452 ELEMENTS OF ELECTRICAL ENGINEERING. 



24. Find the resistance at 2 5 C. of a rectangular copper bus bar 

 50 feet long, 4 inches wide, and y 2 inch thick. Ans. 0.000212 

 ohm. 



NOTE. Square inches X 1,000,000 X 4/ 71 " gives circular mils. 



25. What is the resistance at 25 C. of a steel rail 30 feet long 

 weighing 600 Ibs ? One cubic inch of steel weighs 0.28 Ib. and 

 the specific resistance of the steel is 8 times that of copper. Ans. 

 0.000342 ohm. 



26. What is the resistance at 25 C. of a 20 foot length of 

 wrought iron pipe, I inch inside diameter and I ^ inches outside 

 diameter ? Specific resistance of wrought iron is 7 times that of 

 copper. Ans. 0.001695 ohm. 



27. A transmission line 2,000 feet long (each wire) is to have a 

 total resistance of 0.40 ohm : (a] Find diameter of wire in mils. 

 One cubic inch of copper weighs 0.32 Ib.; (b) find the weight of 

 the above copper wire (bare) ; and (c) find its total cost at 1 5 

 cents per Ib. Ans. (a) 328.6 mils ; (b) 1,303 Ibs.; (c) $195.45. 



28. A given weight of copper is manufactured into 1,000 feet 

 of wire 1 28 mils in diameter having a resistance of 0.629 ohm. 

 The same amount of copper is made into 32 mil wire ; find : (a) 

 its length ; and (b) its resistance. Ans. (a) 1 6,000 feet ; (b) 

 161.02 ohms. 



29. The weight of a cubic inch of copper is 0.32 Ib. and the 

 resistance of an inch cube of copper between opposite faces at 

 20 C. is 0.00000068 ohm. Derive a formula giving the resist- 

 ance at 20 C. of copper wire in terms of its length in feet and 

 its weight in Ibs. Ans. R = 0.00003 133 L 2 / W. 



30. A sample of commercial copper wire three feet long and 

 1 20 mils in diameter is found by test to have the same resistance 

 as 26.2 inches of pure copper wire 100 mils in diameter at the 

 same temperature. Find the conductivity (reciprocal of specific 

 resistance) of the sample expressed in per cent, of the conduc- 

 tivity of pure copper. Ans. 95.42 per cent. 



31. A copper transmission line has a resistance of 5 ohms at 

 20 F. What is its resistance at 90 F.? Ans. 6.236 ohms. 



