CHAPTER EIGHT 



ELECTRICITY 



SECTION 33. Making electricity flow. 



What causes a battery to produce electricity? 

 What makes electricity come into our houses? 



The kind of electricity you get from rubbing (friction) 

 is not of much practical use, you remember. Men had 

 to find a way to get a steady current of electricity before 

 they could make electricity do any work for them. 

 The difference between static electricity when it leaps 

 from one thing to another and flowing electricity is a 

 good deal like the difference between a short shower of 

 rain and a river. Both rain and river are water, and the 

 water of each is moving from one place to another; 

 but you cannot get the raindrops to make any really 

 practical machine go, while the rivers can do real work 

 by turning the wheels in factories and mills. 



Within the past century two devices for making elec- 

 tricity flow and do work have been perfected : One 

 of these is the electric battery; the other is the 

 dynamo. 



The electric battery. A battery consists of two pieces 

 of different kinds of metal, or a metal and some carbon, 

 in a chemical solution. If you hang a piece of zinc and 

 a carbon, such as comes from an arc light, in some water, 

 and then dissolve sal ammoniac in the water, you will 

 have a battery. Some of the molecules of the sal am- 

 moniac divide into two parts when the sal ammoniac 

 gets into the water, and the molecules continue to divide 

 as long as the battery is in use or until it " wears out." 

 One part of each molecule has an unusually large number 



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