r62 ELEMENTS OF ELECTRICAL ENGINEERING. 



indicated by the small arrows in Fig. no, and after the given 

 section of the armature winding has passed the brush, the current 

 in the section is reversed in direction, still, of course, flowing 

 towards the brush. 



The figure shows the instant when the section Fis short-cir- 

 cuited by the contact of the brush with the two commutator bars 

 e and/. This state of affairs lasts for a very brief time, and if an 

 action could be brought about whereby the previously existing current 

 in this section could be reduced to zero and an equal and opposite 

 current established in the section during this brief time, then there 

 would not be the least tendency for a spark to form between the 

 brush and the bar e at the instant they separate. 



Fig. 110. 



(a) As to the establishing of a reversed current in the section 

 V while it is short-circuited, it is to be noted that electromotive 

 forces are induced in the armature windings everywhere except 

 exactly in the neutral axis, as explained in Art. 5 1 . Therefore, 

 if we give the brush shown in Fig. no a forward lead from the 

 neutral axis, the sections as they are short-circuited by the 

 brush will be in a region where they have electromotive forces 

 induced in them in the proper direction to establish the reversed 

 current above mentioned ; and the greater the forward lead, the 

 greater these electromotive forces will be, so that an angle of 





