THE DYNAMO 



497 



FIGURE 169 



in the strength of the permanent bar magnet of that 

 instrument. The rapid variations of strength of this mag- 

 net cause the disk in front of it to vibrate in the 

 same way that the first disk vibrated and thus to throw 

 out sound waves similar to those of the 

 speaker's voice. The sound is in no sense 

 transmitted. The sound waves are trans- 

 formed into electrical impulses which are 

 transmitted to the other instrument, where 

 they are again transformed into sound waves. 



For complicated modern telephone systems, a different 

 instrument is used for transmitting (Figure 169), but the 

 principle involved is the same. The instrument described 



is still used for re- 

 ceiving, except that 

 the bar magnet has 

 been replaced by a 

 U-shaped magnet. 



The Dynamo. - 

 The dynamo is a pro- 

 foundly important 

 result of Faraday's 

 discovery. In the 

 dynamo, coils of wire 

 are revolved between 

 strong magnetic 

 poles, and the cur- 

 rents of electricity 



which are generated are collected and delivered to the line 

 wire to be used wherever desired. In commercial machines, 

 there are usually several pairs of electromagnets and many 



DYNAMO 



