Some characteristics of the Marvin pyrheliometer . radiometric heating and cooling to , for the series of observations made by sighting onthe sun. The change in the temperature of the thermometerduring cooling is therefore identical with the change in the tem-perature of the disk over the 50-se:ond interval. If similar com-putations are made for a heating curve, it will be found that the Foote) The Marvin Pyrheliometer 621 temperature of the thermometer is less than that of the disk byessentially a constant amount over the 50-second interval and thatthe change in temperature of the th
Some characteristics of the Marvin pyrheliometer . radiometric heating and cooling to , for the series of observations made by sighting onthe sun. The change in the temperature of the thermometerduring cooling is therefore identical with the change in the tem-perature of the disk over the 50-se:ond interval. If similar com-putations are made for a heating curve, it will be found that the Foote) The Marvin Pyrheliometer 621 temperature of the thermometer is less than that of the disk byessentially a constant amount over the 50-second interval and thatthe change in temperature of the thermometer is again identicalwith the change in temperature of the disk. A typical heating and cooling curve for a radiometric calibra-tion is shown in Fig. 5. The small circles represent observedpoints. The line A CE represents the corrected heating and coolingcurve. The total change in temperature of the disk in terms ofthe coil resistance is given by the distance CD. This is equal tothe observed change in 50 seconds, PQ or PQ, multiplied by the. Ti fne m> ««ci>n(/«< Fig. 6.—Method of determining lag factor It is also equal to the computed change RE in 50seconds multiplied by the same factor. Another way to obtainthe true change in 60 seconds is to extrapolate the observed curvePQ back to zero time (or to the time of the beginning of a cooling)and take the difference in the ordinates of the points 5 and Q. The graphical method of obtaining the lag for a radiometriccalibration is shown in Fig. 6. The observed points are represented by circles. Throughpoints 2, 3, 4, 5, 6, 7, and 8, 9, 10, 11, 12, 13 pass the best com-puted exponential curves. Extrapolate the cooling curve to thepoint b and move the heating curve up to the position 1 b. Thedistance xy measured along the time axis is twice the lag of thethermometer. 622 Bulletin oj the Bureau of Standards {Vol. 14 In the case of electrical heating, the lag effect is somewhat morecomplicated. During the heatin
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