Figure 7-13: The Magnetic Fields of a Bar Magnet and of Earth (a) This picture was made by placing a piece of paper on top of a bar magnet, then spreading thin iron filings on the paper. Each elongated iron filing acts like a compass needle and aligns with the magnetic field at its location. The pattern of the filings show the magnetic field lines, which appear to stream from one of the magnet’s poles to the other. (b) Earth’s magnetic field lines have a similar pattern. Although Earth’s field is produced in a different way—by electric currents in the liquid portion of our planet’s interior—the field is much the same as if there were a giant bar magnet inside Earth. This “bar magnet” is not exactly aligned with Earth’s rotation axis, which is why the magnetic north and south poles are not at the same locations as the true, or geographic, poles. A compass needle points toward the north magnetic pole, not the true north pole.
(a: Jules Bucher/Photo Researchers)