Electricity 



235 



Connect two heavy insulated copper wires, about No. 12, 

 to the sides of the knife switch just as you connected the 

 fine wires. But this time bring the ends of the copper wires 

 themselves together for an instant, then draw them apart. 

 Hold the ends of the wires over the zinc of the table while 

 you do this, as melted copper will drop from them. 



What happens when an arc is formed. What happens 

 when you form an electric arc is this : As you draw the 

 two ends of the pencils apart, only a speck of the lead 

 in each touches the other. The electricity passing for 

 an instant through the last speck at the end of the pencil 

 makes it so hot that it turns to vapor. The vapor will 

 let electricity go through it, and makes a bridge from 

 one pencil point to the other. But the vapor gets very 

 hot, because it has a rather high resistance. This heat 

 vaporizes more carbon and makes more vapor for the 



FIG. 134. A brilliant arc light is the result. 



