CHAPTER IV 



A. INSTRUMENTS 



For the following observations I used one or more nocturnal com- 

 pensation instruments, pyrgeometers of the type described by K. 

 Angstrom in a paper in 1905. 1 Without going into details, for which 

 I refer to the original paper, it may be of advantage to give here a 

 short description of the instrument. 



Founded on the same principle of electric compensation used in 

 the Angstrom pyrheliometer, the instrument has the general form 

 indicated in figure 2. There are four thin manganine strips (M), of 

 which two are blackened with platinum black, the other two gilded. 

 On the backs of the metal strips are fastened the two contact points of 

 a thermo junction, connected with a sensitive galvanometer G. If 

 the strips are shaded by a screen of uniform temperature, the thermo- 

 j unctions will have the same temperature, and we may read a certain 

 zero position on the galvanometer. If the screen is removed and 

 the strips are exposed to the sky, a radiation will take place, which 

 is stronger for the black strips than for the bright ones, and there 

 will be a deflection on the galvanometer due to the temperature 

 difference between the strips. In order to regain the zero position 

 of the galvanometer, we may restore the heat lost through radiation 

 by sending an electric current through the black strips. Theoretical 

 considerations, as well as experiments made, show that the radiation 

 is proportional to the square of the current used, that is, 



R = ki 2 



where k is a constant that depends upon the dimensions, resistance, 

 and radiating power of the strips. As the radiating power from 

 the strips is difficult to compute, the constant k is determined from 

 experiment with a known radiation. The strips are exposed to 

 radiate to a black hemisphere of known temperature 7\, and the 

 constant is determined by the relation : 



where T is the temperature of the strips. The advantage of this 

 construction over the form used for instance by Exner and Homen, 

 where the effects of conduction and convection are also eliminated, 



1 Nova Acta Reg. Soc, Sc. Upsal., Ser. 4, Vol. 1, No. 2. 

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