2io Common Science 



spinning coil of wire, as shown in Figure 116. Such 

 an arrangement as this is really an extremely simple 

 dynamo. 



You could make a dynamo that would actually work, 

 by arranging such an apparatus so that the coil would 

 spin between the poles of the magnet. But of course 

 the big commercial dynamos are very much more com- 

 plicated in their construction. Figure 116 shows only 

 the general principle on which they work. The main 

 point to note is that by spinning a coil of wire between 

 the poles of a magnet, you can make electricity flow 

 rapidly through the wire. And it is in this way that 

 most of the electricity we use is made. 



The power spinning the coil of wire is sometimes 

 steam, and sometimes gasoline or distillate ; and water 

 power is very often used. A large amount of our elec- 

 tricity comes from places where there are waterfalls. 

 Niagara, for instance, turns great dynamos and generates 

 an enormous amount of electricity. 



Why many automobiles have to be cranked. In an 

 automobile, the magneto is a little dynamo that makes 

 the sparks which explode the gasoline. While the 

 automobile is going the engine spins the coil of wire 

 between the magnets, but at starting you have to spin 

 the coil yourself ; and doing that is called " cranking " 

 the automobile. " Self-starters " have a battery and 

 motor to spin the coil for you until the engine begins 

 to go ; then the engine turns the coil of the magneto. 



How old-fashioned telephones are rung. The old- 

 fashioned telephones, still often used in the country, 

 have little cranks that you turn to ring for central. 



