68 



THE PRODUCTION AND USE OF LIGHT 



fixtures and shades will be chosen which will distribute the 

 light with approximately equal intensity in all directions, 

 Figs. 56 and 57. If, on the other hand, it is desired to 

 illuminate a desk or table standing directly beneath the lamp, 

 another style of fixture and shade should be used, Figs. 58 

 and 59. 



77. Relative Cost of Operating Gas and Electric Lights. 

 Illuminating gas is furnished to the consumer under very 

 slight pressure. Usually gas pressure is sufficient to support 



FIG. 60. The open 

 jet burner. 



FIG. 61. The distribution curv; 

 for the burner. 



a column of water only from 1.8 to 2.8 in. in height, Art. 56. 

 The price charged varies in different localities but usually 

 ranges between 80 cents and $1.25 per 1000 cu. ft. 



Gas may be burned either in the OPEN FLAME jet or in the 

 WELSBACH MANTLE (see Art. 57). The open flame jet (Figs. 

 60 and 61) usually burns about 5 cu. ft. of gas per hour and 

 gives about 20 candlepower of light. Therefore, the cost of 

 this light at $1.00 per 1000 cu. ft. for gas is about .025 of a 

 cent per candlepower-hour. (The student should verify these 

 computations.} 



When gas is burned in a Welsbach mantle it is generally 

 consumed at the rate. of about 3^ cu. ft. per hour and the 

 mantle gives about 60 to 80 candlepower of light. Therefore 

 at $1.00 per 1000 cu. ft. the cost of this light is not more than 



