160 STUDIES IN LUMINESCENCE. 



change of conductivity is the average of at least two measurements, made 

 at intervals of 5 minutes from the end of one measurement to the beginning 

 of the next. It will be noticed that the kathode shows at low voltage a 

 large decrease of resistance due to the light, followed by a considerable 

 increase of resistance as the voltage rises. On the anode the effect at low 

 voltage is just the opposite of that on the kathode and changes sign as the 

 voltage rises as in the case of the kathode. It will be noticed also that the 

 change of sign of the effect occurs considerably earlier in the case of the 

 anode than in that of the kathode. The result is that for a considerable 

 range of voltage both electrodes show a decrease of resistance with the light. 

 As this region happens to include the voltage obtained when two gravity 

 cells were used as the source of current, it is not surprising that a large 

 decrease of resistance was observed in the first experiments, when both 

 electrodes were exposed to the light at once. 



As the curves showed that the effects on the two electrodes were in gen- 

 eral of opposite sign, it was thought advisable to arrange a cell so that one 

 electrode could be studied at considerable length, and with a certainty that 

 no complications would be introduced by stray light reaching the other 

 electrode. 



In order to have the electrodes as far apart as possible, and still to keep 

 the resistance of the cell within reasonable limits, the electrode to be exposed 

 was alone fixed in the front of the cell, clamped to the front wall in front 

 of and very near the edge of a narrow strip of glass. The other electrode 

 was made L-shaped, nearly a centimeter being the length of each leg of the 

 L cross-section. This electrode was placed in the main part of the cell with 

 the base of the L toward the side of the cell away from the light source. 

 This arrangement insured a very large electrode surface and also an average 

 cross-section of the liquid between the electrodes sufficient to make up for 

 the greater distance between them. It also served to concentrate the 

 greater part of the resistance at the electrode to be illuminated. 



As the kathode effect seemed the most promising for further study the 

 current was sent through the cell from the large electrode to the one to be 

 exposed. 



A number of runs were made with this cell and from the results curves 

 were plotted, of which 5, 6, 7, and 8 are typical specimens (Figs. 158, 159, 

 and 160). It will be seen that the effects are considerably greater than 

 those obtained in the former experiments, and also that the form of curve 

 obtained is in general more regular than the others. The peculiar rise of 

 the effect after the large decrease shown in the first part of the curve does 

 not appear to be accidental, but is common to almost all the curves plotted 

 for the kathode effect. 



Curves 7 and 8 were made to ascertain the effect which the previous 

 history of the cell might have on the action of the light. Curve 7 was ob- 

 tained in the usual way, and a run was made immediately after without 

 changing the cell, but beginning with high voltage and reducing the voltage 

 by the same steps that had been used in going up. 



It will be noticed that the curves are very much alike, the one taken on 

 decreasing voltage showing a slight shift toward the right. For the last two 

 points on the return curve it was found impossible to obtain a balance of the 



