ELECTRICAL AND OPTICAL PHENOMENA.

113

 

The earth, like all magnets, possesses a magnetic field.
Lines of magnetic force come out of its north pole, pass
around the earth through the air, and enter the earth at
jts south pole. A magnetic needle, placed in the earth’s
field, if free to move, will come to rest with the earth’s
lines of force passing into its south pole and passing out of
its north pole. That pole of the needle which points to
the geographical north is, therefore, of opposite magnetic
polarity to the earth’s polarity in the Northern Hemi-
sphere. In the United States, the Northern Hemisphere
is regarded as possessing south magnetic polarity; in
France, as possessing north magnetic polarity.

301 Origin of the Earth’s Magnetism—The
exact cause of the earth’s magnetism is unknown.
Currents of electricity circulating around a con-
Electrical currents

ductor render it a magnet.

 

are generated in nearly all substances, when they
are unequally heated. The earth appears to owe
its magnetism to the circulation around it of cur-
rents of electricity, produced, most probably, by
_ the unequal heating of different portions of its
surface by the sun’s rays. These currents would
follow the sun in its apparent motion from east
to west. Since the earth’s magnetism appears to
have its remote cause in the sun’s heat, variations
in the temperature should be followed by corre-
sponding variations in the intensity of magnetism.
This is found to be the case.
Magnetic storms, or unusual variations in
the earth’s magnetism, have been noticed to

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     

 

 

 

 

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Fig. 102, Declination Chart,
(West Declination is represented by the continuous lines; East Declination by the dotted lines; the Agones by the heavy lines.)

correspond with outbursts of solar activity, as
manifested by the unusual occurrence of new
spots.

802. Variations in the Manifestations of Mag-
netic Properties.—The earth’s magnetic poles do
not correspond with its geographical poles. The
magnetic needle, therefore, except in a few local-
ities, does not point to the true geographical north,
but to the east or to the west of it. This deviation
from the true north is called the declination or vari-
ation, and is east or west according as the needle
points to the east or the west of the true or geo-
graphical north. The amount of this variation
differs in different parts of the earth.

The position of the magnetic poles of the earth is not
always the same, but changes slowly from year to year,
thus producing corresponding changes in the declination
of the needle. This change is called secular variation. The
needle, at any place, points more and more to the east,
following the change of the poles. At length, after a long
period, it becomes stationary, and then begins to move
toward the true meridian, which it at length reaches;

\ when, continuing its motion, the declination becomes
west.

~ Isogonal Lines.—Lines connecting places which

. have the same declination, are called isogonal lines.
Lines connecting these places, when the needle
points to the true north, are called agones, or lines
of no declination.

The direction of the isogonal lines is shown in the de-