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CHAPTER XXIX SPEAKERS
The simplest form of loud speaker is the common telephone receiver that anyone can open and examine, and almost everyone has. Equipped with a small horn, that instrument can furnish enough sound for a moderatesized room, and when so used is essentially a magnetictype loud speaker.
(1) If the reader has ever unscrewed the cap of a telephone receiver he has found underneath a disc, or diaphragm, of thin stiff metal. Under this, again, he found a horseshoe magnet, curved to follow the outline of the instrument, and two coils of very thin wire. The two coils of wire are wound around the poles of the magnet. When current flows through them they produce a magnetic field, which either aids or opposes the permanent field of the horseshoe magnet.
This much being clear, the metal diaphragm may be studied more carefully. When in operating position it is held firmly at its circular edge, while its center is pulled inward by the magnetic attraction. The metal of which the diaphragm is made possesses elasticity, and its center, pulled inward by the magnet, has a natural tendency to spring back toward neutral position whenever the magnetic attraction is lessened. If, on the other hand, the magnetic attraction is increased, the center of the diaphragm is drawn still closer to the pole-pieces.
In an instrument of this construction, a. c. or pulsating d. c, fed to the two coils, will cause the center of the diaphragm to vibrate in accordance with the frequency and strength of the current supplied.
Whenever the diaphragm vibrates it acts upon the surrounding air, pushing that out of the way when it moves outward, and leaving a vacuum into which air
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