The GFI works by sensing the return current. If the current into the cirucit doesn't match the current leaving the circuit, then it trips. It trips on unbalanced current. Someone using their body as a potentially lethal short to ground will disrupt the return current, thus tripping the GFI.
That's really one of the beauties of a GFI—that it can protect old wiring. But I agree that a three wire hot/neutral/ground wiring is beneficial too, but not having such wiring doesn't matter to the GFI.
> Actually, a GFI will protect users of a two-wire ungrounded circuit.
Wow! Thanks for that information! I thought I was doing a good deed to some of the others by passing on information about grounds, and instead received some in return.
Though I once knew enough about radios and tvs to repair them (and for a while repaired the old 8-bit computer motherboads professionally), I have no training as an electrician, and have never bothered to read up on GFIs (or GFCIs). I just assumed they needed a ground wire.
I’ve now checked, though, and confirmed with many sources that they’ll work in the old two-wire houses. I’ll put some in my house. Thanks again.