130 ELECTRICITY 



152. How Electrical Energy is produced. We have 

 learned that in order to produce any sort of energy we 

 must transform some other kind of energy into the de- 

 sired sort ( 98). In general we may say that the kinds 

 of energy commonly transformed into electrical energy 

 are heat, chemical energy, and mechanical energy. Elec- 

 trical energy seems also to be developed upon certain 

 bodies by friction, by simple contact, and by dipping 

 into certain liquids. For common uses, the devices for 

 producing electricity are usually the voltaic cell or the 

 dynamo. These will be explained in a later section. 



153. How Electrical Energy is controlled. Electrical 

 energy is easily controlled because electricity passes 

 readily through some substances and not through others. 

 A body through which electricity passes easily is called a 

 conductor; one through which it passes with great diffi- 

 culty, or not at all, is called an insulator. It must be 

 clear that if a conductor is surrounded by an insu- 

 lator, electricity may be kept within the conductor and 

 made to travel long distances sometimes. This gives 

 a partial idea of how electrical energy is controlled; 

 a fuller explanation of conductors and insulators is made 

 in 157. 



Among the better conductors are metals (copper, zinc, 

 iron, etc.), water, animal bodies, the earth, and others. 

 Of insulators, dry air is one of the best; more than 

 any other substance, perhaps, it keeps electricity from 

 spreading about where it would only be lost and might 

 do injury. Other insulators are w*ood, cloth, rubber, 

 and glass. 



