Dear 100 Hour Board,
How do Christmas lights work? And why is it that when one bulb is broken half of the string of lights goes out? You don't have to write this down verbatim, 'cause that was kinda dorky-soundin' and braindead and... yeah.
- Elaine Marley, transcribing for the Interpretive Dancer
p.s.- hey, I'm hungry! You gonna write that down, too?
First, I need to explain a little bit about electric circuits. The way electricity works, you have to have a complete loop going from the power source to the light bulb and back to the power source again. (That's why plugs have two prongs.) If you want to wire two light bulbs on the same electric circuit, you have two choices: you can wire them in series or you can wire them in parallel.
A circuit in series is the easiest to explain: it's just like beads on a string. The power source is at the end of the necklace, then wire, then a light bulb, then more wire, then another light bulb, then wire going back to the power source. Notice that the current has to go through every single light bulb to go through any of them at all. If one of the light bulb was missing or the wire was broken, the current wouldn't be able to make the complete loop.
A circuit is parallel is a bit harder to explain. Imagine a ladder made out of wire; two wires up the sides with wire "rungs" running between them, and a light bulb on each "rung." Notice that the current takes a separate path to go through each light bulb: up one side of the ladder, across the rung and then down the other side of the ladder. Even if one of the rungs was missing or broken, the current could still get to the other ones.
Normally we're used to houses and appliances that are wired in parallel. Just because one light bulb in a fixture is burned out doesn't mean that the others won't work. Old Christmas tree lights used to be wired in parallel as well, but it took a lot of power for that many lights, and the wires got dangerously hot.
Modern Christmas tree lights are wired in series, like beads on a string. This saves a lot of power, but it means that when one light is burnt out, The entire circuit is broken and none of the lights will work. Some kinds of Christmas tree light bulbs have a special shunt in them that will allow the circuit in a burnt-out bulb to stay completed as long as the bulb is physically still in the socket. Some strings of lights have more than one series circuit in them; that's why half of the string won't light when one of the bulbs is removed.
- the Physics Chick