[Electric engineering.] . fy that the greater thedistance, the less is the illumination. This may be alsoexpressed as follows: If x is the illumination producedand / is the brightness of the source of lis;ht, then I_d~* (1.) 16 ELECTRIC LIGHTING. 16 This means, for example, that the illumination of the sur-face will be doubled if the candlepower of the source isdoubled and that it will be one-quarter as great if the dis-tance from the source is doubled. 22. Elementary Photometer.—Suppose, now, that wehave two sources of light, such, for example, as a candle andan incandescent lamp, and that we
[Electric engineering.] . fy that the greater thedistance, the less is the illumination. This may be alsoexpressed as follows: If x is the illumination producedand / is the brightness of the source of lis;ht, then I_d~* (1.) 16 ELECTRIC LIGHTING. 16 This means, for example, that the illumination of the sur-face will be doubled if the candlepower of the source isdoubled and that it will be one-quarter as great if the dis-tance from the source is doubled. 22. Elementary Photometer.—Suppose, now, that wehave two sources of light, such, for example, as a candle andan incandescent lamp, and that we wish to compare thebrightness of these two sources. If the candle A and thelamp B are placed in a dark room, so that there will be noother light to interfere, and a screen C is placed betweenthem, as shown in Fig. 11, one side of the screen will beilluminated by the candle and the other by the lamp. Ifthe candle and lamp were exactly of the same brightness, itis evident that the two sides of the screen would be equally. FIG. n. illuminated when placed midway between them. If thescreen is mounted so that it can be slid along between thelights, a point can always be found where the screen will beequally illuminated on both sides. In the present case, thescreen would have to be moved nearer the candle than thelamp, because the candle is not as bright as the that the screen has been adjusted so that theillumination is equal oh each side, and that the distances dxand d2 have been read off by means of the scale S, dt beingthe distance from the screen to the standard candle and d% § 16 ELECTRIC LIGHTING. 17 the distance from the screen to the light that is beingmeasured. Let x\ be the degree of illumination produced on one sideand x\ that on the other, and f and /„ the candlepowers ofthe standard and the light being measured, from formula 1, we have V=£. and *. = £.; but since the illuminations on the two sides are equal, wemust have L-Ld; ~ d;-
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