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surface. It turns out that this magnetic field emanates from deep within. Its
source is the innermost shell of the earth, a liquid iron core, which hehaves
like a magnetic dynamo.
Even though the magnetic field of the earth (like all magnets) has two
distinct poles, it can be thought of as being cteated by a uniformly magne-
tized sphere. The magnetic field pole positions—today—are 79N, 70W and
79S, 110E. Yet these positions are not fixed on the earth. It has long been
known that the positions of the magnetic poles change. The magnetic field
can be described by three variables: its intensity, or strength at any geo-
graphic position; its declination, which is the deviation of the north mag-
netic pole from the geographic north pole of the earth's rotational axis; and
its inclination, or the steepness of the field at any geographic point (If a com-
pass needle is allowed to move freely, it will dip downward, seeking the mag-
netic pole. The angle of this downward dip is the inclination).
Of these three, declination shows the most movement. The declina-
tion of London was 11E in 1580 and was 24W in 1819—a change of more
than 36 degrees in 240 years. There are only two ways to account fot this
change. Either London moved or the magnetic pole wanders across the face
of the earth through time. This movement is called secular movement.
The secular movement of the magnetic poles was a key discovery of the
nineteenth century. But an even more startling discovery was made in the
early twentieth century: The poles can reverse polarity. In the early 1900s,
two French geologists, using the most primitive equipment, discovered that
the same lava outcrops in France preserved two diametrically opposing di-
rections of polarity. Because the polarity was detected from rocks long since
solidified, these earliest paleomagnetic measurements were looking at fossil
compasses, where the directions of the magnetic poles could be determined.
The fact that two opposing directions were found in the same masses of lava
sparked debate that continued for decades. Finally, after repeated measure-
ments with ever-more-sophisticated equipment, there was but one in-
escapable conclusion: The eatth's magnetic field had somehow switched po-
larity, the positive pole becoming negative, and vice versa. The only
alternative explanation was that the whole earth itself had rotated 180 de-
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