SELENIUM 27 



function of the time, and a is (in both cases) a 

 constant depending on the selenium cell and its 

 characteristics ; a can, in fact, be termed the 

 " sensitiveness " of the cell. 



Professor Korn has shown that where /3 and ;;/ 

 are inertia constants of the selenium cell the change 

 in resistance y can be obtained for an increase 

 in illumination 51 from an equation of the form 



current = y = abl.e ~ & 

 where O < ;;/ < oc ; /3 is the inertia constant and 



FIG. 8. 



m the exponential inertia ; for photo-telegraphic 

 work m should be as small as possible, and this is 

 obtained best by using platinum wire in the 

 preparation of the cell and very pure selenium ; 

 m can be made as low as f . 



The relation between^ time and current, shown 

 as an inertia curve, is seen in Fig. 8. In the 

 figure we see the effect of suddenly illuminat- 

 ing the cell for a time which, expressed as the 

 abscissa, goes as far as the vertical dotted line ; 

 after the time t the illumination is cut off, but the 

 resistance, instead of increasing again to normal 

 instantly, takes a considerable time as indicated ; 



