738 Mr. W. A. Jenkins on the Effect of a 



and Adams gives the following equation as expressing the 

 result 



R 4 m 2 ' 



1 e 2 X 2 



= — - H 2 — — - 



4 m 2 v 2 



= — o H 2 x 10" 16 approximately. 



For a field of H = 1000 we get 



rfR 1 

 E=-3 Xl °- 10 - 



The effect observed is of the order 10 ~ 2 , and although the 

 value of X used is not certain, the difference between the 

 observed and calculated values is far too great to be accounted 

 for by an error in X. To reconcile theory and experiment X 

 would have to be 5 x 10 " 5 cm., i. e. more than five times greater 

 than the mean free path of a molecule in air. Such a result 

 is extremely improbable and we must conclude that the effect 

 observed is not that given by Adams' equation. Moreover 

 the resistance increase is not proportional to the square of the 

 field strength. The effect in iron is so much greater than in 

 other metals as to call for some special explanation. Heaps* 

 has measured the effect in other metals and finds it to be 

 represented by AH 2 where A is of the order 10~ 12 . This 

 gives A, = 10~ 6 approximately which is still very large. A 

 longitudinal effect of the same order is observed in these 

 other metals, and its existance leads one to surmise that the 

 small effect observed in the non-magnetic elements is not the 

 one which has been theoretically deduced. The first term of 

 Adams' equation would give an effect in a longitudinal field, 

 but the fact that the effects in both fields are of the same order 

 of magnitude, and the large value of X which has to be as- 

 sumed to reconcile theory and experiment, points to a common 

 origin for the two phenomena in the case of non-magnetic 

 elements. On the other hand, in the case of nickel the two 

 effects possess different characteristics. The longitudinal 

 effect reaches a maximum with a comparatively small field 

 value while the second part of the transverse effect does not 

 reach a limiting value. Abnormally large values of the effect 

 are observed in the cases of iron, nickel, and bismuth. The 



* Phil. Mao-. 1912. 



