Page:Popular Science Monthly Volume 17.djvu/28

18 stituted. But the spring itself gave out a tone when the diaphragm was in vibration, and was therefore objectionable. To overcome this difficulty thicker wire was used for the spring, and with better results. Trials were made with wires of different thicknesses, and it was found that the results improved as the thickness of the wire was increased, until finally the best results were obtained by using a piece of solid material rigidly secured to the diaphragm and ivory plate. It then occurred to Mr. Edison that, inasmuch as the working of his instrument depended upon changes of pressure only, there would be no need of having a vibrating diaphragm at all. A heavy diaphragm was therefore constructed and rigidly fastened to the carbon disk, so that the loudest tones would produce no vibration in it. With this arrangement the articulation was perfect, and, because the comparatively large area of the inflexible plate produced a greater pressure upon the carbon for a given tone than could be obtained when only the one point of the plate or diaphragm was used, the volume of sound was so magnified that a whisper three feet from the instrument was distinctly intelligible at the other end of the line.

Besides greater simplicity of construction, the carbon telephone possesses advantages over all others. With the telephone, as with an ordinary telegraphic instrument, there is a limit beyond which it fails to be of service, but with the telephone this limit is sooner reached than with the ordinary instruments. For this two causes are assigned: 1. The greater rapidity with which the electric impulses are sent over the line in the use of the telephone allows the line less time for charge and discharge than in Morse circuits where the transmission is done by hand; 2. The inductive action of currents passing through neighboring wires often renders the signals indistinguishable. These disturbances occur with all telephones, but they are least noticeable with the carbon telephone, because with it a stronger current is used, and therefore less sensitive receivers are required. Mr. Henry Bentley, President of the Local Telegraph Company at Philadelphia, made a set of experiments with this apparatus upon the lines of the Western Union Telegraph Company, which were on poles along with other wires through which currents were passing sufficiently strong to render the magneto-telephone useless, and found it entirely successful for a distance of from one hundred to two hundred miles. He has succeeded in using it upon a line seven hundred and twenty miles long. His experiments also show that the instrument can be used in a Morse circuit with a battery and eight or ten way-stations, using the ordinary telegraphic apparatus. It can also be used upon a wire which is at the same time being worked quadruplex.

The carbon telephone is rendered even more efficient when used in connection with the electro-motograph receiver. For the