68 ELEMENTS OF ELECTRICAL ENGINEERING. 



conductors to become loose, especially when an excessive current 

 flows through the armature on account of an accidental short cir- 

 cuit. Dynamo armatures are now constructed with deep slots, 

 as shown in Figs. 41, 46, 48, and 51. In this type of armature 

 the conductors are well protected mechanically, and the side 

 force, which in a smooth core armature acts upon the conductors, 

 acts chiefly on the armature teeth of a slotted armature. Fur- 

 thermore, the air gap, or distance from the iron of the pole face 

 to the iron of the armature core, may be made very much smaller 

 in the slotted than in the smooth core armature. In Chapter VI 

 it is shown that, on account of sparking, the air gap should some- 

 times be greater than mechanical clearance alone would require, 

 so that the possibility of reducing the air gap is not always of 

 prime importance. 



The manner of cutting the lines of force by the conductors 

 which lie in the deep slots of an armature is as follows : Con- 

 sider the tuft of lines of force which emanate from an armature 

 tooth. This tuft moves across the pole face as the armature 

 rotates. This motion of the tuft of lines of force is, in a certain 

 sense, only apparent, since it is due to the addition of new lines 

 of force to the forward edge of the tuft by the very rapid flitting 

 of the lines of force across the slot which is ahead of the tooth in 

 question, and the taking away of lines of force from the backward 

 edge of the tuft by the very rapid flitting of the lines of force 

 across the slot which is behind the tooth in question. The 

 velocity at which the lines of force flit across the slots and cut 

 the armature conductors is much greater * than the mean velocity 

 of the armature conductors. The fundamental equation of the 

 dynamo, see Article 30, applies to smooth core and slotted core 

 armatures alike. 



*In the ratio of the flux density in the slot to the mean flux density in the gap 

 space. The above apparent motion of a tuft of lines of force is similar to the appar- 

 ent motion of a tuft of the bristles of an ordinary brush when the finger is rubbed over 

 it, and the rapid flitting of the lines of force across a slot is similar to the rapid flitting 

 of the bristles across the nearly vacant space behind the ringer. 



