NO. 7 THE PVRANOMETER ABBOT AND ALDRlCIi 5 



shading" the instrument from the sun or sky. A small metal screen, 

 M, subtending' o.ooii hemispheres, is mounted on an equatorial axis 

 operated by a worm-wheel arrangement. This screen is used to 

 shade the sun from the strips, in case it is desired to measure the 

 sky alone, and not the sun and sky in combination. A nickel-plated 

 box, A', enclosing a wood block in which lies the plate E, is provided 

 to keep the copper plate E from external disturbances of tempera- 

 ture by wind currents. Around this box N fits a nickel-plated 

 cover, O, shown in figure 3, for use in observing the sun alone, in 

 making comparisons with the pyrheliometer. When the cover 

 shown in figure 3 is employed, the equatorial mounting of the sun- 

 screen M is removed, and the worm attachment is used for rotating 

 the solar cover box 0, just described. 



The following data were used to determine the constant of Pyra- 

 nometer A. P. O. 6 : 



Cm. 



Length of strips between soldered portions 0.623 



Width of thin strip (mean of 5 places) 0.198 



Width of thick strip (mean of 5 places) 0.201 



Ohms 



Electrical resistance of thin strip 0.2740 



Electrical resistance of thick strip 0.0369 



Electrical resistance in series with thin strip 0.819 



Electrical resistance in series with thick strip 0.364 



Assumed absorption of the lamp black 0.98 



Assumed transmission of the glass hemisphere (allowing 

 for 2 reflections with index of refraction, 1.5) 0.92 



(Thickness of strips determined by weighings approximately 0.00034 and 

 0.0030 centimeters). 



(Resistance of the two thermo-couples in series 30 ohms). 



From these data the current in the thin strip is '^ times the 



1494 



current in the outside electrical heating circuit. Hence the current 



squares are as 0.0719 to unity. Hence the constant of the instrument 



(when glass covered) is 



K = .07 1 9 X. 2740x60 ^ ^ 



.4.. 185 X 0.623 X 0.198 X 0.98 X 0.92 ^'^'^' 



so that the energy of radiation corresponding to a given heating 

 current C measured in amperes is 2.54 C'- calories (15° C.) per cm.- 

 per minute. If used at night without glass for measurement of long- 

 wave rays, the constant should probably be taken at 



92 98 



2.S4X -^ X % =2.41. 



•^^ 100 95 ^ 



The reader will perceive that the instrument may be used for the 



sun alone, the sun and sky in combination, the sky alone by day ; or 



