104 DIRECT-CURRENT DYNAMOS AND MOTORS. 
is 65 X .12 = 7.8 volts, which is 7.8 + 500 = .0156, or 
1.56 per cent. of the voltage supplied, being a moderate 
amount. 
The power loss in the series field coils is 
65? X .12 = 507 watts, 
The cylindrical surface of each coil is: 
(10 + 2 x 14) X zw X 124 = 480 square inches, 
It is proper, also, to consider the ends of the coils as part 
of the cooling surface, each of them being 
114 X wz X 14 = 39 square inches. 
This makes a total surface of 480 + 78 = 658 square 
inches for one coil, or 7,316 square inches for both. The 
specific cooling surface of the field winding is, therefore: 
1,316 + 507 = 2.6 square inches per watt, 
which is within the practical limits. If the number of 
square inches per watt is found below 14, the coil should 
be wound with the next larger size wire, which would 
decrease the power loss and increase the cooling surface, 
thereby augmenting the number of square inches per 
watt. 
%'%. Determination of Series Winding for Bipolar 
Toothed-Armature Machine.—The series winding 
for the toothed-armature core machine will be as follows: 
For the toothed-armature machine AT’ = 9,000 ampere- 
turns, hence the number of turns: 
9.000 
690 
The cores being 9 inches long, the available space for the 
winding will be about 8}inches. Since the diameter of 
No. 6 magnet wire is .172 inch, each core can therefore 
be wound with 8} + .172 = 48 turns of No.6 wire. 
Taking again 3 strands of No. 6 wire, asin Par. '76, the 
number of turns per core is (140 X 3) + 2 = 210, making 
T= = 140. 
