454 ELEMENTS OF ELECTRICAL ENGINEERING. 



force per cell which ranges from 2 volts at the beginning to 1.85 

 volts at the end of the discharge. The battery supplies current 

 to 100 glow lamps (each having 220 ohms resistance) connected 

 in parallel between copper wires 0.325 inch in diameter at a dis- 

 tance of 200 feet from the battery. Find the electromotive force 

 between the terminals of the group of lamps at the beginning and 

 at the end of the discharge of the storage battery. Ans. 105.6 

 volts and 97.5 volts. 



39. The electromotive force of a battery is 15 volts (measured 

 of course on open circuit). The battery terminals are connected 

 by a wire, when it is observed that a current of 1.5 amperes is 

 produced and the electromotive force between the battery termi- 

 nals is 9 volts. Find the resistance of the wire and the apparent 

 resistance of the battery, Ans. 6 ohms and 4 ohms. 



NOTE. When a voltaic cell is called upon to give current, the terminal voltage of 

 the cell falls off, not only on account of the ri drop in the cell, but also on account of 

 what is called polarization. This problem is to be solved on the assumption that the 

 whole of the decrease in terminal voltage is due to ri drop, and the value of the re- 

 sistance as calculated on this assumption is greater than the true resistance of the 

 battery. 



40. Find the total electromotive force that must be induced in 

 a dynamo armature to send a charging current of 100 amperes 

 through a storage battery consisting of 54 cells connected in 

 series. Each cell has an average counter electromotive force of 

 2.3 volts, the resistance of each cell is 0.0004 ohm, the resistance 

 of the dynamo armature is 0.02 ohm, and the resistance of the 

 leads is 0.03 ohm. Ans. 131.36 volts. 



41 . A dynamo having an electromotive force of 1 1 5 volts be- 

 tween its terminals delivers 200 amperes to a group of glow 

 lamps 1,000 feet distant from the generator. Find : (a) the size 

 of copper wire for the mains in order that 95 per cent, of the 

 power output of the generator may be delivered to the lamps ; 

 (b) the electromotive force between the mains at the lamps. Ans. 

 (a) 752,000 circular mils; () 109.25 volts. 



42. What size of copper wire is required to deliver current at 

 no volts to a lo-horse-power motor of 85 per cent, efficiency, 



