upon the Electrical Resistance of Metals. 485 



arrangement, which, by the way, was intended for a different 

 purpose, was no longer employed. The eye-piece was adjusted 

 so that when the slit in the collimator was illuminated the 

 two platinum wires, and a silver wire stretched horizontally 

 in front of the slit, appeared equally distinct in the field of 

 view. When the wires were vertical, and one of them was 

 illuminated from the slit, then half a turn of the telescope 

 about its axis brought the second wire within reach of the 

 rays, and placed the first wire in darkness. The apparatus 

 was covered with a black cloth, so as to prevent any light but 

 that coming from the collimator and prism from falling upon 

 the object-glass of the observing-telescope. The two platinum 

 wires were each 14 millimetres long, and had each an elec- 

 trical resistance of about 13 Siemens's mercury units ; their 

 thickness might therefore be calculated at 0*01122 millim., if 

 we assume the law of propagation of the electrical current to 

 be the same in such thin conductors as it is in thicker ones. 

 The wires were connected with the bridge arrangement ; and 

 their resistances were repeatedly compared while one and 

 then the other was alternately illuminated. 



The results of these experiments, in which sodium-light 

 was employed, showed most clearly a diminution of resistance 

 by the action of light, and also that, in the course of the ex- 

 periments, the sensitiveness to light invariably diminished, 

 until finally it was overbalanced by the heating-effect. If the 

 wires were renewed, or whenever any pause occurred in the 

 experiments, the effect of the illumination came out again 

 more strongly. The mean of the results obtained from 98 

 observations with each wire, gave 0*0125 per cent, as the 

 mean diminution of resistance of each platinum wire owing to 

 the illumination. 



Further experiments, according to the same method, were 

 carried out with two strips of gold leaf, the arrangement of 

 which was as follows : — A rectangular plate of glass, about 

 40 x 70 millims., was coated at opposite ends with tinfoil, so 

 that there was a clear space, 24 millims. wide, right across 

 the glass ; a slip of gold leaf, suitably cut, was then floated on 

 water, and was lifted out by this prepared glass plate in such 

 a manner that it formed a conducting link between the two 

 tinfoil coatings. When the water had evaporated, the gold 

 leaf held firmly to the foil below. If a drop of soft solder be 

 made to fall on the tinfoil it holds firm enough to admit of 

 copper wires being soldered to it ; and after both of the tinfoil 

 coatings had thus been fitted with connecting wires, the under 

 side of the glass was covered with black paper, the black side 

 being turned towards the glass. Two such glass plates were 



