How Compasses Work

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 8,000 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).

Now let's look at how you can create your own compass.

If you don't have a compass, you can create your own in much the same way people did hundreds of years ago. To create your own compass, you will need the following materials:

The first step is to turn the needle into a magnet. The easiest way to do this is with another magnet -- stroke the magnet along the needle 10 or 20 times as shown below.

If you are having trouble finding a magnet around the house, two possible sources include a can opener and an electromagnet that you make yourself (see How Electromagnets Work).

Place your float in the middle of your dish of water as shown below.

The "float on water" technique is an easy way to create a nearly frictionless bearing. Center your magnetic needle on the float. It very slowly will point toward north. You have created a compass!

A magnetic compass like the one created on the previous page has several problems when used on moving platforms like ships and airplanes. It must be level, and it tends to correct itself rather slowly when the platform turns. Because of this tendency, most ships and airplanes use gyroscopic compasses instead.

A spinning gyroscope, if supported in a gimbaled frame and spun up, will maintain the direction it is pointing toward even if the frame moves or rotates. In a gyrocompass, this tendency is used to emulate a magnetic compass. At the start of the trip, the axis of the gyrocompass is pointed toward north using a magnetic compass as a reference. A motor inside the gyrocompass keeps the gyroscope spinning, so the gyrocompass will continue pointing toward north and will adjust itself swiftly and accurately even if the boat is in rough seas or the plane hits turbulence. Periodically, the gyrocompass is checked against the magnetic compass to correct any error it might pick up.

For more information on compasses, navigation and related topics, check out the links on the next page.