Dear Tom and Ray:

I have a question for either car mechanics or geophysicists -- you decide. I often want to know in what direction I'm driving (plus I like gadgets). So I'll buy these stick-on-the-dash compasses. They seem OK when outside the vehicle, but when mounted inside and in use, they give directions, at times, that I know are false. My questions: Do cars have a magnetic field of their own that interferes with the readings, or are the compasses simply too cheap; or is Earth losing its polarity, or are you not the right people to ask this? Thanks. -- Paul

TOM: All of the above, Paul.

RAY: I think scientists have discovered that magnetic north is actually migrating. And they predict that in about 50,000 years, Earth's poles are going to switch, so the north pole will be at the south pole. But I don't think your car will still be around then, Paul.

TOM: The biggest source of electromagnetism in your car is your alternator, which is, essentially, a big electromagnet. But I would think that if it was throwing off your compass, it would throw it off all the time. You say "at times" you know the reading is false. Maybe that means at other times it's also false, but you just don't know it.

RAY: Or it could be that as the output of the alternator increases with the speed of the engine, the magnetic interference increases.

TOM: And, of course, any time a current is going through any wire, you're making magnetism. And there are lots of wires running behind and under the dashboard. So those could be throwing off your cheap little compass. But I'd say it's even more likely to be affected by things outside the car, like overhead power lines.

RAY: So our answers are: Yes, there are sources of magnetism in the car, yes, you're buying cheap compasses, and yes, we are not the right people to ask. If any of our readers have a better answer, drop us a note (you can e-mail us from cartalk.com).