The Direct Indicating Compass — Page 121, Lesson 141

Let’s pick this up right where the magnet system left off. We’ve already talked about how the compass magnet aligns with the earth’s field, and now we’re looking at the two big performance requirements: sensitivity and aperiodicity. First, sensitivity. The magnet system has to seek out the horizontal component of the earth’s field — that’s the part of the field that lies parallel to the earth’s surface, which we call H. This works everywhere except near the magnetic poles, where H is simply too weak to be useful. So the compass is designed to be sensitive enough to respond to that horizontal component. Now, from the magnetism notes, the ability of a pivoted magnet to align itself with an external field — that’s what we call its sensitivity — depends on two things: the strength of the external field, and the magnetic moment of the magnet. The magnetic moment is basically a measure of how strongly the magnet wants to align with a field. We can’t change the external field H at a given place, but we can increase the magnetic moment. And we do that by increasing the magnet’s length and/or its pole strength. But here’s the catch: it’s undesirable to make the magnet longer. So instead, we increase pole strength by using two, four, or six short magnets, or a circular magnet, all made of an alloy that can accept and retain a high degree of magnetism. That alloy choice matters — it has to hold the magnetism well. Sensitivity is also increased by reducing friction, and there are three ways we do that. First, we use an iridium-tipped pivot sitting in a jewelled cup — that’s a very hard, low-friction bearing. Second, we lubricate the pivot with the liquid that fills the compass bowl. And third, we reduce the effective weight of the magnet assembly pressing down through the pivot. That works because the liquid the assembly displaces is denser than air, so it provides buoyancy, effectively lightening the assembly. Now let’s move to aperiodicity. The magnetic assembly needs to be aperiodic, or what we call ‘dead beat’. That means after being displaced by turbulence or manoeuvres, it should settle down quickly onto a steady indication. Any tendency to oscillate — to swing back and forth — must be quickly damped out. We achieve that aperiodicity in a couple of ways. First, we use several short magnets instead of one long one. That keeps the mass of the assembly near the centre, which reduces the moment of inertia. A lower moment of inertia means oscillations are easier to damp out. We also use light alloy wherever possible to minimize the weight of the assembly framework. Second, the primary purpose of the liquid in the compass bowl is to act as a damping liquid on the compass assembly. And here’s an interesting contrast: the grid ring compass dampens oscillations more rapidly than the vertical card compass. That’s because the grid ring compass has additional damping wires. These wires are attached to the magnet assembly and pass through the damping liquid, so they create extra drag and settle the system faster. So to tie it together: sensitivity is about the magnet being able to find H and align with it, and we boost that with more pole strength and less friction. Aperiodicity is about settling quickly after disturbance, and we get that with low moment of inertia and liquid damping. Both are essential for a compass that gives you a stable, reliable heading.