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If I break one bulb on a string of Christmas lights, or the filament goes out, then all of the rest seem to stop working. Why is this?

However, I've also seen the opposite happen in some newer lights -- when one breaks, the others stay on. What makes this one different than the others?

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do you mean the other lights actually break, or just go out? (if you replace the broken one, do the others go back on?) of course, this is very hard to do in practice, since you have no idea where the fault is. – Jeremy Dec 23 '10 at 19:23
up vote 10 down vote accepted

They are in series circuit, so breaking one bulb breaks the circuit itself:

alt text

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...and for the newer one, a parallel circuit is used (which is also why in the mid-price-segment only a some but not all lights turn dark, i.e. not all bulbs have their own parallel circuit) – Tobias Kienzler Nov 26 '10 at 11:30

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