Cost of light obtained from an incandescenttungsten filament lamp, consists of the renewalcost of the lamp and the energy cost consumedby the lamp. The former decreases by using along burning lamp or a low efficiency lamp, while the latter to the contrary. The economicallife can be determined by knowing the lifeefficiencycharacteristic together with the ratioof the renewal cost to the energy cost perK.W.H. and it is shown that the economicallife depends simply on the ratio of the renewalcost to the electrical energy cost per K.W.H. inthe same sence and on the size of the lamp in the inverse sense.
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