upon the Electrical Resistance of Metals. 491 



therefore seemed desirable to test this point by experiment. 

 For this purpose the Wheatstone bridge was employed, and 

 the resistance of a thin metallic plate was compared with a 

 known copper-wire resistance. In this case the electric cur- 

 rent, which was generated by a Daniell's cell, was kept on 

 continuously, and every thing was so arranged that from the 

 commencement there was a small deflection of the galvano- 

 meter-needle. This would necessarily increase whenever the 

 resistance of the lamina increased, while a diminution in the 

 electromotive force of the cell could only slightly diminish 

 the deflection. All the different specimens of thin metallic 

 plates employed in the above experiments agreed with one 

 another in the following phenomena. At first the deflection 

 increased, owing to the effect of the current — considerably in 

 the case of silver, but less so in the case of gold and platinum ; 

 then, when a large copper soldering-iron, which had previously 

 been heated to a dull red by a glass-blower's lamp, w r as 

 brought near the plate under examination, the intense radia- 

 tion from it produced a considerable increase in the deflection, 

 which, on the removal of the hot iron, diminished again very 

 slowly,, at all events much more slowly than did the tempera- 

 ture of the heated metal plate. Hence it follows that, at any 

 rate for ordinary temperatures, the resistance of the thin metal 

 plates increases by heating, just as it does in the case of larger 

 pieces of metal. And, moreover, it appears to follow, from 

 what has just been described, that the heating produces in 

 the thin metal plates an after-effect similar to that produced 

 by the electric current. 



The question as to the relative effect of different kinds of 

 light is naturally connected with these results. In order to 

 elucidate this point, different portions of the solar spectrum 

 were employed as sources of light, in a few experiments 

 wdiich in other respects were arranged exactly like the pre- 

 vious ones. Unfortunately the season and the weather have 

 been very unfavourable for the employment of sunlight, and 

 the author has in consequence been unable to carry out many 

 experiments on this point. As far as they go, however, they 

 appear to indicate that, for very thin layers of silver, the maxi- 

 mum luminous effect is in the blue and violet, the minimum 

 in the green ; for gold leaf the maximum is in the orange, 

 yellow, and violet, and the minimum in the green and blue. 

 At present the author does not consider himself entitled to 

 give a decided opinion, on account of the paucity of experi- 

 mental data on this point; but as soon as the weather and 

 other circumstances permitted, he intended to determine 

 accurately, for as many different metals and different thick- 



