First, the term you need to own is ferromagnetic — Page 106, Lesson 128

Let’s pick this up right where the material takes us: the distinction between ferromagnetic and non-magnetic materials, and why that distinction matters for your compass. First, the term you need to own is ferromagnetic. The excerpt defines it for us: these are metals like iron and steel — and note, steel is simply iron alloyed with other substances such as carbon, cobalt, nickel, chromium, and tungsten. So when you see "steel," think of it as iron that has been deliberately mixed with those elements to change its properties. Each of those alloying elements — carbon, cobalt, nickel, chromium, tungsten — is named in the source, so you should recognize them as the things that get added to iron to make steel. Now, why does this matter in an aircraft? Because ferromagnetic materials can become magnetized. That is the key property. If a piece of iron or steel in the aircraft becomes magnetized, it will produce a magnetic field of its own, and that field will cause deviation in the aircraft's compasses. Deviation, in this context, is the compass error caused by local magnetic fields from the aircraft itself — as opposed to variation, which is the difference between magnetic north and true north caused by the Earth's field. The excerpt is specifically about the local, aircraft-generated error: ferromagnetic parts get magnetized, and that pulls the compass needle away from where it should point. So the danger is clear: any ferromagnetic material in the airframe or equipment can become a little magnet and distort your compass reading. Now the contrast. The excerpt tells us that many materials used in aircraft construction are non-magnetic and therefore do not affect the compass. These are called non-ferrous substances. The source lists them explicitly: aluminium, duralumin, brass, copper, plastic, and paint. Let me define each so you have them straight: - Aluminium — a lightweight metal, widely used in airframes. - Duralumin — this is an aluminium alloy, typically aluminium mixed with copper and other elements to make it stronger. It's still non-magnetic. - Brass — an alloy of copper and zinc. - Copper — a pure metal, good conductor, non-magnetic. - Plastic — a polymer, obviously non-metallic. - Paint — a coating, also non-magnetic. The point is that these materials do not become magnetized, so they do not introduce deviation. That's why aircraft designers can use them freely around the compass without worrying about distortion. So the takeaway, stated precisely: ferromagnetic materials — iron and steel, with steel being iron alloyed with carbon, cobalt, nickel, chromium, or tungsten — can be magnetized and cause compass deviation. Non-ferrous materials — aluminium, duralumin, brass, copper, plastic, paint — are non-magnetic and do not affect the compass. That's the whole relationship here: it's a contrast between two classes of material, defined by whether they can be magnetized, and the consequence of that property for your compass.