406 ELEMENTARY LESSONS ON [CHAP. xn. 



ment consisted of an iron wire fixed upon a sounding 

 board and surrounded by a coil of insulated wire forming 

 part of the circuit. The rapid magnetisation and demag- 

 netisation of the iron core produced audible sounds (Art. 

 113), which, since the pitch of a note depends only on 

 the frequency and not on the form or amplitude of the 

 vibrations, reproduced the pitch of the note sung into 

 the transmitting part. Reis also transmitted speech with 

 this instrument, but very imperfectly, for the tones of 

 speech cannot be transmitted by abrupt interruptions of 

 the current : they require gentle undulations, sometimes 

 simple, sometimes complex, according to the nature of 

 the sound. The vowel sounds are produced by periodic 

 and complex movements in the air ; the consonants 

 being for the most part non-periodic. 



435. Graham Bell's Telephone. In 1876 Graham 

 Bell invented the articulating telephone. In this instru- 

 ment the speaker talks to an elastic disc of thin sheet iron, 

 which vibrates and transmits its every movement electric- 

 ally to a similar disc in a similar telephone at a distant 

 station, causing it to vibrate in an identical manner, and 

 therefore to emit identical sounds. The transmission of 

 the vibrations depends upon the principles of magneto- 

 electric induction explained in Lesson XXXVI. Fig. 

 1 66 shows Bell's Telephone, and its internal parts in 

 section. The disc D is placed behind a conical mouth- 

 piece, to which the speaker places his mouth or the 

 hearer his ear. Behind the disc is a magnet A A running 

 the length of the instrument ; and upon its front pole, 

 which nearly touches the disc, is fixed a small bobbin, on 

 which is wound a coil C of fine insulated wire, the ends 

 of the coil being connected with the terminal screws F F. 

 One such instrument is used to transmit, and one to 

 receive, the sounds, the two telephones being connected 

 in simple circuit. No battery is needed, for the trans- 

 mitting instrument itself generates the induced currents 

 as follows : The magnet A A induces a certain number 



