236 Common Science 



electricity to pass through, and so on. The electricity 

 passing through the carbon vapor makes it white hot, 

 and that is what causes the brilliant glow. Regular 

 arc lights are made exactly like this experimental one, 

 except that the carbons used are much bigger and are 

 made to stand the heat better than the small carbons 

 in your pencil. 



Carbon is one of those substances that turn 'directly 

 from a solid to a gas without first melting. That is 

 one reason why it is used for arc lights. But copper 

 melts when it becomes very hot, as you saw when you 

 made an arc light with the copper wires. So copper 

 cannot be used for practical arc lights. 



Fires caused by arcs. There is one extremely impor- 

 tant point about this experiment with arcs : most fires 

 that result from defective wiring are caused by the form- 

 ing of arcs. You see, if two wires touch each other 

 while the current is passing and then move apart a 

 little, an arc is formed. And you have seen how in- 

 tensely hot such an arc is. Two wires rubbing against 

 each other, or a wire not screwed tightly to its connec- 

 tion, can arc. A wire broken, but with its ends close 

 enough together to touch and then go apart, can cause 

 an arc. And an arc is very dangerous in a house if 

 there is anything burnable near it. 



Wires should never be just twisted together and then 

 bound with tape to form a joint. Twisted wires some- 

 times break and sometimes come loose ; then an arc forms, 

 and the house catches fire. Good wiring always means 

 soldering every joint and screwing the ends of the wires 

 tightly into the switches or sockets to which they lead. 



