i^ocfaling Jfunb 



Till-: SMITJISONIAN STANDARD 

 I'^'kIIELIOMETRY 



P.Y C. G. AP.P.OT 

 Rcsciirilt .Issociatc, Smithsonian Inslitnlion 



Since 1910 nearly a hundred copies of the silver-disk pyrhehometer 

 have been prepared at the Smithsonian Institution, ihey are in use 

 in many CDuntries. Observers, even those using other types (jf ])yr- 

 hehometer, often express their results in terms of "the Smithsonian 

 standard scale" which is carried to them by these silver-disk instru- 

 ments, standardized against the water-llow pyrhehometer. Aidrich 

 and Abbot, in 1947, made a painstaking comparison at Mount Wilson 

 between two silver-disk instruments and the water-flow pyrhehometer. 

 They obtained within one part in a thousand the same result as in 

 1934 and earlier.^ Various observers have investigated old silver-disk 

 instruments and find no evidence that there has been a change of their 

 sensitiveness since 1910. 



So the question of the standard scale depends on the adequacy of 

 tlie water-flow pyrhehometer as a standard. (Originally this instrument 

 comjirised a single deep test-tube-like blackened chamber of metal 

 with hollow walls. In these walls, in the extreme rear wall, and in 

 the walls of a hollow cone not quite at the rear, on which all the sun's 

 rays fell directly, a current of water constantly flowed to carry off 

 the solar heat as fast as absorbed. An electrical thermometer, meticu- 

 lously calibrated by means of an extremely delicate standard mercury 

 thermometer, registered the rise of temperature between the entrance 

 and the exit of the stream of water. A carefully gaged diaphragm 

 admitted the solar rays to the chamber. Other diaphragms of slightly 

 larger diameter, along the vestibule and within the chamber, served 

 the double purpose of opposing air currents, and of obstructing the 

 entrance or the escape of stray light. The rate of flow of the water 

 was determined by frequent weighings. 



As all of the entering beam of sunlight fell upon the hollow black- 

 ened cone near the extreme rear of the chamber, over 95 percent of 



• Aldricli, L. B., and .\bbot, C. G., Smithsonian pyrliclionictry and the 

 standard scale of solar radiation. Smithsonian Misc. Coll., vol. 1 10. No. 5, 1948. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 110, NO. 11 



