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Treebeard's Stumper
7 January 2000

Christmas Lights

I thought I understood Christmas tree lights. If the bulbs are wired one after the other in series, they all go out when any one does. If they are wired in parallel like rungs on a ladder, the other bulbs stay bright, but they tend to run hot. The new lights on our tree have three wires, not two. If a bulb burns out, the others stay lit. Then I noticed a bigger mystery as I played with them. If I gently twist a bulb in its socket, it goes out and the others stay on. But if I twist too much or remove the bulb, they all go out. How do Christmas lights work?

Parallel Circuit
Series Circuit


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