It can reproduce what it is called upon to reproduce, and if human nature demands the commonplace, the instrument will be made to satisfy the demand.  On the other hand, speeches of famous men, national songs, magnificent opera selections, and other pleasing and instructive productions can be reproduced fairly accurately.  In this way the phonograph, perhaps more than any other recent invention, can carry to the “shut-ins” a lively glimpse of the outside world and its doings.

[Illustration:  FIG. 195.—­A vibrating tuning fork traces a curved line on smoked glass.]

The phonograph consists of a cylinder or disk of wax upon which the vibrations of a sensitive diaphragm are recorded by means of a fine metal point.  The action of the pointer in reporting the vibrations of a diaphragm is easily understood by reference to a tuning fork.  Fasten a stiff bristle to a tuning fork by means of wax, allowing the end of the point to rest lightly upon a piece of smoked glass.  If the glass is drawn under the bristle a straight line will be scratched on the glass, but if the tuning fork is struck so that the prongs vibrate back and forth, then the straight line changes to a wavy line and the type of wavy line depends upon the fork used.

In the phonograph, a diaphragm replaces the tuning fork and a cylinder (or a disk) coated with wax replaces the glass plate.  When the speaker talks or the singer sings, his voice strikes against a delicate diaphragm and throws it into vibration, and the metal point attached to it traces on the wax of a moving cylinder a groove of varying shape and appearance called the “record.”  Every variation in the speaker’s voice is repeated in the vibrations of the metal disk and hence in the minute motion of the pointer and in the consequent record on the cylinder.  The record thus made can be placed in any other phonograph and if the metal pointer of this new phonograph is made to pass over the tracing, the process is reversed and the speaker’s voice is reproduced.  The sound given out in the this way is faint and weak, but can be strengthened by means of a trumpet attached to the phonograph.

[Illustration:  FIG. 196.—­A phonograph.  In this machine the cylinder is replaced by a revolving disk.]

CHAPTER XXX

ELECTRICITY

280.  Many animals possess the five senses, but only man possesses constructive, creative power, and is able to build on the information gained through the senses.  It is the constructive, creative power which raises man above the level of the beast and enables him to devise and fashion wonderful inventions.  Among the most important of his inventions are those which relate to electricity; inventions such as trolley car, elevator, automobile, electric light, the telephone, the telegraph.  Bell, by his superior constructive ability, made possible the practical use of the telephone, and Marconi that of wireless telegraphy.  To these inventions might be added many others which have increased the efficiency and production of the business world and have decreased the labor and strain of domestic life.