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Question #41417 posted on 12/17/2007 3:01 a.m.

Dear 100 Hour Board,

What determines the tempo of a car's blinker? Is this a constant tempo for each individual car? (I know its not the same across the board for every car) Or can it change depending on outward influences?

- mj

A: Dear mj,

There's a good explanation here, but I'll still talk you through what you're seeing.

When you activate your turn signal, current flows through a wire which is wrapped around a curved piece of steel. As the current flows through the wire, the steel heats up, expands, and straightens out, pushing the wire towards a contact which is attached to the light bulb in your turn signal. Once the contacts touch each other, the current starts to flow to the light instead of through part of the wire. This lets the wire cool down which allows the steel to cool down and bend until the contacts no longer touch, at which point the whole process starts again. This happens 1-2 times a second.

There are a lot of factors which can affect the frequency of a turn signal. (As an aside, I can tell you're a musician, because you talked about the signal "tempo.") The basic variables are the rates at which the wire heats up and cools down, and the rate at which the steel expands and contracts in response to the change of temperature. These variables, in turn, could be affected by the composition of the wire and the steel, the dimensions of both, the number of times the wire is wrapped around the steel, the resistance of the light bulb, the ambient temperature, etc., etc. No doubt there are industry specifications which set minimum and maximum frequencies, but there's plenty of room for variation within those tolerances, and yes, you could have some variation from outside factors (such as temperature), although my experience is that it doesn't have a significant effect.

- Katya the physics chick

P.S. See also this.