Figure 76. — Patent drawing of Runnings' 

 carbon granule telephone transmitter. From 

 U.S. patent 246512 (August 30, 1881). 



ther refined his approach by basing his experimenta- 

 tion upon the fact that the apparent resistance of semi- 

 conductors varied considerably with the pressure, and 

 a few months later he created a successful transmitter. 

 (See figs. 71, 72.) 



Edison filed the applications for the patents''^ on 

 the first of his many forms of the carbon transmitter 

 on April 1 and 27 and July 20, 1877. Basically 

 Edison's transmitter consisted of a mass of carbon in 

 various shapes and textures, against which a vibrat- 

 ing diaphram pressed. Change of pressure on the 

 diaphram brought a change of resistance of the car- 

 bon and so modulated the current. Edison's device 

 soon proved to be better than Bell's magneto trans- 

 mitter; and although it was insensitive by modern 

 standards, shouting was no longer mandatory in 

 order to carry on a telephone conversation. The 

 Edison transmitter was rugged, and it gave a better 

 quality of reproduction than Bell's instrument. 



Figure 77. — Diagram of White's "solid-back" 

 telephone transmitter. Reprinted (with per- 

 mission) from F. Rhodes, Beginnings of Telephony, 

 Harper and Brothers, New York, 1929, p. 81. 



In England the carbon transmitter was suggested 

 independently by David Hughes in 1878.*^ Hughes 

 revived the term "microphone" to describe his var- 

 iable contact transmitter with its remarkable sensi- 

 tivity. Hughes' microphone (fig. 73) was constructed 

 of several pieces of carbon that rested loosely on or 

 against one another. This whole mass of carbon 

 was mounted on a sounding box. When Hughes 

 announced his discovery, he disclaimed any inten- 

 tion of taking out a patent because his laboratory 

 model, although as sensitive as it was simple, was too 

 erratic in performance to be practical. 



In America applications for patents on other mod- 

 ulating transmitters were submitted, and it was soon 

 found that there was duplication in their principles 

 of operation and design. Because of various delays, 

 Berliner's patent was not issued until 1891, and Edi- 

 son did not receive his until 1892. Although the 

 Patent Office originally ruled in Berliner's favor, 

 in 1894, 1895, and 1901 successive courts declared 

 his patent void because of the delays and because of 



55 Thomas Edison, U.S. patents 474230 and 474231 (May 

 3. 1892) and 203016 (April 30, 1878); Prescott, op. cit. (footnote 

 48), pp. 218-234; Bell's Electric Speaking Telephone, New York, 



1884, pp. 126-174. Patent Office models of some of Edison's 

 microphones are preserved in the Museum of History and 

 Technology of the Smithsonian Institution. 



SI David Hughes, "On the Physical Action of the Micro- 

 phone," Pkilosophical Magazine, 1878, vol. 6, pp. 44-50. 



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



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