Artificial Lighting 171 



In the usual incandescent lamp bulb there is a vacuum. 

 But in some of the latest products the bulb is filled to about 

 two thirds of normal atmospheric pressure with nitrogen. 

 If air were present, the electric current would heat the fila- 

 ment to kindling temperature, when it would burn out. In 

 modern lamps the aim is to reach a natural standard of 100 

 per cent efficiency. This standard is set by the glowworm or 

 firefly, which produces light without heat. But at present 

 artificial lamps fall far below this level, the carbon filament 

 being about 3 per cent efficient ; the tungsten 10 per cent ; 

 the electric arc 17 per cent ; and the mercury-vapor glass tube 

 about 6 per cent. The gas light with the Welsbach mantle is 

 5 per cent efficient, which is also the grade of the acetylene 

 light. 



66. Comparative life, cost, and efficiency of lamps. It is 

 useful to compare the life, cost of operation, and efficiency of 

 the carbon and tungsten filament lamps. The life of the 

 carbon filament in a 16 candle power lamp is rated at 800 

 hours maximum. The metal filament is rated at 1600 hours 

 maximum. The carbon lamp consumes about 3.1 watts per 

 candle power, or 3.1 X 16 or 49.6 watts per lamp. During 

 its life of service it consumes 49.6 X 800 or 39,680 watts. 

 A thousand watts for one hour make one kilowatt hour, so 

 that the carbon lamp consumes 39.6 kilowatt hours of electric 

 current. If electric current costs 10 cents per kilowatt hour, 

 the cost for the operation of the lamp is $3.96. 



The metal filament lamp is rated to consume 25 watts per 

 lamp. Multiply its life term of 1600 hours by this, and the 

 product is 40,000 watts or 40 kilowatt hours. This at the 

 same rate costs $4, for the electric current. But the life 

 of the tungsten is twice that of the carbon, which would make 

 one tungsten equal two carbon lamps and puts the cost for 

 current for the one, $4, against the cost for current for the 

 other two, $7.92. The saving by the metal filament is worth 

 while. 



