= 
a 
DIRECT-CURRENT DYNAMOS AND MOTORS. 77 
yoke will be 17.4 + 8 = 2.2, which we will raise to 2} 
inches to allow for rounding the corners of the yoke- 
section. 
With the same armature flux, the radial depth of the 
_armature core is one half as great in Fig. 26 as it is in 
Fig. 25, since in the four-pole design the armature flux is 
‘ 
‘ 
\ 
\ 
> 
ad 
© 
Fig, 26.—MAGNETIC CirRCUIT OF FouR-POLE MACHINE, 
split upinto four circuits, while in the bipolar frame there 
areonly two flux pathsthrough the armature (see Par. 52). 
Therefore PQ, for a smooth core, and RS, for a toothed 
armature, is made 3{ + 2 =1,% inches, which determines 
the internal diameter. The air gap is left the same as 
before, being 4 inch for a toothed armature and 4 inch 
for a smooth ring core. 
61. Form of Magnet Cores.—In the preceding paragraphs 
it has been assumed that the field cores were simple cylin- 
ders having a straight axis and a circular cross-section. 
This is ordinarily and preferably the case. 
