168 



ELECTRICITY 



vacuum; if air were admitted, the hot carbon would 

 quickly burn up. 



In the arc lamp a current having very great electro- 

 motive force is made to pass through two carbon pencils 

 placed end to end. Because these pencils 

 just loosely touch each other, great resist- 

 ance is offered to the current at that point. 

 The current flowing against this resistance 

 heats the ends of the pencils to white heat. 

 Tiny bits of the carbon are detached from 

 one pencil and pass, in a glowing condition, 

 to the other pencil. Now the two points are 

 drawn apart to a distance of about a quarter 

 of an inch, the space between being filled 

 with these glowing particles (Fig. 125). 

 This space filled with the particles is called 

 an arc, and the current is conducted through 

 it. But the resistance is now even higher 

 than at first; thus the two carbon points and the arc 

 between are all very highly heated, so that they glow 

 and give a bright light. 



Arc lamps are used in lighting streets, halls, and 

 large rooms. They are arranged in series on the circuit 



FIG. 125 



IL_H_J 



FIG. 126 



(Fig. 126), and need a current of high potential. Each 

 lamp contains a device for keeping the carbon points the 

 proper distance apart as they are consumed. 



