440 Profs. J. Elster and H. Geitel on the Alleged 



charged with positive or negative electricity, the plate parallel 

 to it must become charged with negative or positive electricity. 

 If now the earth connexion of the electrometer was removed, 

 then as soon as a passage of electricity took place between 

 the gauze and the plate, the change of potential in the latter 

 could be read off on the electrometer. It is to be observed 

 that with a positive charge of the gauze, the plate to be tested 

 has negative electricity on its surface, and that therefore from 

 this clean metallic surface in ultra-violet light a free discharge 

 towards the gauze was to be expected and a consequent 

 positive charge of the plate. We used for the experiment a 

 disk of amalgamated zinc, also similar pieces of ziuc covered 

 with a thin layer of paraffin or tallow, and also a plate of 

 wood covered with tallow. According to Herr Branly, such 

 surfaces covered with paraffin or tallow suffer a greater loss 

 of positive than of negative electricity when exposed to light. 

 "We observed the deflexion of the electrometer-needle 

 which took place in one minute, both with positive and with 

 negative charge of the gauze, and both in the dark and when 

 illuminated with light from the spark. The Wagner's hammer 

 was so arranged that it came into action of itself upon closing 

 the current, it was therefore only necessary to keep the 

 current closed for one minute. The results of a series of 

 measurements are brought together in the following table. 

 The numbers give the change of potential of the plate, 

 measured in volts, which took place in one minute : each 

 number is the mean of two readings : — 





Amalgamated 

 Zinc Plate. 



Paraffined 

 Zinc Plate. 



Zinc Plate covered 

 with Tallow. 



Wood Plate 



covered with 



Tallow. 



Dark. 



Illumi- 

 nated. 



Dark. 



Illumi- 

 nated. 



Dark. 



Illumi- 

 nated. 



Dark. 



Illumi- 

 nated. 



Gauze T 

 positive J 



Gauze \ 

 negative J 



+0-40 

 -016 



+138 

 (+123) 



-0-40 



+0-54 

 -0-22 



+0-69 

 -0-16 



+0-79 

 -009 



+0-38 

 -006 



+0-58 

 -0-52 



+0-52 

 -052 



As was to be expected from what has been said, we have in 

 this series of observations evidence of the great photoelectric 

 dispersion from a plate of amalgamated zinc charged with 

 negative electricity. 



We were not able to expose the plate for a full minute to 



