The Direct Indicating Compass — Page 121, Lesson 140

Let’s start with the direct indicating compass. This is the classic magnetic compass you’ll see on the panel — a pivoted magnet that must align itself, and stay aligned, with the horizontal component of the earth’s magnetic field. Now, that phrase “horizontal component” is the key. The earth’s total magnetic field isn’t horizontal — it dips down into the ground. So the compass magnet only senses the part of that field that lies in the horizontal plane. For the compass to work properly, three requirements must be satisfied. The magnet system must be horizontal, sensitive, and aperiodic. Let’s take horizontality first, because that’s what this passage develops. Why does the magnet need to be horizontal? Because we measure direction in the horizontal plane. If the magnet were freely suspended, it would align with the earth’s total field — which means it would only be horizontal at the magnetic equator. Everywhere else, the field dips, and the magnet would tilt with it. That’s useless for measuring heading. So how do we force the magnet to stay horizontal? We suspend it pendulously. That means the centre of gravity of the magnet assembly is lower than its supporting pivot. Look at Figure 10.2 — you’ll see the pivot at the top, and the weight of the assembly hanging below it. Here’s the physics. The vertical component of the earth’s field — we call it Z — tries to tilt the magnet. In the northern hemisphere, Z pulls the red, north-seeking end of the magnet downward, and pushes the blue end upward. That’s one turning couple acting on the magnet. But the pendulous suspension creates an opposing couple. The weight W acts downward through the centre of gravity, which is displaced because of the tilt. And the reaction R acts upward through the pivot. These two forces — W down at the centre of gravity, R up at the pivot — form a couple that opposes the tilt from Z. So you have two turning couples in balance. The tilting effect of Z is opposed by the weight of the assembly. The result is equilibrium — but not perfect horizontality. There’s a small residual tilt. In mid-latitudes of the northern hemisphere, the north-seeking end sits down by about 2°. In the southern hemisphere, it’s the south-seeking end that dips down by that same small amount. So the magnet is never perfectly horizontal — but 2° of residual tilt is acceptable for measuring direction. That’s the horizontality requirement, and the pendulous suspension is how we achieve it. Now, the other two requirements — sensitivity and aperiodicity — are mentioned here as essential, but this passage develops horizontality in detail. Sensitivity means the magnet must respond readily to small changes in heading, and aperiodic means it must settle to its final position without oscillating back and forth. We’ll build on those as we go. For now, hold onto this: the direct indicating compass uses a pendulously suspended magnet, with its centre of gravity below the pivot, so that the vertical component Z of the earth’s field is opposed by the weight of the assembly, leaving only a slight residual tilt of about 2° in mid-latitudes.