Consciously or
unconsciously Edison accomplished the feat. With the hardihood of
genius, he attempted to devise a telephone which would speak out loud
enough to be heard in any corner of a large hall.
In the telephone of Bell, the voice of the speaker is the motive power
which generates the current in the line. The vibrations of the sound
may be said to transform themselves into electrical undulations. Hence
the current is very weak, and the reproduction of the voice is
relatively faint. Edison adopted the principle of making the vibrations
of the voice control the intensity of a current which was independently
supplied to the line by a voltaic battery. The plan of Bell, in short,
may be compared to a man who employs his strength to pump a quantity of
water into a pipe, and that of Edison to one who uses his to open a
sluice, through which a stream of water flows from a capacious dam into
the pipe. Edison was acquainted with two experimental facts on which to
base the invention.
In 1873, or thereabout, he claimed to have observed, while constructing
rheostats, or electrical resistances for making an artificial telegraph
line, that powdered plumbago and carbon has the property of varying in
its resistance to the passage of the current when under pressure.
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