Satgen494  Magnetism Pt1  by GM4IHJ   (BID SGEN494)    1998-09-12

There are two well established ways to produce a magnetic field :-
a. Any electric current carries with it a magnetic field
b. Some materials have the property of being attracted by , or , 
   acquiring the properties of a magnet.
The earth is a magnet of the first type, in that its central core is
electrified and it is in motion. A changing electrical field cannot
exist without magnetism, ie magnetic forces and electrical forces are
attributal to one field, as two different aspects of the same thing.
So the earth is a powerful magnet, and we talk about it as having
magnetic poles at its surface. Although this is not absolutely correct
, the poles are down on the core, we just encounter the manifestation
of them at the surface.

Just to complicate things a little more, the earth is not a simple
dipole magnet. The dynamo effect producing magnetism in the core is
rather like a multipolar machine. There is a strong dipole pair but in
addition there are at least 3 more pairs of poles, albeit a lot weaker
than the main poles. So the earth is perhaps better described as
octopolar. Even worse the dynamo action is not fixed . So the poles
move relatively slowly with time. Seen from Scotland the magnetic pole
is moving slowly into line with the rotation pole and could line up in
about 26 years time . Please note however this simply means north from
Scotland will briefly be the same for magnetic bearing and rotation
pole bearing. The two poles will be in line as seen from Scotland but
they will not be coincident, they will simply lie at different ranges
along the same azimuth bearing. As viewed from other locations they
will not be in line . So users of magnetic compasses must check with
their local authority (Eg Ordnance Survey in UK, US Geological Survey
in USA ,the British name is a relict from the fact that Army Gunners
were the first to make useful maps, back before there was a USA).

When contemplating long periods of earth history the problem of earth
magnetism is much more complex. In the last 4 million years the field
has reversed completely several times. So that some one using a
compass needle mmagnetised north south, would find that unlike now
when we use the north pole of a compass needle to point towards what
we call the north magnetic pole ( actually the north attracting pole),
at a time of reverse earth magnetism the polarities were completely
opposite to the present line up. A subject most folks can happily
ignore for much of the time , but which can have very obvious local
effects for us if we locate near old rocks heavily magnetised millions
of years ago, and still heavily magnetised today thereby producing
strong local magnetic anomalies. About which more in later parts of
this discourse. For now it is enough to know that when your magnetic
compass needle points at the pole it is not pointing at the earth
surface magnetic pole. It is in fact trying to point at the pole deep
down in the earths core. Hence the fact that knowing the azimuth of
the earth surface magnetic pole from your station does not give you
the azimuth at which your compass needle is pointing - so you must get
the variation angle separating compass heading and rotation pole
azimuth from your local authority Survey .