I bought Mrs. R one of those ball-type compasses for her new Suzuki Aerio. The other day while I was riding with her I noticed that it seemed that we were always driving southwest; being a trained professional, it seemed to me that it was impossible to drive to town and back from town and be going southwest both ways.
So I bought her a new compass, carefully checking it in the store. I checked it in the parking lot. I opened the door of the car, got in, and whaddya know, the compass pointed southwest again. We played with it a bit, and it seems that whenever the compass is inside the car, it points southwest. Wow, we thought, that’s wild; the car is magnetized.
So when we got home, I tried it with my car (also a new Suzuki Aerio) (hey, we liked the car, so we bought two) and it does the exact same thing.
Questions for the Dopers:
Y’all ever heard of this sort of thing before?
Will the magnetism of the car go away with time? I recall that you can demagnetize a bar magnet by dropping it; will the constant jiggling as one drives produce the same effect eventually?
I suspect that the answer is going to rely on your getting a better brand of compass. All cars have magnetic attraction because (big surprise), most still contain a fair amount of metal.
Having had to rely on a compass for my line of work, one quickly learned to recognize metalic attraction for accurate readings.
While I don’t understand how those compasses like that DO work, I’d think it would be cheaper to replace your compass with a better model than dropping your car.
Add-on car compasses have calibration adjustments. I strongly suspect you have not adjusted the compass to its mounting environment in the car. There should be instructions that came with the compass.
The ones that can compensate probably work in much the same way as ship’s compasses used to (and maybe they still do, I don’t know): balls of ferrous metal are placed near the compass at right angles to the external influence they are to cancel out. The size and distance of the metal balls is chosen to exactly balance out the influence of the metal in the ship (or car).
What QED is talking about is a binnacle. The green and metal balls are moved out or in to compensate for the metal of the ship.
(I couldn’t find anything on if they still use them though)
First I’d ask you if this is a novelty or necessity. If it’s a novelty, check installation directions and calibration. If it’s a necessity (your wife gets lost alot, or your checking up on Rand McNally) I’d invest in a GPS.
I have seem the instructions on some fancy car compasses & they say to drive the car in a full circle first. I got a cheap compass that doesn’t require that.
Okay, based on the responses here, we bought a different brand of compass (“Sherrill”, available from JC Whitney; it has adjusting screws (one marked E-W and one marked N-S) (–I wonder what those abbreviations mean?) on the bottom. Installed it, adjusted it per the directions, works fine, end of problem.
I bet that the “N” is pointed towards one of the speakers. The speakers have strong magnets in them. Alternatively, perhaps the magnet in the compass may be right next to a large hunk o’ metal.
