Fig. 48. - Relative cost of producing a given amount of light by various illuminants at usual prices. Costs are based on the following prices: Candles, 12 cents a pound; kerosene, 15 cents a gallon; gas, $1 for 1000 cubic feet; electricity 10 cents a kilowatt hour. The solid lines represent cost of fuel or of current, the shaded parts the cost of the mantles and bulbs. Where prices are different from those given above, costs will be correspondingly different.
The total economic gain which is made possible by doubling the efficiency of production of light is almost incalculable. To consider only one item, it has been estimated by good authorities that the liberal use of the newer lamps might reduce the yearly consumption of coal in the United States by $8,000,000. The general tendency has been, however, not to reduce the amount of power used, but rather to use more light. The ability to produce so much more light without increasing the cost has encouraged the development of methods of lighting in which attention is given primarily to artistic appearance and to the comfort of the user rather than to getting as much light as possible out of a fixture.
* U. S. Bur. Standards, Circ. 55.
The cost of lighting by any method depends to some extent on local conditions, and the statements of cost given below will apply only approximately in any particular case. The cost will naturally depend on the candle power of lamps used and the time the lamps burn. In order to make comparisons between different kinds of lamps, it is convenient to consider a definite amount of lighting, which is obtained by multiplying the candle power of the lamps burned by the number of hours they burn. For example, 1,000 candle-hours of lighting may be obtained by burning a 10-candle lamp 100 hours or a 50-candle lamp 20 hours, but if the lamps are of the same kind the cost will be about the same. Calculations of the cost of producing 1,000 candle-hours by different lamps are sometimes useful in choosing between lamps, but of course it does not necessarily follow that the lamp for which this cost is lowest is most economical for household use.