15G Royal Society: — Prof. W. G. Adams on the 



With, the lid of the box on, the resistance of the selenium was 

 measured, and was found to increase slowly and regularly in con- 

 sequence of the heating by the current. In most of the experi- 

 ments a battery of 30 Leclanche cells was employed. 



It was found that the higher the battery-power the less is the 

 resistance of the selenium. Experiments with 5, 30, and 35 cells 

 gave the following results : — 



Eesistance E with 5 cells 5400 ohms. 



„ 35 „ 4400 „ 



5 „ 5400 „ 



„ 30 „ 4600 „ 



After some hours : — 



Resistance E with 30 „ 4800 „ 



j> •>■> *5 ,, 5750 „ 



This diminution of resistance with increased battery-power may 

 be accounted for in part by leakage from the rheocord ; and there 

 may be also an opposing electromotive force similar to polarization 

 brought into action in the selenium when the current is passing 

 which increases with the current. 



Exposure to light diminishes the resistance of selenium. 

 This may be accounted for by either of two hypotheses : — 



(1) That light acting on the selenium sets up a polarization 

 current in it which opposes the battery-current passing through it, 



(2) That light makes the selenium a better conductor of electri- 

 city by producing a change in its surface similar to the change 

 which it produces on the surface of a phosphorescent body, by which 

 that body is enabled to give out light after it has been exposed. 



With the same battery-power, an increase in the temperature 

 causes an increase in the resistance of the selenium. 



In December, before the extreme cold, the resistance E with 

 30 cells was about 5200. Throughout the extreme cold, from 

 December 18 to January 4, the resistance E was about 4400 

 This was the value of E at 1 p.m. on January 1st, a bright cold day 

 On January 5 the temperature out of doors changed to 44° E. ir 

 the shade at 12 o'clock, and the value of E was found to be 5400 

 These values were obtained before the box was opened, and wer< 

 the first experiments made on the days named. 



When first exposed after being closed up for some days or eve: 

 hours, the selenium is more sensitive to light : this sensitivenes 

 increases with the time during which the selenium has been kep 

 in the dark ; hence the first experiment is generally not compara 

 ble with the others. 



On exposure to light the resistance is diminished ; but on bein 

 again eclipsed, the selenium returns in a very few minutes nearly 

 to its previous resistance. 



The change of resistance produced by exposure to daylight some 

 times amounts to one fourth of the whole resistance of the sele 

 nium. 



The experiments with various absorbing media seem to sho 



