18 THE POPULAR SCIENCE MONTHLY. 



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 instru- 

 ment 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 arrange- 

 ment 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 neigh- 

 boring wires often renders the signals indistinguishable. These dis- 

 turbances 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 ren- 

 der the magneto-telephone useless, and found it entirely successful for 

 a distance of from one hundred to two hundred miles. He has suc- 

 ceeded 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 follow- 



* For a description of the motograph the reader is referred to Edwin M. Fox's article 

 in " Scribner's Monthly," June, 18T9. 



