Thread: compass in a car

This may be an old question, but let's see what opinions are out there.

You have probably seen these little compasses sold as car accessories,
meant to be attached to the windshield or the dash board. Knowing about
the effect of a Faraday cage, does anyone anctually believe they work
properly? Has anyone actually bought one?

I know for a fact that the good compass I use for hiking ceases to work inside my
car. Depending on position it's either completely useless or at least very
unreliable, even close to the windshield. So what's up with those seemingly
useless car compasses? Are they only for convertibles? Or maybe for plastic cars?

Your car is sort of not a perfect faraday cage. A compass normally works inside one for basic directions unless you have a *REALLY* sensitive compass. I used to know someone who had one in a very old truck, and it worked fairly well. It wasn't perfectly accurate, but it was fine.

You should have that car checked out. Something is screwy if it throws a compass off that much.

I thought a farady cage was to do with radio waves not magnetic fields on their own. if your car is fully degaussed (de-magnetised) then the lines of flux of the earth's mag field will flow through it and the compass will react to that. Any magentic field in your car will almost certainly swamp a compass. YOu ought to be able to check whether the car is nuetral by walking towards it with your compass as you get closer if the needle swings at all then your car is magnetic. Iron ships had all sorts of problems with this effect and magnetic sea floor mines relied on their weak magnetic field to set themselves off.

A compass normally works inside one for basic directions unless you have a *REALLY* sensitive compass.

Any good compass is sensitive. Otherwise it'd be no good. It's true, though that the compass is completely useless only in the center of the car. Near the windshield, it's just unreliable, but not completely, completely useless. Well, I guess if all you want to do is roughly distinguish north from south, and it works 70% of the time... but then you could use the sun and stars as guides (to more accuracy, actually).

I thought a farady cage was to do with radio waves not magnetic fields on their own.

Radio waves are electromagnetic, and so is the Earth's magnetic field.
Do I have a magnetic car? I hope not. My other car shows the same symptom. Maybe I am magnetic. Or maybe my compass is just too sensitive. Hey, I got it: Take a compass that doesn't work anyway, and it won't be affected. That must be the answer. :wink:

Originally Posted by M

Radio waves are electromagnetic, and so is the Earth's magnetic field.

Could you tell me what the frequency is then, since frequency is a component of all electromagnetic waves.

Originally Posted by M

Do I have a magnetic car? I hope not.

Yes you do. It may only have a very slight magnetic field but it it there.

Originally Posted by M

My other car shows the same symptom. Maybe I am magnetic.

Yep, you have traces of iron in you and so again, you will have a slight magnetic field!

Originally Posted by M

Or maybe my compass is just too sensitive. Hey, I got it: Take a compass that doesn't work anyway, and it won't be affected. That must be the answer. :wink:

Well I can't force you to accept what I say..

You really should get that car checked out. A car doesn't block magnetic waves; in fact they penetrate through a lot of places. Which means your car is emitting a magnetic field, most likely higher than it ever should.

By the way that cumpass was in the center of the car, and it was accurate. It was only about 5-10% flawed due to it's crappy design, though.

Could you tell me what the frequency is then, since frequency is a component of all electromagnetic waves.

You probably mean "wavelength", since we're talking about a static field, not dynamic, right? Well, I would say the dominant wavelength of the Earth's magnetic field should be in the order of the Earth's diameter. Does that help?

Have you guys ever heard of magnetic shielding? It's an issue on airplanes as well as on ships, why not in cars? well, of course, navigation on ships is more a precision task than in a car.

It's possible that the effect on the compass is as much caused by induction from nearby metal as from shielding, but that doesn't change my point. A compass in a car is inevitably surrounded by a mass of metal, unless your car is made of plastic or composite materials. I figure it makes sense to sell those crappy compasses for cars, after all. Since they can't work with much precision anyway. But that still renders it useless by definition, which is kind of comical.

No I mean frequency since C/wavelength = Freq. If you reckon the diameter of the earth as 6800KM as a wavelength that would mean a frequency of 48Hz - The earth's magnetic core does not give out electromagnetic waves.

