Physics for Geologists, Second edition

78 Electricity and magnetism
Figure 8.2 The field between two electrodes in a homogeneous, isotropic
medium. Lines and surfaces of equal energy, called equipotential
lines/surfaces, are normal to the flow paths. (This is a map of part of
a field, not of a block.)
energy or potential by virtue of the elevation to which water would rise
unassisted in the boreholes (the static water level). A contour map of the
elevation of the water surface in boreholes drilled to the same aquifer is a
map of the energy of the aquifer, as we saw in Figure 3.7. The contours
are lines of equal energy (equipotential lines) and flow lines are normal to
them. The surface contoured is a notional surface called the potentiometric
surface.
Magnetism
Magnetism is one of the fundamental forces of the universe. The mineral
magnetite, FeO . Fe203, commonly with titanium oxide, Ti02, has been
known for more than 2500 years for its properties of attracting iron and
orientating itself in the Earth's magnetic field. A bar magnet has a magnetic
axis, the ends of which are called poles. The end that seeks the geographical
North when freely suspended is called the North pole or North-seeking pole
(or +); the other is the South or South-seeking pole (or -).
We can magnetize bars of ferromagnetic metals (iron, nickel, cobalt and
some of their alloys) by putting them in a magnetic field; by cooling them
when aligned N-S; by stroking them with another magnet (the new pole at
the beginning of the stroke is the same as the stroking pole); and by hammer-
ing them when oriented in a magnetic field. This last is the reason why steel
ships and boats become magnetized during their construction, particularly
Copyright 2002 by Richard E. Chapman