COMPUTING THE COST OF LIGHTING 



377 



from the light to the screen be squared and divided by the distance 

 of the candle from the screen squared, the candle power of the light 

 may be obtained. 



Cost of Light. There are many different types of lights on the 

 market. Some of them produce light at a very low cost, but 

 others produce light at a very great cost. 



Fig. 260 will show the comparative cost of different materials 

 for lighting. It will be seen that candles are the most expensive, 

 and that gas mantles are much cheaper than the open gas flame 

 or kerosene. However, people may live in localities where it is 

 impossible to obtain the cheapest form of lighting. If kerosene 



Inverted Hw* Open Flame Upright Mantle 



rboaLamp 



_ 20 Cand/epower 



FIG. 261. Amount of light received from 

 different lights. The same amount of 

 gas used in each case . Why is it 

 cheaper to use mantles than open 

 flame? 



= iO Can<//cfio*er 



FIG. 262. How much more light 

 do you receive from a Tungsten 

 lamp than from a carbon lamp if 

 the same amount of electricity is 

 used in each case? 



only can be used, the kerosene mantle lamp is very much cheaper 

 than the ordinary kerosene lamp. 



Use of Mantles. Wherever mantles can be used, they make it 

 possible to obtain the same amount of light much more cheaply 

 than by the use of open flames. Usually a mantle will use about 

 one-fourth as much gas in giving the same amount of light as an 

 open flame. In addition to the actual saving in gas, the mantle 

 burners have a great advantage in furnishing a steady light. In 

 some cases, also, there is considerable reduction of the risk of fire, 

 since the mantle burners are protected by chimneys so effectually 

 that inflammable objects cannot reach the flame. There appears 

 to be no good reason for the use of open flame burners in any 

 place unless there are conditions which cause excessive breakage of 

 mantles. 



