1.05 The Earth's magnetic field.
As discussed previously, if iron filings are sprinkled on a piece of card which is placed above a bar magnet,
they form into magnetic field lines. This is because each individual iron filing becomes a tiny magnet and
then rotates so that it's north pole points along the direction of the field lines.
The Earth possesses its own magnetic field, which is similar to the field around a simple bar magnet. The field lines
run from the South geographical pole to the north pole. If a magnet is suspended freely above the Earth's surface
it will rotate to point along the Earth's magnetic field lines . The north pole of the magnet will point to
the geographical north pole of the Earth.
Note that this actually means that a south magnetic pole is located at the Earth's north
geographical pole!
Magnetic dip.
As stated above, a freely suspended magnet will align itself with the field lines of the Earth,
with the north magnetic pole pointing towards the north geographical pole. This
principle has been exploited for many centuries with navigational compasses. From the diagram
we can see that at the equator, the field lines are parallel to the surface of the Earth. However
at the geographical poles the field lines are vertical. If a compass needle was free to rotate vertically
and perfectly balanced on its pivot point, then its needle would be parallel to the ground at the equator
and vertical at the magnetic poles.
So if the compass was moved in a northerly direction from the equator, then the north pole of the compass needle
would begin to dip towards the ground, dipping more and more the further we moved north. (The same is true for the south pole
of the compass needle, the further south of the equator it is moved). This action is called magnetic
dip and the magnetic dip of a compass can indicate the latitude north or south of the
equator.
Note the diagram above does not show the Earth's magnetic field accurately enough, to reflect the
magnetic dip at latitudes between the equator and the poles. As stated above, the magnetic dip
increases gradually as we move from the equator towards the poles.
