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Magnetic
compass
S

FIGURE 6.20 These lines are a map of the magnetic field
around a bar magnet. The needle of a magnetic compass will follow
the lines, with the north end showing the direction of the field.

shaped and oriented as if there were a huge bar magnet inside
Earth (Figure 6.21). The geographic North Pole is the axis of

Earth’s rotation, and this pole is used to determine the direction
of true north on maps. A magnetic compass does not point to
true north because the north magnetic pole and the geographic
North Pole are in two different places. Th e difference is called

the magnetic declination. The map in Figure 6.22 shows approxi-

mately how many degrees east or west of true north a compass

FIGURE 6.18 Every magnet has ends, or poles, about which the needle will point in different locations. Magnetic declination
magnetic properties seem to be concentrated. As this photo shows, must be considered when navigating with a compass. If you are
more iron filings are attracted to the poles, revealing their location. navigating with a compass, you might want to consider using

an up-to-date declination map. The magnetic north pole is con-
tinuously moving, so any magnetic declination map is probably
N S a snapshot of how it used to be in the past.

Geographic North Pole
N S N S
Magnetic
north
pole
N S N S N S N S

FIGURE 6.19 A bar magnet cut into halves always makes new,
complete magnets with both a north pole and a south pole. The
S
poles always come in pairs, and the separation of a pair into single
poles, called monopoles, has never been accomplished.

point in various places in the magnetic field. Arrowheads indi- N

cate the direction that the north pole would point, thus defi ning
the direction of the magnetic fi eld. The strength of the magnetic

field is greater where the lines are closer together and weaker

where they are farther apart. Figure 6.20 shows the magnetic
field lines around the familiar bar magnet. Note that magnetic

fi eld lines emerge from the magnet at the north pole and enter FIGURE 6.21 Earth’s magnetic field. Note that the magnetic
north pole and the geographic North Pole are not in the same
the magnet at the south pole. Magnetic field lines always form

place. Note also that the magnetic north pole acts as if the south
closed loops. pole of a huge bar magnet were inside the earth. You know that it
The north end of a magnetic compass needle points north must be a magnetic south pole, since the north end of a magnetic

because Earth has a magnetic field. Earth’s magnetic fi eld is compass is attracted to it, and opposite poles attract.
6-17 CHAPTER 6 Electricity 155