Travel Reference
In-Depth Information
an age for the Cretaceous of California, and in so doing I took the first major
step toward finding the age of Sucia Island. This tool had been widely used
for other time periods and other places, but never before in western North
America.
All magnets exhibit the curious property of being bipolar; they all have
positive and negative poles. In an experiment beloved of sixth-grade science
teachers, iron filings sprinkled around a magnet obediently orient themselves
in concentric patterns around these poles. Less obvious, however, is that the
poles are producing forces that differ markedly in direction. Why does a
compass needle always seek the North Pole rather than simply pointing to-
ward either the north or the south pole? A compass, after all, is nothing
more than a small magnet in a closed box, a magnet whose positive pole al-
ways seeks the direction of a negative magnetic pole. When placed together,
two magnets either cling to or repel each other, depending on whether their
positive and negative poles are in contact. Although the forces emanating
from these poles are of equal intensity, something about them is markedly
different.
The magnets with which everyone is familiar, be they horseshoe-
shaped, rods, or the small disks placed on refrigerators, are all quite consis-
tent in theit polarity: Their positive and negative poles are always in the
same place. But other, more complicated magnets can be made to act in a
more variable manner: Their polarities sometimes can be traded, so that the
positive pole becomes negative and the negative pole positive.
Magnets come in all sizes. The largest we are acquainted with (though
there may be many larger out in space) is the earth. The earth acts as though
it had a huge bar magnet in its interior, aligned roughly north-south. At first
glance, the earth would seem to be an unlikely magnet: Why does it have
magnetic properties at all? Perhaps metal on the earth's surface creates the
earth's magnetic field. But closer scrutiny casts doubt on this possibility.
Most of the eatth's crust is made up of
nonmetallic
matetial, and thus non-
magnetic material. Thete clearly isn't enough metal to produce the enor-
mous magnetic field surrounding the earth, a field so strong that it can de-
flect cosmic rays and solar radiation that would otherwise strike our planet's
