ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 97 
cussion, enabling the caleulated results to be cheeked by 
those of the actual tests. 
In Fig. 46, two sets of curves are given, one of which indi- 
cates the values of the permeabilities for various magnetio 
densities; and the other set shows how many ampere turns 
are required to drive the magnetic lines of flux, corre- 
sponding to the density, through a path length of one inch... 
By the application of the Jatter set of curves, the calculation 
of the necessary ampere turns is greatly facilitated. 
169. Example 22.— Assume, for instance, that the length of 
a pole core is 8 inches and the section of the core 88 square 
inches. Then for a flux of 8.3 x 10° the density in the 
; ; 8.3 10° 7 : 
section will be ae = 94,000 lines per square inch 
and the corresponding number of ampere turns 70, pro- 
vided that the material of the core is sheet iron. 
The ampere turns required in this particular ease for the 
8 inch pole, are 8 x 70 = 560. 
170. Correct Leakage Factor.—The second condition in — 
the proper determination of the no-load characteristic is te. 
caleulate the correct leakage factor. This may be accom- 
plished by the calculation indicated in Par. 155. 
171. Estimation of Sectional Areas.—The proper estima- 
tion of the various sectional areas and the length of 
' magnetic path, is based on the data of average permissible 
densities, on condition of the economical use of material 
and the provision of the necessary space for the copper 
winding. 
172. Assumption of Form Factor.—tThe correct assump- 
tion of the form factor is an important matter, because its 
value varies as much as 50% and more, for the different 
kinds of winding on the armature. 
