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VII.
Field Intensity
The
study of the intensity of the earth's magnetic field is valuable from the
points of view of pure science and of engineering, and also for geological
prospecting for mineral and energy resources. Intensity measurements are
made with instruments called magnetometers, which determine the total
intensity of the field and the intensities in the horizontal and vertical
directions. The intensity of the magnetic field of the earth varies in
different places on its surface. In the temperate zones it amounts to
about 0.6 oersted (the oersted is a unit of measurement of a magnetic
field , of which 0.2 oersted is in a horizontal direction.
A.
Paleomagnetism
Studies
of ancient volcanic rocks show that as they cooled, they "froze"
with their minerals oriented in the magnetic field existing at that time.
Worldwide measurements of such mineral deposits show that through
geological time the orientation of the magnetic field has shifted with
respect to the continents. The north magnetic pole 500 million years ago,
for example, lay south of Hawaii, and for the next 300 million years the
magnetic equator lay across the United States. To account for this,
geologists believe that the outer crust of the earth has gradually shifted
around, even though the axis on which the earth spins has remained the
same. If this were the case, the climatic belts would have remained the
same, but the continents would have drifted slowly through different
"paleolatitudes."
B.
Magnetic Reversals
Recent
studies of remanent (residual) magnetism in rocks and of magnetic
anomalies on the floors of the oceans have shown that the magnetic field
of the earth has reversed its polarity at least 170 times in the past 100
million years. Knowledge of these reversals, which can be dated from
radioactive isotopes in the rocks, has had a great influence on theories
of continental drift and the spreading of ocean floors.
VIII.
Terrestrial Electricity
Three
electrical systems generated in the earth and in the atmosphere by natural
geophysical processes are known. One of them is in the atmosphere, and one
is within the earth, flowing parallel to the surface of the earth. The
third, which transfers an electric charge continuously between the
atmosphere and the earth, flows vertically. See Electricity.
Atmospheric
electricity, except for that associated with charges within a cloud and
lightning, results from the ionization of the atmosphere by solar
radiation and from the movement of clouds of ions carried by atmospheric
tides .
Atmospheric tides result from the gravitational attraction of the sun and
the moon on the earth's atmosphere ,
and, like the oceanic tides, they rise and fall daily. The ionization, and
consequently the electrical conductivity, of the atmosphere close to the
surface of the earth is low, but it increases rapidly with increasing
altitude. Between 40 and 400 km (25 to 250 mi) above the earth, the ionosphere
forms an almost perfectly conducting spherical shell. The shell reflects
radio signals back to earth and absorbs electromagnetic radiations
approaching the earth from space. The ionization of the atmosphere varies
greatly, not only with altitude but with the time of day and the latitude.
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