fine on DC too), but otherwise nothing.

However, there are battery powered versions of the same thing, which use a button cell, integrated circuit, and LED, and these will work at lower voltages than the 90V required for an neon lamp to light.

Most are neons with a very low current so they can usually light with just the body on tother end oft, neon. I don't think anyone makes neon screwdrivers any more, Health and safety etc.

Brian

I had some green neons that struck at 50 volts.

Brian

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Brian

plenty showing on Google - usually called mains tester.

Geoff

There's more than one way to make a green neon, but they usually rely on a fluorescent coating, and the actual discharge is a dim white colour (but it's UV which excites the green phosphor).

The normal trouble is that they require a significantly higher voltage, and cannot be made to run on 120V mains. They also have a significantly shorter life. I've never seen a 50V one.

There are also green neon indicators which have a regular orange neon on them, and generate a very disappointing dim brown light when lit and viewed through the green lense.

There's no way to make a green neon.

Yes, OK! I think he knew what I meant...

Add krypton to the neon gas inside. Technically, it's no longer a neon, I know, but it works the same way.