MOTION MOUNTAIN

36 1 electricity and fields
F I G U R E 14 Visualizing magnetic fields around magnets and coils – with iron fillings, with compass
needles and with computer graphics and with iron filings (© Wikimedia, MIT).
net has two points of highest magnetization, and he called them poles. He found that
even after a magnet is cut, the resulting pieces always retain two poles: when the stone is
left free to rotate, one points to the north and the other to the south.
⊳ All magnets are dipoles.
The two poles are called thenorth pole and thesouthpole. Maricourt also found that
⊳ Like poles repel, and unlike poles attract.
As a consequence, the magnetic north pole of the Earth is the one near the south pole,
and vice versa.
Magnets are surrounded by magnetic fields. Magnetic fields, like electric fields, can
be visualized with field lines. Figure 14 shows some ways to do this. We directly note the
main difference between magnetic and electric field lines: magnetic field lines have no
beginning and no ends, whereas electric field lines do. (However, magnetic field lines are
usually not closed; this only happens in very special cases.)The direction of the field lines
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net