Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . Fig. 38 b. Fig. 37.—Contrasting elements. Figs. 38 a and 33 b.—Balanced and unbalanced condition. construct a practical instrument have failed to yield satisfactoryresults. The disturbing influences are mainly the moisture andheat of the air and a gradual alteration in the condition of theselenium itself. One serious objection to this photometer is thelong time it takes the resistance to assume a value correspondingto a given illumination. This is particularly true of the decreaseof

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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912