upon the Electrical Resistance of Metals, 489 



leaf upon glass ; platinum, as a very thin film, burnt in on 

 plate-glass, appearing of a grey colour by transmitted light ; 

 and silver in layers of varying thickness on plate glass, the 

 layers being of a bluish colour by transmitted light. The 

 platinum held so firmly to the glass, that the thin connecting 

 wires could be soldered to it directly. In the case of silver it 

 was necessary first to cover the points intended for the con- 

 nexions with a galvano-plastic layer of copper ; and then the 

 connecting wires could be soldered on to the layer of copper 

 thus produced. The plates were then fixed to suitable stands, 

 so that the longer side, which was parallel to the line joining 

 the points of connexion, was vertical. The illumination 

 invariably took place on the metallic side. 



The dimensions of the gold plates employed were 24 x 9 

 millims.; and they had a resistance of 3 mercury units. 

 Those of the platinum plates were about 50 x 16 millims., 

 with a resistance of 175 units ; while the dimensions of the 

 silver plates were about 65 x 17 millims., with a resistance 

 of 4*853 units. 



The experiments consisted in determining, by the above 

 method, the resistance of the metallic plate, first when unex- 

 posed, and then when exposed to light. Exposure to light 

 was thus found to produce a diminution of resistance, the 

 maximum of which amounted to 3, 4, and 5 per cent, for the 

 respective resistances of the platinum, gold, and silver plates. 

 These values, moreover, did not by any means remain con- 

 stant in different experiments, although these were carried 

 out, as far as possible, under similar conditions. On the con- 

 trary, whenever, in order to ensure similarity in the external 

 conditions, several experiments were made in quick succession 

 with the same metallic plate, it always became less sensitive 

 to light ; in fact, in a few instances the opposite effect was 

 eventually produced, just as if the rays were now only able to 

 generate a rise of temperature. The experiments always suc- 

 ceeded best when the particular metallic plate had not been 

 experimented upon for some days ; and when, in such cases, 

 the observations were continued for any length of time, it 

 was very evident that, owing to the duration of the experi- 

 ments, the sensitiveness to light diminished, but again in- 

 creased after every pause. The same thing had also been 

 noticed, but less distinctly, in the experiments with the 

 Wheatstone bridge. It might appear open to question, 

 whether the cause of this phenomenon, which has come out 

 in the course of the experiments, is to be sought for in the 

 electrical currents, or in the illumination, since both exerted 

 their influence simultaneously ; but, on comparing the experi- 



