MAGNETISM AND ELECTRICITY 



203 



bulb from which air has been almost exhausted. When 



the current passes through the filament, it is heated to 



the point of incandescence. It does 



not burn because of lack of oxygen 



(Figure 172). 

 At the present time tungsten and 



tantalum filaments have largely re- 

 placed the carbon 

 for use in incandes- 

 cent lamps, since 

 they are nearly three 

 times as efficient as 

 the carbon lamps. 



The electric 

 " arc " light is pro- 

 duced by placing 

 two carbon rods end 

 to end in a strong 



electric current. If the carbons are sepa- 

 rated slightly after the ends are heated 

 red hot, the current will continue to flow. 

 The conducting layer of incandescent 

 vapor between the ends of the carbon is 

 called the electric arc (Figures 173-174). 

 The temperature in this arc is the hottest 

 that man has been able to produce. It 

 will vaporize all known substances. 

 When the electric arc is inclosed in a box 



FIG. 174. TheEiec- (Figure 175) made of high heat-resisting 

 trie Arc Lamp. materials, it is called an electric furnace. 

 Magnetic Effects of Currents. If we hold a wire 



carrying a current near a compass, the needle will be 



deflected. If the current is moving from south to north 



FIG. 173. The Electric 

 Arc. 



