Electrical Phenomena provoked by Radiation. 315 



phenomena of contact-electricity between metals. I will give 

 here a brief account of the first results obtained. 



(a) A vertical metallic disk A is put more or less close 

 to a metal net B stretched parallel to the disk. Such an ar- 

 rangement was adopted in order to be able to illuminate those 

 parts of one of the metals which are very near to the other. 

 One of the two metals, A for instance, is in connexion with 

 one of the quadrants of an electrometer of convenient sensi- 

 bility (a volt corresponding to 130 divisions of the scale); 

 the other, B, is in connexion with the other quadrants and 

 with the earth, while the needle of the instrument is kept at a 

 constant potential (with 100 cells copper-water-zinc). If A 

 is put to earth for an instant, and soon after is conveniently 

 illuminated, a deflection is obtained, which reaches its maxi- 

 mum in a time which is shorter the nearer the irradiating 

 source, and the larger the surfaces of the two metals. The 

 deflection is negative if A is zinc and B brass. The same 

 final value is obtained if A is so initially charged as to have 

 a greater deflection. 



If A is very near B, the complete deflection does not change 

 if A is suddenly withdrawn from B, which proves that the 

 radiation has reduced the two metals to the same potential. 

 It follows that this deflection measures the difference of the 

 potential of contact between A and B. If B is placed in 

 connexion with the electrometer instead of A, the deflection 

 is of the contrary sign. 



The system of the two metals, when receiving the radia- 

 tion, behaves then as a voltaic cell, and can be called a 

 photoelectrical cell. 



The sun's light does not produce the above mentioned effect, 

 at least in a well marked way; the magnesium light is more 

 active; and the light of the voltaic arc gives remarkable 

 results. If the arc be formed between carbon and zinc, the 

 phenomenon shows the greatest intensity, for the electro- 

 metric deflection takes place in a few seconds. This fact 

 leads to the belief that the ultra-violet rays are the most 

 active, which is confirmed by the fact that a plate of glass 

 intercepts the action, while one of quartz diminishes it very 

 little. 



(b) Four photoelectrical cells, each formed by a disk A and 

 a metal net B, are joined in series and then illuminated. 

 Such a kind of photoelectrical battery shows the well-known 

 electrostatic phenomena which are obtained by a battery with 

 open circuit, just as if the metals which form it were plunged 

 in a vessel of water. 



(c) If the net in experiment (a) is taken away, and the 



