Terrestrial Magnetism — Page 106, Lesson 126

Let’s start with the very idea of making a magnet, because that’s the foundation of everything we’ll do with the compass. I want to walk you through the four methods of magnetization, then the three ways we demagnetize, and finally what makes a material magnetic at all. First, the simplest method: stroking. If you take an unmagnetized bar of iron and stroke it repeatedly in the same direction with one end of a magnet, you induce magnetism. Here’s the key detail — the end of the bar that was last touched by the red end of the magnet is left as a blue pole. So the last point of contact determines the polarity. Figure 9.3 shows this process and the resulting polarity of the 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. Think about it — an aircraft being built 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. The induced polarity is such that there’s continuity in the pattern of lines of force — they always point into a blue pole and out from a red pole. Figure 9.4 shows this. Third method: for iron, simply subjecting it to a magnetic field. The induced polarity is again shown in Figure 9.4. Fourth method: placing the specimen inside a solenoid — that’s a cylindrical coil of wire — carrying a direct current. This is the most satisfactory method, because 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. But note this limitation: the amount of magnetism is not unlimited. At a certain level, the iron becomes magnetically ‘saturated’ — it simply can’t hold any more. Figure 9.5 shows the polarity induced in the bar inside the solenoid. And if you reverse the current flow, the induced magnetic polarity reverses too. Now, demagnetization. There are three ways to remove most or all of the magnetism. First, 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. Second, heat. If you heat the specimen to about 900°C, it loses its magnetism, and importantly, it does not return as the specimen cools. That’s a permanent loss. Third, electric current. 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. So 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 are the metals we call magnetic. That’s the core of terrestrial magnetism as it applies to our instruments. Let’s look at the figures to see the polarity patterns clearly.