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- GENERATOR MOTOR 



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 



trrnnnH wirp Thiss ic PvapfKr FlG - 105 ' Diagram to show gener- 



und wire, inis is exactly ator and motor connected . the electric 



what One finds in the Opera- current is measured by galvanometer 



tion of motors of street cars ^ G)< 



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 

 takes off the current gener- 

 ated in the revolving coils in 

 only one direction, as illus- 

 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- 



CONTROLLEB 



FIG. 106. Diagram of course of elec- 

 tric current from generator along trolley 

 or third rail to motor of a street car and 

 from this through the rails to the power 

 station. 



