The Spark Hits the Fan
RAY: Tommy's right. You didn't need to know a whole lot about, or anything maybe about, cars, but when I pulled that wire off --
TOM: All you needed to know was Faraday.
RAY: Yeah, and we're going to get letters from all the engineers in the group, who are going to say that this can't happen.
TOM: Oh yeah, we'll say. Oh yeah.
RAY: Just like they all said, you can't have a DC motor that runs backwards, if you change the poles, which is not true either. We know that. But what happened when I disconnected that wire is the fan didn't just stop turning it immediately, it was slowing down, but it was still, like any fan that's unplugged, it continues to turn. While it was turning, it was acting as a generator.
TOM: Of course.
RAY: And it was making electricity, hence the spark. And it also explained why when I turned the key off, the thing didn't shut off right away, because, believe it or not, the electric fan's spinning now. Even though the key is off and there's no power being supplied to the fan, the fan is supplying electricity, which is running the car's ignition system.
TOM: Isn't this sweet?
RAY: And as the fan slows down, the engine begins to stumble and miss and finally, the fan is going too slowly to supply enough current to keep the engine running, and the thing --
RAY: -- stops running.
TOM: What a beautiful puzzler.
RAY: Do we have a winner this week?
TOM: Do we ever. The winner this week is Michael Faraday.
RAY: From jolly old England.TOM: No, the winner is Christina Stolberg from Denver, Colorado.
[ Car Talk Puzzler ]