It is
said to render a whisper audible at a distance of 1000 yards; but its
very sensitiveness is a drawback, since it gathers up extraneous
sounds.
To the same category belongs the 'aerophone,' which may be described as
a gigantic tympanum, vibrated by a piston working in a cylinder of
compressed air, which is regulated by the vibrations of the sound to be
magnified. It was designed to call out fog or other warnings in a loud
and penetrating tone, but it has not been successful.
The 'magnetic ore separator' is an application of magnetism to the
extraction of iron particles from powdered ores and unmagnetic matter.
The ground material is poured through a funnel or 'hopper,' and falls in
a shower between the poles of a powerful electro-magnet, which draws the
metal aside, thus removing it from the dress.
Among Edison's toys and minor inventions may be mentioned a 'voice
mill,' or wheel driven by the vibrations of the air set up in speaking.
It consists of a tympanum or drum, having a stylus attached as in the
phonograph. When the tympanum vibrates under the influence of the
voice, the stylus acts as a pawl and turns a ratchet-wheel. An
ingenious smith might apply it to the construction of a lock which would
operate at the command of 'Open, Sesame!' Another trifle perhaps worthy
of note is his ink, which rises on the paper and solidifies, so that a
blind person can read the writing by passing his fingers over the
letters.
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