134 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
22%. Value of Space Factor.—The available winding space 
is properly utilized, that is, there is no superfluous insula- | 
tion material used, as concluded from the value of the 
space factor, 0.32, where the air section of the slot is 
0.9 x 2.05 = 1.84 square inches and the copper section 
0.586 
2 = 0.52. 
1.84 Ge 
0.586 square inch, which in their ratio 
228. Calculated Cupper lLoss.—Regarding the specific 
utilization of the armature copper, it is found, that for 
66 amperes normal current, a section of 2 X 0.01629 = 
0.0326 is provided in which the current density will be 
66 
0.0326 
average value, corresponding to a_.loss of 10 watts per 
pound of copper, see Fig. 31. 
= 2,030 amperes per square inch. This is a good 
229. Section and Form of Coils.—The necessary section 
of conductor in the present design, is obtained by using 
two No. 9 S.W.G. wires in parallel, as one wire of a large 
gauge will not permit the same favorable slot shape and, 
what is still more important, the winding of such a thick 
wire is a difficult job and would tend to increase the labor 
costs of the armature. 
The slots being open, former wound coils of double cotton 
covered, high conductivity, round copper wire are used. 
230. Average Length of Mean Turn.—For a voltage of 
3,900 between phases and 2,040 volts per phase, it is 
necessary to allow an air distance of about 1.75 inches 
between the coils of any two phases and as much between 
the projecting part of the coil and the nearest part of 
the iron body. Under these conditions the average length 
of mean turn in the armature coils is equal to 77 inches. 
231. Section of Teeth.—In order to check the dimensions of 
