
CHAPTER 4
Inductance
4.1 Magnets and Electromagnets
If a piece of paper is laid on a straight bar magnet, and iron filings
are sprinkled on the paper, they are seen to arrange themselves in a
pattern something like Fig. 4.1a. Gently tapping the paper helps the
filings to take up the pattern. The lines show the paths along which
the attraction of the magnet exerts
itself.
(Compare the lines of
electric force in Fig. 3.2.) As a whole, they map out the magnetic
field,
which is the sphere of influence, as it were, of the magnet. The
field is most concentrated around two regions, called poles, * at the
ends of the bar. The lines may be supposed