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fuel is high priced, or where other caused operate to make the cost of generating current high, it is specially important to use lamps of the maximum efficiency, for we have seen from the above curves that where the cost of current is high, the use of lamps whose efficiency is a little above or below the maximum efficiency attainable under the conditions of operation, makes a very marked increase in the operating expenses of the plant.
If in any plant the lamp bills are only 10 per cent, of the total expenses, then increasing the efficiency of the lamps by increasing their candle-power does not diminish the total cost of operating the plant. In order to reduce the total cost, the lamps must he replaced by lamps of the same candle-power but of higher efficiency. If the efficiency of the lamps is increased by raising their candle-power, the cost of operating the plant per unit of light produced is reduced, but the total cost is increased. A plant which is paying less than 15 per cent, of its total expenses for lamps, and which brings
