OF ARTS AND SCIENCES. 77 



V. 



EESEARCHES IN TELEPHONY. 

 By Professor Dolbear. 



Presented Nov. 13, 1878. 



I began my experiments in Telephony in August, 1876, my first 

 attempts being with a Helmholtz interruptor, using forks whose vibra- 

 tions were 64, 128, and 256 per second respectively. For a receiver 

 a coil of wire about a small rod of iron, as in Page's experiment in 

 1837, in which he used an automatic interruptor. I also used several 

 forms and sizes of electro-magnets as receivers. 



With all of these arrangements the reproduction of the sound was 

 well marked. The next step was to cause the vibrations produced by 

 the voice to make and break the circuit in a manner analogous to that 

 of the vibrating forks. To this end a platinum wire was made fast 

 to the end of an opeidoscope tube having a membrane of stretched 

 rubber over the end. The wire was bent at right angles at the middle 

 of the membrane, and projected about half an inch from it. This wire 

 was coupled in circuit with the instruments mentioned above as re- 

 ceivers, and the tube being placed at the mouth while the tip of the 

 wire on the membrane dipped into mercury, also into water acidulated, 

 some of the sounds were very audible ; but it was necessary to make 

 the membrane vibrate with amplitude enough to break completely the 

 circuit, or no sound whatever could be heard. From this I concluded that 

 the difference of resistance interposed by the vibrations was not great 

 enough. I therefore sought for some means to increase the difference, 

 and made for that purpose a small cone of iron, which was soldered to 

 the wire upon the opeidoscope tube, for the reason that, when the cone 

 was plunged into the mercury, a slight motion of the membrane would 

 materially vary the cross-section immersed and consequently the inter- 

 posed resistance. With this arrangement no better results were ob- 

 tained, as it was found that the mercury bounded away from the cone 

 when the latter was vibrating. I therefore sought for other means for 

 varying the current strength. 



