RESISTANCE AND ELECTROMOTIVE FORCE. 55 



At what rate does the fan do work in delivering this air ? Ans. 

 2,160 foot pounds per second. 



Note. Reduce the pressure-difference to pounds per square foot and then the 

 unit in terms of which the result is expressed will be at once evident. 



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 horse-power to drive 

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

 tenance of the 1 50 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. In the electrolytic refining of copper an electromotive force 

 of 0.3 of a volt suffices to send the current through the electro- 

 lytic cell in which the pure copper is deposited. Calculate the 

 number of kilowatt-hours required to deposit a ton of pure 

 copper. Ans. 230 kilowatt-hours. 



Note. See data on pages 23 and 24. 



38. In the electrolytic manufacture of aluminum by electrolysis 

 an electromotive force of 5.5 volts suffices to send the current 

 through the electrolytic cell in which the metallic aluminum is 

 deposited. Find the number of kilowatt-hours required for the 

 production of one ton of aluminum. Ans. 14,8 10 kilowatt-hours. 



Note. One kilowatt continuously for one year costs from $20 to $40 when 

 developed on a large scale by water power. See data on pages 23 and 24. 



39. When electrical energy costs 1 5 cents per kilowatt-hour 

 how much does it cost to operate, for 10 hours, a glow lamp 

 which takes J an ampere from no-volt mains? Ans. 8| cents. 



40. An electric motor which delivers 5 horse-power at its belt 

 has an efficiency of 85 per cent. This motor is supplied with cur- 

 rent from no-volt mains. What current does it take? Ans. 

 39.89 amperes. 



