442 Profs. J. Elster and H. G-eitel on the Alleged 



dark, which always indicated a loss in the charge of the 

 gauze, show that the instrument is too delicate for experi- 

 ments such as these, in which a thin plate of air is exposed to 

 a fall of potential of more than 100 volts per millimetre. We 

 have therefore repeated these experiments with the much less 

 sensitive aluminium-leaf electroscope, and were able to make 

 the charge of the gauze and the time of exposure twice as 

 great as before. But then also the photoelectric discharge 

 took place only when the gauze had a negative charge. 

 Thus the charge of an amalgamated zinc plate rose in fivo 

 seconds to 400 volts, of an oxidized zinc plate in two minutes 

 to 190 volts, and with greased or paraffined zinc or wood 

 plates the potential remained at zero, irrespective of the sign 

 of the charge of the gauze. 



From the result of this experiment we concluded that in 

 the experiments of Herr Branly some unsuspected source of 

 error must have existed. In order to discover what this may 

 have been, we have repeated the experiment of Herr Branly, 

 essentially according to his arrangement as far as was possible 

 from the data which he gives. The sparks of the induction- 

 coil were taken within a box of sheet-iron connected to 

 earth, in the side of which there was a quartz window. 

 Opposite to this was the insulated and electrified plate con- 

 nected with an aluminium-leaf electroscope. Since there 

 was no gauze placed in the way of the light rays, any elec- 

 tricity escaping from the plate must be lost in the air or partly 

 pass to the side of the iron box, and from there pass to the 

 earth. 



So long as the plate was some distance (about 50 centim.) 

 from the quartz window, we also observed with this arrange- 

 ment an increase of electric dispersion in light with a negative 

 charge. But if the plate is brought to within a few centi- 

 metres of the window, and, consequently, near to the box, it 

 may happen, if the surface of the plate is covered with tallow 

 or with paraffin, that a positive charge decreases in light more 

 rapidly than a negative charge. But here, as in the above 

 discussed analogous case, it is to be remembered that the 

 positive charge of the plate collects negative electricity upon 

 the side of the box turned towards it by induction ; if there- 

 fore this surface is struck by the ultra-violet light reflected 

 from the surface of the layer of fat, a passage of negative 

 electricity from it to the plate must result, and give the 

 same effect as if a photoelectric dispersion of positive elec- 

 tricity from it had taken place. 



This suspicion was converted into certainty by the obser- 

 vation that the phenomenon is dependent upon the nature of 



