284 



On the Electrical Resistance of Selenium. 



[May 8, 



considerably, as indicated by the rapid movement o£ the spot of light on 

 the galvanometer-scale. 



Experiment 2. — The transition from darkness to the light given by an 

 ordinary gas-burner (conditions as before), caused a slight and barely 

 perceptible fall in the resistance. 



Experiment 3. — The bar of selenium was next tried in the solar spec- 

 trum on a very bright cloudless day (conditions as before), except that 

 more battery-power was used (10 cells Daniell). 



The diffused daylight could not be cut off ; so the trial was made in the 

 most shaded part of an ordinary room, the spectrum being superimposed 

 on the ordinary diffused daylight. 



The resistances were very carefully balanced in each case, with the 

 following results : — 



Resistance in darkness 330,000 



„ violet 279,000 



„ „ red 255,700 



„ „ orange 277,000 



„ „ green 278,000 



„ „ blue and indigo 279,000 



,, ,, centre of red 255,000 



Resistance just on the outside edge of red, 



red side 220,000 



Resistance in dark rays clear of red ...... 228,000 



Resistance in diffused daylight only 270,000 



Resistance taken in the dark immediately 



after exposure (resistance rising) 310,000 



The indications were very clear ; and the bar of selenium was so sensi- 

 tive to the action of the spectrum, that a slight movement of the prism 

 produced a corresponding movement in the spot of light on the galvano- 

 meter-scale. 



It is to be noted that in this experiment the reflecting galvanometer 

 was placed on a heavy masonry pillar insulated from the floor for obser- 

 vatory purposes, and that the battery, bar of selenium, and resistance- 

 coils were in another room, being connected by long and carefully in- 

 sulated leads with the galvanometer. 



Experiment 4. — The diffused light was cut off as much as possible by 

 screens ; and the resistances were again balanced in the solar spectrum. 



Conditions as in the last case. 



Resistance of selenium in red 240,000 



„ just outside red 240,700 



in blue 270,000 



„ in such diffused light as came 



through screens 290,000 



Light cut off by lid of box (resistance rising) 310,000 



