WHAT MAKES THE TROLLEY CAR GO. 



417 



Electric currents must always circulate in closed paths — that 

 is, the current that starts out from a generator must return to it, 

 and the amount coming back is the same as that which leaves. 

 The action of an electric generator can be understood by compar- 

 ing it with that of a water pump pumping into a pipe which runs 

 around from the delivery end to the suction. With such an ar- 

 rangement it can be seen that the action of the pump would be to 

 keep the water in circulation, but the same water would be pumped 

 through the pump and the pipe all the time. "With an electric 



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Fig. 22.- 



-DlAORAM ILLUSTRATING THE Ma>'NER IN WHICH THE ElECTRIO CURRENT FLOWS 



FROM THE Generators to the Cars upon the Tracks. 



generator the action is the same, and in Fig. 22 the current flow- 

 ing along any one of the tracks follows the course indicated by the 

 arrows. The currents pass out to the several tracks through the 

 trolley wires T T T T, and return through the tracks R R R R. 

 The bus bar A is connected with a plate D, which is imbedded in 

 the ground, and is also connected with the ends of the rails 

 R R R R. Suppose for a moment that the two lower generators 

 are out of service, their switches a a being turned so as to discon- 

 nect them from the bus A, and, further, suppose that the three 

 lower h switches are open, so that the current can only pass to the 



