[Illustration: Fig. 19. Extension of a Permanent Magnet]

"Why the ringer rings" may be gathered from a study of Figs. 18 to 21.
A permanent magnet will impart temporary magnetism to pieces of iron
near it. In Fig. 18 two pieces of iron are so energized. The ends of
these pieces which are nearest to the permanent magnet _1_ are of the
opposite polarity to the end they approach, the free ends being of
opposite polarity. In the figure, these free ends are marked _N_,
meaning they are of a polarity to point north if free to point at all.
English-speaking persons call this _north polarity_. Similarly, as in
Fig. 19, any arrangement of iron near a permanent magnet always will
have free poles of the same polarity as the end of the permanent
magnet nearest them.

A permanent magnet so related to iron forms part of a polarized
ringer. So does an electromagnet composed of windings and iron cores.
Fig. 20 reminds us of the law of electromagnets. If current flows from
the plus towards the minus side, with the windings as drawn,
polarities will be induced as marked.

[Illustration: Fig. 20. Electromagnet]

[Illustration: Fig. 21. Polarized Ringer]

If, now, such an electromagnet, a permanent magnet, and a pivoted
armature be related to a pair of gongs as shown in Fig. 21, a
polarized ringer results. It should be noted that a permanent magnet
has both its poles presented (though one of the poles is not actually
attached) to two parts of the iron of the _electro_-magnet. The result