We're starting a brand-new chapter today — Chapter 9, Terrestrial Magnetism — Page 106, Lesson 122

We're starting a brand-new chapter today — Chapter 9, Terrestrial Magnetism. This is the foundation for everything you'll later learn about the compass, which is one of the most basic yet critical instruments in the cockpit. So let's build this from the ground up. First, we begin with the magnet itself. A magnet is a piece of material that produces a magnetic field — that's the region around it where its influence can be felt. Every magnet has two poles: a north pole and a south pole. Now, here's a convention you'll see throughout aviation: we call them the red pole and the blue pole. The red pole is the north-seeking pole, and the blue pole is the south-seeking pole. Don't get confused by the colours — they're just a standard way of marking them. Then we have the fundamental rules of attraction and repulsion. Like poles repel — north repels north, south repels south. Unlike poles attract — north attracts south, and south attracts north. This single rule governs how magnets behave, and it's the basis for how a compass needle aligns itself. Next, we look at how magnets are made and unmade. There are methods of magnetization — ways to turn a piece of material into a magnet — and methods of demagnetization — ways to remove that magnetism. I'll go into the specific techniques shortly. Now, an important distinction: magnetic and non-magnetic materials. Some materials can be magnetized, and some cannot. Among those that can, we split them into two categories: hard iron and soft iron. Hard iron retains its magnetism once it's been magnetized — it holds onto it. Soft iron loses its magnetism easily — it can be magnetized temporarily, but it doesn't keep it. This distinction is absolutely crucial for compass work, because the aircraft itself is made of materials that become magnetized, and they affect the compass reading. Then we move to the Earth itself. Terrestrial magnetism is simply the Earth acting as a giant magnet. The Earth has a magnetic field, and it behaves as if there's a huge bar magnet inside it. But here's the key point: the Earth's magnetic poles do not coincide with the geographic poles — the ones we use for navigation. That difference gives us magnetic variation, which is the angular difference between true north and magnetic north. This is something you'll correct for constantly in navigation. Related to that is magnetic dip. At the magnetic poles, the Earth's magnetic field lines point straight down into the ground. At the magnetic equator, they're horizontal. Everywhere in between, the field lines dip at an angle. That angle is the magnetic dip, and it affects how a compass needle behaves — it wants to tilt downward, which is why compasses are weighted and balanced to compensate. Then we have field strength — the intensity of the Earth's magnetic field at any given location. And from that, we get directive force. This is the horizontal component of the Earth's magnetic field that actually acts on a compass needle to align it with magnetic north. The stronger the directive force, the more strongly the compass points north. Near the poles, the directive force is weak, and the compass becomes sluggish and unreliable. Finally, we have changes in the Earth's magnetism. Some are regular and predictable — these are the regular changes in Earth magnetism. Others are unpredictable — sudden, irregular disturbances. Both affect the compass, and you need to know which ones you can compensate for and which ones you simply have to be aware of. Let me pause here and go deeper into the magnetization and demagnetization methods, because those are practical and you'll see them referenced in maintenance and in how the compass is handled. Let's look at the figures I have for this section. — that shows the basic magnet and its field. and — those illustrate the magnetization methods. For magnetization, the standard methods include stroking — rubbing a magnet along the material in one direction repeatedly — and induction, where you place the material in a strong magnetic field. There's also the electrical method, where you pass a current through a coil wrapped around the material. Each of these aligns the magnetic domains within the material, turning it into a magnet. For demagnetization, the opposite happens. You want to scramble those domains. The common method is to subject the material to an alternating magnetic field and gradually reduce its strength — this is often done with a coil carrying alternating current. You can also heat the material above a certain temperature, or hammer it, which disrupts the alignment. The key is that demagnetization removes the magnetism, returning the material to a non-magnetic state. Now, the hard iron and soft iron distinction ties directly into this. Hard iron, once magnetized, is difficult to demagnetize — it stays magnetized. Soft iron is easy to magnetize but also easy to demagnetize — it doesn't hold its magnetism. In an aircraft, the structure contains both types, and they create what we call deviation — a compass error caused by the aircraft's own magnetism. That's a later topic, but the foundation is right here. Let me also clarify the Earth's magnetic field more precisely. The Earth behaves as if there's a bar magnet at its centre, but the axis of this magnet is tilted relative to the Earth's rotation axis. That's why the magnetic poles are offset from the geographic poles. Magnetic variation is the angle between true north — the geographic pole — and magnetic north — where the compass points. This varies from place to place and changes over time, which is why navigation charts show variation values and their annual rate of change. Magnetic dip, as I said, is the angle the field lines make with the horizontal. At the magnetic equator, dip is zero. At the magnetic poles, dip is 90 degrees — the field is vertical. This dip is what causes a compass needle to tilt, and it's why compasses have a dip-correction weight to keep the needle horizontal. Field strength is the total intensity of the Earth's magnetic field at a location. It's not uniform — it's stronger near the poles and weaker near the equator. The directive force is the horizontal component of that field strength. It's this horizontal component that acts on the compass needle. Where the directive force is strong, the compass responds quickly and accurately. Where it's weak — near the poles — the compass is sluggish and unreliable. Finally, the changes. Regular changes in Earth magnetism are predictable — they follow cycles, like daily variations and seasonal variations. These are small and can be accounted for. Unpredictable changes are things like magnetic storms — sudden, irregular disturbances caused by solar activity. These can cause significant compass errors and are not predictable in advance. As a pilot, you need to know that these exist and that they can affect your compass. That's the full sweep of this chapter's content. We've covered the magnet, its poles, the attraction and repulsion rules, magnetization and demagnetization, magnetic versus non-magnetic materials, hard and soft iron, terrestrial magnetism, variation, dip, field strength, directive force, and the regular and unpredictable changes. Each of these builds toward your understanding of the compass, which we'll tackle next.