Terrestrial Magnetism — Page 115, Lesson 129

Let's start with the very foundation of this chapter: the difference between hard iron and soft iron. The words "hard" and "soft" here have nothing to do with how physically tough the metal is. They describe magnetic behavior. Hard iron is hard to magnetize. It needs a strong magnetizing field to reach magnetic saturation — that's the point where the material can't hold any more magnetism. But once it's magnetized, it stays that way. We call that "permanent" magnetism. The material, typically steel containing cobalt or chromium, remains magnetized for an indefinite period even after it's removed from the magnetizing field. That's why hard iron is suitable for permanent magnets. Soft iron is the opposite. It's easy to saturate magnetically with only a weak magnetizing field, but it retains little or no magnetism when the field is removed. We call that "temporary," "transient," or "induced" magnetism. Nearly pure iron behaves this way. There's a third category too. Some materials sit between the two. They can be magnetized, but this "sub-permanent" magnetism is lost partly or wholly over a period of time. Let me give you the table that summarizes this. For hard iron, the metals are cobalt and tungsten steel. Ease of magnetism: hard. Retention of magnetism: considerable length of time. For soft iron, the metals are silicon iron and pure iron. Ease of magnetism: easy. Retention: practically nil. Now, the big picture. The earth behaves as though a huge permanent magnet were situated near its centre, producing a magnetic field over the surface. But here's the key point: the poles of this hypothetical earth-magnet do not lie on the earth's spin axis. That lack of symmetry is what gives rise to magnetic variation. And the magnetic poles aren't stationary. They're 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. Now let's talk about magnetic variation itself. 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. And here's the precise definition: the magnetic meridian is the direction of the horizontal component of the earth's field at a point on the earth's surface. So, magnetic variation is the angle, measured in the horizontal plane, between the magnetic meridian at a point and the true meridian at that same point. Variation is designated west or east, depending on whether the magnetic pole lies to the west or to the east of true north. Let me make sure that's clear. The true meridian points to geographic north — the spin axis. The magnetic meridian points to magnetic north — where the compass needle points. The angular difference between those two, measured in the horizontal plane, is the variation. If the magnetic pole is west of true north, we call it west variation. If it's east, east variation. That's the core of terrestrial magnetism as it applies to navigation. The earth is a giant magnet, its poles don't line up with the geographic poles, and that misalignment is what we correct for as magnetic variation.