Another way to answer your question: air serves as a dialectric in the spark plug. High voltage (you DO know about the secondary on that coil, right?) causes the dialectric to break down, permitting current flow.This creates a plasma arc, which we interpret as the "spark", and which emits visable light. The frequency of light that the arc emits is directly variable with the amount of energy that produces the plasma arc. In other words, all other factors being equal, a healthy ignition system creates plasma which tends to radiate a relatively greater amount of light toward the ultra-violet end of the spectrum, which looks blue. A weaker system that cannot provide as much energy creates a plasma arc that tends to emit lower-frequency light, closer to the infra-red part of the spectrum, which looks orange-ish. This is one reason why I like to leave the points in my Farmall. When a civillian freind sees the Farmall, and they ask "What the heck is THAT?," I can look them in the eye and say, "It's a gasoline-powered plasma generator from the 'fifties..."
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