Chemistry. — ''Tke Use of the Thermopile of Dr. W. M. Moll 

 for Absolute Measurements ^ By N. H. Sievvertsz van Reesema. 

 (Communicated by Prof. J. Böeseken). 



(Communicated in the meeting of Sept. 29, 1917). 



For a number of Photochemical researches, carried out in the 

 Phys. Chem. Lab. of the Technical University of Delft, use was 

 made of the Thermopile and the Galvanometer of Dr. Moll. 



In order to be able to express the measured light absorptions in 

 an absolute measure, it proved necessary to gauge the thermopile. 

 What follows here is a preliminary communication about the 

 measurements referring to this, the particulars of which will be 

 published in my Thesis for the Doctorate. 



The idea to gauge the thermopile by means of a Hefner-lamp or 

 another normal lamp or also by an incandescent lamp tested e.g. 

 by the "Physikalisch Technische Reichsanstalt" was relinquished. 

 A direct method was preferred, (without use being made of auxiliary 

 light sources, Hefner-lamp or other normal lamp), which could be 

 carried out by the investigator himself in a simple way, independent 

 of the measurements of others made in other laboratories. 



Bevsides it would be possible, as will appear, to avoid the 

 measurement of illuminated surfaces (here therefore a thin line of 

 light on the thermopile) and the measurement of the distance from 

 illuminated surface to light source, which becomes necessary in the 

 use of a normal lamp in the indirect method. 



At first it was my intention to make use of the compensation 

 Pyrheliometer of Angstrom or of a simplified application of the 

 principle on which it is based. 



The course of procedure would have been as follows. A quantity 

 of light in the form of a thin streak of light is made to fall on the 

 platinum plate of the Pyrheliometer, and the electrical equivalent by 

 compensation is measured. An electric current is namely conducted 

 through another plate of the same shape. Behind the plates there 

 are found thermo-elements, which have been adjusted in such a way 

 that their electric forces work in opposite directions. Thermo-elements 

 and plates are of the same shape. If the quantity of absorbed light 

 in one plate is equal to the quantity of heat generated by the 



