4 ELECTRICAL ENGINEERING PROBLEMS 



meter 12 \ volts are wanted; how many ohms of the resistance 

 of the wire and what length must be included between the ter- 

 minals of the voltmeter? (The voltmeter is supposed to have an 

 infinite resistance.) (1 min.) 



7. Two resistances of 50 and 100 ohms are connected in parallel 

 between two points A and B, between which the e.m.f. is 100 volts; 

 what current will flow in each circuit? If the 100-ohm circuit 

 be reduced to 0.05 ohm, what current will flow in each? (2 min.) 



8. Circuits P, Q and R are in parallel, as are also circuits S 

 and T; these two groups are in series. If 270 volts are applied 

 to the outside terminals of the combination, find the conductance 

 and resistance of each group and the current in each circuit, the 

 resistances of the circuits being 3, 5, 7J, 20 and 30 ohms respec- 

 tively. (7 min.) 



9. A resistance of 42 ohms between A and H is tapped at 

 six equidistant points, B, C, D, E, F and G; four switches are 

 placed between A, C, E and G, and one bus bar, and four between 

 B, D, F and H, and the other bus bar. Find the conductance and 

 resistance between the bus bars with each of the following six 

 groups of switches closed A, B and G ; A, C and H ; A, D and 

 G; A, B, C and H; A, B, C, G and H; all closed. (8 min.) 



10. Four points A, B, C and D are at the successive corners 

 of a square and resistances are connected between them as fol- 

 lows: AB, 2 ohms; BC, 6 ohms; CD, 12 ohms; BD, 100 ohms. 

 What resistance must be placed between D and A in order that 

 no current shall flow through the circuit BD when 10 volts, is 

 applied between A and C? What if 100 volts be applied? (2 min.) 



11. The field of a 15-kw. shunt motor has a resistance of 25 

 ohms; what current will flow when it is connected in on a con- 

 stant-potential 125-volt circuit? If the armature, the resistance 

 of which is 0.04 ohm, was connected in parallel (not running), 

 what current would flow through it? What would then be the 

 current through the field? What would happen to the armature? 

 If a 5-ohm rheostat were included in the armature circuit, what 

 would be the total current taken by the motor? (4 min.) 



12. A 70-cell storage battery, designed to give 100 amperes 

 for 8 hours, has a resistance of 0.0005 ohm per cell. When the 

 charging of the battery is nearing completion, the cells have an 

 electromotive force of 2.5 volts each. If a 220-volt generator is 

 being used for charging at 100 amperes, what voltage must be 



