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the separation of the two magnetism and afterwards their recom
position is called the coercive force (thus the coercive force in
steel is greater than in soft iron. 414).
§ 244. 2) As to the earth's magnetic action we see that a
magnetised needle placed on a pivot sets itself in a position
more or less north and south. Thus we distinguish between
magnetic poles and magnetic meridians of the earth, which
do not coincide with the geographical poles and meridians.
The magnetic southern pole of the earth is in the north and
the northern pole in the south of the earth. Magnetic meridian
we call the plane laid through the direction of the magnetic
needle and the centre of the earth.
§ 245. The difference in any place between the direction
of the magnetic needle and the geographical meridian is called
the declination of that place. Places on which the magnetic
needle shows the same declination may be pointed out and con
nected by lines on a map; such lines are called isogonic lines.
The magnetic needle was known as early as the 13th century,
the first maps showing its declination were drawn by Halley
(1683).
§ 246. The mariner's compass consists of a star or rote
drawn on a paper for pointing out the different directions and
a magnetic needle resting on a steel pivot in the centre of the
rote. By means of this instrument the declination of a place,
if the meridian is known, may be easily determined, or, if the
declination is known, the meridian may be found out.
§ 247. There are also compasses to show the magnetic in
clination or dip (Inclination compass) of the earth, that means:
they show the different strength of the earth's magnetism, or the
digression of the magnetic needle from the horizontal direction.
As this power increases, the more we proceed either to the
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