Figure 59. — Reproductions of Bell's harmonic multiple 

 telegraph transmitter and receiver. (USNM 208211, ■^08212; 

 Smithsonian photo 1^204.) 



Ontario, to Boston, Massachusetts, Bell started experi- 

 menting with such a telegraph. He first considered 

 using tuning forks to interrupt the circuit at the 

 transmitting end, for a tuning fork could produce a 

 response only in another electrically driven tuning fork 

 of the same frequency. A number of forks of different 

 frequencies could thus be used for multiple telegraphy 

 on a single telegraph line. In November 1873 

 Bell replaced the tuning forks with steel reeds. How- 

 ever, difficulties in putting his concepts into practice 

 caused Bell to drop his experimentation for a while. 



In the meantime, however, Bell did not cease 

 developing his ideas. During the summer of 1874 

 it occurred to him that if a magnetized reed were 

 vibrated before a coil of wire it would induce a fluctu- 

 ating current in which the vibrations would corre- 

 spond exactly to the sound waves causing the current. 

 If this undulatory current could actuate at the end 

 of the line an instrument similar to the one producing 

 the current, such a receiver would produce a re- 

 sponse — but only in a receiver tuned to the same fre- 

 quency as that of the transmitter. If those conditions 

 were met, much of the auxiliary apparatus used in 

 most electromagnetic communication devices to 

 interrupt and power the circuit could be eliminated. 

 Moreover, with such a device there would be an exact 

 reproduction of the sound waves transmitted in the 

 form of an undulatory current rather than by the set 

 of pulses produced by a vibrating reed. 



Bell also speculated that if at one end of a line 

 there were a set of magnetized reeds of varying 

 lengths (like the reeds in a harmonica or Aeolian 

 harp) acting as armatures for an electromagnet and 

 a similar instrument at the other end of the line, 

 such a "harp" apparatus (fig. 60) would be capable 



of transmitting and reproducing complex tones in the 

 same manner that Helmholtz compounded complex 

 sounds with his tuning forks. Indeed, a pair of such 

 instruments might even be capable of transmitting 

 human speech over a telegraph wire. 



Further speculation suggested to Bell that his 

 complex "harp" apparatus might be reduced to a 

 single vibrator. Among the acoustical instruments 

 that Professor Charles Cross of the Massachusetts 

 Institute of Technology showed Bell during the winter 

 of 1873-1874 were Koenig's manemetric capsule, 

 Scott's phonautograph, and possibly Reis' telephone. 

 Bell's consideration of these instruments brought to 

 his attention the similarity between the mechanical 

 motions of the diaphragms used in them and the 

 motion of his vibrating reeds. He thought that 

 perhaps, if he attached a inagnetized armature to 

 the center of a membrane that vibrated under the 

 influence of a human voice, currents could be in- 

 duced that might reproduce the human voice in 

 a similar instrument installed at a distance. This 

 first form of Bell's magneto telephone is described 

 in his patent of 1876. However, when Bell first made 

 these speculations, he rejected them as impractical, 

 for he held that not enough current could be induced 

 either in the "harp" instrument or in the membrane 

 instrument to actuate a receiver. 



Instead of completing work on his telephone. Bell 

 continued to experiment with his harmonic multiple 

 telegraph, for which he obtained a U.S. patent 

 (161739) on April 6, 1875 (fig. 59). Bell successfully 

 demonstrated his telegraph to Western Union officials 

 in March 1875, showing how two messages could be 

 sent simultaneously over 200 miles of telegraph line. 

 Work on the telephone was still in progress, however. 



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



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