DIRECT-CURRENT DYNAMOS AND MOTORS. 
Par. 58. The leakage factor, however, is smaller for a 
multipolar dynamo than for a bipolar machine of equal 
output; see Pars. 54 and 55. For a four-pole machine 
of the size under consideration the average value of A is 
about 1.25 instead of 1.40, which it was in the bipolar 
design. The field flux required for the four-pole frame 
will therefore be about 
®' = 1.25 X 5,000,000 = 6,250,000 lines. 
Each field core carries one-half of the total flux, because all 
the useful lines enter the armature from the two pole- 
pieces of north polarity, and pass to the two south-poles; 
hence each core must carry 3,125,000 lines. If composed 
of wrought iron or cast steel, a flux density of 90,000 lines 
per square inch may be allowed, requiring a sectional area 
of 
g = 3:125,000 
=~ 90,000 
The corresponding diameter is: . 
Dm = V 34.7 X 1.273 = 6% inches. 
The length of the magnets in case of a smooth-armature 
machine, according to Par. 56, is approximately: 
Im = 6% X .85 = 52 inches, 
= 34.7 square inches. 
and in case of a toothed-armature machine: 
Un = 58 X .7 = 4 inches. 
In Fig. 26, the field cores for the toothed armature are 
drawn to scale at BI, at the left side of the center line, 
and for the smooth armature at DJ, at the right side, as 
in Fig. 25. 
The yoke in the four-pole design carries but half the lines 
of each magnet, hence its cross-section need only be one- 
half the area of one magnet core, or 17.4 square inches, 
Making the frame 8 inches wide, which gives a flange, or 
overlap, of 44 inch on either side of the cores in the direc- 
tion of the shaft, the radial thickness JM or JN of the 
