Figure 55. — Reis telephone. Transinitter (top); 

 detail of diaphragm contact {middle); and 

 receiver. 



Western Electric Manufacturing Company. Some- 

 time during the winter of 1873-1874, he began to 

 construct instruments that could transform mechanical 

 vibrations into electrical signals and then change the 

 signals back again into vibrations. 



The transmitters that Gray used consisted of 

 electromagnets that caused a metal reed to vibrate, 

 thereby interrupting, at a definite rate, the flow of 

 current in the circuit. By May 1874 Gray had 

 constructed a transmitter of eight such vibrators 

 corresponding to the eight notes of a diatonic scale. 

 By July he had an instrument that could produce 

 two such octaves of alternating current. 



At the same time that Gray was working on his 

 transmitters, he was also trying various kinds of 



receivers. He devised two main types. One was 

 based upon the fact that if electricity flows between 

 two solids in rubbing contact, the friction between the 

 two bodies will change with the voltage applied 

 across them. The other type of receiver was based 

 upon the phenomenon already used by Helmholtz 

 and Laborde: the mechanical vibrations produced in 

 the armature of an electromagnet carrying an alter- 

 nating current correspond to the frequency of the 

 current. Gray found the latter method to be more 

 useful, although the former was not too impractical, 

 for Edison's chalk telephone was subsequently to be 

 based upon it. Gray decided in the spring of 1874 

 that the receiver should be a circular metal dia- 

 phragm — either partially clamped along its edge or 

 entirely clamped around its circumference — driven 

 by an electromagnet. One of his experimental dia- 

 phragms finally was a metal washbasin (USNM 

 214296) and the other a metal cover of a shoe-polish 

 can (visible in fig. 57). 



If one struck out a tune on the keyboard of Gray's 

 transmitter, then the receiver at the end of the tele- 

 graph line would play the tune. Gray demonstrated 

 his device to officials of the Western Electric Com- 

 pany in New York City in May 1 874 and to Joseph 

 Henry at the Smithsonian Institution in the following 

 month. He demonstrated it in London in December 

 of the same year. By January 1875 he had worked 

 out a patentable system for his electric organ ^^ 

 (fig. 57). 



Gray found he could also apply his experimentation 

 to multiple telegraphy. If there were several stations 

 connected to the same telegraph line, each station 

 with its own transmitting frequency, then signals 

 from the different stations could be detected only if a 

 given receiver was tuned to the appropriate fre- 

 quency. He applied for a patent on a harmonic 

 multiple telegraph system (fig. 56) on June 28, 1875, 

 and he received the patent on July 20, 1875.*^ He 

 also applied his method of transmitting tones of 

 different frequency to the invention of a printing 

 telegraph. ^^ 



In 1875, while working on the transmitter for his 

 multiple telegraph system. Gray realized that if a 

 number of tones could be sent at the same time over a 



« U.S. patents 165728 (July 20, 1875), transmitter; 166094 

 (July 27, 1875), receiver; 166095 (July 27, 1875), diaphragm 

 receiver; 166096 Quiy 27, 1875), transmitter. 



« U.S. patent 173460. 



" U.S. patent 179549. 



316 



BULLETIN 228: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



