208 Prof. Minchin's Experiments in Photoelectricity. 



Silver Plates. 



Let two strips of cleaned silver foil be each fastened on a 

 plate of glass, by means of pitch or other suitable substance ; 

 take some finely powdered chloride of silver and shake it up 

 well in a test-tube containing collodion, the emulsion being, 

 of course, screened from light by a covering of black paper 

 on the test-tube ; pour a thin coating of the emulsion over 

 both silver plates in the dark ; and when the films have set, 

 immerse both plates in a glass vessel containing distilled 

 water with a few grains of common salt; screen one plate 

 completely ; and then connect the plates with the terminals 

 of a Thomson galvanometer. A disturbing current will, of 

 course, be produced ; but if the plates are very nearly alike, 

 this current will be small and will soon almost disappear. 



Now, on exposing the unscreened plate to light, a current 

 will be produced, and the exposed plate is negative to the un- 

 exposed (i. e., as copper to zinc). 



The direction of the disturbing current is not in any way 

 related to the direction of the current produced by light — as 

 is, indeed, a priori evident. 



If the plates are coated with an emulsion of bromide of 

 silver, the liquid being distilled water with a few grains of 

 bromide of potassium, the exposed plate is made also negative 

 with respect to the unexposed by the action of light. 



If the plates are coated with iodide of silver by first pouring 

 a layer of iodized collodion over them, and then immersing 

 them in a nitrate of silver bath, the liquid being water with a 

 few grains of iodide of potassium, we obtain a reversal of the 

 nature of the exposed plate ; i. e. the action of light makes 

 the exposed plate positive with respect to the unexposed. 



By placing coloured glasses in front of the exposed plate 

 in each of these cells, it was found that the red rays produced 

 comparatively feeble currents, while the currents produced by 

 the blue and violet rays were very great ; but the directions 

 of the currents were the same for all rays. 



A somewhat different result was obtained when the plates 

 were coated with an emulsion of sulphide of silver, the liquid 

 being water with a few grains of sulphate of potash. The 

 exposed plate is, as in the case of iodide of silver, positive. 

 In this cell the directions of the currents were the same for 

 rays passing through all the coloured glasses ; but the strength 

 of the current was very much less for rays passing through 

 the green glass than for the rays at each side of the green. 



When the plates were coated with an emulsion of nitrate 

 of silver and gelatine, the water containing a few grains of 