Megabrain,

the Earth's magnetic field is a static electromagnetic field. With which part of this do you disagree, or what is your trouble with this?

http://en.wikipedia.org/wiki/Magnetic_field

Originally Posted by M

Radio waves are electromagnetic, and so is the Earth's magnetic field.

My point is merely that this statement I reproduce implies the earths magnetic field somehow has the same properties as a radio wave.
IT does not. It has niether wavelength nor frequency.

Whatever the underlying mechanism... induction through metal, induction through electrical circuits, magnetic shielding... a conventional compass in a car is inherently imprecise, unless you're driving a Corvette with a glas fiber body. That was my point.

Originally Posted by M

Whatever the underlying mechanism... induction through metal, induction through electrical circuits, magnetic shielding... a conventional compass in a car is inherently imprecise, unless you're driving a Corvette with a glas fiber body. That was my point.

It is only imprecise if the car is magnetised. If the car is not magnetised the earth's lines of magnetic flux will flow through it and a compass will still work. This is the same reason why ship's compasses work. Ship's are de-gaussed from time to time to minimise stray fileds. You may well know that if you point a nail along the earth's flux lines and subject it to shock the nail will become magnetised. In a car the same thing happens due to vibration, it is particularly noticeable in engines where, parts made of iron may be heated and subjected to shock. Incidentally radio waves are low energy photons. 8)

I haven't heard anyone mention anything about the speakers in a car that have fairly large magnets in the back of them, and in modern cars are in every door, on the dashboard, and even in the seats, you would think that would have an effect. I heard somewhere that the military procedure for navigation is to get 30 meters away from the vehicle before taking a compass reading. Not sure if thats true or not.

I did a magnetometer survey over the summer as part of my degree, we set up a cheap (£4,000) base magnetometer on the first day to monitor diurnal variation. Unfortunately when we came back from surveying we found the base data was full of spikes presumably caused by cars passing by in the road which was about 20m away. We moved the base further from the road and got clean data; so either there was a big magnetic storm or cars interfered (which was more likely by looking at occurrence of spikes against time of day). Cars do interfere with the magnetic field by 100s to 1000s (or more) of nanoTeslas, and that's just when you're outside (okay the sensitivity is ridiculously high compared to the background field at high latitudes - so your compass should still work i suppose). Don't know anything about Faraday cages but am intrigued and will look into them :wink:

I haven't heard anyone mention anything about the speakers in a car that have fairly large magnets in the back of them

That's a good point. In that case you'd actually be worried about ruining the compass.

Originally Posted by billiards

I did a magnetometer survey over the summer as part of my degree, we set up a cheap (£4,000) base magnetometer on the first day to monitor diurnal variation. Unfortunately when we came back from surveying we found the base data was full of spikes presumably caused by cars passing by in the road which was about 20m away. We moved the base further from the road and got clean data; so either there was a big magnetic storm or cars interfered (which was more likely by looking at occurrence of spikes against time of day). Cars do interfere with the magnetic field by 100s to 1000s (or more) of nanoTeslas, and that's just when you're outside (okay the sensitivity is ridiculously high compared to the background field at high latitudes - so your compass should still work i suppose). Don't know anything about Faraday cages but am intrigued and will look into them :wink:

Well a true faraday cage is used to screen electromagnetic radiation, radio waves, it can be made from any conducting material, the door in your microwave is part of a farady cage to keep the EM inside, bouncing around till it hits the food where the low frequency photons interact with water molecules to heat them up. they are nothing to do with magnetic screening.

A Faraday cage eliminates any electrostatic field. It does not have to be dynamic. Any standing electric wave will be canceled inside a Faraday cage. Now, that may not apply to static magnetic fields even though they are subsets of the electrmagnetic fields. It may have to be electric, or in case of magnetism, dynamic (a moving magnetic field is equivalent to an electric field). A car moving through the static magnetic field would see a dynamic magnetic field (relative to the car), and there may be a Faraday effect but it's probably small, so don't get hung up on the Faraday comment. We have come up with plenty of reasons why a compass is unreliable inside a car.

I have driven some cars that are actually standard equipped with a compass, showing a digital display somewhere near the rear mirror, but I am not sure if that's a conventional compass and where it's located. Also, these displays only show S, SW, W, NW... and so on, which implies an uncertainty of 45 degrees! I guess that's all you can expect.