THE DESIGN OF DIRECT-CURRENT 
DYNAMOS AND. MOTORS. 
DESIGN OF FIELD MAGNET. 
51. General Form of Field Magnet.—With a given 
size (diameter and length) of armature the value of the 
total flux, ®, does not necessarily depend upon the form 
of the field magnet. This is shown by comparing Figs. 
20, 21, and 22,in which armatures of equal size are repre- 
Fics. 20, 21, AND 22.—ARMATURES OF EQualL SIZE IN BIPOLAR, 
: FOUR-POLE, AND EIGHT-POLE FIELDs. 
sented in bipolar, four-pole, and eight-pole fields. <As- 
suming that in all three cases the polepieces cover a like 
portion of the armature periphery, then the total number 
of magnetic lines entering and leaving the surface of the 
armature is the same in the three designs. This is evi- 
dent when it is considered that by the above assumption 
the total number of square inches covered by the pole- 
pieces in Figs. 21 and 22 is equal to those in Fig. 20, and 
that each square inch contains the same number of lines 
at the given flux density. In practice the flux density is 
usually less in bipolar than in multipolar machines, but 
