68 DIRECT-CURRENT DYNAMOS AND MOTORS. 
of one south pole to one of the north poles, as in Figs. 20 
2Np 
inches, if one-half the lines of each south pole are carried — 
by it to the next north pole on one side, and the other haif 
to the next north pole on the other side, as is the case with 
the types shown in Figs. 21, 22, and 24. 
and 23; or it should have a sectional area of square 
Fic. 23.—Four-POLE TYPk. IN Fic. 24.— BIPOLAR TYPE, IN 
WHICH THE YOKE CARRIES WHICH THE YOKE CARRIES 
ALL THE LINES FROM HALF THE J.INES FROM 
ONE POLE. ONE POLE. 
55. Factor of Magnetic Leakage.—In Table 24 the usual 
values of the leakage factor for the types of machines 
most commonly employed for direct-current dynamos are 
compiled. The higher values for each size refer to the 
most unfavorable conditions as regards magnetic leakage. 
namely, to a smooth-core armature, to a massive magnet 
frame with cast tron polepieces, and to high speed. 
- The lower values in each case are for the mest favorable 
magnetic conditions brought about by the employment of 
a toothed armature, a wrought tron or cast steel field 
frame, and luw speed. With toothed armatures, the 
length of the air gap is much less than for smooth-core 
armatures, hence the gap reluctance is smaller than for a. 
corresponding machine having a smooth armature, and 
therefore the leakageis proportionately diminished. The 
use of wrought tron or cast steel for the field frame 
has the effect of materially reducing the areas of the 
leakage paths without much affecting their lengths; the 
leakage of a wrought iron or cast steel machine is conse- 
quently less than that of a cast iron machine, other con- 
