Page:Electricity (1912) Kapp.djvu/185

Rh Twice during each complete period the lamp receives alternately a maximum of current and no current at all. Will this produce a disagreeable flicker? The answer to this question depends on two things; first the frequency and then the thermal storage capacity of the lamp filament. The light is due to the high temperature of the metallic filament, and that is due to the current: A strong current produces more light than a weak one, but the emission of light does not instantly follow the variation in current strength. Time is required for heating and for cooling, and provided the intervals between heating are sufficiently short as compared to the heat which the lamp can radiate in the time, its temperature will not materially change and there will be no flicker. Obviously the thinner the filament and the greater its radiating power, the higher must be the frequency at which flickering is no longer noticeable. There is also a personal element in the observation of flickering; some persons observe it sooner and feel it more unpleasantly than others. From experiments I have made with various lamps and assisted by various observers, I found that at a frequency of 25 no observer could detect flickering when