Page:Incandescent electric lighting- A practical description of the Edison system.djvu/135

 And the cost of lamps from this formula:

Cost of lamps =

$cost of one lamp × 100 × 1,000⁄Life at given efficiency$

The curve marked total cost shows the total cost of running 100 16 c.p, lamps 1,0U0 hours, the efficiencies of the lamps varying between 2.5 and 4.25 watts per candle. The efficiencies are shown by the vertical lines, referring to the scale at bottom. The value of the total cost at any point of the curve is shown by the horizontal line through the point, referring to the scale at the left of the diagram.

The lowest point of the curve shows the point where the total cost is lowest. This is the minimum cost of operating these lamps under the given conditions. The mark at the lowest point of the curve shows this minimum cost to be $783, and a vertical line through this point to the scale at bottom of the diagram shows that this total cost is a