MISCELLANEOUS PRACTICAL APPLICATIONS. 



337 



perceptible extent. When, however, electric waves act upon the 

 antenna, the slight current which is produced in the antenna is 

 forced through the filings and produces what seems to be a 

 welding together of the particles of the filings at the points 

 of contact. At any rate, as long as a slight amount of current 

 is forced through the filings from the antenna, the filings form a 

 good conducting path for the battery current and the sounder is 

 excited, but, the moment the electric waves cease, the vibratory 

 motion of the metal filings causes them to become detached from 

 each other and the battery current ceases. 



Figure 40 shows the trend of the electric lines of force in the 

 electric waves as they approach the receiving antenna. The 



magnetic lines of force are horizontal and perpendicular to the 

 plane of the paper. 



39. Electric lighting. One of the most extended applications 

 of the electric current is in the production of artificial illumination. 

 This is usually accomplished by the heating to incandescence of a 

 high resistance portion of a circuit, by the electric current. The 

 high resistance portion of the circuit, together with its mounting, is 

 called an electric lamp. Two types of electric lamp are in gen- 

 23 



