442 ALTERNATING CURRENTS 



Example. In the polar diagram of Fig. 396, 1 in. radial distance equals 

 20 candlepower. The diameter of the circle and the line ab have a length of 

 4 in. The area of abge'a is 8 sq. in. The lamp takes 60 watts and the length 

 of the line oh is 2.5 in. Determine: (a) The mean spherical candlepower. 

 (6) The output of the lamp in lumens, (c) The mean horizontal candle- 

 power, (d) The efficiency of the lamp in watts per mean horizontal and 

 per mean spherical candlepower. (e) The spherical reduction factor of the 

 lamp. 



8 X 20 

 (a) m.s.cp. = i = 40 cp. Ans. 



(6) 40 X 12.57 = 502.8 lumens. Ans. 



(c) The mean horizontal candlepower = the length of the line oh X 20 = 

 2.5 X 20 = 50 cp. Ans. 



60 



(d) ^Q = 1.20 watts per m.h.cp. Ans. 



60 



^ = 1.5 watts per m.s.cp. Ans. 



4u 



40 



(e) The spherical reduction factor = en = 0.80. Ans. 



203. Reflectors. It is impossible, except in a general way, to 

 design a lamp so that it will distribute its illumination in a 

 desired manner. The distribution of light from light sources is 

 readily controlled, however, by reflectors, the type of reflector 

 being determined by the existing conditions. In "white way" 

 illumination, for example, as used in the business districts, it is 

 desirable that considerable light be delivered upward so as to illu- 

 minate the front of the buildings. Ornamental fixtures simi- 

 lar to that shown in Fig. 397 can be used. On the -other hand, 

 light sent upwards in the residential districts is either lost or 

 is undesirable from the point of view of the residents, because 

 such light comes in at their windows. Hence, some type of 

 fixture with a reflector similar to the pendant type shown in 

 Fig. 398 should be selected for this type of illumination. When 

 it is desirable to throw the light into the lower hemisphere, a 

 prismatic refractor is very efficient. This consists of a glass 

 cylinder in which prisms are cut. A cross-section of such a 

 refractor is shown in Fig. 399 (vertical section). The prisms 

 bend the light beams downward by refraction, as shown. Often- 

 times a second refractor is used in which the prisms are cut 

 vertically, as shown in Fig. 399 (horizontal section). This 



