139 



musical tone is made up of a succession of 

 sounds repeated at equal intervals, and that 

 the pitch of the tone is determined by the num- 

 ber of sound-impulses per second. Applying 

 this law to the sparks, you will be able to see 

 how the switchboatd played tunes for the 

 operator. 



In the foregoing experiments in transmit- 

 ting musical tones telegraphically, I used a 

 great many different varieties of receivers. 

 Some of them were designed with metal dia- 

 phragms mounted over single electromagnets, 

 not unlike the receiver of an ordinary tele- 

 phone. These instruments would both trans- 

 mit and receive articulate speech when placed 

 in circuit with the right amount of battery to 

 furnish the necessary magnetism. However, 

 they were not used in that way at the time 

 they were first made in 1874. These I called 

 common receivers, as they were designed to 

 reproduce all tones equally well. I designed 

 and constructed another form of receiver, 

 based somewhat upon the theory of the har- 

 monic telegraph. 



This consisted of an electromagnet of con- 

 siderable size, mounted upon a wooden rod 

 about ten feet long. Mounted upon this rod 

 were also resonating boxes or tubes made of 

 wood of tin- right si/o in luivo ilioir air-cavi- 

 ties correspond with the various pitches of the 

 mitting-reeds, so that each tone would be 



