DIRECT-CURRENT DYNAMOS AND MOTORS, | __ oee 7. 
more or less inclined to the latter, owing to the distortion 
of the field. We therefore have: 
at, = .313 X 30,500 X 14 = 11,930 ampere-turns. 
The ampere-turns needed to overcome the armature reluct- 
ance are obtained from (43). In order to find the aver- 
age length, /,”, of path for the lines in the armature, the 
longest line, #’G', and the shortest line, TU, are drawn as 
indicated by dotted lines in Fig. 25. Theaverage length, 
VW, is then laid off midway between them. In Fig. 25 
the lines FG, TU, and VW are not continuous, because 
the armature shown to the left, being a toothed armature, 
has a smaller internal diameter than that on the right, 
which is a smooth armature. In the present case the 
right side of Fig. 25 is to be considered only; in speaking 
of lines FG, TW, and V W, therefore, the left half of the 
armature is supposed to be symmetrical to the right half, 
so that the dotted lines in the two halves meet at the 
center line. By measuring the continuous line VW so 
obtained, the average length of the path in the armature 
is found to be about 11 inches. The flux density in the 
armature core was:taken at 80,000 lines per square inch 
(see page 75); the specific magnetizing force for wrought 
iron having this density is H,” = 31 ampere-turns per 
inch, according to Fig. 27 or to Table 26. Consequently, 
the armature magnetizing force: 
at, = 31°x 11 = 340 ampere turns. 
The ampere-turns needled to overcome the field magne? 
reluctance are calculated by substituting in (43) the 
respective values of /,," and H,," for the magnet frame. 
The length of path, XH, Fig. 25, is found by meas- 
urement to be about 59 inches. The flux density B,,’ 
was fixed at 90,000 lines per square inch (see page 72), 
for which the specific magnetizing force, by Table 26, 
is H,,” = 51, the material of the cores and yoke being 
wrought iron. Hence, 
atn = 51 X 59 = 3,010 ampere-turns. 
