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



his results are in fair agreement for small field values with 

 those now obtained. The loop in the curve is peculiar, but 

 its presence seems certain as Dr. Swann has previously 

 noticed it, and it was observed in every experiment which 

 the author carried out. 



Fig. 11, Pl.X. shows the elongation effect under a gradually 

 increasing magnetic field. The large field values are taken 

 from Bidwell's results. It will be observed that the maxi- 

 mum elongation effect occurs at a field strength of about 850, 

 which is not far removed from the field strength which gives 

 the maximum resistance increase effect. 



Fig. 12, PL X. shows a remarkable relation between the two 

 effects, which holds for all small field values. The points on 

 the curve are obtained by noting the corresponding values 



of -75- and j for gradually increasing field values, and 



then plotting-^- against —a/— . The graph is a straight 

 line at 45° to the axes for all small values of H^ After 

 H = 80, -5- increases more quickly than does a/ _ . 



Discussion of Results. 



In his ' Corpuscular Theory of Matter ' Thomson put& 

 forward two theories of the method of conduction in solids. 

 On the first of these the current is carried by free electrons 

 while on the second it is carried by a transmission of 

 electrons from atom to atom which are supposed to be 

 orientated by the electric field. On the first theory the 

 following is deduced : 



Resistance = 



4*fl 



/3ne 2 \v 



or R = — — , where K is a constant. 



n\v 



Thus any alteration of resistance on switching on the 

 magnetic field must be due to an alteration of n, \ or v. Of 

 these three an alteration of X seems the most probable. 

 Adams * has shown that in a transverse field, 



d_R__dT l nl _£» 

 R " T 4 m 2 



* Physical Eeview, 1907. 



