436 



GENERAL SCIENCE 



to cause an iron membrane or disc to vibrate when sound waves 

 struck it; and if the vibrations of this membrane could cause elec- 

 trical currents to pass over a wire and make another membrane 

 vibrate exactly like the first membrane, a person listening at the 

 other end would be able to hear the membrane producing sounds 

 like the human voice. The telephone is made on this principle 



FIG. 330. A section of a record produced by a soprano voice. Magnified. 



In other words, a solid metallic body takes up from the air count- 

 less varieties of vibrations produced by speech, and these vibra- 

 tions are carried along the wire by means of electricity, and 

 are reproduced exactly by another metallic disc on the other end of 

 the wire. Of course it is not the sound which actually travels over 

 the wire. Electrical currents are caused by the vibration of the disc 



