The Science of Christmas Lights

Happy December!  As everyone festoons their houses with holiday decorations, we thought we'd talk a little bit about electricity.  This might even help you fix your broken Christmas lights.

Let's talk about holiday lights.  In a simple circuit, electricity travels through a closed circuit, passing over a filament, which causes it to glow brightly.  The more current that passes over the filament, the brighter it will burn and the quicker it will burn out.  If the circuit is broken, the electricity will not be able to pass over the filament, it will not be able to light.  If the current is too high, the filament will blow out of melt, breaking the circuit.

A lot of us probably remember detangling a whole string of lights only for it to not light because one bulb has blown out.  Finding and replacing that broken bulb suddenly makes the whole string of lights work again.  When this happens, it means that the lights on the string have been attached in a series.  This means that the electricity passes from the power source to the first light, and then from light to light until it returns to the power source.  Any single blown bulb breaks the circuit, making it impossible for electricity to reach any of the lights.

Another way to wire lights is in parallel.  When lights are attached in parallel, each light is on its own circuit.  If one light burns out, the remaining lights continue to shine because they continue to be in a closed circuit with the power source.

Holiday lights nowadays tend to be several series of lights strung together in parallel.  This means that, if a light burns out, some of the lights on the string may go out because their circuit has been disrupted, but the rest of the lights continue to glow.  Of course, they might not, and that is because of something called a shunt.

A shunt is a device that allows current to continue flowing through a circuit by creating a path of lower resistance than the original path.  In incandescent holiday lights, shunts are small wires wrapped beneath the filament.   When the lights are constructed, the shunts are coated with a substance that makes them an insulator.  Electricity will not pass across the shunt while the filament exists, because the coating gives the shunt a higher resistance than the filament.  The electrical current will avoid the shut because electricity seeks the path of least resistance.

If the filament burns out, the high temperature created from the burnout melts the coating off of the shunt.  Now the shunt stops being an insulator and becomes a conductor.  Current passes along the shunt, keeping the circuit open, allowing the rest of the lights in the series to keep burning.

In this blog post, we've talked about the electricity science behind incandescent holiday lights.  If you want to learn more or read about LED lights, check out this post.

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