Remote Indicating Magnetic Compass — Page 220, Lesson 257

Let’s pick up right where the flux valve’s legs are doing their work. I want to walk you through what happens inside those legs, because this is the heart of how the remote indicating magnetic compass senses the earth’s field. First, a key definition: magnetic flux is a measure of the density of the lines of force. Think of it as how tightly packed the magnetic field lines are in a given area. In Figure 17.6, we see the flux in legs A and B drawn as red and blue lines. These two fluxes are at the same frequency and the same amplitude — meaning they oscillate identically in strength — but they are in anti-phase. That means when one is at its positive peak, the other is at its negative peak. If you add component A and component B together, the resultant flux — the green line — is zero. And because there’s no changing flux, no current is induced in the pick-off coil. That’s the baseline condition. Now, here’s where the earth’s field comes in. If the earth’s magnetic field were present as a background, the positive and negative flux would start from a different baseline — not zero. That’s shown in Figure 17.7. So the earth’s field shifts the whole oscillation up or down. But there’s a physical limit. The metal used in the flux valve legs magnetically saturates at a certain level. That means the metal will not magnetize further beyond a certain point — it tops out. In this case, the saturation curve tops out at a limiting saturation level, which flattens the upper ends of the blue and red lines. When you add the flux levels together to get the resultant, the total flux follows the path of the green line — and that green line now has dips in it, changes in flux density. This all works according to Faraday’s Law of Electromagnetic Induction. Let me give you the exact statement, because it’s the governing principle: if the number of lines of force threading a circuit is changing, an induced electromotive force — an EMF — will be set up in the circuit, and the magnitude of that EMF is proportional to the rate of change in the number of lines of force threading the circuit. So the faster the flux changes, the bigger the induced voltage. That’s exactly what the secondary winding picks up. The secondary winding — the one coloured red in Figure 17.4 — will pick up the change in magnetic flux density, those dips in the green line, as an EMF. This is detected as an AC signal, because the flux is oscillating. Now, the direction matters. Figure 17.9 shows that if the flux valve leg is in line with the earth’s field, the EMF induced is at its maximum value. The secondary winding is aligned with the leg, so it sees the full effect. If the flux valve leg is at right angles to the field, the EMF induced is zero. So the EMF varies as the cosine of the magnetic direction of the flux valve leg. But here’s the problem — and this is why we can’t just use a single leg. The cosine relationship means that, except for 0° and 180°, there are two possible values of heading for each value of voltage. A single voltage reading is ambiguous. Worse, any slight change of input voltage would give an altered output voltage, resulting in a different measured heading. So a single leg is useless for heading. That’s why the 3-leg system is used. The output from each leg is fed to one of the 3 legs of a stator. This re-creates the earth’s field relative to the direction of the flux valve, as shown in Figure 17.10, around the direct drive shaft from the gyro to the heading indicator compass card. So the three legs together reconstruct the earth’s field in the instrument, and that field orientation relative to the aircraft gives you the heading. Let me make sure the picture is complete. The flux valve senses the earth’s field, the legs saturate to create those tell-tale dips, the secondary windings pick up the changing flux as an AC EMF, and the three legs feed a stator that re-creates the field around the gyro-driven compass card. That’s how a remote indicating magnetic compass turns the earth’s magnetism into a heading you can read.