NO. 7 THE PYRANOMETER — ABBOT AND ALDRICH 7 



O. 9 is found for all altitudes of the sun when the pyranonieter 

 readings are reduced to vertical incidence. This confirms the accu- 

 racy of the instrument for observations of the entire sky. 



PYRANOMETER A. P. O. 5 



As stated above, we employed but one sensitive strip in Pyranometer 

 A. P. O. 5, embedding the cool junctions of the thermo-elements in 

 the copper plate E. This form is several times as sensitive as Pyra- 

 nometer A. P. O. 6, so much so that we employed with it a poten- 

 tiometer current to bring the very large galvanometer deflections to 

 zero, and then balanced the potentiometer current by heating the 

 strip. Unfortunately, a defect of this pyranometer is a secondary 

 deflection, caused by the warming of the portion of the plate E vmder 

 glass as soon as the shutter is opened. This secondary deflection was 

 found very large, sometimes even as great as a c[uarter of the pri- 

 mary one. Its direction was sometimes in one sense, sometimes in 

 the other, for reasons that we have not fully understood. There is, 

 however, a method of reading whereby this source of error is very 

 nearly eliminated. It was noticed that when heating the strip with 

 the electric current no secondary deflection occurred, and the primary 

 deflection was complete in 20 seconds. When heating by radiation a 

 nearly complete temporary halt of the deflection occurred at about 20 

 seconds after exposure, before the secondary deflection appreciably 

 manifested itself. Hence we balanced the radiation deflection exactly 

 on the 20th second by the potentiometer current, closed the shutter, 

 waited two full minutes for restored zero conditions, and then bal- 

 anced the potentiometer deflection by the heating current.' Under 

 these conditions the error is practically negligible, and on account of 

 its great sensitiveness Pyranometer A. P. O. 5 is regarded as a valu- 

 able instrument. 



Its constant is determined as follows : Length of strip, 0.628 cm. ; 

 width, 0.294 cm.; electrical resistance, 0.300 ohms. Radiation = 



/v'C^ where A'= ^ . .°-^°''''^° - « =^S-9. 



4.185x0.628x0.294x0.98x0.92 



We have employed this Pyranometer A. P. O. 5 in numerous 

 measurements of radiation from the sun, sun and sky, sky alone, and 



^ The secondary plate heating effect is not wholly absent from tlie two strip 

 form of Pyranometer No. 6, but it is very greatly reduced in its percentage 

 importance. To entirely eliminate it, however, we have found it necessary to 

 close the shutter 30 seconds after opening, and then to wait at least one minute 

 before balancing with the electric current. 



