. Laboratory exercises in elementary physics. ETER. 199 the relations of distance and degree of radiation ? 2. IfD and D be any two distances, and / and 1 the corre-sponding intensities, can you combine them in a formula ?Remember that for each distance the same amount of lightwill spread over a surface whose area would be proportionalto the squares of the width of the square of light, and thatthe intensity of the light will become less in proportion tothe surface illuminated. EXERCISE 4. CANDLE-POWER BY THE RUMFORD PHOTOMETER. Preliminary.—The unit used in comparing the light-in-tensities is
. Laboratory exercises in elementary physics. ETER. 199 the relations of distance and degree of radiation ? 2. IfD and D be any two distances, and / and 1 the corre-sponding intensities, can you combine them in a formula ?Remember that for each distance the same amount of lightwill spread over a surface whose area would be proportionalto the squares of the width of the square of light, and thatthe intensity of the light will become less in proportion tothe surface illuminated. EXERCISE 4. CANDLE-POWER BY THE RUMFORD PHOTOMETER. Preliminary.—The unit used in comparing the light-in-tensities is called one candle-power, written c. p. Candle-power is measured by determining the distances at whichthe source of light to be tested, and a standard candlespecified by law, give lights of equal intensity. For ex-ample, if the light to be measured and the standard candlemust be at the same distance in order to give light ofequal intensity, the light of required intensity would be1 c. p. The form of photometer used in this exercise is shown. Fig. 101. in Fig. 101, and is called the Rwnforcl Photometer. Twometer-sticks MM carry two blocks LL, one of whichsupports the standard reference-candle, the other the lightto be measured, represented here by two candles. These 200 LIGHT. blocks can be moved to any position on the card-board screen S is placed between them, and an up-right rod R at the left-hand ends of the meter-sticks. Eachlight will cast a shadow from this rod upon the screen atthe left. When these shadows are equally black, both lightsare of equal intensity at the screen. EXPERIMENT. Apparatus.—Rumford photometer as in Fig. 101; candle; light tobe tested; scissors. Object.—To determine the candle-power of a light bythe Rumford photometer. Manipulation.—Place one candle on block Z/, light it,and set the block 30 cm. from the rod. Place on L thelight to be tested. Allow the lights to burn for a fewmoments, and then move L towards the screen until the
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishe, booksubjectphysics
