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Today magnets can be found in almost every
home: refrigerator magnets, compasses, screwdrivers, cow magnets,
bar magnets, and hard drives in computers. Long ago, compasses
were the only commonly used magnets. Compasses helped guide travelers
because the needle on a compass would always point north. People
used compasses long before anyone knew that a compass was magnetic,
and long before anyone knew about magnetism.
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| Earth's
magnetic field is very similar to that of a bar
magnet, with a south and a north pole. The red
lines represent the invisible magnetic lines
of force with which other magnets and charged
particles interact. |
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In the late 1500's, William Gilbert
realized that the compass was a tiny magnet and it was interacting
with a larger magnetic field in order to point north. In 1600,
he published "De Magnete" explaining that "the
globe of the earth is magnetic, a magnet," Chapter 17,
Book 1.
If you take two bar magnets and play with them for a while, you will realize
that each end of the bar magnet will either repel or attract one end of the
other bar magnet. Each end of a bar magnet is called a magnetic pole with one
end as a north pole and one as a south pole. Like poles (e.g. south-south)
repel, and opposite poles (e.g. south-north) attract. Because the bar magnet
has two poles, its magnetic field is called a dipole magnetic field. It appears
that all magnetic fields have an even number of poles, as scientists have been
looking for a single magnetic pole for over 50 years now but have never found
one.
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| In
1716, Sir Edmund Halley pointed out that aurora
is aligned with Earth's magnetic field. And in
1741, Olof Hiorter noticed that the magnetic
field measured on Earth's surface would change
when the aurora passed overhead. Somehow the
aurora and the magnetosphere are connected. |
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Relatively close to Earth (out to
about 8 Earth Radii or 50,000 km at the Equator), Earth's magnetic
field looks approximately like that of a bar magnet,
as shown in the figure. There are some strange places on Earth that make
the dipole different that in the picture. And if we go further
out into space,
Earth's magnetic field gets weaker and is affected by charged particles in
space. We will talk more about this in the Sun-Earth Connection page. We
call Earth's magnetic field and the particles and space within
this field, Earth's "magnetosphere."
Why are we talking about magnetic fields?
These pages are about the aurora and auroral substorms! Yes,
but Earth's magnetosphere plays a very important
role in the aurora and in auroral substorms.
If you would like to measure magnetic fields around different magnets, here
is an activity
(Measuring Magnetism -
 PDF Format - 180kb) that shows you how!
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