. The Astrophysical journal. T)e H, having a resistance of 50 ohms and a sensi-tivity of microvolt. The critical damping resistance of suchan instrument is 120 ohms, and is obtained by using external resist-ance in series with galvanometer and thermopile. With diaphragmNo. 60 and a photographic plate of normal density, these conditionsafford deflections conveniently large and within the limits of thescale. To obtain corresponding deflections with smaller dia-phragms the external resistance is necessarily reduced, increasingthe sensitivity of the galvanometer and lengthening its period. TH
. The Astrophysical journal. T)e H, having a resistance of 50 ohms and a sensi-tivity of microvolt. The critical damping resistance of suchan instrument is 120 ohms, and is obtained by using external resist-ance in series with galvanometer and thermopile. With diaphragmNo. 60 and a photographic plate of normal density, these conditionsafford deflections conveniently large and within the limits of thescale. To obtain corresponding deflections with smaller dia-phragms the external resistance is necessarily reduced, increasingthe sensitivity of the galvanometer and lengthening its period. THERMOPILE IN PHOTOGRAPHIC PHOTOMETRY 261. 262 IIAKLAX TKLE STETSON As few connections as possible should be made between thegalvanometer and thermopile, care being taken to avoid unlikemetals, to use ordinary means for maintaining uniform temperatureand freedom from draughts, and to protect all binding-posts andconnectors with cotton wool. METHODS OF MEASUREMENT AND REDUCTION In measuring a plate for reduction, a diaphragm somewhatlarger than the stellar images to be measured is selected and putin place upon the stage of the instrument. Since the constructionof the diaphragm is such that it projects shghtly above the level ofthe stage-platform to insure contact with the film, the plate isalways placed film side down with care. The stellar image inquestion is placed over the diaphragm, and brought to the centerof the latter by viewing its image upon the thermopile by meansof the finding eyepiece. As soon as the galvanometer has cometo a steady deflection the reading is noted. The stellar imageis then moved just off the diaphra
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
