40 DIRECT-CURRENT DYNAMOS AND MOTORS. 
For machines having a very low voltage, such as electro- 
plating machines, battery motors, etc., or for generators 
in which the amperage is relatively very high, as in 
incandescent generators of large outputs, the field 
density is usually taken about two-thirds or three-fourths 
of the density employed under similar conditions for 
ordinary machines. 
33. Length of Armature Core.—Having computed the 
required length of active wire, /,, from (19), while the 
number of inductors, N, has been found in accordance 
with Pars. 26 to 30, the length of the armature core, 
I,, in inches, may be obtained by dividing the length of 
active wire, expressed in inches, by the number N, or in 
symbols: 
The above method of obtaining the length of the armature 
by finding the length of active wire, necessitates the as- 
sumption of the field density. Another plan is to com- 
pute the total flux from the number of inductors, as will 
be shown in Par. 34, and to adjust the length of the 
armature so that the final density attains a proper value. 
The latter method is simpler, because it is independent of 
exact dimensions and involves only total values. 
34. Preliminary Determination of Flux in Arma- 
ture.—With a given number of inductors, N, the tota] 
flux, ®, passing through the armature must be such as to 
give HE, the E. M. F. generated, at the specified number | 
of revolutions per minute, n,. Remembering that the 
lines of flux are cut twice per revolution by each inductor, 
-and that the number of inductors in series’ to generate 
E volts is =, when n, denotes the number of pairs of 
P 
parallel branches in the armature, we have: 
