THE STANDARD SCALE OF SOLAR RADL\TION 



By C. G. abbot and L. B. ALDRICH 



Smithsonian Institutian 



We published about 2 years ago" a description of the double- 

 chamber compensation water-flow pyrheliometer No. 5 and the results 

 of comparisons made between it and silver-disk pyrheliometer ST.5bis. 

 These results indicated that the Smithsonian radiation scale of 1913 

 was 2.5 percent too high, but on account of certain defects in the 

 sensitive parts of the water-flow pyrheliometer No. 5 we felt that 

 further comparisons were needed before accepting that correction. 



The thermoelectric junctions, which were imperfectly soldered in 

 1932, have been removed and replaced by others securely melted to- 

 gether. Every care was used in reassembling the instrument to guard 

 against all conceivable sources of error. We have employed the in- 

 strument on Mount Wilson in July 1934 to restandardize silver-disk 

 pyrheliometer S.LSbis- Besides the diaphragms within the boxed in- 

 strument shown in plate i, figure 3 of our former publication, we 

 invariably used in addition in 1934 the double polished screen de- 

 scribed in lines 34-40, page 4, of our former publication, whereby the 

 water-flow pyrheliometer No. 5 was limited to observe precisely the 

 same sky area as the silver-disk pyrheliometer S.L5bis- All other ar- 

 rangements were precisely like the best ones described in our former 

 paper. During most of the days observed in 1934 the sky was of ex- 

 ceptional clearness, and observations were made only at high sun. The 

 temperature of the different days varied so much that on some com- 

 parisons silver-disk pyrheliometer S.L5bis was read at 46° C, while on 

 some other occasions its readings were as low as 26°, with all inter- 

 mediate temperatures between these extremes represented in the 

 several series. We were unable to detect any difi^erences in results 

 showing any influence of this large range in temperatures. 



As stated in our former paper, the entire measurement made with 

 the compensation water-flow instrument consists in a determination of 

 the energy of the compensating electric current. Equivalence between 

 solar and electric heating in the two opposed chambers is indicated by 



' Smithsonian Misc. Coll., vol. 87, no. 15, 1932. 



Smithsonian Miscellaneous Collections Vol. 92, No. 13 



