Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ILLUMINATION parison lamp carnage, R an adjustable rheostat, W a screw-driven cursor on slide wire in bridge, K the knob for the com-parison lamp drive. The troublesome feature in all small port-able photometers is the comparison lamp. In the new designthe large resistance-temperature coefficient of the tungstenfilament is taken advantage of. The lamp is connected so as toconstitute one arm of a Wheatstone bridge. The other arms arecomposed of small coils of manganin wire. Between one lamptermin
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ILLUMINATION parison lamp carnage, R an adjustable rheostat, W a screw-driven cursor on slide wire in bridge, K the knob for the com-parison lamp drive. The troublesome feature in all small port-able photometers is the comparison lamp. In the new designthe large resistance-temperature coefficient of the tungstenfilament is taken advantage of. The lamp is connected so as toconstitute one arm of a Wheatstone bridge. The other arms arecomposed of small coils of manganin wire. Between one lampterminal and one coil a portion of this wire is extended to form aslide wire, and a scale is arranged to indicate the position of thecursor on this wire. The coils have such a resistance that whenthe lamp is receiving its proper current its resistance will besuch that the balance point of the bridge will come within therange of the cursor on the slide wire. Any change in the currentthrough the lamp is accompanied by a corresponding change inits resistance and the bridge is thrown out of balance. As a. Fig. 57.—The small Sharp-Millar photometer. convenient means of detecting the point of balance of the bridge,a low-resistance telephone receiver is used. In series with thereceiver is an interrupter consisting of a wheel into the peripheryof which pieces of insulating material have been set at frequentintervals. The two bridge arms which are in parallel withthe lamp and its corresponding arm have approximately fivetimes the resistance of the latter, so that the amount of addi-tional current which they require produces no undue drain onthe battery. The telephone receiver is provided with a headband so that both hands are left free for the operation of rotatingthe contact wheel and adjusting the external rheostat until acondition of silence in the telephone is attained. The adjust-ment of current in this way can be made to within about 1 milli-ampere (one-third of 1 per cent.), which is
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
