Pete Verdon presented the following explanation :
How are they supposed to work?

I tagged one of these onto the end of a TLC order just to get an ok
electrician's-sized screwdriver. I don't intend to use the alleged "testing"
facility, but I'm curious how it's supposed to work. It looks like it's meant
to be touched to a live terminal and have the current flow up the shaft,
through a capsule which I assume is the bulb, and then go via a brass end-cap
*into my hand*. The last part of that I'm not particularly keen on.

I assume that, while they're not well-regarded by most here, neon
screwdrivers don't actually electrocute their owners during intended use. Is
it just that the current flow is very small, and almost all the voltage is
across the neon? Can anyone with a better grasp of electrical theory explain?

The neon draws a relatively tiny current and to restrict it even more
there is an high value resistor in series with it. The idea is that the
current passes through your body to ground. Problem - you may not
always be at ground potential so it may not light up or lights up
dimly. So one finger on the top and make sure you are touching
something else with a reasonable ground on it.

Problem 2 - if the neon should become damp or faulty internally, you
may be electrocuted.

Problem 3 - They are dim and may not be be seen, which may lead you to
believe something is dead when it is not.

Solution - Don't use it, buy a volt stick which responds to the field
around a live cable and no current needs to flow through you. But test
it before every use.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk