Found this screwdriver in bottom of auction box. It was dirty,rusty and not eyecatching except for the conical transition between handle and shaft. Looking it up it is listed as (Neon Tube Screwdriver round shank) Usage is listed as (used to check current to spark plugs) Is this more scientific than wetting your fingers and grabbing plug wire ? My question : what is half moon slot in handle for ? Has screwdriver slot in butt end but probably not removable without a lot of penetrating oil. It is listed as screwdriver but rounded tip looks more like something to use as a contactor rather than a screwdriver. Guess I should have trashed it as I'm sure I've wasted more time than this tool is worth but inquiring minds need to know.

I think you lay the plug wire in the half-moon slot, place the tip near a ground, and if current is flowing, you'll see a spark jump.
OR- there might be a neon tube in the handle which would light up when current passes through with the tip grounded?

answer B, neon tube in the handle

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don't do the lick your finger think anymore, they have real coils these days