Terrestrial Magnetism — Page 106, Lesson 124

Let's begin our study of terrestrial magnetism. I want to start with the fundamental object—the magnet itself. For thousands of years, people have observed that an oxide of iron called magnetite attracts small pieces of iron. That property is known as magnetism. But magnetite had a second, even more remarkable property: it was north-seeking. If you mounted a piece of magnetite 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 materials, meaning iron and steel—could be given these same properties. Bars of such magnetized material are called magnets. Now, what is a magnetic field? The field of a magnet is the space around it in which its magnetic influence is felt. You can visualize this by placing a piece of card over a bar magnet and scattering iron filings on it. When you tap the card, the filings line up along the field pattern, tracing out what we call lines of force. Those lines converge toward small areas near the ends of the magnet. Those two areas are the poles of the magnet—the places where the properties of magnetism are most strongly displayed. Every magnet, whatever its shape, always has two poles. A unit pole cannot exist. If you cut a magnet into two pieces, each piece will have two poles of its own. Now, let's talk about the red and blue 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 the north-seeking pole, also called 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. For convenience, we often divide the magnet into two halves—one half containing the red pole, the other containing the blue pole. Finally, the attraction and repulsion rules. If you place two bar magnets end to end so that the blue pole of one faces the blue pole of the other, you feel a repulsion. Turn both magnets around so red faces red, and 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: like poles repel each other; unlike poles attract each other. That's the foundation. Next, we'll look at how the earth itself behaves as a giant magnet.