Fig. 18. Extension of a Permanent Magnet
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The alternating current from the generator is received and converted into sound by means of the polarized ringer, a device which is interesting as depending upon several of the electrical, mechanical, and magnetic actions which are the foundations of telephone engineering.
Fig. 19. Extension of a Permanent Magnet
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"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.
