How does a compass work?

Last week, we explored magnets and how they work. This week we will explain how a compass works.

When you put a magnet near metal, an invisible force pulls the metal to the magnet. This invisible force is called magnetism. In many ways, Earth acts like it has a big bar magnet buried inside it.

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. 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. The bar magnet does not run exactly along the Earth’s axis, but is slightly off center. This skew is called declination.

A compass is really just a detector for very slight magnetic fields. Since the needle of a compass is magnetic, the invisible pull of Earth’s magnetic field causes the needle of a compass to point north. Once the compass helps you find north, you can navigate east, west, and south, too.

The magnetic field of the Earth is fairly weak on the surface, but considering the Earth is almost 8,000 miles in diameter, it is understandable. The magnetic field around the Earth has to travel a long way to affect your compass. That is why a compass must 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 analogy that there is a big bar magnet buried in the Earth helps to explain why the Earth has a magnetic field, but not really what is happening. Scientists do not know for sure, but they have a theory. The Earth’s core is thought to be made up of mostly molten (liquid) iron. At the inner core, the pressure inside is so large that the molten iron is “squeezed” into a solid. As the heat radiates out from the solid inner core and the Earth spins, it causes the liquid iron of the outer core to move in a rotational pattern. Scientists think that this rotational force in the liquid iron causes the magnetic forces around the Earth.