THE OPERATION OF THE DYNAMO AS A MOTOR. II 



made upon a motor-driven lathe turning a piece of steel 3f inches 

 in diameter. The speed, which has been reduced to a low value 

 by an armature rheostat, rises from 60.8 to 210 feet per minute 

 when the depth of cut is reduced from */ inch to zero ; whereas, 

 when the speed has been reduced to the desired low value by 

 using E x = 60 volts instead of E x = 236 volts, the speed rises 

 only from 60 to 71 feet per minute when the depth of cut is 

 reduced to zero. 



The Ward Leonard system of speed control is perhaps more 

 expensive than the multivoltage system, but it is the most com- 

 plete method of all, giving as it does a finely graded control of 

 speed from full speed down to zero speed. This system is used 

 to a considerable extent for operating guns and turrets on war- 

 ships where completeness and fineness of control is of prime 

 importance, and where the question of cost is secondary. 



A disadvantage which is common to all methods of speed con- 

 trol of an electric motor, where the motor is to supply approxi- 

 mately the same amount of power at all speeds, is that the motor 

 must be very large to be able to deliver the requisite amount of 

 power when it runs at the lowest speed, and that the capacity of 

 the motor is not fully utilized when it runs at higher speeds. A 

 much smaller and cheaper motor could take the place of the 

 large variable speed motor, if the small motor were driven at 

 constant full speed, and if the variations of speed of the driven 

 machine were accomplished by mechanical means ; and the 



