DEVELOPMENT OF THE TELEPHONE 



served by an operator standing there. It seemed to me 

 that in this way a number of distinct telegraph messages 

 might be sent simultaneously from the tuning-fork piano 

 to the other end of the circuit, by operators, each ma- 

 nipulating a different key of the instrument. These 

 messages would be read by operators stationed at the 

 distant piano, each receiving operator listening for 

 signals of a certain definite pitch, and ignoring all others. 

 In this way could be accomplished the simultaneous 

 transmission of a number of telegraphic messages 

 along a simple wire, the number being limited only 

 by the delicacy of the listener's ear. The idea of in- 

 creasing the carrying power of a telegraph wire in this 

 way took complete possession of my mind, and it was 

 this practical end that I had in view when I commenced 

 my researches in electric telephony." 



Bell then entered into a brief discussion of telephonic 

 currents, with graphic illustrations, and continued : 



"Nine varieties of telephonic currents may be distin- 

 guished, but it will only be necessary to show you six 

 of these. The primary varieties designated are 'inter- 

 mittent,' ' pulsatory,' and 'undulatory.' 



" Sub- varieties of these can be distinguished as 'direct' 

 or 'reversed' currents according as the electrical im- 

 pulses are all of one kind or are alternately positive and 

 negative. 'Direct' currents may still further be dis- 

 tinguished as 'positive' or 'negative,' according as the 

 impulses are of one kind or of the other. 



"An intermittent current is characterized by the al- 

 ternate presence and absence of electricity upon the 

 circuit; 



