[Electric engineering.] . the standard from the screen. Example.—Suppose, in Fig. 11, that A is a standard candle giving1 candlepower and that B is an incandescent lamp. The screen ismoved until a point is found where the two sides are equally illumi-nated. The reading on the scale then shows that the distance from thestandard is 20 inches. The total distance between the lamps is100 inches. What is the candlepower of B ? 18 ELECTRIC LIGHTING. 16 Solution.—If the total length of the photometer is 100 inches, thedistance from the lamp to the screen must be 100 — 20 = 80 candlepower of
[Electric engineering.] . the standard from the screen. Example.—Suppose, in Fig. 11, that A is a standard candle giving1 candlepower and that B is an incandescent lamp. The screen ismoved until a point is found where the two sides are equally illumi-nated. The reading on the scale then shows that the distance from thestandard is 20 inches. The total distance between the lamps is100 inches. What is the candlepower of B ? 18 ELECTRIC LIGHTING. 16 Solution.—If the total length of the photometer is 100 inches, thedistance from the lamp to the screen must be 100 — 20 = 80 candlepower of the standard is 1; hence, substituting in formula 2, /i = lX 80*202 16 c. p. Ans. 24. The Biinsen Photometer.—The Bunsen photom-eter has been more largely used than any other. It isvery simple and is capable of giving good results if usedproperly. The arrangement of the different parts is essen-tially the same as that shown in Fig. 11, but the distin-guishing feature of this photometer lies in the style of screen. used. It would be a difficult matter to tell when a simplescreen like that shown in Fig. 11 was illuminated equally onboth sides, and to overcome this difficulty Prof. Bunsendevised the screen shown in Fig. 12. The screen is madeby taking a piece of good quality of white paper and makinga grease spot in its center, as indicated by the star in .such a screen be held so that the front side is more strongly § 16 ELECTRIC LIGHTING. 19 illuminated than the back side, the grease spot will appeardark on the white ground of the paper, as shown in (a). If,however, the screen be more brightly illuminated on theback side, as, for example, if it were held between the eyeand a window, the grease spot will appear light on a darkground, as shown in (/;). If such a screen is mounted inplace of the screen B in Fig. 11, and arranged so that bothsides may be seen at once, the grease spot will disappearalmost entirely when the two sides of the screen are equallyillum
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