52 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



15. The field coil of a dynamo contains 25 pounds of copper 

 (specific heat 0.094), weight of cotton insulation negligible. The 

 resistance of the coil is 100 ohms, (a) At what rate does the 

 temperature of the coil begin to rise when a current of 0.5 

 ampere is started in the coil ? () How long would it take for 

 the temperature of the coil to rise to 20 C. if no heat were 

 given off from the coil by radiation ? Ans. (a) o.o'2345 centi- 

 grade degree per second ; (b) 14.2 minutes. 



16. A given spool wound full of copper wire 60 mils in diameter 

 has a resistance of 3.2 ohms. An exactly similar spool is wound 

 full of copper wire 1 20 mils in diameter. What is its resistance ? 

 Ans. 0.2 ohm. 



17. What is the resistance at 20 C. of 2 miles of commercial 

 copper wire 300 mils in diameter? Ans. 1.22 ohms. 



18. What is the resistance at 20 C. of one mile of a conductor 

 consisting of seven copper wires each 40 mils in diameter? 

 Ans. 4.9 ohms. 



19. Find the resistance at 20 C. of a copper conductor 100 

 feet long, having a rectangular section 0.5 x 0.25 inch. Ans. 

 0.00653 ohm. 



20. A sample of commercial copper wire 3 feet long and 1 20 

 mils in diameter is found, by test, to have at the same tempera- 

 ture a resistance equal to that of 26.2 inches of pure copper wire 

 100 mils in diameter. Find the ratio of the specific resistance of 

 the sample to the specific resistance of pure copper. Ans. 1.048. 



21. What is the resistance at 20 C. of a steel rail 30 feet long 

 weighing 900 pounds? The specific gravity of the steel is 7.8. 

 Ans. 0.000191 ohm. 



22. What is the resistance at 20 C. of an iron pipe 120 feet 

 long having I inch inside diameter, and I \ inches outside diam- 

 eter ? The pipe has seven joints, and each joint is assumed to 

 have the resistance of one foot of pipe. Specific resistance 

 assumed to be the same as rail steel. Ans. 0.01536 ohm. 



