ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 



159 



60 

 50 



3 30 



= 20 







-IO 

 <D 



00-20 



c 

 ro -30 



O -40 



2- -50 

 -60 



.4 .8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 



Potential difference in voits at cell terminals 



Fig. 156. 



the fall of potential across the cell terminals was nearly 86 per cent of the 

 voltmeter reading. 



The change of resistance due to the illumination was estimated in two 

 different ways, depending on the magnitude of the effect. If the galvanom- 

 eter reading remained on the scale when the cell was illuminated, a reading 

 of the deflection was taken. The illumination was then stopped and a 

 second reading was taken and the two were averaged. A plug was then 

 inserted in the io,ooo-ohm box, thechangeof galvanometer reading obtained, 

 then the plug was removed, a second reading taken, and the two averaged. 

 Assuming that the galvanometer deflection was proportional to the change 

 of resistance in the io,ooo-ohm box, the number of scale divisions correspond- 

 ing to a change of one ohm or 

 one part in ten thousand was 

 easily obtained. As increase or 

 decrease in the apparent resis- 

 tance of the cell would produce 

 the same change in the potential 

 difference at the galvanometer 

 terminals as the same percen- 

 tage decrease or increase of the 

 resistance in R iy it was easy 

 to get quite satisfactory esti- 

 mates of the change produced 

 by the illumination. If the 

 effect of the light was great 

 enough to throw the galvanom- 

 eter off the scale, a balance 

 was restored by changing the 

 resistance in Ri. The light was 

 then shut off and a second 

 balance was obtained, and 

 from the average of the two 

 changes in R\ the change of 

 resistance of the cell was cal- 

 culated. 



Curves were plotted, using 

 potential differences at the cell 

 terminals as abscissas, and F'g- !57- 



change of conductivity, i. e., reciprocals of resistance changes, as ordinates. 

 Eight of these curves are shown in the accompanying figures. Each value 

 of the conductivity change is the average of at least two readings, taken 

 at intervals of 5 minutes time. These curves are typical of a large number 

 that were obtained in a similar manner. 



Owing to the variation in the intensity of the arc light used, and to the 

 difficulty of setting up the cells in exactly the same manner each time, the 

 magnitude of the effects obtained varies considerably in different curves. 

 Nevertheless the character of the effects obtained did not vary greatly. 



In curves 1 and 2, 3 and 4 (Figs. 156 and 157) the kathode and anode were 

 illuminated alternately throughout the experiment. Each value of the 



> 



60 

 50 

 40 



-40 



c 



(0 



o -50 





60 



,4. .8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 

 Potential difference in volts at cell terminals. 



