Figure 56. — Gray's harmonic multiple telegraph of 1875. 

 Above, transmitter; below, receiver. By tuning each pair to 

 the same frequency and different pairs to different fre- 

 quencies, as many telegraph messages as there were pairs 

 could be sent over the same line. From U.S. patents 

 165728 (July 20, 1875) and 166094 (July 27, 1875). 



Figure 57. — Gray's telegraph for transmitting 

 musical tones, 1875. A single receiver could 

 respond to any frequency over a broad range. 

 From U.S. patent 166095 (July 27, 1875). 



telegraph vt'ire, then so could the human voice. At 

 first he tried to devise an instrument capable of 

 separately reproducing each of the most common 

 tones of the human voice. This was necessarily a 

 difficult task because a diflferent unit was contem- 

 plated for each of the main parts of human speech. 

 Late in 1875, however. Gray came upon the so-called 

 "lover's telegraph," which consisted of a short 

 cylinder of metal or wood that was open at one end 

 and had a membrane across the other end. The 

 centers of the membranes of two such instruments 

 were connected by a taut wire. When a person spoke 

 into one cylinder, the speech could be heard in the 

 other. This device showed Gray that the vibrating 

 diaphragm receiver he had already invented (fig. 57) 

 should be capable of repeating speech transmitted to 

 it, and that consequently part of his task had already 

 been accomplished. He had only to devise a trans- 

 mitter, which he did a short time later. 



The transmitter that Gray designed was a cylinder, 

 at one end of which was a diaphragm with a light 

 metal wire fastened to the side facing away from the 

 cylinder. The cylinder was placed over a container 



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



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