348 On an Explanation of Hall's Phenomenon, [Feb. 21, 



experiment with it. As I expected, the sign of the "rotational 

 coefficient" ..proved to be + like that of iron, zinc, and cobalt. 

 It is probable, therefore, that the specimen of aluminium used by 

 Mr. Hall differed in its chemical or physical constitution from mine. 

 Both my specimens were supplied by Messrs. Johnson and Matthey 

 as pure. 



With regard to cobalt, since it could not be obtained in the form 

 either of wire or of sheet, it was necessary for the observation of 

 the thermoelectric effect to treat it in a somewhat different manner 

 from that previously described. A rod of the metal about 8 millims. 

 in diameter and 7 centims. long was firmly held by one end in a vice 

 and both ends were connected by wires with the terminals of the 

 galvanometer. The middle of the rod was gripped with gas-tongs, 

 which had been previously heated in a Bunsen flame, and stress was 

 produced by twisting. It was found that while the stress continued a 

 current passed from the free to the strained portion of the rod 

 through the hot part. The direction of the current was the same 

 whether the torsion was clockwise or counter-clockwise, and it ceased 

 the moment the stress was discontinued. For the sake of comparison 

 a copper rod was treated in a similar manner, and the resultin g 

 current was in the reverse direction. There can be little doubt that 

 in experiments of this kind torsion is perfectly equivalent to traction, 

 both tending to draw asunder the molecules of the metal. 



I venture to think that the considerations and experiments detailed 

 in this paper render it abundantly evident that the phenomenon 

 described by Mr. Hall involves no new law of nature, but is merely 

 an example of certain thermoelectric effects which had been observed 

 nearly thirty years ago. 



