Terrestrial Magnetism — Page 115, Lesson 129

Let’s start with the material itself, because everything else in this chapter hangs off it. I’m talking about hard iron and soft iron. Ferromagnetic material — that’s material that can be magnetized — is broadly divided into two classes: hard iron and soft iron. Now, the words “hard” and “soft” here do not refer to the physical properties of the material, like how tough or bendy it is. They refer to the magnetic characteristics. That’s a distinction you must keep straight from the start. Hard iron requires a strong magnetizing field to produce magnetic saturation. Magnetic saturation is the point where the material can’t take on any more magnetism — it’s as magnetized as it can get. Hard iron needs a strong field to reach that point. And hard iron magnetism is said to be “permanent.” That means the material — typically steel containing cobalt or chromium — remains magnetized for an indefinite period after it has been removed from the magnetizing field. So once you magnetize it, it stays magnetized. That’s why such a substance is suitable for permanent magnets. Soft iron magnetism, on the other hand, is called “temporary,” or “transient,” or “induced.” The substance is easy to saturate magnetically with only a weak magnetizing field, but it retains little or no magnetism when the field is removed. Nearly pure iron behaves this way. So soft iron picks up magnetism easily and loses it easily. Now, between those two extremes, some materials exhibit magnetic characteristics that lie somewhere in between. These substances can be magnetized, but this “sub-permanent” magnetism is lost partly or wholly over a period of time. So it’s not fully permanent like hard iron, and not fully temporary like soft iron — it fades, either partially or completely, with time. Let me give you the comparison table, because it lays it out cleanly. For hard iron, the typical metals are cobalt and tungsten steel. The ease of magnetism is “hard” — meaning difficult, it takes a strong field. And the retention of magnetism is considerable, over a considerable length of time. For soft iron, the typical metals are silicon iron and pure iron. The ease of magnetism is “easy” — a weak field will do it. And the retention of magnetism is practically nil. So the key contrast: hard iron is hard to magnetize but holds it; soft iron is easy to magnetize but doesn’t hold it. Now let’s move up to the big picture — terrestrial magnetism itself. The earth behaves as though a huge permanent magnet were situated near the centre, producing a magnetic field over the surface. That’s the model we work with. Now, here’s the crucial point. Figure 9.6 shows that the poles of this hypothetical earth-magnet do not lie on the earth’s spin axis. This lack of symmetry is what gives rise to magnetic variation. And the magnetic poles are not stationary — they are currently moving at between 6 and 25 nautical miles per year. The north magnetic pole is moving faster than the south magnetic pole. As of 2015, the north magnetic pole is located north of Alaska at 86°N 153°W, and the south magnetic pole is south of Australia at 64°S 136°E. So those are the current positions — worth knowing, and worth knowing they drift. Now, magnetic variation. The direction of the earth’s field at any given point can be indicated by a freely-suspended magnet. Such a magnet will align itself roughly in a north-south direction, with its red pole towards the north magnetic pole. The longitudinal axis of that magnet defines the direction of the magnet meridian at that point. So let me define that term precisely: the magnetic meridian is the direction of the horizontal component of the earth’s field at a point on the earth’s surface. Note the word horizontal — we’re only talking about the horizontal component of the field here. Then we define variation. The angle, measured in the horizontal plane, between the magnetic meridian at a point and the true meridian at that same point, is known as the magnetic variation. So it’s the angular difference, in the horizontal plane, between where the compass points and where true north is. And variation is designated west or east, depending on whether the magnetic pole lies to the west or to the east of true north. So if the magnetic pole is west of true north, you have westerly variation; if it’s east, you have easterly variation. That’s the core of terrestrial magnetism for this chapter. Hard iron versus soft iron — how they magnetize and how they hold magnetism. Then the earth as a giant magnet with poles off the spin axis, drifting at 6 to 25 nautical miles per year. And finally variation — the horizontal angle between magnetic meridian and true meridian, designated east or west depending on which side of true north the magnetic pole sits.