Magnetic Field on Metallic Resistance. 735 



where T is the mean free period o£ the corpuscle and conse- 

 quently = - . In a ] 

 disappears and we get 



quently = . In a longitudinal field the second term 



dR _ d\ dv 



As is shown in figure 11 the switching on of a magnetic 

 field causes a dimensional change and it seems probable that 

 this change is accompanied by a change of mean free path. 

 From Nagaoka and Honda's experiments we know th.it the 

 volume alteration is negligibly small compared with the 

 length and cross-section changes. Calculating the cross- 

 section changes on the assumption that the volume change is 

 zero, it is possible to obtain a rough estimate of the mean 

 free path changes in different directions, corresponding to 

 certain dimensional changes. Such calculations show that a 

 dimensional change give a variation of mean free path of the 

 same order. The resistance change and dimensional change 

 are not, however, of the same order. Even should R var 



11. 

 as — 2 or r-g the resistance change would only be —2 or — S 



times that of the mean free path change. It does not seem 

 probable therefore that the alteration in length is the cause 

 of the conductivity change. There is, however, probably 

 some relation between the two, as figure 12 indicates. 

 The following also point to some connexion. 



As saturation point is approached both effects diminish 

 rapidly. 



There is little difference between the field strengths which 

 give maximum effects in both cases. 



An increase in temperature gives a diminished resistance 

 effect for a given magnetic field while Bidwell observed a 

 similar effect with respect to the elongation effect. Some 

 such connexion as both being dependent on a third factor 

 would explain the similarities of the two effects. This view 

 is strengthened by the following considerations. In a longi- 

 tudinal magnetic field nickel shows a continuous decrease in 

 length and a continuous increase in resistance. Iron shows 

 an increase in length at first, after which it commences to 

 decrease until a saturation value is reached. Its resistance 

 change, however, shows a decrease at first and afterwards 

 an increase. 



Such a thing as an alteration in configuration of the 

 atom might both alter the mean free path of the corpuscles 

 and diminish the number of corpuscles which carry the 



3C2 



