upon the Electrical Resistance of Metals. 487 



being the same as before. The electromotor consisted of a 

 Weber's magneto-inductor. The bar magnet, by whose 

 motion the electric currents were induced, was fixed in an 

 upright, cylindrical, wooden frame, at the upper and of which 

 was a string, leading over a pulley to the observer's place, 

 who was thus able to raise or depress the bar magnet at will. 

 The bar magnet was thus made to move in the direction of 

 its axis, which coincided with that of a fixed spiral of copper 

 wire surrounding it. The induced currents were observed by 

 means of a multiplier, wound with thick wire, which had a 

 powerful copper damper and a reflecting scale, its oscillating 

 magnet, which w r eighed one pound, being suspended by an 

 iron wire from the lid. The resistance of the inductor coil 

 was equal to 2*328 mercurial units, and that of the multiplier 

 1*223 units ; and the two together with the metallic body 

 under examination, and the necessary connecting wires, com- 

 pleted the circuit. 



The mode of carrying out an observation is due to Weber, 

 and has been fully described by him*. 



The resistance of one of the pieces of gold leaf was then 

 determined several times in succession, during which it was 

 alternately unexposed and then exposed to the light of a 

 sodium-flame ; and the general results of these experiments 

 may be summed up as follows : — 



There was a difference in the resistance of about 0*035 per 

 cent, to be accounted for by the exposure to light ; but there 

 was also a permanent increase of resistance developed in the 

 gold leaf by the passage of the electric current. For instance, 

 it was found, at the conclusion of one series of experiments, 

 that the resistance of the unexposed gold leaf had increased 

 by 1*25 per cent, of its original value, owing to the passage 

 of the electric current ; but on resuming the experiments the 

 next day with the same piece of gold leaf, this after-effect had 

 apparently disappeared, and the gold leaf had regained its 

 original resistance. 



Now this increase of resistance during the passage of elec- 

 tricity cannot be regarded as due to any heating by the elec- 

 tric current ; for the change went on increasing uniformly 

 during the whole period of the first set of observations, which 

 extended, with several interruptions, over a period of nine 

 hours. Moreover any heat developed in the strip of gold 

 leaf must have been communicated very quickly to the glass- 

 plate which was immediately connected with it ; and there is 



* Weber, Elektrodynamisehe Maasbestimnmngen, JX , insbes. iiber 

 Widerstandmessungen, Abh, d. kgl, sacJis. Gesellschaft d. Wissenschaften, 

 Bd. I. S. 351. 1852. 



