DIRECT-CURRENT DYNAMOS AND MOTORS. 37 
the bottom of the slot, the available depth of the slot 
becomes 1.15 — .0381 = 1.119 inch, and the available 
width .35 — (2 x .031) = .288 inch. 
The diameter of No. 9 D. C. C. wire is .129 inch; the depth 
of the slot will therefore take 1.119 + .129 = 9 wires, and 
the width will take .288 + .129 = 2 wires, giving a total 
of 18 wires per slot. In order to accommodate 9 wires in 
height, the depth of the slot must be slightly increased, 
the size required being (9 xX .129) + .031 = 1.2 inch. 
On the other hand, the width of the slot is not completely 
filled by the ? wires, hence a more economical arrange- 
ment will be obtained by using a correspondingly greater 
number of narrower slots. 
The width required for 2 wires and 2 slot insulations is 
2x (.129 + .081) = .382; hence, we may have 
Bias 2B) aL 7.5 additional slots. Taking 8, the 
nearest even whole number, the corrected number of slots 
is 80 + 8 = 88, and the total number of wires that can 
actually be placed on the armature is 88 X 18 = 1584. 
31. Approximate Length of Active Armature In- 
ductor.—The length, of armature inductor required to 
generate a given K. M. F. can be computed from the 
fundamental fact that 1 volt KE. M. F. in a generator, or 
counter EK. M. F. in a motor, is generated when lines of 
force are cut at the rate of 100,000,000, or 10°, lines per 
second. 
Accordingly, each foot of inductor, when moving at the 
rate of J foot per minute in a magnetic field having a 
density of Z line per square inch, since it moves across 1 
square foot (= 144 square inches) of field, and therefore 
cuts 144 lines of force per minute, or 144 + 60 = 2,4 lines 
per second, generates 
2.4 
100,000,000 
If there are /, feet of inductor on the armature, the voltage 
, or 2.4 X 10-° volt. 
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