We have a set of out door lights - a string of (12V * 20) E12 lights in series giving 240V and running off mains electric. No transformer required.
Having just tested the bulbs of an out door light set with a meter (continuity beep) and sorted out 24 working ones (using some spares and some from a second set) then a day later seen a couple of the lights out but the string still working made me ponder.
The instructions say that with a lamp out the string will still work, but that you should change the bulb a.s.a.p. to avoid long term damage.
So presumably when the filament fails there is some kind of continuity pass through in the base which keeps the string alive.
However if this is the case then a bulb can pass the "beep" test but not light.
Cue the memory of the childhood hobbyist things where you had a battery and a light bulb and some circuitry. Often screwed to a bit of fibre board.
The sockets and bulbs I remember were almost certainly E12 and they just seemed to be an everyday thing.
Now I am looking for an E12 socket to wire up a simple bulb tester.
eBay lists them but the seem to come from China. No UK stockists.
Just grumbling and feeling old because things weren't like that when I were a nipper.
I will wander down to the oldest electrical shop I know in town, in case they have one in stock, but it may well be cheaper just to stock up on spare bulbs.
Cheers
Dave R