PROBLEMS. 453 



32. The field coil of a dynamo (copper wire) has a resistance 

 of 30 ohms after it has been standing for a long time in a room 

 at a temperature of 68 F. After the machine has been running 

 fully loaded for several hours the resistance of the field coil is 

 35 ohms, what is the temperature of the coil? Ans. I49F. 



33. An ordinary 1 6-candle-power no-volt glow lamp has a 

 resistance of 400 ohms at o C. and 220 ohms at a white heat, 

 say, I5OOC. What is the average temperature coefficient of its 

 resistance between o C. and 1500 C.? Ans. 0.0003 P er cen ~ 

 tigrade degree. 



34. The specific resistance of commercial copper is 1,600 c.g.s. 

 units at o C., find from this : (a) the resistance at o C. of a 

 copper wire I mil in diameter and one foot long ; and () the re- 

 sistance of a copper bar one inch long and one square inch in 

 sectional area. Ans. (a) 9.63 ohms ; (ft) 0.00000063 ohm. 



35. When a certain electric generator is giving out no current 

 it takes 1,75 horse-power to drive it. When the generator de- 

 livers a current of 150 amperes it takes 25 ho e-power to drive 

 it. Assuming that the increased power is all used in the main- 

 tenance of the 150 amperes of current, find the electromotive 

 force of the generator. Ans. 115. 7 volts. 



36. An incandescent lamp takes 0.6 ampere when the electro- 

 motive force between its terminals is 1 10 volts. Find the power 

 delivered to the lamp in watts and in horse-power. Ans. 66 

 watts or 0.0884 horse-power. 



37. A so-called 2,000 candle-power arc lamp has a current of 

 9.6 amperes flowing through it, and an electromotive force of 47 

 volts between its terminals. (#) Find how many kilowatt-hours 

 and how many horse-power-hours of work are delivered to the 

 lamp during a run of eleven hours, (b) If the charge for a run of 

 eleven hours is 27 cents, find the price of power per kilowatt- 

 hour. Ans. (a) 4.96 kilowatt-hours or 6.66 horse-power-hours ; 

 (b) 5.43 cents. 



38. A storage battery consisting of 54 cells connected in series 

 has a resistance of 0.0002 ohm per cell, and an electromotive 



