492 



ELEMENTS OF ELECTRICAL ENGINEERING. 



each wire assuming rubber insulated wire to be used ; (c) show 

 the location and mark the current rating of every fusible cut-out 

 and branch block required by the National Electrical Code. 



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Fig. 167/>. 



CHAPTER X. PHOTOMETRY AND ELECTRIC LIGHTING. 



168. The intensity of illumination at a distance of four feet from 

 a i6-candle lamp is sufficient for easy reading of ordinary book 

 type, (a) Find the distance from a 2O-candle lamp at which the 

 lamp gives the same intensity of illumination ; (b) express this in- 

 tensity of illumination in spherical-candles of light falling on each 

 square foot of illuminated surface ; and (c) express this intensity 

 of illumination in luxes. Ans. (a) 4.47 feet ; (b) 0.0796 spherical- 

 candles per square foot; (c) 12.21 luxes. 



Note. One lux is the intensity of illumination produced by a Hefner lamp at a 

 distance of one meter. 



169. The glow lamp which is used as a standard in a Bunsen 

 photometer has a candle power of 16.8 candles in the direction 

 towards the photometer screen. Another lamp, B, is placed at the 

 other end of the photometer bar ; and when the screen is adjusted 



to equality of illumination on both sides, it is 2.61 meters from 

 the lamp, B, and 1.80 meters from the standard lamp. What is 



