DIRECT-CURRENT DYNAMOS AND MOTORS. 41 
N n 
SAE SO Se ia hd —8 
EH =2@x In, x 60 x 10 volts, 
60X10°x2n,xX EH 6xX10°xXn, XH 
o@— p ps Pp 
from which = Taba Wx, ete? 
The value of ® can also be found when the flux density, B,, 
in the air gap, the length, L,, of the armature, and its 
diameter, D,, are known. If in this case the average per- 
centage of the idle inductors is taken as 1/7, see Par. 31, 
the flux in the armature is obtained as follows: 
Piewis, < 08D, 7 XL, = 1:3 x B, x Da X Bs. ..(22) 
If B,. is not given, it may be assumed from Table 16. 
As stated in Par. 33, however, this indirect method is 
not as satisfactory as the direct method represented in 
(21), but it may be used as a check on the latter. 
Equations (21) and (22) apply to multipolar as well as to 
bipolar machines. In case of bipolar dynamos the total 
flux, ®, inthe armature is the number of lines which enter 
it from the polepiece of North polarity, while in a multv- 
polar field it is the swm of the lines that enter it from all 
of the North poles. The same lines pass out through the 
South pole-faces; hence, % is one-half of all the lines enter- 
ing and leaving the armature. For this reason, the factor 
2 is used in (21), and the factor 4 in (22). The student 
should be careful to avoid confusion on this point, espe- 
cially as the flux per pole and the flux per magnetic cir- 
cuit are often used in calculations, instead of the total 
flux. 
Before the flux is computed by (21) or (22), the approxi- 
mate size of the armature should be determined by the 
methods given in Pars. 13 to 18, otherwise the resulting 
flux density in the armature eore might either be absurd- 
ly high, that is, above the practical magnetic saturation 
point, or else far below the normal value. It must be 
borne in mind that all of these preliminary calculations 
are tentative, the object being to reconcile and adjust the 
various elements, so that in the end a harmonious result 
will be obtained. 
