The Telephone, subsequently simplified by Professor Bell, is shown in the two following illustrations (figs. 192, 193). The voice strikes against the diaphragm, and it begins to vibrate. The sound is not conveyed by the wire; the motion is communicated, and the vibrations become sound waves again. The Telephone consists of a cylindrical magnet encircled at one end by a bobbin, on which is wound a quantity of fine insulated copper wire. The magnet and coil are contained in a wooden case, the ends of the coil being soldered to thick copper wire, which traverse the “wooden envelope,” and terminate in the binding screws. In front of the magnet is a thin circular iron plate, in which is the mouthpiece. The drawings will explain the instrument.
Fig. 192.—External appearance of Bell Telephone.
Fig. 193.—a. Bobbin of coil wire round magnet. b. Diaphragm. c. Mouthpiece. d. Permanent magnet. e. Wires to binding screws. f. Both wires as one for convenience. g. Adjusting screw-holding magnet.
Mr. Edison also invented a Telephone like Gray’s, and made the discovery, that when properly prepared, carbon would change its resistance with pressure, and that the ratio of these changes corresponded with the pressure. This solved the problem of the production of speech. The carbon is placed between two plates of platinum connected in the circuit and near the diaphragm, and the carbon receives the pressure from it by means of the mouthpiece.
When we come to Magnetism and Electricity we may have something more to say respecting the mysteries of the Telephone and its later developments. At present we are only concerned with it as a sound conveyer, and it answers its purpose admirably, although somewhat liable to attract other sounds or vibrations from neighbouring wires.
The Phonograph, a mechanical invention of Mr. Edison, does not make use of electricity, although the vibratory motion of the diaphragm is utilized. It, in a simple form, consists of a diaphragm so arranged as to operate upon a small stylus, placed just opposite and below the centre, and a brass cylinder, six or eight inches long, by three or four in diameter, mounted upon a horizontal axis, extending each way beyond its ends for a distance about its own length.
“A spiral groove is cut in the circumference of the cylinder, from one end to the other, each spiral of the groove being separated from its neighbour by about one-tenth of an inch. The shaft or axis is also cut by a screw thread corresponding to the spiral groove of the cylinder, and works in screw bearings; consequently when the cylinder is caused to revolve, by means of a crank that is fitted to the axis for this purpose, it receives a forward or backward movement of about one-tenth of an inch for every turn of the same, the direction, of course, depending upon the way the crank is turned. The diaphragm is firmly supported by an upright casting capable of adjustment, and so arranged that it may be removed altogether when necessary. When in use, however, it is clamped in a fixed position above or in front of the cylinder, thus bringing the stylus always opposite the groove as the cylinder is turned. A small, flat spring attached to the casting extends underneath the diaphragm as far as its centre and carries the stylus, and between the diaphragm and spring a small piece of india-rubber is placed to modify the action, it having been found that better results are obtained by this means than when the stylus is rigidly attached to the diaphragm itself.
