GENERATING ELECTRICITY COMMERCIALLY 199 



regulation is desired. In starting any direct current motor, 

 it is necessary to put a considerable resistance in series with 

 the armature. Otherwise, the very low resistance of the 

 armature would permit the flow of an enormous current, 



FIG. 91. 



which would blow fuses or overheat armature coils and cause 

 excessive sparking. As the machine increases in speed this 

 resistance is cut out by using a starting box with each motor. 



242. Electric Railway Motors. The work of the electric 

 railway requires a special type of motor of great flexibility. 

 For example, the current demanded by a motor in starting 

 a car is always in excess of the current afterwards required 

 to run the car at full speed on a level track. The rating of a 

 railway motor is the horse-power output it will deliver during 

 a one-hour run at a rated voltage at the brushes with a 



