402 Note on Hall's Phenomenon. 



to mechanical traction of varied amount, would assist in deci- 

 ding the question as to whether purely mechanical stresses 

 are to be regarded as producing the phenomenon. For it has 

 been shown* that with this metal the maximum diminution of 

 resistance following from longitudinal traction is at 100° C. 

 less than one half of that at 15°; and, further f, that whilst 

 traction not exceeding a certain limit produces diminution of 

 resistance, increase of stress beyond this limit is followed by 

 increase of resistance. Hall has remarked, in the i Philoso- 

 phical Magazine } for September 1881, that the transverse 

 current obtained with a nickel strip is much increased, other 

 conditions remaining unchanged, by rise of temperature. But 

 his experiments in this direction are, I think, hardly conclu- 

 sive; and, indeed, an examination of the numbers given in the 

 column in which the values of the " rotational coefficient " are 

 recorded shows, in the first ten out of the eighteen experiments, 

 evidence that rise of temperature causes a decrease of the 

 transverse current. Should Hall's phenomenon prove to result 

 from mechanical strain, I incline to the belief that a rise of 

 temperature from 15° to 100° C. will produce decrease in the 

 transverse current. 



The experiments of Mr. Bidwell tend very largely to prove 

 that the " Hall effect " can be explained by the joint action of 

 mechanical strain and certain " Peltier effects." Should this 

 be so, we ought, I think, to be able, by means of two thermo- 

 elements connected with each other and with a delicate gal- 

 vanometer, one element being in a cooled region and the other 

 in a heated region, to put this theory to the test. Or, again, 

 we might cover the strip with some material of bad thermal 

 conductivity and watch for a gradual increase of transverse 

 current with time when the electromagnet is excited, and 

 gradual decrease when the electromagnetic stress is removed. 

 Indeed in the actual arrangement adopted by Hall and Bid- 

 well, it would seem that this gradual increase or decrease 

 should occur ; for though the strips themselves would rapidly 

 cool if unmounted and exposed to the air, the nonconducting 

 material of the mounting would take some little time both to 

 gain and lose the heat imparted to it by the strip. 



* Loc. cit. p. 125. f Loc. cit. 



