164 



THE MAGNETIC CIRCUIT 



[ART. 52 



The polarity of the brushes cannot be indicated without knowing 

 the actual connections in the winding. It is preferable, therefore, 

 for our purposes to designate the brushes as E and W (east and 

 west), according to their position with respect to the poles of the 

 machine, the observer looking from the commutator side. The 

 whole interpolar regions to the right of the north poles can be called 

 the eastern regions, those to the left the western regions; the same 

 notation can be also applied to the commutating poles. 



The armature currents exert a two-fold action upon the main 

 field of the machine: they partly distort it, and partly weaken it. 

 For the purposes of theory and calculation it is convenient to 

 separate these two actions, the same as in the case of the synchro- 



FIG. 42. The direct and transverse armature reactions in a 

 direct-current machine. 



nous machine in the preceding chapter. Let the sheets of current 

 be divided into parts denoted by the letters D and T with sub- 

 scripts corresponding to their location with reference to the poles 

 and brushes. The belts denoted by D are comprised within the 

 space d, one each side of the geometrical neutrals; those denoted 

 by T are (%T d) centimeters wide. 



The belts D exert a direct demagnetizing action upon the poles. 

 Namely, the belts D n D n can be considered as two sides of a coil the 

 axis of which is along the center line C n C n '. The m.m.f. of this 

 coil opposes that of the field coil on the north pole. In the same 

 way, the m.m.f. of the coil D 8 D 8 opposes the action of the field coil 

 on the south pole. The foregoing is true no matter what the 

 actual connections of the armature conductors are, provided that 

 the winding-pitch is nearly 100 per cent. With a fractional-pitch 



