70 THE PRODUCTION AND USE OF LIGHT 



.03 of a cent per candlepower-hour, or about the same as the 

 open flame gas jet. (Verify these computations.) 



The TUNGSTEN FILAMENT lamp is also made in several sizes; 

 the sizes most commonly used for resident lighting are the 

 25-watt lamp giving about 20 candlepower of light, the 40-watt 

 lamp giving about 32 candlepower of light, and the 60-watt 

 lamp giving about 50 candlepower of light. Therefore, at 10 

 cents per kilowatt-hour for current, these lamps cost about 

 .012 of a cent per candlepower-hour. This is about % 

 the cost of operating the carbon lamps and about .two times 

 the cost of operating the Welsbach mantle gas lamps per 

 candlepower-hour. (Verify these calculations.) 



FIG. 63. Dial of the watt-hour meter. The reading is 1581 kilowatt-hours. 



Although modern electric lighting using tungsten lamps is 

 as yet generally more expensive than gas lighting using 

 Welsbach mantles, still electric lighting affords so many 

 advantages that it is rapidly displacing gas where both are 

 available. Some of these advantages are: (1) Greater con- 

 venience; (2) smaller and more numerous units are easily 

 provided a very desirable feature; (3) to a certain extent 

 gas lights consume the oxygen in the air and give off carbon 

 dioxide; electric lights do not; (4) to a certain extent gas 

 lights tend to blacken the ceiling and walls of the room; electric 

 lights do not; (5) while gas may be 'put to many uses other 

 than lighting in the home, notably cooking and heating, 

 modern invention makes it possible to use the electric current 

 in the home in a multitude of ways (Figs. 46 and 47). 



