124 



Trans. Acad. Sci. of St. Louis. 



has been stopped (i. e., n = 0) the current is maintained by 

 an impressed e. m. f . which is equal to the intercept of the line 

 on the e. m. f. axis.* 



The resistance of the machine is then this value divided 

 by the current. 



120 



8 



Fig. 10 

 Series Dynamo Test. E. M. Fs. Computed from those Observed. 



From the observed line 



e + ri = nc 

 55.3 + ri= 1159 c 

 1.6 + ri = 454 c 



ri = 32.9 since i = 9.7 r = 3.39 



* S. P. Thomson in " Dynamo-Electric Machinery, pp. 88 and 206, fifth 

 edition, apparently disregards the fall of potential through the machine and 

 so finds that the e. m. f. is not proportional to the speed. The correction is 

 made by subtracting from the speed that gotten from the intercept on the 

 n axis. He calls these rev. pr. min. the " dead turns." 



