Earth Magnetism — Page 56, Lesson 63

Let's pick up with the Earth's magnetic field and look at something called the Angle of Dip. Picture the lines of magnetic force curving around the Earth's surface. If you imagine an aircraft at three different positions — call them A, B, and C — the lines of force will meet the horizontal at a different angle at each spot. That angle, between the total line of force and the horizontal, is the Angle of Dip. Now, the Earth's total magnetic field runs along that total line of force, which we label T. We can break T down into two components: a horizontal component H and a vertical component Z. Think of it as resolving one vector into two perpendicular parts. Let's talk about the vertical component Z first. It's of no use for finding horizontal direction — in fact, it's undesirable, for two reasons. First, it pulls the needle of a direct-reading magnetic compass down from the horizontal, making it dip. We partially correct for that with pendulous suspension, but the needle still hangs down somewhat. That means the centre of gravity is no longer directly below the centre of suspension, and that's what leads to the well-known turning and acceleration errors. Second, the vertical component induces vertical soft-iron magnetism in the aircraft itself, which increases the deviation — the error caused by the aircraft's own magnetic material. Now the horizontal component H — this is the useful one. It's the part the compass needle detects to find magnetic north, and we call it the directive force. At the magnetic equator, H approaches the full value of T, while Z approaches zero, and so does the angle of dip. As you move toward the poles, the directive force H decreases as the angle of dip increases — they trade off against each other. When you get near either magnetic pole, H approaches zero strength, and Z approaches the value of T. We measure magnetic field strength in microteslas — that's the unit, abbreviated µT. The generally accepted figure at which the horizontal component becomes too small for a compass to detect is 6 microteslas. In practice, the actual detection threshold depends on the design of the particular compass, but 6 µT is the notional figure normally quoted. And the maximum possible angle of dip is 90°, which occurs directly overhead the North and South Magnetic Poles. So the key relationship to hold onto: H is the directive force that steers the compass, Z is the troublesome vertical pull, and the angle of dip tells you how tilted the field is at your latitude.