124 ELECTRICAL MACHINERY 



perpendicular to that of the main field; they constitute 

 the cross-magnetizing turns. 



The field m.m.f. is shown in Fig. 77 by OE, the armature 

 m.m.f. by OF. The resultant m.m.f. (which actually pro- 

 duces the magnetic field through the armature) is obtained 

 by adding OE and OF vectorially; it is shown at OK. The 

 armature m.m.f. may be divided into its two components, 

 OH, the cross-magnetizing force and, OG, the demagnetizing 

 force. It is seen that this demagnetizing force is produced 



FIG. 78. Showing Pole Tips with Half as Many Laminations as the 

 Main Part of the Pole. 



by all conductors in an angle equal to twice that through 

 which the brushes have been shifted. 



Reduction of Field Distortion. The amount that the 

 main field actually twists under the influence of the arma- 

 ture reaction depends upon how nearly saturated the 

 pole tips are, how long the air gap is and other factors. 

 If, without any distortion at all, the trailing pole tips are 

 saturated, then the field flux cannot crowd any more into 

 these pole tips even if the cross m.m.f. is very large. 

 Laminated pole pieces are generally constructed with the 

 laminations so shaped that only every other one extends 



