Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ves, one sleeve over or outside of the other. By making aspiral slot in one sleeve and a longitudinal slot in the other andby placing a pin through them into the shaft, the size of the open-ings between the sections of the disks may be easily increasedor decreased by sliding the sleeves along the shaft in theproper direction. This change in adjustment may be made 82 LIGHT, PHOTOMETRY A^D ILLUMINATION while the disk is in operation by placing the end of a lever in thegroove of the collar C which
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ves, one sleeve over or outside of the other. By making aspiral slot in one sleeve and a longitudinal slot in the other andby placing a pin through them into the shaft, the size of the open-ings between the sections of the disks may be easily increasedor decreased by sliding the sleeves along the shaft in theproper direction. This change in adjustment may be made 82 LIGHT, PHOTOMETRY A^D ILLUMINATION while the disk is in operation by placing the end of a lever in thegroove of the collar C which is attached to the sleeves. Theother end of the lever may be placed above a scale calibrated toindicate by the position of the lever the value of the openings inthe disk. With this arrangement the intensity can only bevaried through a range of about 50 per cent. The minimum speed of rotation is that at which no flickereffect is experienced. Higher speeds give the same results. It is obvious that where the flicker is produced by a rotatingsectored disk, it would be more pronounced than if due to. Meter Candles 1000 Fig. 25.—Frequency at which flicker becomes perceptible for difierentintensities of illumination. (Sectored disk.) alternating current supplied to the illuminant. Investigationsof the critical speed of the disk were conducted by Ferry* andPorter^ where light of known intensity and quality were passedthrough a rotating sectored disk. The critical frequency or thefrequency at which a flicker was just imperceptible is indicatedby the curve in Fig. 25.^ It was found that the vanishing flickerfrequency appeared to be a function of optical intensity aloneand independent of the color value of the light. Porter also found that the critical frequency was a linear func-tion of the logarithm of the intensity. For intensities greaterthan meter-candles. * American Jour, of Science, Vol. 44, p. 192, Roy. Soc, Vol. 70, p. 313, 1902.^Bull. Bur. Stand., Vol. 5
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
