61 



sign as the field-poles which they ap- 

 proach, and the armature has therefore a 

 tendency to push back some of the lines 

 emanating from the field-poles. To over- 

 come this tendency the exciting current 

 must be increased, the effect being 

 similar to, though not identical with, that 

 produced by self-induction. When the 

 machine is working as a motor, the effect 

 of the armature current is generally, but 

 not always, to strengthen the field-poles. 

 The author has selected the dynamic 

 characteristic at 30 amperes, for the de- 

 termination of the coefficient of self- 

 induction ; because the great distance 

 between this curve and the static charac- 

 teristic made it probable that the result 

 would be more accurate than with one of 

 the nearer curves. The coefficient of 

 self-induction was found to be L = 0.955 

 in C. G. S. measure, and 0.0955 in prac- 

 tical units. Taking this as a basis, it is 

 now an easy matter to find, by means of 

 the graphic construction shown in Fig. 

 10, the motor characteristic for any given 

 current. A current of 10 amperes was 



