118 Trans. Acad. Sci. of St. Louis. 



from (4) and (11) 



R , 6Ri 



n = - H 



a a 



(12) n = 1000 + 4.0 i 



n = -+ - E 

 a a 



(13) n = 1000 + 0.572 E 



See fig. 6 JF, 



See fig. 6 JK, 



Equations (11), (12) and (13) are the equations of the 

 projections upon the co-ordinate planes of a line on the surface 

 which gives the simultaneous variation of E, i and n. 



It is instructive to note that projections on the En plane 

 for different resistances are parallel, while the projections on 

 the ni plane are not parallel, but vary with R as is shown by 

 the co-efficient of i containing R in one case and not in the 

 other. 



It will be observed that from equations (12) and (13) the 

 constants of the equation for the surface can be easily gotten, 

 should it be experimentally easier to vary n than R. In the 

 laboratory it is generally easy to vary R but frequently in 

 practice n can be varied by throttling the steam, where a 

 change of R is impracticable. See figs. 14 and 16. 



By the means outlined above it is a simple matter to fore- 

 tell the result of any change in the conditions of operation. 



lb. 

 Series Dynamo and Series Motor. 



The foregoing discussion of the series dynamo characteris- 

 tic surface, as has been pointed out, applies equally to the 

 series motor, when E a is plotted instead of the E of the 

 dynamo. 



As has been shown the counter electromotive-force — E a — 

 of the motor is analogous to the total electromotive-force of 

 the dynamo. 



