390 ELEMENTS OF ELECTRICAL ENGINEERING. 



microfarad capacity is connected through 50,000 ohms non- 

 inductive resistance to 6o-cycle mains. Find the percentage 

 error* of the readings of the instrument, and state whether it 

 indicates too high or too low. Ans. 0.0064 per cent, too low. 



49. The above electrostatic voltmeter is connected in series 

 with an inductance of 2 henrys, resistance negligible. Find the 

 percentage error of the voltmeter readings when a frequency of 

 60 cycles is used, and state whether it indicates too high or too 

 low. Ans. 0.0017 per cent, too high. 



50. An electrodynamometer having an inductance of 0.02 

 henry and a resistance of 1,500 ohms gives the same deflection 

 for a certain 6o-cycle electromotive force as it does for 127.5 

 volts direct electromotive force. What is the effective value of 

 the 6o-cycle electromotive force? Ans. 127.5016 volts. 



51. An electrodynamometer has a resistance of 500 ohms. 

 When used as an alternating voltmeter at a frequency of 60 

 cycles per second its percentage error is ^ per cent. What is 

 its inductance ? Ans. 0.0593 henry. 



52. An alternator delivers current to two receiving circuits in 

 parallel. One receiving circuit takes 20 amperes and its power 

 factor is 0.9, and the other receiving circuit takes 25 amperes 

 and its power factor is 0.7. What is the power factor of the 

 combination? Ans. 0.801. 



53. An alternator delivers I oo amperes at I , I oo volts and 60 

 cycles per second to a receiving circuit of which the power factor 

 is 0.85. What capacity condenser would be required to com- 

 pensate for lagging current ? What number of leaves of paraf- 

 fined paper 22 by 27 centimeters would be required for this con- 

 denser, thickness of paraffined paper being 0.08 centimeter, 

 allowing I centimeter margin beyond the tinfoil ? Take induc- 

 tivity of paraffined paper equal to 2. Ans. 127.0 microfarads, 

 114,800 leaves. 



* In this and subsequent problems the percentage error of an instrument is under- 

 stood to mean the difference between the instrument reading and the true value of the 

 quantity measured, divided by the true value of the quantity. 



