50 



at least two similar machines, using the 

 one as generator and the other as motor, 

 and during the test the field strength of 

 both machines would have to be regu- 

 lated, so as to keep the current at its 

 predetermined value. Thus, a " motor 

 characteristic " would be obtained, lying 

 wholly above the •' static characteristic." 

 As the last experiment is, however, only 

 possible if two machines can be tested at 

 the same time, and is also somewhat 

 delicate, another method of investigation 

 is desirable. This can be done by using 

 diagram Fig. 1(>, to deduct the motor 

 characteristic from the two other curves, 

 the experimental determination of which 

 presents no difficulties whatever, and 

 would, indeed, be made as a matter of 

 course in the ordinary test of the ma- 

 chine before it is sent out from the 

 manufacturer. The static characteristic 

 gives the strength of Held for every ex- 

 citing power. From the measured re- 

 sistance of the armature it can easily be 

 found how much of the effective electro- 

 motive force has been absorbed in over- 



