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FIGURE 6.22 This magnetic declination map shows the approximate number of degrees east or west of the true geographic north that a
magnetic compass will point in various locations.

Note in Figure 6.21 that Earth’s magnetic field acts as if isolated units of positive protons and units of negative electrons.
there were a huge bar magnet inside Earth with a south mag- An object becomes electrostatically charged when charges are

netic pole near Earth’s geographic North Pole. This is not an separated and the object acquires an excess or defi ciency of
error. The north pole of a magnet is attracted to the south pole negative charges. You might wonder, by analogy, if the poles of a

of a second magnet, and the north pole of a compass needle magnet are similarly made up of an excess or deficiency of mag-

points to the north. Therefore, the bar magnet must be arranged netic poles. The answer is no; magnetic poles are diff erent from
as shown. This apparent contradiction is a result of naming the electric charges. Positive and negative charges can be separated

magnetic poles after their “seeking” direction. and isolated. But suppose that you try to separate and isolate the

The typical compass needle pivots in a horizontal plane, poles of a magnet by cutting a magnet into two halves. Cutting a

moving to the left or right without up or down motion. Inspec- magnet in half will produce two new magnets, each with north

tion of Figure 6.22, however, shows that Earth’s magnetic fi eld is and south poles. You could continue cutting each half into new
horizontal to the surface only at the magnetic equator. A com- halves, but each time the new half will have its own north and
pass needle that is pivoted so that it moves only up and down south poles (Figure 6.19). It seems that no subdivision will ever
will be horizontal only at the magnetic equator. Elsewhere, it separate and isolate a single magnetic pole, called a monopole.
shows the angle of the field from the horizontal, called the mag- Magnetic poles always come in matched pairs of north and south,

netic dip. The angle of dip is the vertical component of Earth’s mag- and a monopole has never been found. The two poles are always

netic field. As you travel from the equator, the angle of magnetic found to come together, and as it is understood today, magnetism
dip increases from 0° to a maximum of 90° at the magnetic poles. is thought to be produced by electric currents, not an excess of
monopoles. The modern concept of magnetism is electric in ori-

gin, and magnetism is understood to be a property of electricity.
THE SOURCE OF MAGNETIC FIELDS The key discovery about the source of magnetic fi elds was

The observation that like magnetic poles repel and unlike mag- reported in 1820 by a Danish physics professor named Hans

netic poles attract might remind you of the forces involved with Christian Oersted. Oersted found that a wire conducting an
like and unlike charges. Recall that electric charges exist as single electric current caused a magnetic compass needle below the
156 CHAPTER 6 Electricity 6-18