How does compass needle work

One end of the needle is often marked "N," for north, or colored in some way to indicate that it points toward north. On the surface, that's all there is to a compass. The reason why a compass works is more interesting. It turns out that you can think of the Earth as having a gigantic bar magnet buried inside.

In order for the north end of the compass to point toward the North Pole , you have to assume that the buried bar magnet has its south end at the North Pole, as shown in the diagram at the right. If you think of the world this way, then you can see that the normal "opposites attract" rule of magnets would cause the north end of the compass needle to point toward the south end of the buried bar magnet.

So the compass points toward the North Pole. To be completely accurate, the bar magnet does not run exactly along the Earth's rotational axis. It is skewed slightly off center. This skew is called the declination , and most good maps indicate what the declination is in different areas since it changes a little depending on where you are on the planet. The magnetic field of the Earth is fairly weak on the surface.

After all, the planet Earth is almost 8, miles in diameter, so the magnetic field has to travel a long way to affect your compass. That is why a compass needs to have a lightweight magnet and a frictionless bearing. Otherwise, there just isn't enough strength in the Earth's magnetic field to turn the needle. The "big bar magnet buried in the core" analogy works to explain why the Earth has a magnetic field, but obviously that is not what is really happening.

So what is really happening? No one knows for sure, but there is a working theory currently making the rounds. As seen on the above, the Earth's core is thought to consist largely of molten iron red.

But at the very core, the pressure is so great that this superhot iron crystallizes into a solid. Convection caused by heat radiating from the core, along with the rotation of the Earth, causes the liquid iron to move in a rotational pattern.

It is believed that these rotational forces in the liquid iron layer lead to weak magnetic forces around the axis of spin. It turns out that because the Earth's magnetic field is so weak, a compass is nothing but a detector for very slight magnetic fields created by anything.

That is why we can use a compass to detect the small magnetic field produced by a wire carrying a current see How Electromagnets Work. This made it very difficult to travel to far or unknown destinations. Compasses were one of the key breakthroughs that made such voyages a reality. So how does a compass work? The Earth has an iron core that is part liquid and part solid crystal due to gravitational pressure.

Essentially a compass is a light weight magnet, generally a magnetized needle, on a free rotating pivot. This allows the needle to better react to nearby magnetic fields.

This is how navigators are able to discern north. You may have seen this demonstrated by a pair of simple bar magnets or refrigerator magnets pushed end to end. The Earth is a magnet that can interact with other magnets in this way, so the north end of a compass magnet is drawn to align with the Earth's magnetic field. Because the Earth's magnetic North Pole attracts the "north" ends of other magnets, it is technically the "South Pole" of our planet's magnetic field.

While a compass is a great tool for navigation, it doesn't always point exactly north. This is because the Earth's magnetic North Pole is not the same as "true north," or the Earth's geographic North Pole. The magnetic North Pole lies about 1, miles south of true north, in Canada.