Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . running lengthwise of the box, by means of a cord which 118 LIGHT, PHOTOMETRY AND ILLUMINATION is attached to the supports of the lamp and passes throughpulleys and around a drum which is operated by the knob light from this lamp falls upon a milk glass plate at K whichis viewed through the optical device. The intensity of lightupon this plate is made to vary inversely as the square of thedistance from the comparison source by making the inner sur-face of the instrument a dull black and by
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . running lengthwise of the box, by means of a cord which 118 LIGHT, PHOTOMETRY AND ILLUMINATION is attached to the supports of the lamp and passes throughpulleys and around a drum which is operated by the knob light from this lamp falls upon a milk glass plate at K whichis viewed through the optical device. The intensity of lightupon this plate is made to vary inversely as the square of thedistance from the comparison source by making the inner sur-face of the instrument a dull black and by interposing a systemof moving screens as shown in Fig. 55. These screens are offiber, supported on brass rods and have apertures of sufficientsize to permit the direct rays from the lamp to pass the same time they are of sufficient size to intercept anylight reflected from the sides of the box. The scale upon which the indications of the photometer arer^^d, is made of translucent celluloid, graduated with an inverse-square scale, and placed in a longitudinal opening in the side. Fig. 55.—Plan of Sharp-Millar photometer. of the box. This scale is equipped with a shutter which may belowered or raised by means of an external knob, and which maybe used to exclude external light from the interior of the facilitate taking readings in dark places, a slit is cut in theside cf the housing containing the comparison lamp, so thatwhen the shutter is raised the shadow of the pointer, which isinside the box, is cast upon a brightly illuminated scale. Onthe other side of the housing is a small tube and cross-hairwhich, when the comparison lamp is properly placed, throws aline in the middle of a spot of light on the side of the box. Inthis way the proper position of the lamp is determined. Beneath the scale and toward the right of Fig. 54 is shown aresistance for varying the current of the lamp. At the rear endof the box are four terminals, two for the supply line and
Size: 3131px × 798px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
