256 THE DIRECT-CURRENT MOTOR ('II. XI 



When we compare the energy expended by the two 

 methods, we see that the results of the test give a much 

 greater expenditure than that indicated as necessary by 

 the calculations. The maximum speed attained in the test 

 was 35 feet per second, the kinetic energy is thus 380 x 10 4 

 foot-pounds. The train resistance is 900 pounds, giving an 

 expenditure of energy of 174 x 10 4 foot-pounds throughout 

 the distance of 1,930 feet. Hence the total energy ex- 

 pended as work is 554 x 10 4 foot-pounds. 



The area of the dotted current curve in the figure 

 represents the total expenditure of energy in the experi- 

 ment ; this area measured with a planimeter is found to 

 represent 963 x 10 4 foot-pounds. The difference between 

 the work done and the observed expenditure of energy, 

 amounting to 409 x 10 4 foot-pounds, is mainly represented 

 by the energy lost in heating the resistances. If we 

 allow a mechanical efficiency averaging 85 per cent., the 

 heat loss amounts to 311 x 10 4 foot-pounds. 



The maximum speed for the calculated curve is 33'5 

 feet per second, giving 350 x 10* foot-pounds of kinetic 

 energy ; the energy required to overcome the train 

 resistance is 174xl0 4 foot-pounds, giving a total of 

 524 x 10 4 foot-pounds of work done. Assuming an average 

 efficiency of 85 per cent, we get a total torque loss of 

 92 x 10 4 foot-pounds. We have already seen how to 

 estimate the heat loss, and know that it is represented, 

 within a small error, by the area of the current curve 

 above the line oa in the figure ; this area is 67 x 10 4 

 foot-pounds, hence the total energy required, as ob- 

 tained by calculation, is 683 x 10 4 foot-pounds, as 

 compared with 963 x 10 4 obtained with the existing 

 motors. The area of the calculated current curve in 



