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A
Fig. 8 Photometer and Screens Case A — Screen at left of balance point Case B — Screen at right of balance point Case C — Screen at balance point
lation between the intensities of illumination that the two lamps being compared are able to produce, we can reason back as to the relation between the candle-powers given, respectively, by lamps A and5 (see Fig. 8). The scale of the photometer shows us that at a distance of 60 inches the lamp A produces an illumination equal to that which the lam_p B can produce at a distance of 40 inches. At first thought we
might say that A must give ^ as great a candle-power as 5, but recalling the inverse square law referred to on page 14, which states that the intensity of illumination varies inversely as the square of the distance, or what is the same thing, that the candle-power necessary to produce a given illumination varies as the square of the distance from the source of the plane, we see that the ratio of the candle-power of ^ to that oi B instead of being 3 to 2 is 3' to 2 or 9 to 4. The horizontal candle-power of yf, therefore, is 1% times that oi B. If 5 is a lamp of some known candle-power, the candle-power of A is determined by multiplying the candle-power of B by 2^. The general rule, then is that the candle-power of two lamps on a photometer are to each other as the square of the distances from each to the screen are to each other. For accurate photometry, the grease spot screen is no longer in use, but the newer and more accurate photometers are the same in principle.
The simple bar photometer measures candle-power in one direction only. If the lamp being measured is rotated about its vertical axis, its mean horizontal candle-power is obtained. In like manner, if the vertical axis of the lamp being measured is tipped and the lamp rotated on this axis, the average candle-power at any angle can be determined.
x^n instrument called the foot-candle meter as mentioned last year, has been designed to measure foot-candle intensities quickly and with