5 8 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



47. The electromotive force of a battery is 1 5 volts (measured 

 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 terminals 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, uot 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. The value of the 

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

 battery. 



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

 a dynamo armature to send a charging cufrent 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. 



49. A dynamo having an electromotive force of 115 volts 

 between its terminals delivers 200 amperes to a group of glow 

 lamps i ,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 ; 

 (ft) the electromotive force between the mains at the lamps. 

 Ans. (a) 792,300 circular mils; (ft) 109.25 volts. 



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

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

 the motor being 2,000 feet from the generator, and the electro- 

 motive force between the generator terminals being 125 volts. 

 Ans. 221,200 circular mils. 



51. A motor receiving 100 kilowatts of power is at a distance 

 of 1 5 miles from the generator. Line wires 200 mils in diameter 

 are to be used. The line loss is to be 10 per cent, of the gener- 

 ator output. Find : (a) the current ; (ft) the voltage at the gen- 

 erator, and (c) the voltage at the motor. Ans. (a) 16.44 amperes ; 

 (ft) 6,763 volts ; (c} 6,087 volts. 



