DIRECT-CURRENT DYNAMOS AND MOTORS. . 103 
which is slightly less than the sectional area of No. 1 
B. & S. wire. As this size of wire would ordinarily be 
rather clumsy to handle, itis better to use several smaller 
conductors in parallel. For example, we find in Table 
28, page 102, that three No. 6 wires have.a combined 
sectional area of 3 X 23,244 = 78,732 circular mils, which 
is slightly smaller than found above and gives about 
1,210 circular mils per ampere. No. 6 §.C.C. wire has a 
diameter of .172 inch. 
Since the magnet core for the smooth-armature machine is 
13$ inches long, the actual length of the winding space 
will be about 124 inches, when allowing for end discs and 
insulation of the spool ends. The number of turns per 
~ layer is therefore 12} + .172 = 72, and, since for three 
wires in parallel the total number of turns is 3 X 272 = 
816, or 408 on each core, there must be 408 + 72 = 6 
layers of No. 6 wire in each coil. The height of winding 
will therefore be 6 X .172 = 14 inches, allowing for core 
insulation. 
The diameter of the core being 10 inches, the average length 
of one turn is: 
L,” = (10 + 14) X wz = 39 inches, or 2.92 feet. 
There are 408 + 3 = 136 turns of each wire on each core- 
the length of each No. 6 wire per core, therefore, is 136 X 
2.92 = 397 feet, and its resistance at 20° C., from Table 
28, is 397 X .000394 = .156 ohm. Since there are three 
of these wires in parallel and two coils in series, the com- 
bined resistance of the two magnets at 20° C. is 
(.156 + 3) xX 2 = .104 ohm. 
The working temperature may be taken as 60° C., that is, 
25° C. plus the average rise of 35° C., and since the 
increase in resistance is about .4 per cent. for each rise of 
one degree C., the resistance will become 35 X .4 = 14 
per cent. greater, that is, 
104 X 1.14 = .12 ohm. 
The current being 65 amperes, the drop in the field winding 
