Terrestrial Magnetism — Page 106, Lesson 126

Let’s start with the very idea of making a magnet, because everything in aircraft compass work hangs off this. I want to walk you through the four methods of magnetization first, then the three ways we get rid of magnetism, and finally what makes a material magnetic at all. First, the simplest method: stroking. Take an unmagnetized bar of iron and stroke it repeatedly in the same direction with one end of a magnet. The key detail here is the polarity rule — the end of the bar that is last touched by the red end of the magnet is left as a blue pole. So if you finish your stroke with the magnet’s red end at the right-hand end of the bar, that right-hand end becomes blue. Figure 9.3 shows the process and the resulting polarity of the iron bar. Second method: aligning the bar with the lines of force of a magnetic field and subjecting it to vibration or hammering. This agitation during manufacture, in the earth’s magnetic field, is the main cause of aircraft magnetism. Now, the polarity that results is such that there is continuity in the pattern of lines of force — the lines are directed into a blue pole and out from a red pole, as usual. Here’s the aircraft analogy that matters for you: an aircraft being manufactured on a northerly heading in the earth’s field will acquire a permanent red pole in the nose and a blue pole in the tail. That’s why aircraft pick up magnetism just from being built. Figure 9.4 shows the polarity of the induced magnetism. Third method: for iron, simply subjecting it to a magnetic field. The induced polarity is again shown in Figure 9.4. Fourth method, and the most satisfactory one: placing the specimen inside a solenoid — that’s a cylindrical coil of wire — carrying a direct current. The current flowing in the coil produces a concentrated magnetic field along the axis of the coil, so a high degree of magnetism can be induced in the iron. Figure 9.5 shows the polarity of the magnetism induced in the bar inside the solenoid. Two important notes here. First, the amount of magnetism that can be induced is not unlimited — at a certain level the iron becomes magnetically ‘saturated’, meaning it simply cannot take any more. Second, if you reversed the direction of the current flow, the induced magnetic polarity would reverse as well. Now, demagnetization — removing most or all of the magnetism from a magnetized item. There are three ways. Shock. Place a magnetized bar of iron at right angles to the earth’s magnetic field and hammer it. The physical jarring disrupts the alignment of the magnetic domains. Heat. Heat the specimen to about 900°C and it loses its magnetism — and crucially, that magnetism does not return as the specimen cools. So heating past that temperature permanently demagnetizes it. Electric current. This is the most controlled method. Place the component inside a solenoid carrying alternating current, and gradually reduce the amplitude of that current to zero. The strong alternating magnetic field keeps reversing the direction of magnetization — that is, the polarity — in the specimen. Not only is the polarity being reversed, but the intensity of magnetization is being reduced as the current is reduced. The result is that the specimen’s magnetism is very quickly reduced to zero, or very nearly zero. Finally, magnetic and non-magnetic materials. Magnetic materials are ‘ferrous’ metals — iron and steel. Steel is iron alloyed with substances such as carbon, cobalt, nickel, chromium, and tungsten. These metals are called… and that’s where the passage cuts off, but the point is that these ferrous metals are the ones that can be magnetized, and they’re the ones that matter for aircraft construction and for compass installation. So the whole picture: magnetization can be done by stroking, by vibration or hammering in a field, by simple exposure to a field, or most effectively by a direct-current solenoid — with saturation as the limit. Demagnetization can be done by shock, by heating to about 900°C, or by an alternating-current solenoid with decaying amplitude. And the materials that respond to all of this are the ferrous metals — iron and steel, with steel being the alloyed form.