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Fig 6 



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Figure 62. — Drawings from Bell's famous patent of 1876, showing how his 

 invention could be used to transmit and receive both continuous and dis- 

 continuous sound waves. From U.S. patent 174465 (March 7, 1876). 



Bell's backers deemed his progress sufficient for a 

 demonstration in June 1876 of several forms of his 

 membrane telephone (fig. 64) together with his 

 multiple telegraph at the Philadelphia Centennial 

 Exposition. It was at this exposition that the famous 

 Dom Pedro incident occurred, which is now so 

 familiar that it need not be repeated again. 



Bell was so discouraged, however, by the poor 

 efficiency of his membrane telephones that he felt 

 it necessary to drop work on his multiple telegraph 

 in order to devote all his time to the telephones. 

 They would work moderately well over a short line, 

 but the apparatus was delicate and did not articulate 

 sounds distinctly enough for practical use. Bell 

 never eliminated the necessity of shouting into his 

 magneto telephone, but he did succeed in improving 

 it somewhat. Early in October of 1876 Bell replaced 

 the steel reed that he had been using on the membrane 



by a steel plate almost as large as the membrane 

 and glued to it. About the same time Watson re- 

 placed the soft iron core of the electromagnet with 

 a compound permanent magnet. After these changes 

 were made. Bell and Watson tested the new apparatus 

 between two rooms in the building where their 

 laboratory was located and found they had no dif- 

 ficulty in carrying on a "sustained conversation." 

 On October 9, 1876, "sustained conversation was 

 successfully carried on for the first time upon a real 

 line several miles in length." 



These improvements inspired Bell a week later to 

 replace the membrane by an all-metal diaphragm 

 clamped around the edge. After placing the resulting 

 structure in a box as large as a professional photog- 

 rapher's camera of the time. Bell and Watson tested 

 this "box" telephone (fig. 65) over a line 4 or 5 miles 

 long and found thev could maintain a conversation. 



PAPER 29: DEVELOPMENT OF ELECTRICAL TECHNOLOGY IN THE 19TH CENTURY: II 

 576628—62 4 



321 



