THE PROGRESS OF THE ELECTRIC LIGHT n 



by the crater. The long flame arc allows the carbons to be 

 arranged side by side pointing downwards, so that the arc may 

 burn across the lower ends. In this way the crater is fully 

 exposed and the whole of the light emitted by it is rendered 

 available. 



The flame arc is more than twice as efficient as the open 

 arc, the consumption being about 0*4 watt per candle. This 

 is the figure for the yellow flame arc ; the white and red are 

 less efficient, consuming about 07 watt per candle. The 

 determination of the cost of lighting with arc lamps is a more 

 difficult matter than in the case of incandescent lamps. In 

 the latter case it is only necessary to know the average con- 

 sumption of watts per candle and the average useful life; 

 the calculation of the cost per 1,000 candle hours is then a 

 simple matter. With the arc lamp the prime factors in the 

 cost of lighting are naturally also the consumption in watts 

 per candle and the cost of carbons ; but in addition the 

 maintenance costs are considerable and unfortunately so 

 variable from one installation to another that it is impossible 

 to make any estimate that is of general application. Trimming 

 costs will vary immensely according as the lamps are scattered 

 over a wide area or grouped closely together : the extent to 

 which the lamps are depreciated in value will also vary very 

 greatly according as they are burning under cover or in the 

 open. It is better therefore to give the cost taking into 

 account only the cost of energy and carbons, which will be 

 independent of the nature of the installation ; on this basis 

 the costs are as follows : 



Cost of Lighting per 1,000 Candle Hours 



If these figures are compared with those already given for 

 incandescent electric lamps it will be seen that the tungsten 



