Did you know that the compass on your boat may not work properly in other parts of the world? I didn’t, until just recently. Inspired by something that I read in the book My Old Man and the Sea, and a comment by our friend Wade on this post, I did a bit of research on the subject of how compasses react to changes in latitude. What I came across was a Wikipedia article that talked about Magnetic Dip.
Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth’s magnetic field lines. This angle varies at different points on the Earth’s surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement, and negative values indicate that it is pointing upward. The dip angle is in principle the angle made by the needle of a vertically held compass, though in practice ordinary compass needles may be weighted against dip or may be unable to move freely in the correct plane.
I’m going to admit it, I didn’t know any of this!
Both the guys in the book that I mentioned, and the folks that our friend Wade came across, were ultimately able to fix their compass problems. This explains the problem a bit more:
Because the Earth’s magnetic field’s inclination and intensity vary at different latitudes, compasses are often balanced during manufacture so that the dial or needle will be level, eliminating needle drag which can give inaccurate readings. Most manufacturers balance their compass needles for one of five zones, ranging from zone 1, covering most of the Northern Hemisphere, to zone 5 covering Australia and the southern oceans. This individual zone balancing prevents excessive dipping of one end of the needle which can cause the compass card to stick and give false readings.
I believe the problem can be fixed by opening the compass and adding a small, calibrated weight to the needle. How specifically, I have no idea. At this point, I don’t know anything beyond what I’ve written. I wonder if our friends who have travelled to the southern latitudes have experienced anything like this. I’ll need to talk to them.