567 



electric current in the other plate, the two E.M.F. must neutralize 

 each other, and then we may write: I = i^ r, in which / represents, 

 the quantity of light absorbed per time unit, / the electric current, 

 r the electric resistance in the plate. When / has been measured 

 in this way, the pyrheliometer is removed and the thermopile is 

 put in its place. 



If Ui is the deviation that the galvanometer gets in consequence 

 of this, then : 



I V 



kUj= I =i^ r , hence k = — - , 



k is a constant. In this way tlie pile and the galvanometer are 

 gauged, k indicates the quantity of energy falling on the thermopile, 

 which causes the unity of deviation. 



In a conversation on this subject Dr. Moll suggested a great 

 simplification in December 1915. Instead of the pyrheliometer a 

 blackened platinum plate might be placed immediately before the 

 pile. Then no special apparatus is required for the gauging, but a 

 plate is simply slid before the pile, which plate is subsequently 

 illuminated, then an electric current is passed through it, while the corre- 

 sponding deviations in the galvanometer are measured. 



Dr. Moll was so kind as to give me a quantity of Wollaston 

 plate, for which I express my indebtedness to him here, and through 

 which he enabled me to work out and apply his excellent idea. 



The mode of procedure was now as follows: 



A plate was slid before the pile, an electric current was conducted 

 through it. 



Let us then call the deviation in the galvanometer belonging to 

 the thermopile, (V 



The strength of the current led through the plate, i. 



Its resistance r. 



We then write: 



Uik' = Pr . (1) 



Then a beam of light originating fronti some constant source of 

 light or other, for which in my case a Nernst-lamp served, was 

 thrown on the plate. 



Let us call the deviation of the galvanometer U'l and put the 

 quantity of light = /, then : 



u'ik'=r (2) 



Now the quantity of light has been gauged, k' is eliminated from 



(1) and (2) and we get : 



