Physics for Geologists, Second edition

Electricity and magnetism 8 1
defined as the strength of the field at a point where a unit pole is repelled
with a force of 1 dyne. The SI unit is the amperelmetre (A m-I), which equals
12.566 x lop3 oersteds.
The origin of magnetism lies in electric currents, that is, moving electric
charges, as first shown by Ampkre. In magnetic materials, the currents are
due to electrons and protons bound to the atoms. The Earth's magnetism is
more complicated.
For treatment at greater depth, see Bleany (1984) and Feynman et al.
(1963).
Terrestrial magnetism
The Earth has a magnetic field that is similar to that which would be gener-
ated by an axial bar magnet near its centre. This has been known for about
400 years. We can measure the Earth's magnetic field with great precision.
If we suspend a magnetized needle so that it can freely orientate itself in the
magnetic field, we find that it is, in general, not horizontal but has a dip or
inclination below the horizontal towards the nearer pole and a declination
or variation from geographic north (Figure 8.5). The force can be resolved
into three components: x, horizontal in the direction of geographic North;
y, horizontal towards geographic East; and z, vertical. The force is vertical
near the poles, horizontal near the Equator, but we find that the magnetic
poles are not at the geographic poles.
The South Magnetic Pole in 2000 was in Latitude 64.63"S, Longi-
tude 138.36"E: the North Magnetic Pole, in Latitude 80.68"N, Lon-
gitude 109.12"W - so they were not even diametrically opposite each
other, but separated from the projected pole by 1400 nautical miles,
or 2600 km. However, the geomagnetic poles are the poles of what is
called the centred geomagnetic dipole, which is the dipole that gives
the observed field most closely. In 2000, the geomagnetic poles were in
Copyright 2002 by Richard E. Chapman
Figure 8.5 Magnetic declination or variation (d) and inclination or dip (i).