on Hall's Phenomenon. 



401 



of mechanical stress on the specific electrical resistance of 

 metals. A comparison of the two tables given below shows 

 that, with the exception of platinum, the metals stand in nearly 

 the same order in both, and that iron and nickel are very con- 

 spicuous, the one at the top and the other at the bottom of 

 the lists. The last-named metal is especially worthy of notice, 

 because in it longitudinal traction, when not carried beyond a 

 certain limit, diminishes the electrical resistance in spite of the 

 increase of length and diminution of section which ensues. 



Name of Metal. 



Eotational 

 coefficient*. 



Temporary alteration 



of specific resistance 



per unit produced by 



temporary increase of 



length per unit. 

 + signifies increase of 

 resistance on applica- 

 tion of stress. 



Iron 



+ 78-0 

 + 15-0 



- 0-2 

 



- 24 



- 8-6 



- 10-0 



- 50-t) 

 -120-0 



+2-618 

 +2-113 

 +1-630 

 + 1-613 

 +2-239 

 +1-617 

 +1-005 

 -0420 

 -8-860 



Zinc 



Tin 



Lead 



Platinum 



Silver 



Copper 



Aluminium 



Nickel 





It might then be suspected that the results obtained by Hall 

 are capable of receiving an explanation, in the fact that the 

 electrical resistance of the strips of metal used by him would 

 be altered by the mechanical strain consequent on their en- 

 deavour to move across the lines of magnetic force. Mr. 

 Shelford Bidwell has, however, in his recent interesting com- 

 munications to the Eoyal Society and the Physical Society, 

 pointed out that " Hall's phenomenon " cannot be thus ac- 

 counted for. Nevertheless, as it seems not improbable that 

 there is some indirect connexion between the alteration of 

 electrical resistance produced by mechanical stress and the 

 " Hall effect," I have little doubt but that experiments on the 

 "rotational coefficient" of nickel, in which the temperature 

 is varied up to 100° C, or in which the strips are subjected 



* A + sign attached to a metal signifies that the effect is in a direction 

 the same as that which the conductor itself hearing the current would 

 follow if free to move across the lines of magnetic force under the action 

 of the ordinary u ponderomotive " force. 



