THE PYRANOMETER— AN INSTRUMENT FOR MEASUR- 

 ING SKY RADIATION 

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



The instruments we are about to describe are the result of 

 investigations begun under a grant from the Hodgkins Fund in 

 1913. They are derived in principle from the highly successful pyr- 

 heliometer of K. Angstrom.' In that instrument there are two 

 strips of blackened manganin, one of which is shaded, from the solar 

 radiation, the other exposed. The shaded strip is heated by an elec- 

 tric current whose strength can be graduated until the temperatures 

 at the back of the two strips are equal, as shown by means of thermo- 

 elements attached to the rear of the two strips. When the equality 

 of temperature is brought about, as shown by zero deflection of the 

 galvanometer, it is assumed that the energy of radiation absorbed in 

 the exposed strip is equal to the energy of the electric current dissi- 

 pated in the shaded one. To eliminate errors the uses of the strips 

 are reversed, so that the formerly shaded strip is exposed to radiation, 

 and the formerly exposed strip is shaded and heated by the electric 

 current. 



In another instrument of K. Angstrom, called by his son the 

 pyrgeometer,' a pair of blackened manganin strips alternate with a 

 pair of polished gold-plated ones, and the whole grid of four strips, 

 arranged centrally nearly in the plane of the surface of a nickel-plated 

 box, is exposed to the night sky. The bright strips lose very little 

 heat by radiation, while the black ones lose comparatively a good 

 deal, and so the effect is to cool the blackened strips with respect to 

 the bright ones, and this state of affairs is indicated by means of 

 thermo-elements attached to the series of strips. It is provided that 

 the electric heating current can be used to warm the blackened strips 

 until their temperature is restored to that of the bright strips as 

 indicated by the zero of the galvanometer. This instrument was not 

 regarded by its inventor as a primary instrument, and following his 



^ Astrophysical Journal, Vol. 9, page 332. 



' Smithsonian Misc. Coll., \'ol. 65, No. 3, page 28. 



Smithsonian Miscellaneous Collections, Vol. 66, No. 7. 



