Re: What will happen to a compass if you are at Magnetic North(Hudson's Bay)?

Hi Mark!

Here's how to think about this problem. We need to be aware of two important issues:

• Compasses try to align with the "lines" of magnetic field, and if those lines are tilted down into the ground, then the compass needle will try to tilt too.

• If the Earth was like a huge bar-magnet, then the ends of that magnet would be thousands of kilometers deep, down inside the Earth's iron core.

If I used a good, expensive compass, then the needle would be on a little axel and would not be able to tilt. Here in Seattle with this type of compass, the "twisting force" on the needle that is caused by the Earth's field is a little less than normal. The field is tilted downwards, so it doesn't have its full effect on the needle. If I went to the Earth's equator where the magnetic field is nearly parallel to the ground, then the alignment force would be greater. Or, if I stayed here in Seattle and tilted my compass down so the north end of the needle could move down like it wants to, then the alignment force would become greater.

If I held this compass flat (parallel to the ground), and then started walking to the north magnetic pole, the force on the compass needle would become weaker and weaker. Unless the compass needle had a frictionless bearing, it would soon stop moving at all, even when I hadn't reached the north pole. As long as I kept it held flat, the needle would follow the compass body, as if the needle was not even magnetized.

However, the Earth's field is still there up north, even if it is pointing vertically downwards. Therfor, if I should TILT my compass the least little bit, then the compass needle is going to go zow!, and spin around and point in the direction that I tilt the compass. If I tilt one edge slightly downwards, the north end of the needle will whip around and point in the direction of the downwards tilt. If I arrive at the north magnetic pole up in Canada (way north of Hudson Bay, actually,) then I can't tell which way is north anymore. The compass needle will point in any direction, depending on how I tilt the compass.

If instead I had been tilting my compass more and more as I walked north, so that the needle could point in the direction that it "wants", then at the North Magnetic Pole it would be pointing vertically downwards. The compass would become useless for navigation, since it tells me that north is down, and south is up! But which way is Russia, or Europe, or the USA?

There is another type of compass besides the normal, everyday type. These compasses are called "dip needles," and are positioned vertically. Their needles rotate downward, not sideways as with an everyday compass. Also, a Dip Needle must be manually aligned so that it's on a north/south line. The needle will tilt diagonally, depending on how close you are to the north magnetic pole. When you are right over the pole, the Dip Needle will point preceisly downwards.

In answer to your original question, here is an experiment you can do. If you have a good compass (the alcohol-filled kind, with the needle on a little axel), then try tilting it. In some positions the needle will align more strongly. In other positions the needle will start to wander, and will swoop around if you tilt it slight amounts. Guess which behavior is similar to a compass held flat at the magnetic north pole. The second one, obviously. If you hold your compass so that the needle spins around when you tilt it differently, then you are seeing what an Arctic Explorer would see. But if you live near the equator, then you are actually holding your compass vertically tilted, and aligned along an east/west line.

If you go and stand at the Geographic north pole, what will happen to your compass? Magnetic North is down towards Canada, and is not aligned with the Earth's axis of rotation. Therefor your compass would point south, towards the location of the North Magnetic Pole. At least it will TRY to. Don't forget, the magnetic field up there is almost vertical. A cheap compass will have bottomed out already. An expensive compass will be deflecting very weakly, and you will have to place it on a perfectly level surface, otherwise it will point in the direction in which you tilt it.

Is there a south magnetic pole? Yep. It is very similar to the North. It is not aligned with the South Geographic Pole, instead it is offshore of Antarctica, south of Australia.

If you check out an earth globe, you'll find that the South Magnetic Pole is in the wrong place! The pole in the north is about 15degrees from the Earth's axis, while the pole in the south is more like 25degrees away. And if you find the north magnetic pole and then look for the opposite position on the other side of the Earth, you would expect to find the south pole on the Antarctic continent, below India. Instead it is out in the ocean, below Australia. Which pole is "right"? Neither. The poles are thought to be caused by electric currents and the roiling motion of the liquid iron core of the earth. The motions of the core aren't aligned with the earth's spin. Also, besides the two strong north/south magnetic poles, there are four weaker poles in the shape of a tetrahedron, a triangular pyramid, and these weak poles distort the positions of the much stronger north/south poles. We're lucky that the Earth's magnetic poles are usable for navigation at all. If the physics was a little different, then the Earth's magnetic poles might wander crazily around from month to month, or there might be lots of weak north and south all over the place

Here are a couple of links to my Misconceptions page:

North is actually South!

Earth's magnetic "poles" are weak