176 ALEXANDER GRAHAM BELL 



in order to discriminate the signals; and secondly, that the 

 signals could only pass in one direction along the line (so that 

 two wires would be necessary in order to complete communi- 

 cation in both directions). The first objection was got over 

 by employing the device which I term a ''vibratory circuit 

 breaker," whereby musical signals can be automatically re- 

 corded. . . . 



I have formerly stated that Helmholtz was enabled to 

 produce vowel sounds artificially by combining musical tones 

 of different pitches and intensities. Tuning forks of different 

 pitch were placed between the poles of electromagnets, and 

 are kept in continuous vibration by the action of an inter- 

 mittent current from one fork. Resonators are arranged so 

 as to reinforce the sounds in a greater or less degree, accord- 

 ing as the exterior orifices are enlarged or contracted. 



Thus it will be seen that upon Helmholtz 's plan the tun- 

 ing forks themselves produce tones of uniform intensity, the 

 loudness being varied by an external reinforcement; but it 

 struck me that the same results would be obtained, and in 

 a much more perfect manner, by causing the tuning forks 

 themselves to vibrate with different degrees of amplitude. 

 I therefore devised the apparatus which was my first form of 

 articulating telephone. A harp of steel rods was employed, 

 attached to the poles of a permanent magnet. When any 

 one of the rods is thrown into vibration an undulatory cur- 

 rent is produced in the coils of the electromagnet, and the 

 electromagnet attracts the rods of the harp with a varying 

 force, throwing into vibration that rod which is in unison 

 with that vibrating at the other end of the circuit. Not 

 only so, but the amplitude of vibration in the one will deter- 

 mine the amplitude of vibration in the other, for the intensity 

 of the induced current is determined by the amplitude of the 

 inducing vibration, and the amplitude of the vibration at the 

 receiving end depends upon the intensity of the attractive 

 impulses. When we sing into a piano, certain of the strings 

 of the instrument are set in vibration sympathetically by the 

 action of the voice with different degrees of amplitude, and a 

 sound, which is an approximation to the vowel uttered, is pro- 

 duced from the piano. Theory shows that; had the piano 



