48 DIRECT-CURRENT DYNAMOS AND MOTORS. 
70 — 20 
At 70° C., the resistance is aera 20 per cent. higher, or 
r,! = 1.20 X .0020 = .0024 ohm. 
41. Example of Ring Armature.—find the resistance 
at 150° F. of a 12-pole ring armature which is 
wound with 107 sections, each of 6 turns of 12 
strands of No. 7 B. & S. wire, tf tts dimensions are 
as follows: Diameter, 106 inches; length, 26 inches; 
radial thickness, 54 inches; winding depth, % inch. 
Solution.—The total number of inductors in this arma- 
ture is 107 X 6 = 642; hence by formula (23): 
2x2#@+2xd54+2xz 
12 
From Table 13, the resistivity of No. 7 wire is .000497 
ohm per foot; the value of /;, therefore, as there are 12 wires 
in parallel in this case, is .000497 + 12 = .0000414. The 
number of bifurcations in a multipolar machine being 
equal to the number of pairs of poles, we have from (27%): 
_ 8,500 % 9000414 
Ly = 643 X z= 3,500 feet. 
Figo xe = .001006 ohm at 68° F. 
To find the resistance at 150° F. we must increase this 
value by Ne ee 18,2 per cent.; hence we have: 
43 
r, = 1.182 X .001006 = .00119 ohm. 
42. Check on Armature Calculation with Refer- 
ence to Heating Limit.—The dimensions and the 
winding of the armature having been determined, it will 
be well, before going into the design of the magnet frame, 
to investigate whether the armature as designed fulfills 
the requirements as to its heating limit. 
For this purpose we must determine the power losses due to 
the resistance of the winding, to hysteresis of the iron, — 
and to the production of eddy currents in the core. The 
total loss being found, its ratio to the cooling surface of 
