The Direct Indicating Compass — Page 121, Lesson 141

We’re now into the direct indicating compass, and I want to pick this up at the point where we’re looking at how the magnet system behaves in equilibrium. So, picture the compass magnet suspended in its liquid. When the aircraft tilts, the magnet tilts with it, and we get couples—turning forces—acting on it. For equilibrium, the magnet takes up the amount of tilt necessary to make those couples balance. There’s a third, very weak couple produced by the horizontal component of the earth’s field, which we call H, and that opposes the tilt. For simplicity, we’ve omitted that one from the analysis. So the balance we’re talking about is between the tilt-producing couples and the restoring effect, and the magnet settles at the angle where they cancel. Now, let’s talk about sensitivity. The magnet system is required to seek the horizontal component H of the earth’s field in all areas except near the magnetic poles, where the horizontal component is inadequate. That’s a key limitation—near the poles, H is too weak for the compass to work properly. From the notes on magnetism, the ability of a pivoted magnet to align itself with an external field—that’s its sensitivity—depends on two things: the strength of the external field and the magnetic moment of the magnet. Now, the weak external field H at a given place cannot be changed. But the magnetic moment of the magnet can be increased, and we do that by increasing the magnet’s length and/or its pole strength. However, it’s undesirable to increase the magnet length. So instead, we increase pole strength by using two, four, or six short magnets, or a circular magnet, made of an alloy which will accept and retain the high degree of magnetism required. So the design choice is multiple short magnets rather than one long one. Sensitivity is further increased by reducing friction, and this is achieved in three ways. First, by using an iridium-tipped pivot in a jewelled cup. Second, by lubricating the pivot with the liquid which fills the compass bowl. And third, by reducing the effective weight of the magnet assembly acting down through the pivot, because the liquid that the magnet assembly is displacing is denser than air. That buoyancy effect lightens the load on the pivot. Now, aperiodicity. The magnetic assembly is required to be aperiodic, or ‘dead beat’. That means it should settle down quickly on a steady indication after being displaced by turbulence or manoeuvres. Any tendency to oscillate must be quickly ‘damped out’. So we want it to come to rest promptly, not swing back and forth. The desired aperiodicity is achieved as follows. Several short magnets are used instead of one longer one. This keeps the mass of the assembly near the centre, so reducing the moment of inertia and consequently making any oscillations easier to damp out. Light alloy is utilized wherever possible in order to minimize the weight of the assembly framework. The primary purpose of the liquid in the compass bowl is to act as a damping liquid on the compass assembly. So the liquid isn’t just for lubrication—its main job is damping. The grid ring compass dampens oscillations more rapidly than the vertical card compass, due to the addition of damping wires. These wires are attached to the magnet assembly and also pass through the damping liquid. So the wires increase the drag through the liquid, which speeds up the damping. So to summarise the key points: sensitivity is about the magnet’s ability to align with H, improved by increasing pole strength and reducing friction. Aperiodicity is about settling quickly, achieved by keeping mass near the centre, using light alloy, and using the liquid as a damping medium, with extra damping wires in the grid ring compass.