The Direct Indicating Compass — Page 500, Lesson 499

Let’s pick up with the direct indicating compass, and I want to focus on what makes it work well and how we keep it serviceable. First, sensitivity. The compass magnet needs to be strong enough to respond to the Earth’s magnetic field. We can increase the magnetic moment of the magnet — that’s the strength of its magnetic effect — by increasing the magnet’s length and/or its pole strength. But we don’t want a long magnet, because that makes the assembly bulky and harder to damp. So instead, we increase pole strength by using two, four, or six short magnets, or a circular magnet, made of an alloy that will accept and retain the high degree of magnetism required. Sensitivity is further increased by reducing friction. There are three ways. First, an iridium-tipped pivot in a jewelled cup — iridium is a hard metal, and the jewel cup gives a low-friction bearing. Second, lubricating the pivot with the liquid that fills the compass bowl. Third, 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 — so the liquid provides buoyancy, lightening 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. How do we achieve that? Several short magnets are used instead of one longer one. This keeps the mass of the assembly near the centre, reducing the moment of inertia — that’s the resistance to rotational change — and consequently making any oscillations easier to damp out. Light alloy is used wherever possible 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. 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 liquid does double duty — it lubricates the pivot and it damps the oscillations. Now, serviceability checks. On the compass body, check that there is no obvious damage such as dents or cracks. Any lighting system should be checked, as should the efficacy of the luminous paint — that’s the paint that glows so you can read the compass at night. On the compass liquid, it should be checked and be free from sediment and discolouration — either of which would indicate corrosion, which would result in increased pivot friction. It should also be free from bubbles — which would probably indicate a leaking seal. Turbulence and manoeuvres would cause any bubbles to move about, creating eddies which could disturb the magnet system. Finally, the accuracy limit under CS-OPS1 is ±10°. That’s the tolerance within which the compass indication must be accurate for the operation to be legal. So, to tie it together: sensitivity comes from strong magnets and low friction; aperiodicity comes from short magnets, light alloy, and the damping liquid with damping wires; and serviceability is about checking the body and the liquid for damage, corrosion, and leaks. And the whole thing must hold ±10° accuracy.