– [Voiceover] A magnet, free to rotate, will align itself with
the local magnetic field. We often use these as compasses to help us find north and south. If we have another magnet nearby, or a ferrous material, that might vary the local magnetic field, so it’s not just that of the Earth, and change which direction
our compass is pointing. But a compass isn’t
completely free to rotate. It can spin around this way, but it can’t, for instance, spin this way. If the local magnetic
field goes into or out of the plane, the compass is somewhat constrained in how it can deal with that. If we instead had something
with multiple axes, and as we probed the magnetic field, we could see more variation instead of just rotation and one axis. Now, the magnetic field of the Earth can loosely be approximated
as a bar magnet embedded in the Earth, and so, if we now look at something free to rotate as it moves around that, we can see oh, at this point, near the equator, the magnetic field is roughly parallel to the surface, but up here, the magnetic field is perpendicular to the surface, and perpendicular to the surface, and in those in-between parts, it points a little bit into the surface, not completely parallel to the surface. As a side note, the red
north end of this magnet, pointing towards the south pole here, so your north magnetic pole would actually be the south geographic pole, and your north geographic pole is actually a south magnetic pole. That’s just an aside. For now, what we’re interested in is how much that dip, that non-parallel to the surface, shows up. And to more accurately measure that, we’ll use a dip meter. Now this is in effect a two step compass. To start off, I’ll have the compass, and make sure that I’ve aligned it with north south, and then I can rotate 90 degrees, so that it’s free to
spin in this direction. So first it was free to spin this way, and then I found the alignment of that spin in this direction, and then I changed the angle it can, or
the orientation it can swing through, and then
it swings this way, and I let it settle down,
and I want to measure the angle from horizontal that this new needle makes. It’s making an angle of about 70 degrees. And I’d want to let that settle down to get my final measurement, but that’s the measurement of the
dip, so the magnetic field of the Earth here is
not along the surface, as we often assume, but instead pointing pretty significantly into the surface of the Earth.