Remote Indicating Magnetic Compass — Page 220, Lesson 257

We're starting a new topic now: the remote indicating magnetic compass. This is the system that gives the pilot a heading readout without relying on a simple magnetic needle swinging in a bowl of liquid. Let me walk you through the heart of it. The key component is the flux valve — sometimes called a magnetic detector unit. It's positioned in a part of the aircraft least affected by on-board electrical interference, often out on the wing. The flux valve has three legs, and each leg is wound with a primary and a secondary winding. The primary winding is excited with AC, and the secondary winding is what we read the signal from. Now, the physics. The magnetic flux — and by flux I mean a measure of the density of the lines of force — flows through the legs. In legs A and B, the flux is at the same frequency and amplitude, 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 those two components together, the resultant flux is zero. And because there's no changing flux, no current is induced in the pick-off coil. That's the null condition. But the earth's magnetic field is always present as a background. If that background flux is added, the positive and negative flux no longer start from a zero baseline — they start from a shifted baseline. That's the effect of the earth's background magnetism. Here's where the material properties come in. The metal used in the flux valve legs has a physical characteristic: it magnetically saturates at a certain level. Beyond that point, the metal will not magnetize further — the saturation curve tops out at a limiting saturation level. So when we add the flux levels together, the total flux follows a path that's no longer a simple sine wave; it's distorted by that saturation limit. This operation is governed by Faraday's Law of Electromagnetic Induction. Let me give you the exact statement: 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 secondary winding — the one coloured red in the diagram — picks up the change in magnetic flux density, the dips in the green line, as an EMF. That EMF is detected as an AC signal. Now, the direction sensitivity. If the flux valve leg is in line with the earth's field, the induced EMF is at its maximum value. If the leg is at right angles to the field, the induced EMF is zero. And in between, the EMF varies as the cosine of the magnetic direction of the leg. Here's the problem with a single leg: the cosine function is not one-to-one. Except for 0° and 180°, there are two possible heading values for each voltage value. And worse, any slight change in input voltage would give an altered output voltage, resulting in a different measured heading. So a single leg can't give you an unambiguous heading. That's why the system uses the three-leg arrangement. The output from each leg is fed to one of the three legs of a stator. This re-creates the earth's field relative to the direction of the flux valve, around the direct drive shaft from the gyro to the heading indicator compass card. So the three legs together resolve the ambiguity — the stator reconstructs the earth's field vector, and that gives you a unique heading. Let me show you the geometry of the flux valve legs and how the stator connects. And here's the flux field relationship at A and B with the resultant. So the whole idea: the flux valve senses the earth's field direction, the saturation creates a detectable signal, Faraday's law turns the changing flux into an EMF, and the three-leg stator resolves the cosine ambiguity to give a clean heading to the compass card.