65<) THi: INVENTORS OF THE TELEGRAPH AND TELEPHONE. 



inability to produce, with any approacli to iiccuracy, the necessary 

 variations of loudness and (quality. 



To i)roduce not only the fre«iuency of vibration, but also the loudness 

 and quality of the sounds evidently required a transmitter and a 

 receiver which did not depend for its action on simple interruption 

 of the current, but which varied it in an undulating manner, similar to 

 the variations of ijressure to which the diaphragm receiving the sound 

 vibrations was subjected due to the sound waves. Such an apjiaratus 

 of a very x>erfect type was produced by Graham Bell in 187(!, who, in 

 the descriptions of his apparatus given in his patent specifications and 

 elsewhere, shows that he thoroughly understood what had to be done. 

 We all know from actual experience that the instrument a\ hich he pro- 

 duced did it. Since the publication of Bell's invention a great many 

 modifications have been produced. Most of them have, however, been 

 held to embody the same essential principle as that of Bell, the varia- 

 tion being simply one of mechanical arrangement. One field of inves- 

 tigation has, however, been fruitful of improvement. In the original 

 patent of Bell, and also in a caveat filed almost simultaneously by 

 Elisha Grayjit is pointed out that the variations of the current may be 

 l)roduced by causing the vibrations of the diaphragm to vary the 

 resistance of the circuit. This idea has proved of great value in increas- 

 ing the loudness of the sounds given out by the Bell telephone when 

 used as a receiver. A great many forms of these "-microphone" trans- 

 mitters have been invented. Among those who have made im- 

 j)ortant contributions we may mention Berliner, Blake, Edison, Gower, 

 Gray, Huglies ami Hunnings. 



Another form of telephone has been ])ro})osed by Prof. Dolbear. 

 In this telephone system one diaphragm of the receiver is made to 

 form one plate of an electric condenser, and tiie varying electric force 

 on this plate, due to the fluctuations of the charge, causes it to vibrate 

 in res]>onse to the varying electro-motive force produced by the trans- 

 mitter. This condenser telephone can evidently be used either as a 

 transmitter or as a receiver, and, as Dolbear has pointed out, may be 

 rendered sensitlv<' by keeping one plate of the condenser at a high 

 potential. 



Another interesting discovery in this subject should be mentioned, 

 namely, the transmission of speech from one place to another by means 

 of beams of light or radiant heat. This was based originally on the 

 discovery by May and Smith of the variation of the electric resistance 

 of selenium when exposed to light or radiant heat. Many otlier sub- 

 stances have since been found to have the same property in a greater 

 or less degre<'. The experiments of Bell and Sumner Tainter have 

 shown that if a beam of light be reflected fr(»m a thin mirror, and, by 

 means of lenses oi' otherwise, made to pass as a i>arallel beam from the 

 transmitter to the receiving station, and there received mi a bar or 

 series of bars, or a coil of a substance having the pro])erties of selenium, 



