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If I take a battery, some wire, and a light bulb and hook them all in a circuit that would create a current and light the bulb. If I turned the battery around so the terminals were switched then I would have a current in the opposite direction, but the light bulb would still light.

So why doesn't that principle hold for more complicated devices like a flashlight?

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    It's true that any RLC circuit works backwards, but modern electrical devices use more complicated, nonlinear components. For example, even something as simple as a diode treats current asymmetrically. –  May 06 '16 at 03:50
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    We could, in principle, make devices that can work with both polarities, but why bother? –  May 06 '16 at 04:02
  • Does this question belong to [electronics.se]? – Qmechanic May 06 '16 at 04:57
  • @knzhou I'm referring to some of the older more simple devices like my 90's era flashlight (see my below comment). –  May 06 '16 at 22:42
  • @CuriousOne for the same reason people are developing type C reversible USB. For convenience. (also check my comment below). –  May 06 '16 at 22:47
  • @Qmechanic if this question belongs in Electrical Engineering, is there a way to move it? –  May 06 '16 at 22:47
  • @Nova It might be more complicated. Apparently, incandescent bulbs last longer if you feed them AC power, because the filament will be 'boiled off' on both sides equally. So there might be a DC to AC converter in there. There might also be safeguards against voltage spikes or drops, or maybe a DC step-up? –  May 06 '16 at 23:08
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    @knzhou: I have never heard of a flashlight with incandescent lamp that uses a dc/ac converter. If it's an LED flashlight, then the polarity matters. –  May 07 '16 at 00:20
  • @Nova: The reversible USB sounds more like a patent play. :-) –  May 07 '16 at 00:21

1 Answers1

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UPDATE : My explanation might not be correct. Please read comments below.

Although knzhou gives a good general reason, I suspect that in this particular case your flashlight does not contain any non-linear components or diodes and the explanation is much more banal. I suspect that the contacts in the flashlight are designed for the batteries to be a particular way round - ie with the spring at the bottom of a cell (-) and the tip of the bulb (or a strip of metal) at the top (+). Then if you reverse the batteries the contacts will not make an electrical connection between the batteries and the bulb, because the spring does not touch the small + electrode of the cell.

sammy gerbil
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  • I don't think so(just tried it). Springs are used to have maximum contact with the flattened end (-ve terminal). The spring force results in better stability of the battery inside the holder. – Shubham May 06 '16 at 13:12
  • @Shubham : Yes, that is what I am saying - that the springs are designed to make contact with the flattened end of the cell (-ve terminal). But what if you turn the cell around in the batter holder? Does the spring make contact with the +ve terminal? And does whatever is at the other end of the holder (a metal plate?) make contact with the flattened -ve terminal? –  May 06 '16 at 13:35
  • @Shubham : What kind of flashlight is it? Does it use LEDs instead of an incandescent bulb? Does it allow you to choose different flashing patterns? If so it is an 'electronic device' as you describe and knzhou's explanation is probably correct. I am sorry : I assumed that you were using the term 'electronic' incorrectly and that your 'flashlight' is the older type which (despite the name) didn't actually flash - it had only on/off settings. –  May 06 '16 at 13:36
  • Yes, I was talking about like one of those old flashlights with only an on/off switch. What's the point of there being a spring and bulb tip. Why not just have a battery just be a cylinder that you can slide into a slot with two springs. So the direction of polarity doesnt matter. It would save everyone a few seconds from having to make sure they are lining up + with + or putting the battery in the "right" way... –  May 06 '16 at 22:40
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    @Nova the point of the batteries not being symmetrical is exactly because it dóes matter which way round the battery goes for a lot of devices. By not having a symmetrical battery, it is easier for users to see which way the battery should go in. – Dolf Andringa May 07 '16 at 03:29
  • To this day, Sammy is still waiting for that check mark. – NoName Jul 07 '18 at 14:39