I’ve never seen this before. A standard 75 Watt incandescent hall light. (Basically a bare light bulb in a pretty fixture). I flick on the wall switch and it flickers rapidly on and off (almost like a flourescent). Thinking there’s a bad connection somewhere in the wiring I turn off the wall switch. It keeps flickering until I turn it off and doesn’t appear to flash out or anything. I sniff the wall switch to make sure I don’t smell anything burning. But it seems to be ok. When I take out the bulb there’s a barely perceptible dark spot which makes me think it’s burnt out, but I don’t hear a loose filament so I try it in another lamp. It doesn’t work so the bulb’s dead. I try another bulb in the same fixture and it’s fine, so nothing’s wrong with the fixture.
So - what’s going on? Bulb flickers until I shut off the switch and then won’t go on again. Filament sounds intact. I’ve never seen a bulb blow out that way. Any thoughts on what happened? I’d like to have some idea since I don’t really want to use that hall light again if there’s anything wrong with the wiring. On the other hand since the problem seems to be the bulb itself I don’t really want to call an electrician either for something I’m pretty sure is a bulb malfunction.
The filament in an incandescant lamp is actually a length if wire of a certain resistance path to current flow, the problem is that to get that much wire into the glass envelope, it has to be folded and folded and folded in a sort of spiral.
The filament is folded back on itself so much that its several times the length that it appears at first glance.
Sometimes, when the filament fails, it partly shorts out, some of the filement folding unravels and touches another part of it, bypassing some of its length.
When this happens, the resistance to current flow reduces, hence the current increases greatly, and the remainder of the filament glows very much more brightly, but not usually for long.
On other occasions, the filament breaks, but this break is so fine, that the current arcs across the gap, usually bypassing part of the filament and this gives a characteristic ‘arc glow’.This will usually ficker slightly, because along with the increased current, there is an increased magnetic field, which physically moves the ends of the broken filament slightly thus changing the arc gap.
Last of all, the filament can overheat during failure and this causes it to become so hot that it droops and stretches out all those folds and turns. It lengthens so much that it droops from the supports and rests against the glass envelope, and forms a conductive path.
Glass + hot conductive path = not very good, the buld can shatter or the current path is robust enough to carry much more current than even the design failure amount, and it takes out the fuse.
I’ve seen them go this way - with a sustained, regular flicker, then die completely when they are turned off; I assume it’s some kind of feedback effect; the filament fails and breaks the circuit, thermal contraction happens to move it in such a way as to close the circuit again - current flows, lighting the filament and thermal expansion moves the filament, breaking the circuit… repeat etc. When turned off, the lamp is dead because the filament is already broken.
Thanks for the detailed explanations. Really interesting. It’s responses like these that make the Dope so great. **Mangetout ** I think it’s what you described because the flicker wasn’t slight - it was on and off - and regular. Although maybe a change in magnetic field like **casdave ** described ciould set up the same oscillation as a thermal contraction?
Ooh I’m so glad you explained this in advance. Because, as rational as I try to be, if ever my lightbulb shattered for no reason, I’m pretty sure I’d assume some sort of malevolent supernatural force.