CHAPTER XXXV 



HOW ELECTRICITY IS MADE ON A LARGE SCALE 



318. The Dynamo. We have learned that cells furnish 

 current as a result of chemical action, and that the substance 

 usually consumed within the cell is zinc. Just as coal within 

 the furnace furnishes heat, so zinc within the cell furnishes 

 electricity. But zinc is a much more expensive fuel than 

 coal or oil or gas, and to run a large motor by electricity pro- 

 duced in this way would be very much more expensive than 

 to run the motor by water or steam. For weak and infre- 

 quent currents such as are used in the electric bell, only small 

 quantities of zinc are needed, and the expense is small. But 

 for the production of such powerful currents as are needed 

 to drive trolley cars, elevators, and huge machinery, enormous 

 quantities of zinc would be necessary and the cost would be 

 prohibitive. It is safe to say that electricity would never 

 have been used on a large scale if some less expensive and 

 more convenient source than zinc had not been found. 



319. A New Source of Electricity. It came to most of us 

 as a surprise that an electric current has magnetic properties 

 and transforms a coil into a veritable magnet. Perhaps it 

 will not surprise us now to learn that a magnet in motion 

 has electric properties and is, in fact, able to produce a current 

 within a wire. This can be proved as follows: 



Attach a closely wound coil to a sensitive galvanometer 

 (Fig. 237); naturally there is no deflection of the galvanom- 



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