412 Mr. R. Sabine on some Electrical Experiments 



Therefore the effects of light and heat upon the surface of 

 crystalline selenium are identical. Both heat and light render 

 the contact-surface between crystalline selenium and water 

 more electronegative ; and therefore we may surmise (although 

 the experiment does not amount to a proof) that the surface 

 does not become more metallic, as has been assumed as ac- 

 counting for the higher conductivity of selenium in the light. 



It is worthy of remark, that the end which was the more 

 sensitive to light was also more thermoelectric. The differ- 

 ence may be due to inequality in the selenium plate, one end 

 being better annealed than the other ; and it indicates still 

 further the similar behaviour of heat and light in this expe- 

 riment. 



Effect of Light on Conductivity. — The object of the following 

 experiments was to determine whether the effect of light upon 

 a plate of selenium when in a galvanic circuit, in increasing 

 the current, is due to a photo-electromotive force in the same 

 direction as the battery current, or to a decrement of resistance. 



Let the resistance between the wires of the selenium plate 

 in the dark and of the galvanometer be r, the electromotive 

 force of the measuring battery be E, and the observed cur- 

 rent c. On admitting light the current increases to c f , and one 

 of two things must have happened. Either (1) the increment 

 of current is due to a decrement (%) of resistance, in which 

 case 



c = — has changed to c' = 



r ° r—x 



*=E^=- e ; ....... (I.) 



or (2) the increment of current is due to a photo-electromotive 

 force (y) in the selenium, in which case 



c — — has changed to c' = -•> 



y=E^=- c (ii.) 



With a single measurement it is, of course, impossible to 

 discriminate between the two cases ; but by a known augmen- 

 tation of battery and of resistance, it is easy to find which 

 supposition affords the better agreement. 



I increased E to 2E, and inserted a known resistance fa) 

 as nearly equal to r as I could make it. The resulting cur- 

 rent, when the selenium was in the dark, was c Y ; and it in- 

 creased to c 2 when subjected to the same source of light as 



