WIRES, WIRE TABLES, ETC. 7 



7. 2000 feet of No. 10 wire, 102 mils in diameter, has a resist- 

 ance of 2 ohms at 20 C. What is the resistivity of the material? 

 What might the material be? (1 min.) 



8. 3500 feet of No. 25 wire of a certain material used for con- 

 ductors has a resistance at 20 C. of 175 ohms. Its area is 320 

 circular mils. What is the resistivity and what is the material? 

 (2 min.) 



9. Given the temperature coefficient of copper as 0.004, and 

 the resistance of a circular mil-foot at 25 degrees as 10.55; required 

 the resistance of a No. 18 wire 150 feet long at 55 C. (2 min.) 



10. At the working temperature of 70 C., the field of a shunt 

 dynamo has a resistance of 100 ohms; how many feet of No. 14 



B. & S. German silver wire, whose resistivity is 290, must be inserted 

 in series to keep the field current the same, when the machine is 

 started at a room temperature of 10 C.? (5 min.) 



11. Required the e.m.f. necessary to send 25 amperes through 

 one mile of No. 10 copper wire. If a generator supplied 500 volts 

 at one end of this circuit, what e.m.f. would be available to run 

 a motor at the other? Would this be an economical transmission? 

 (4 min.) 



12. A lighting plant is, during the day, supplying two 250- 

 ohm incandescent lamps connected in series, at a distance of 500 

 feet from the station, with current at 250 volts, the conductors 

 being one inch in diameter. At evening, 500 additional pairs of 

 two lamps (in series) are placed in parallel with the pair already 

 burning. Required the current flowing through the two lamps 

 during the day, the total amount of the night load in amperes arid 

 the change in the current through the two original lamps; what 

 would this change be if the resistance of the feeding wires were 

 zero? (6 min.) 



13. A voltmeter has a resistance with the leads of one ohm, 

 and is to be used at a distance of 25 feet, and connected with u 

 copper wire having a temperature coefficient of 0.004 per degree 



C. Required the area and size of wire necessary in order that an 

 increase of 8 degrees from 24 C. may not cause an error of over 

 J per cent. Take resistivity for 24 degrees as 10.5. Note that 

 the voltmeter is calibrated with the leads in series, and also that 

 the readings are independent of the temperature of the instru- 

 ment. (5 min.) 



