There are a hundred or more lines of the trace in
the width of an inch, and they are hardly visible to the naked eye.
Only with a magnifying glass can the undulations caused by the vibrating
stylus be distinguished. This tube of wax is filed upon a metal barrel
like a sleeve, and the barrel, which forms part of a horizontal spindle,
is rotated by means of a silent electro-motor, controlled by a very
sensitive governor. A motion of translation is also given to the
barrel as it revolves, so that the marking stylus held over it describes
a spiral path upon its surface. In front of the wax two small metal
tympanums are supported, each carrying a fine needle point or stylus on
its under centre. One of these is the recording diaphragm, which prints
the sounds in the first place; the other is the reproducing diaphragm,
which emits the sounds recorded on the wax. They are used, one at a
time, as the machine is required, to take down or to render back a
phonographic message.
The recording tympanum, which is about the size of a crown-piece, is
fitted with a mouthpiece, and when it is desired to record a sentence
the spindle is started, and you speak into the mouthpiece. The tympanum
vibrates under your voice, and the stylus, partaking of its motion, digs
into the yielding surface of the wax which moves beneath, and leaves a
tiny furrow to mark its passage.
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