

Properties of Selenium Blocks. 371 



Selenium Bridges can be divided into two groups : — 



(A) Containing preparations illuminated in a direction 



perpendicular to that in which the resistance is 

 measured. 



(B) Containing preparations illuminated in a direction 



parallel to that in which the resistance is measured. 



It has generally been supposed that the main phenomena 

 of Selenium Bridges may be explained by the change in 

 conductivity produced by the penetration of light to a small 

 depth at the illuminated surface. That is, the effect of light 

 on a bridge of the first type is to change the conductivity of 

 one of two conductors in parallel, and on bridges oE the 

 second type, to change the conductivity of one of two 

 conductors in series. 



Thus it was anticipated that, in order to prepare an 

 appreciably sensitive "bridge of type B," the thickness of 

 the selenium in the direction of the incident light must be 

 exceedingly small. For instance, the first bridge of this 

 type, made by Uljanin *, consisted of a thin layer of cry- 

 stalline selenium between two plates of glass covered with 

 semi-transparent metal films. Bighi f covered a metal plate 

 with selenium, and placed it with the selenium side on a 

 sheet of wire-gauze, through which the selenium could be 

 illuminated. 



While experimenting with bridges of this type, it was 

 found by the author that it was unnecessary to use ex- 

 ceedingly thin selenium films, the bridge being appreciably 

 sensitive when the thickness was several millimetres. This 

 unexpected phenomenon could be utilised to make a bridge 

 capable of being used as either of the two types by suitably 

 choosing the direction of illumination. The use of this 

 " double bridge " was to be two-fold : — 



(1) To compare the two types of bridges. This has 

 never been performed previously owing to the peculiarities 

 exhibited by selenium, and the consequent difficulty of 

 obtaining two identical preparations. 



A theoretical comparison was deduced from mathematical 

 considerations by Sperling $, who assumed an exponential 

 change of conductivity throughout a thin illuminated layer, 

 and concluded that " the bridge of the first type is more 

 sensitive than that of the second, the conditions being 



* Wied. Ann. xxiv. p. 241 (1838). 

 t Ann, d. Phys. xii. p. 683 (1888). 

 % ' Beitrage zur Kenntnis tier Seleuzellen/ Gottingen, 1908. 



2B2 



