DIRECT-CURRENT DYNAMOS AND MOTORS. 63 
(3) With multiple circuit armature windings (Figs. 14 and 
15), the flux must be the same for each pole, otherwise 
the E. M. F. of the circuits will be unequal, producing 
wasteful currents in them, which, in turn, cause excessive 
sparking and heating. This difficulty is not present in 
two-circuit windings (Fig. 16), using. only two sets of 
brushes. 
The saving in material is clearly shown by comparing Figs. 
20 and 21. In the former, it will be seen that the arma- 
ture must carry 4 the totai flux between its shaft and 
periphery, while in Fig. 21 the armature cross-section is 
only required to carry 4 the total flux, hence the radial 
depth B of the armature core in Fig. 21 need be only one- 
half as great as A in Fig. 20. This allows the core of the 
four-pole machine to be hollowed out as shown in Fig. 21, 
thus reducing its weight. It will be observed that the 
bulk of the field magnet is also less for the multipolar 
than for the bipolar construction. 
The difference is still more marked when we compare the 
eight-pole machine in Fig. 22 with the bipolar type in 
Fig. 20. In Fig. 22 the radial thickness of the neld ring 
is only 4 as much as in Fig. 21, and 4 as great as the 
thickness of the yoke in Fig. 20, and the armature may 
be hollowed out to a radial thickness, C, of only } that of 
the bipolar armature, A, as shown, the tvutal flux being 
the same for all three designs. The external diameters 
of the armatures being the same, it follows that an equal 
number of similar inductors can be carried in each case. 
In short, the capacity of the multipolar machines in Figs. 
21 and 22, considered either as generators or motors, is as 
great as that of the bipolar form in Fig. 20, although the 
volume of material is decidedly less for the multipolar 
types. 
53. Proper Number of Poles for Multipolar Field 
Magnets.—In the case of a machine of considerable 
size, for which a multipolar field would almost surely be 
adopted, the next step is to decide upon how many poles 
