488 



ENGINEERING ON THE FARM 



Fig. 378. 



After International Harvester Co. 



horse power stationary 

 gas engine 



2% 



work, while only 25 per cent was done by the tractors. All 



the reports emphasized the fact that the great advantage of 



the tractor lies in its ability to save time at critical seasons 



when time is precious and 

 the success or failure of 

 the entire crop hangs on 

 the speed with which it is 

 handled. 



Electric motors. Elec- 

 tric motors are economical 

 only where current may 

 be obtained from an out- 

 side source at low rates. 

 Where it is necessary to 



use a gas engine to generate the electricity the work itself 



could better be done by the engine. 



A device for converting electrical energy back into 



mechanical energy was soon demanded after the perfection 



of the electric generating machine. 



Generators are rated in kilowatts and motors in 



horse power. 



Kilowatts are 



supplied to the 



motor and horse 



power is deliv- 

 ered. One horse 



power is, theo- 

 retically, 746 



watts ; a kilowatt , 



1,000 watts. A 



kilowatt, then, is 



approximately 



Fig. 3:<). 



After International Harvester Co. 



Cream separator operated by gas engine 



i}/3 horse power. 

 The efficiency of motors varies greatly . The smaller the motor, 

 the more loss. Also, the lower the voltage used, the greater 

 will be the loss. Motors vary from 40 to 9 5 per cent efficiency. 



