506 ALTERNATING CURRENTS 



wire; (6) the generator voltage; (c) the line regulation; and (d)the generator 

 power-factor. 



153. Given transmission distance 20 miles; load, 5,000 kw.; load voltage, 

 33,000 volts between conductors; system, three-phase; frequency, 60 cycles 

 per second; load power-factor of 0.85, lagging current; spacing of conductors, 

 48 in.; permissible line loss, 10 per cent, of receiver power. Find (a) line 

 regulation; (6) generator power-factor. Neglect the charging current. 



154. Repeat problem 153 for a power-factor of 0.85 leading current. 



155. A hydroelectric station transmits 50,000 kw., three-phase, 60 cycles, 

 a distance of 120 miles over a line consisting of three 0000 copper conductors 

 spaced 13 ft. apart. The voltage between line wires at the sub-station is 

 140,000 volts; the power-factor of the load is 0.9, lagging current. Find (a) 

 line regulation; (6) efficiency of transmission. The line charging current 

 should be taken into consideration. 



156. Repeat problem 155 for a power-factor of 0.9, leading current. 



QUESTIONS ON CHAPTER XIII 



1. How may light be described? What is illumination? Photometry? 



2. What is luminous intensity? In what units is it measured? What 

 is the objection to the use of the candle as a photometric standard? What 

 photometric standards are used at the present time? 



3. Define a unit solid angle or "steradian." What are its geometrical 

 properties? How many solid angles exist about a point? 



4. In what way may light flux be considered? In what way is it com- 

 parable to magnetic flux? What is a lumen? Why is a given cone of 

 light flux confined to the solid angle? How many lumens would a standard 

 candle emit if its intensity in all directions were the same as its horizontal 

 intensity ? 



5. W T hy were carbon lamps rated in mean horizontal candlepower? 

 What is the objection to this method of rating? Why is it highly desirable 

 at the present time to rate lamps in lumens? What is the relation between 

 lumens and mean spherical candlepower? 



6. What is illumination? What is the unit ? To what does it correspond 

 in magnetism? 



7. What is the law of inverse squares? How is this law proved geometric- 

 ally? What assumption may introduce error into the application of this 

 law? What is the magnitude of this error and how may it be made 

 negligible? 



8. What is meant by "absorption?" What types of surfaces reflect 

 the largest proportion of incident light? What types reflect the least? 

 What determines the color of a surface? What is meant by the "coefficient 

 of absorption"? 



9. Why does the intensity of light emanating from light sources usually 

 vary in different directions? How does light intensity in general vary in 

 horizontal planes or zones? In what way may this distribution be 

 represented? In what regions is the light from an incandescent lamp of 



