374 



Mr. G. W. White on the 



The resistance was always measured by means of! a Post 

 Office Box, using a fairly high voltage (4 to 20 volts) in 

 order to make the method sensitive for such high resistances. 

 The resistance of the illuminated bridge was observed in 

 all cases after being illuminated for 90 seconds, and having 

 therefore practically attained its equilibrium state. 



In comparing the two types of bridges, the effect of the 

 platinum film on the light intensity in " bridge B" was com- 

 pensated for by illuminating " bridge A " by light from the 

 same source, but which had passed through a platinum film 

 exactly similar to that used in making the bridge. 



The results of observations on three such "double bridges" 

 are given in Table I. The selenium was illuminated by an 

 Osram Lamp (105 v., 17 w.) at a distance of 30 cm. in 

 cases I. and II., and at 20 cm. in case III. 



Table I. 





Resistance measured in Direction 



Perpendicular to that of Light. 



(Bridge of Type " A.") 



Resistance measured in Direction 



Parallel to that of Light. 



(Bridge of Type " B.") 



Res. in Dark. 

 Ohms. 



Res. in Light. 

 Ohms. 



Per cent. 

 Res. Change. 



Res. in Dark. Res. in Light.! Per cent. 

 Ohms. Ohms. Res. Change. 



1 — II — 1 t— t 



34,900 



1,110,000 



280,000 



33,800 



1,004,000 



255,000 



3-1 



9-6 



8-9 



37,300 30,000 197 

 950,000 840,000 ll-o 

 280,000 287,000 15-4 



Thus with the above three bridges, the resistance change 

 was greater when the electrode was illuminated or the bridge 

 was used as one of type " B." 



There was the possibility that this unexpected result might 

 be due to the illumination of two different varieties of 

 selenium, since the selenium illuminated in bridge "A" had 

 been crystallized in contact with air, and that in bridge " B " 

 in contact with platinum. It became necessary, therefore, 

 not only to use the same block of selenium in both obser- 

 vations, but also to illuminate the same surface. Accordingly 

 a cube of selenium was crystallized in a brass mould. In 

 measuring its resistance in a direction parallel to the illumi- 

 nation (as bridge "B"), it was pressed between electrodes of 

 fine copper gauze. To measure its resistance in a perpen- 

 dicular direction (bridge "A"), the electrodes were removed 

 and pressed on two other opposite faces of the cube, and the 

 surface used in the first measurement again illuminated. As 



