ON THE USE OF THE PYRANOMETER 

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



\\ e described the pyranometer in an earlier paper. ^ Its purpose is 

 to measure the radiation, originally a part of the solar beam, which 

 reaches a horizontal surface by scattering from all parts of the sunlit 

 sky : or to measure at night the loss of heat of a blackened horizontal 

 surface by reason of its long-wave radiation toward the whole sky. 

 The pyranometer may be used al^ to measure solar radiation. We 

 published in our earlier paper a comparison between observations of 

 solar radiation with the pyranometer and the pyrheliometer, and 

 these showed almost exact agreement. As these measurements were 

 made at various altitudes of the sun from about io° upward, they 

 indicated strongly the probable accuracy of the pyranometer for 

 sky radiation, which comes from all angles. 



Further use of the instrument has not diminished our confidence 

 in its accuracy, but we have found it necessary to alter the method 

 of reading with it. As stated in our earlier paper, the pyranometer 

 is developed from the plan of the Angstrom electrical compensation 

 pyrheliometer. A thermo-electric couple connected to a moving coil 

 galvanometer indicates differences of temperature between the two 

 blackened metallic strips which form the receiving surfaces for radia- 

 tion ; and a heating current of electricity furnishes to these strips a 

 quantity of energy whose measured intensity is the index of the 

 intensity of the absorbed radiation. In our first observations we found 

 that nearly 30 seconds must elapse after throwing on a heating 

 current before a new state of temperature equilibrium becomes com- 

 pletely established, as shown by a steady state of the galvanometer. 

 Accordingly we adopted the custom of waiting 30 seconds after each 

 exposure to radiation, in order that the steady state might be reached, 

 before recording the galvanometer deflection. At Mount Wilson 

 our attention was drawn to a phenomenon unnoticed at Washington. 

 When exposing the instrument to sunlit sky, through the glass 

 hemisphere used to cut oft' long-wave rays, the deflection of the 

 galvanometer came to a maximum within 5 seconds, and then con- 



* Smithsonian Misc. Coll., Vol. 66, No. 7. 



Smithsonian Miscellaneous Collections, Vol. 66, No. 1 1 



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