"my preferences for lightbulbs is that they should produce white light when switched on, and not produce light when switched off."
Odd phenomenon seen the other day that I've still not got to the bottom of.
We have 3 light fittings each of which take 3 bulbs or, as sparky BiL insists, lamps.* These have been are SES golf-ball types. Incandescents last fairly well but go out in spectacular style, invariably tripping the consumer unit & sometimes ejecting the glass bit & leaving the bit of screw-threaded tin in place to be extracted with the aid of a cut finger or two. QIs fail undramatically but more frequently. I'm attempting to replace them with LEDs.
LEDs are a problem of their own. The good-quality golf-ball format jobbies have a large opaque chunk occupying the bottom of the globular bit and just don't work with the shades on these fittings so I've resorted to the stick types. These are inevitably cheap (not necessarily to buy) and nasty and (a) often fail to achieve the claimed equivalence in brightness to incandescents and (b) come in a range of strange colours only vaguely matching an incandescent of the alleged colour temperature. So over the past few months I've bought a considerable selection trying to find something acceptable.
On the last pass of this I'd removed one of the bulbs from one of the fittings to change with a newly bought one and noticed that, despite the fitting being switched off, one of the other bulbs was glowing very dimly! I removed that & the third bulb started to glow. I've no reason to think that the switch is faulty or that there's anything strange in the wiring. I suspect that the switch-mode power supplies in the bulbs are able to harvest energy from some form of coupling in the wiring, either induced or capacitively coupled and that whichever of them was the more efficient grabbed whatever was going until I unscrewed it and the other took over. But is this capacitively coupled mains? Or is it coming from the local VHF transmitter churning out over 1 MW ERP a couple of miles away? Whatever it is I've got at least two LED units capable of producing allegedly white light when switched off.
*"Bulbs grow, lamps glow." Until we catch him out saying "bulbs" like the rest of us.