The Direct Indicating Compass — Page 121, Lesson 140

Let’s start with the big picture. The direct reading magnetic compass is the simplest heading instrument you’ll ever fly with, and its whole job is to let a pivoted magnet align itself with the horizontal component of the earth’s magnetic field — and stay aligned. That’s the definition I want you to hold onto: the compass measures direction in the horizontal plane, so it only cares about the horizontal part of the earth’s field, not the total field. For that to work, the magnet system has to satisfy three requirements, and these are the three pillars of the whole design. The magnet system must be horizontal, it must be sensitive, and it must be aperiodic. Let me unpack each one, because they’re not just checkboxes — each one solves a specific physical problem. First, horizontality. To measure direction in the horizontal, the magnets have to lie as nearly as possible in the horizontal plane during normal straight and level flight. Now here’s the catch: if you just hung a magnet freely, it would align itself with the earth’s total magnetic field — not the horizontal component. And the total field is only horizontal at the magnetic equator. Everywhere else, the field dips down into the earth, so a free magnet would tilt, and the compass would be useless for measuring horizontal direction. The solution is what we call pendulous suspension. The magnet assembly is suspended like a pendulum — its centre of gravity is lower than its supporting pivot. That’s the key geometry, and it’s shown in Figure 10.2. Because the weight hangs below the pivot, the assembly resists tilting. Now let’s look at the physics of why that works. The earth’s vertical component of the field — we call it Z — tries to tilt the magnets. In the northern hemisphere, Z exerts a downward force on the red, north-seeking end of the compass magnet, and an upward force on the blue end. That creates a turning couple — a pair of equal and opposite forces that tries to rotate the magnet assembly. That’s the tilting effect we have to oppose. Opposing it is the weight of the assembly. The weight W acts downwards through the centre of gravity, which is displaced because of the tilt, and the reaction R acts upwards through the pivot. That’s the second turning couple. So you have two couples fighting each other: Z’s couple trying to tilt the magnets, and the weight-versus-reaction couple trying to hold them level. The equilibrium between these two is achieved at the cost of only a very slight residual tilt of the magnets — the north-seeking ends dip down by about 2° in mid-latitudes in the northern hemisphere. And in the southern hemisphere, it’s the south-seeking end that dips down instead. So the compass never sits perfectly horizontal, but that 2° residual tilt is a small price to pay for being able to measure direction at all. So that’s horizontality: pendulous suspension, centre of gravity below the pivot, Z’s couple opposed by the weight couple, with a small residual tilt as the compromise. Now, the other two requirements — sensitivity and aperiodicity — aren’t in this excerpt, so I’ll leave them for when we get to them. But I want you to remember the structure: horizontal, sensitive, aperiodic. We’ve just covered the first one in full. Take a look at Figure 10.2 to see the two turning couples drawn out — the Z couple on the magnet ends, and the W-versus-R couple through the centre of gravity and pivot. That diagram is the whole story of horizontality in one picture.