The Norsemen, or "northmen", used primitive compasses to orient their ships on foggy nights. They used a lodestone or "leading stone" to guide their ships since they realized that lodestones always pointed north.

In 1269, Pierre de Maricourt mapped out directions taken by a needle when it was situated on different places on a spherical natural magnet. He concluded that every magnet, regardless of its shape, has two poles, one north and one south. These poles exert forces on each other (like poles repel and unlike poles attract). When we speak of a magnet, we say that it has a "north-seeking" pole and a "south-seeking" pole.

The earth, too, acts as a huge magnet. When a magnet is used as a compass, its north magnetic pole will be attracted to the south magnetic pole of the earth, and vice versa. In 1832, the north magnetic pole of the earth was found to be located just north of the Hudson's Bay. (People are often surprised to learn that the magnetic north is not in the same place as the geographic north, which is at the north pole!)

The source of the earth's magnetism is believed to be a result of movements in the earth's core. Charged ions or electrons circle around in the liquid interior of the earth, producing a magnetic field. Furthermore, there is evidence to indicate that the strength of a planet's field is related to the planet's rate of rotation. For example, Jupiter rotates faster than the earth, and recent space probes have indicated that Jupiter has a stronger magnetic field than the earth.

As an interesting side note, there is evidence to suggest that the earth's magnetic field has reversed direction several times in the last million years. One set of evidence is contained in a type of rock called basalt. Basalt is continually being spewed out of volcanoes in places like the mid-ocean ridge in the Atlantic Ocean. When the basalt cools, it retains a signature of the direction of the magnetic field at that time. From the patterns of the magnetic signatures in the basalt on the ocean floor, we can determine when the magnetic fields of the earth switched poles.

• yogurt container
• magnet
• cork piece
• thread
• pin or needle
• markers
• compass
• button

1. The night before, magnetize the needles by laying the "eye" end over a magnet. Leave overnight.
2. Force the end of the magnetized needle through a piece of cork so that it is balanced roughly in the middle.

Day 2:

1. Tie three pieces of string to a button.
2. Tape the strings to the outside of the yoghurt container to suspend the button in the middle of the container. See diagram at right:
3. Put the stick pin through the remaining hole in the button and stick it into the middle of the cork with the magnetized needle. This will ensure that the magnetized needle stays in the centre of the container.
4. Put enough water in the container to float the cork with the stick pin head above the button.
5. Using a compass, find out which end of the needle is pointing north.
6. Mark the directions on your home-made compass.

You know that the magnetized needle on your compass in pointing north-south. To find out which end is pointing north, stretch your arms out and align them north-south with the needle. If it is before noon, and the sun is in front of you, your left hand will be pointing north. Once you know which way is north, you can identify which end of the needle is pointing north. How does this work? Since the sun rises in the east, if you're facing the sun in the morning then you're facing east. If you think of the directions on a compass or a map, you'll remember that north is to the left of the east. This is why your left arm will be pointing north. Similarly, if it was afternoon and the sun was setting in the west, you would know that your right arm was facing north.

The second way to find out which end of your compass is pointing north involves using a "shadow stick". Find a place that will be in direct sunlight for several hours. Stick a pole, approximately 30 cm long, into the ground so that it is standing straight up. Find a small rock and place it at the end of the shadow of the pole. Leave it there for an hour, giving the shadow a chance to move. An hour later, go back and place a second small rock at the end of where the shadow is now resting. Now place your left foot on the first rock and your right foot on the second. You will be facing north. How does this method work? The thing that makes this procedure confusing is the fact that we automatically assume the shadow moves in a circle. The shadow, however, does not go in a circle, but rather changes length as the sun goes around the earth. The sun travels east to west, so the shadow will travel west to east. When you put your left foot on the first rock and your right foot on the second rock, you're putting your left foot to the east of your right foot. This means that you end up facing north.