Terrestrial Magnetism — Page 115, Lesson 131

I want to walk you through terrestrial magnetism now, and we're starting right at the sharp end: variation. Variation can have any value from zero up to 180 degrees. That maximum of 180 degrees occurs on the true meridian that links the north geographical pole with the north magnetic pole, and the same applies in the southern hemisphere. So think of it this way — when you're standing on that particular line, your compass is pointing exactly opposite to true north, a full half-turn of error. Now let's move to magnetic dip, because this is the behaviour of a freely-suspended magnet in the earth's field. Except near the magnetic equator, where the lines of force run parallel to the surface, one end of that freely-suspended magnet will dip below the horizontal, pointing toward the nearer pole. North of the magnetic equator, the magnet's red pole sits lower; south of it, the blue pole sits lower. The angle, measured in the vertical plane, between the axis of the magnet and the horizontal — that is the angle of dip. Now, what exactly is the magnetic equator? It fairly closely follows the geographical equator, generally within about 10 degrees of latitude of it. On a chart, we represent it as a line joining all the points on the earth where the angle of dip is zero. So at the magnetic equator, the magnet hangs perfectly horizontal. Move that freely-suspended magnet either north or south of the magnetic equator, and the dip gradually increases. In the United Kingdom it reaches about 66 degrees. Over the earth's magnetic poles, the dip is 90 degrees — the magnet stands perfectly vertical. Finally, 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 T is the full strength of the field at that location, and its direction is exactly the direction your magnet points. Now, the total force, the angle of dip, and the magnetic variation at a point are together known as the magnetic elements for that place. And it's convenient to resolve this total force T into two components: a horizontal component H and a vertical component Z. That resolution is what Figure 9.8 demonstrates — the total force T split into its horizontal and vertical parts. So to tie it together: variation gives you the angular error between true and magnetic north, dip tells you how the field tilts relative to the horizontal, and the field strength T resolves into H and Z, which are the components your instruments actually work with.