Electric Current in longitudinally Magnetized Conductors. 425 



the point of view now suggested we have, in the first case, a 

 further diminution of longitudinal magnetization by the inter- 

 mittent current, and in the second case a partial restoration 

 of it; and these changes would account for the observed facts. 

 An examination of Prof. Hughes's paper will show that his 

 other experiments bearing on this point admit equally well 

 of the same explanation. 



On the other hand, we have a reason for anticipating that 

 the path of the electric current in a magnetized wire will be 

 found to possess some helical quality. 



The circular magnetization due to the current, together 

 with the existing longitudinal magnetization, gives a helical 

 magnetization which will be right-handed (like a common 

 screAv) when the current enters at the S. pole. It is known 

 that magnetization develops an Eeolotropic difference of elec- 

 trical conductivity along and across the lines of force. If 

 then the conductivity be greater across than along the direc- 

 tion of magnetization, the path of a current entering at the 

 S. pole will be a left-handed helix. The inductive effects 

 will in that case be of the same sign as those observed by 

 Prof. Hughes. 



Sir William Thomson (Phil. Trans. 1856, part 3) has con- 

 cluded that in iron the effect of magnetism on conductivity is 

 to give greater resistance along than across the lines of induc- 

 tion. This would give a helical quality of the kind supposed 

 by Prof. Hughes ; but the difference in conductivities is so 

 small as to make it highly improbable that this action accounts 

 for the very considerable inductions observed by him. In steel 

 the effect is more doubtful (cf. Thomson, loc. cit.; Tomlinson, 

 Proc. Roy. Soc. June 17, 1875 ; F. Auerbach, Phil. Mag. 

 1879, Nos. 46-48). _ 



These considerations indicate that the induced currents in 

 the coil are, probably, due in chief part to changes of longi- 

 tudinal magnetization which accompany the passage and stop- 

 page of the current in the wire ; also that there may be some 

 effect caused directly by the solenoidal component of helical 

 currents, whose helical path is attributable to the development 

 of a difference of conductivities by helical magnetization. 

 Further, it is obvious that if (from this or any other cause) 

 there is a solenoidal quality in the path of the current, the 

 change of magnetic moment and the induced currents will be 

 indirectly increased by the magnetizing action of this solenoid 

 on those parts of the metal which may be considered to lie 

 within it. 



The method used by Prof. Hughes does not enable us to 

 distinguish between the part of the action due to change of 



