496 



MAGNETISM AND ELECTRICITY 



at the end by a coil of fine wire. In front of the magnet, 

 and almost touching it, is mounted a thin iron disk. Above 

 this a concave rubber cap with a hole in the center com- 

 pletes the case. The ends of the coil of wire are 

 connected with the wires from the coil of another 

 instrument of the same kind. One of the wires 

 from each coil may be connected with the ground. 

 The sound waves from the voice (or from any 

 other source) cause the disk to vibrate back and 

 forth in front of the magnet. These rapid vibra- 

 tions of the disk result in correspondingly rapid 

 changes in the strength of the magnet, and momentary 

 electrical currents are induced in the coil of wire. These 

 electrical impulses flow to the coil of wire in the other 

 instrument, where they cause correspondingly rapid changes 



FIGURE 168 



TELEPHONE STATION IN THE TRENCHES DURING THE WORLD WAR 



