Terrestrial Magnetism — Page 115, Lesson 131

Let’s pick this up right where the idea of variation left off, because the passage opens with a striking fact about it. Variation — the angular difference between true north and magnetic north — can take any value from zero right up to 180 degrees. That maximum of 180 degrees occurs when you are on the true meridian that links the north geographical pole with the north magnetic pole, and the same holds true in the southern hemisphere. So on that particular line, true north and magnetic north point in exactly opposite directions. Now I want to introduce you to a brand-new concept: magnetic dip. Imagine a magnet that is freely suspended, so it can swing in any direction. Except near the magnetic equator — where the lines of force run parallel to the earth’s surface — one end of that freely-suspended magnet will dip below the horizontal, and it will point toward the nearer magnetic pole. So if you are north of the magnetic equator, the magnet’s red pole — that’s the north-seeking pole — will be the one that is lower. If you are south of the magnetic equator, the blue pole, the south-seeking pole, will be lower. The angle, measured in the vertical plane, between the axis of the magnet and the horizontal, is what we call the angle of dip. Let me be precise about the magnetic equator, because it’s easy to confuse with the geographical equator. The magnetic equator is a line on a chart joining all the points on the earth where the angle of dip is zero. It fairly closely follows the geographical equator, staying within about 10 degrees of latitude of it in the main. So it’s not exactly the geographical equator, but it’s near it. Now, what happens as you move away from that magnetic equator? If you take your freely-suspended magnet and move it either north or south of the magnetic equator, the dip gradually increases. In the United Kingdom, for example, the dip reaches about 66 degrees. And over the earth’s magnetic poles themselves, the dip is 90 degrees — the magnet is then perfectly vertical, pointing straight down. Let me show you what that looks like with the resolution of the earth’s field. Now let’s talk about field strength. The total force T exerted at a point by the earth’s field acts in the direction taken up by a freely-suspended magnet influenced only by the earth’s field. So if you had a magnet with nothing else acting on it, it would align itself along the direction of T. Now, the total force T, together with the angle of dip and the magnetic variation at a point, are sometimes known as the magnetic elements for that place. Those three quantities together fully describe the earth’s magnetic field at a given location. It’s convenient to resolve this total force T into two components: a horizontal component, which we call H, and a vertical component, which we call Z. Figure 9.8 demonstrates this resolution — you can see T split into its horizontal and vertical parts. So to tie it all together: the earth’s magnetic field at any point is described by the total force T, the angle of dip, and the variation. And when we need to work with that field for navigation or compass work, we break T down into its horizontal component H and vertical component Z. The horizontal component is particularly important, because that’s the part that actually acts on a compass needle to make it point north.