Transportation on Land 



327 



from a generator some distance from the motor. In either case 



the connections are as indicated in Fig. 105. The electromotive 



force produced by the generator is transmitted by a conducting 



wire to the motor. The pres- 



sure of the current is shown 



by the galvanometer G placed 



in the circuit. The electric 



current is made to operate the 



motor and then it returns to 



the generator by way of the 



ground wire. This is exactly 



current is measured 



galvanometer 



what one finds in the opera- 

 tion of motors of street cars 

 or electric locomotives. The current is transmitted by a 

 trolley wire or third rail to the motor and thence to the gen- 

 erator by way of the rails and the ground (Fig. 106). How is 

 the circuit completed in the case of the telegraph and telephone ? 

 144. Direct and alternating current generators. Electric 

 generators are of many different kinds, but they may be grouped 

 conveniently into two classes according to the kind of current 

 they produce. These are the direct or continuous current (des- 

 ignated as D. C.) and the 

 alternating current gener- 

 ators (designated as A. C.). 

 The direct current gener- 

 ator has a commutator that 

 FIG. 106. Diagram of course of elec- takes off the current gener- 



tric current from generator along trolley ated in the rev olving Coils in 



or third rail to motor of a street car and ... 



from this through the rails to the power Only One direction, as lllUS- 



station - trated in Fig. 107. The 



commutator is divided (Fig. 104) into two parts or segments 

 which are insulated from each other on the actual generator, 

 and not separated as shown in the diagram. The brushes are 

 placed so exactly in contact with the commutator that each 

 passes from one segment to the other at the moment the cur- 



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