Terrestrial Magnetism — Page 106, Lesson 124

Let's start with the very foundation of this whole subject — the magnet itself. For thousands of years, people noticed that a certain oxide of iron, called magnetite, would attract small pieces of iron. That property is what we call magnetism. But magnetite had a second, even more remarkable property: it was north-seeking. If you mounted a piece of it on wood and floated it in water, it would swing around and align itself in a roughly north-south direction, acting as a primitive compass. Later, we discovered that some metallic elements and alloys — mainly ferrous ones, meaning iron and steel — could be given these same properties. A bar of such magnetized material is what we call a magnet. Now, let's talk about the magnetic field. The field of a magnet is the space around it in which its magnetic influence is felt. You can see this for yourself: place a piece of card over a bar magnet and scatter iron filings on it. When you shake or tap the card, the filings line up along the field pattern. That's Figure 9.1. Look at that figure and you'll notice something important: the lines of force traced by the iron filings converge towards small areas near the ends of the magnet. Those two areas are called the poles of the magnet, and they're where the properties of magnetism are most strongly displayed. Magnets come in various shapes, but every magnet always has two poles. And here's a key point: a unit pole cannot exist. If you cut a magnet into two pieces, each piece will have two poles. You can never isolate a single pole. Now, let's talk about how we name those poles. A freely suspended bar magnet — or a compass needle — in the earth's magnetic field will align itself roughly north-south. The end that points north is known as the north-seeking pole, or the red pole. The other end is the south-seeking pole, or the blue pole. By convention, magnetic lines of force are directed out from the red pole and back in to the blue pole. That's Figure 9.2. For convenience, we often divide the magnet into two halves — one half containing the red pole, the other containing the blue pole. That just makes it easier to talk about what happens when magnets interact. And that brings us to the attraction and repulsion rules. If you place two bar magnets in a line, end to end, so that the blue pole of one faces the blue pole of the other, you'll feel a repulsion. If you turn both magnets around so that red pole faces red pole, again the ends try to move apart. But if you place the blue pole of one magnet close to the red pole of the other, you feel an attraction. The rule is simple and absolute: like poles repel each other, unlike poles attract each other. That's the whole foundation of terrestrial magnetism — the magnet, its field, its poles, and the rules of attraction and repulsion. Everything else in this chapter builds on these basics.