568 



'=''^ü; (3) 



Then the plate is removed, and the same / is thrown directlj on 

 the thermopile. We then get : 



l=üik (4) 



if U^ denotes the deviation which the galvanometer now gets. 

 From (3j and (4) now follows : 



, iV U'l 



k= (5) 



in which k again represents the quaniitj of energy required to 

 impart the unity of deviation to the thermopile. 



Now the thermopile and the galvanometer have been gauged. 



As now the sensitiveness of Dr. Moll's thermopile is variable over 

 the width of the pile, it must be defined at the gauging of the pile 

 which spot has been gauged. For the same reason it is not indiffer- 

 ent with what width of beam of light we work. To obtain an idea 

 on both points, the sensitivity of the pile was determined as function 

 of the width of the pile, and at the same time of the length of 

 the pile. 



For this purpose a very narrow beam of light, 0.2 mm. wide and 

 5 mm. long emitted by a Nernst-lamp, was thrown on the pile. 



The rays fell at right angles to the latitudinal direction of the pile. 

 The latter stood on a heavy block of wood, which could be displaced 

 in a direction normal to the direction of the rays. A simple arran- 

 gement was applied to measure the displacements. 



At every position of the piece of wood a number of readings were 

 made of the deviation given to the galvanometer, when the beam 

 of light fell on the pile. Then the pile was shifted vertically, so that 

 another spot in the longitudinal direction of the pile could be treated. 

 Also for the new longitudinal position the function of the latitude 

 was determined. 



At last the lines were obtained which are adjoined here. 



The thermopile was made in the year 1914 by the firm Kipp 

 and Son at Delft (May, year 1914). 



The thermoelements were made of copper-constantan. The pile was 

 ± 20 mm. long and 8 mm. wide. The pile had to be repaired once, 

 but showed still a sufficient uniformity. 



It follows from the steep course of the curves how greatly the 

 sensitiveness of the thermopile depends on the latitudinal direction 

 of the pile. 



