We begin with the magnet itself — Page 106, Lesson 122

This is the start of Chapter 9, Terrestrial Magnetism. This is the foundation for everything you'll do with compasses and magnetic heading later, so let's build it properly from the ground up. We begin with the magnet itself. A magnet is simply a piece of material that produces a magnetic field. Now, what is a magnetic field? It's the region of space around the magnet where its magnetic influence can be detected — where it can exert a force on other magnetic materials. You can't see it, but it's there, and it's what makes a compass needle swing. Every magnet has two poles. These are the two ends where the magnetic field is strongest. We call them the north-seeking pole and the south-seeking pole. But in this book, we also refer to them as the red pole and the blue pole. Conventionally, the red pole is the north-seeking pole, and the blue pole is the south-seeking pole. So when you see "red" and "blue" in this chapter, think north and south. Now, the fundamental rule of magnetism — attraction and repulsion. Like poles repel, and unlike poles attract. So a red pole and a blue pole pull toward each other. But two red poles push apart, and two blue poles push apart. This is the single most important rule for understanding how a compass works. Next, how do we make a magnet? There are methods of magnetization. The basic idea is that you align the magnetic domains — the tiny internal magnetic regions — within a material so they all point the same way. And conversely, there are methods of demagnetization, which scramble those domains back into random alignment, destroying the net magnetic field. This leads us to materials. We classify materials as magnetic or non-magnetic. Magnetic materials are those that can be magnetized — they respond to a magnetic field. Non-magnetic materials don't respond. Within the magnetic materials, we have a critical distinction for aviation: hard iron and soft iron. Hard iron retains its magnetism once magnetized — it's like a permanent magnet. Soft iron, on the other hand, is easily magnetized but also easily demagnetized — it doesn't hold its magnetism when the external field is removed. This distinction is absolutely crucial for aircraft compass compensation, because the aircraft structure itself contains both types of iron, and they affect the compass differently. Now we get to the heart of the chapter — terrestrial magnetism. The Earth itself behaves like a giant magnet. It has a magnetic field that surrounds the planet, and this field is what a compass needle aligns with. But here's the catch: the Earth's magnetic poles do not coincide with the geographic poles — the ones defined by the axis of rotation. This offset creates what we call magnetic variation. Variation is the angular difference between true north — the geographic pole — and magnetic north — where the compass points. This is something you'll correct for constantly in navigation. Then there's magnetic dip. Because the Earth's magnetic field lines are not horizontal — they dip into the Earth — a freely suspended compass needle will tilt. At the magnetic equator, the dip is zero. As you move toward the magnetic poles, the dip increases, until at the poles the needle points straight down. This is a major problem for aircraft compasses, because the dip causes turning errors. We also have field strength — the intensity of the Earth's magnetic field at any location. And related to that, we have directive force. This is the horizontal component of the Earth's magnetic field that actually acts on the compass needle to align it with magnetic north. The stronger the directive force, the more strongly the compass seeks north. Near the poles, the directive force weakens, and the compass becomes sluggish and unreliable. Finally, we consider changes in the Earth's magnetism. There are regular changes — predictable, periodic variations in the field that occur over time. And there are unpredictable changes — sudden, irregular disturbances, like magnetic storms, that can't be forecast. Both affect your compass, but you can compensate for the regular ones and you just have to be aware of the unpredictable ones. Let me show you the geometry of this — the poles, the field lines, and how dip and variation relate to the Earth's surface. That's the structure of the whole chapter. We're going to go through each of these in detail, starting with the magnet and its field.