He attached the pin or stylus to the centre of the mica, and
brought its point to bear on a cylindrical surface of prepared chalk.
The undulatory current from the line was passed through the stylus and
the chalk, while the latter was moved by turning a handle; and at every
pulse of the electricity the friction between the pin and chalk was
diminished, so that the stylus slipped upon its surface. The
consequence was a vibration of the mica diaphragm to which the stylus
was attached. Thus the undulatory current was able to establish
vibrations of the disc, which communicated themselves to the air and
reproduced the original sounds. The replica was loud enough to be heard
by a large audience, and by reducing the strength of the current it
could be lowered to a feeble murmur. The combined transmitter and
receiver took the form of a small case with a mouthpiece to speak into,
an car-piece on a hinged bracket for listening to it, press-keys for
manipulating the call-bell and battery, and a small handle by which to
revolve the little chalk cylinder. This last feature was a practical
drawback to the system, which was patented in 1877.
The Edison telephone, when at its best, could transmit all kinds of
noises, gentle or harsh; it could lift up its voice and cry aloud, or
sink it to a confidential whisper.
Pages:
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279