Page:Incandescent electric lighting- A practical description of the Edison system.djvu/151

 the lamp bills up to 15 per cent, by increasing the candle-power of the lamps, does not decrease the cost per lamp of operating the plants but does decrease the cost per candle of light furnished. If they are paid for the increased light given by the lamps, the efficiency of the plant is made a maximum for the existing conditions; but if they are not paid for the additional light furnished, the efficiency of the plant is reduced.

This law enables any one operating an incandescent lamp plant to determine whether or not he is using the most suitable lamps for his plant. If the conditions of operation of a new plant are all known, and the quality of the lamps made by any lamp maker is known, we can determine before starting what is the most economical lamp to use.