88 DIRECT-CURRENT DYNAMOS. AND MOTORS. 
The compensating ampere-turns, since the average density 
in the wrought iron polepieces is less than 80,000 lines 
per square inch, are, according to formula (45), 
125 XK 240 — 225 
“ge X eg = 2,350 ampere-turns. 
It is here assumed that the machine under consideration 
is a generator of about 125 volts and 240 amperes output, 
the number of armature conductors being determined by 
transposing formula (21), page 41, thus: 
6 xX 10° X Mm X H_ 6 X 10° X 1 XK 125 _ 
xX n, ~ 5,000,000 xX 1200 — 
The pole space angle, in laying out Fig 25, was assumed 
to be 180° — 135° = 45°, half of which is taken as the 
angle of brush lead in the above formula. 
Summing up the above magnetizing forces, according to 
(3'7), we obtain: 
at, i330 
125. 
oe —- 
at, = 11,930 ampere-turns for air gaps; 
at, = 340 ms ** armature; 
Qi = “oa * ‘* magnet frame; 
ai; =. 2 a0 nS ‘* armature reaction. 
AT = 17,630, total ampere turns required. 
6%. Calculation of Ampere-Turns for Bipolar 
Toothed-Armature Dynamo.—tThe calculation in 
Par. 66 applies to a smooth armature; if a toothed core 
armature were adopted, the required magnetizing force 
would be less, as follows: 
The air gap is smaller in a toothed machine, being $ inch in 
the present case instead of 4 inch. The length of the 
path in air, therefore, is 4 inch for the toothed armature. 
The gap density, however, is greater if the armature flux 
remains the same, owing to the concentration of the lines 
by virtue of the teeth. Adding about 50 per cent. to the 
previous value of the gap density, we obtain B,” = 45,000 
lines per square inch. Hence, we find that the gap mag- 
netizing force for the toothed armature is: 
at, = .313 X 45,000 X 4 = 3,520 ampere-turns. 
