RATINGS AND GUARANTEES. 1 55 



be seen by applying the rule which correlates direction of current 

 in a coil and direction of magnetizing action of the coil. To sum- 

 marize, we have in Fig. 100 a generator of which the brushes 

 have a forward lead from the line AB and the armature has a 

 demagnetizing action. In a generator with brushes having a 

 backward lead from the line AB, the armature has a magnetizing 

 action which helps the passage of the flux < through the arma- 

 ture. In a motor, on the other hand, forward lead of brushes 

 gives magnetizing action, and backward lead of brushes gives 

 demagnetizing action of the armature. 



The effect of the cross-magnetizing action in the armature of a 

 dynamo may be made more easily intelligible with the help of 

 Figs. 103, 104 and 105. Fig. 103 shows the magnetic flux 

 through armature and field poles due to the magnetizing action 



Fig. 103. 



of the armature alone (field excitation zero), Fig. 104 shows the 

 magnetic flux due to the action of the field coils alone (armature 

 current zero), and Fig. 105 shows the composite effect due to 

 field coils and armature acting together. At the pole tips a and d 

 the flux is in the same direction, in Figs. 103 and 104, and there- 

 fore the resultant flux density under these tips is great, as shown 

 in Fig. 105. At the pole tips b and c the flux is in opposite 



