AN IMPROVED WATER-FLOW PYRHELIOMETER AND 

 THE STANDARD SCALE OF SOLAR RADIATION 



By C. G. ABBOT and L. B. ALDRICH, 



Smithsonian Institution 



(With One Plate) 



In 1927, V. M. Shulgin ' suggested an improvement of the water- 

 flow standard pyrheliometer." He pointed out, and our experience 

 abundantly confirms it, that fluctuations of the rate of flow and of the 

 temperature of the water lead to irregular drift and wiggle of the 

 galvanometer record in the use of such an instrument, for instance, 

 as water-flow pyrheliometer No. 3. These irregularities make up a 

 principal part of the total error in using the instrument as described 

 by us in Volumes 3 and 4 of the Annals of the Astrophysical Observa- 

 tory. Shulgin's improvement consists in duplicating the instrument 

 so as to have two chambers instead of one. He divides the current 

 of water nearly equally between them. Solar radiation is introduced 

 in one chamber, and compensating electrical energy in the other. Thus 

 a null method is substituted for the deflection method, and the ir- 

 regularities of the water current are eliminated as a source of error 

 because they affect both chambers equally and simultaneously. 



We wholly approved of Shulgin's principle but felt that a more 

 favorable mechanical and electrical expression than his could be given 

 to it. Accordingly, by the skill of A. Kramer, mechanician, we du- 

 plicated last winter the water-flow pyrheliometer No. 3, described 

 at page 52 of Volume 3 of the Annals. We combined the two chambers 

 thus made available to form a new standard water-flow pyrheliometer 

 No. 5. In combining the two instruments, in order to control sur- 

 rounding conditions, we added a common enclosing metallic case whose 

 hollow walls were bathed by a separate current of tap water. We also 

 arranged a common entrance pipe for the distilled water used in the 

 measurements. This entering water stream was protected by a closely 

 surrounding Dewar vacuum enclosure and was well stirred by a baffle 

 circulatory system. To determine the equality of temperatures of the 



' Monthly Weather Rev., August, 1927. 



" See description of this instrument, Ann. Astrophys. Obs., Smithsonian Inst., 

 vol. 3, p. 52, 1913. 



Smithsonian Miscellaneous Collections, Vol. 8/, No. 15 



