of Iron and Nickel Wires, 253 



clusively that elongation cannot occur at any early stage of 

 the magnetization. 



Here then we have three classes of phenomena, which if 

 Maxwell's explanation were correct would enable us to predict 

 certain variations in the torsional effects observed by Wiede- 

 mann. 



Let us consider first the case of nickel. According to 

 Maxwell, a spirally magnetized iron wire is twisted in a cer- 

 tain manner, because the iron expands in the direction of the 

 magnetization. Since, then, nickel contracts in the direction 

 of the magnetization, we should expect a nickel wire to twist 

 oppositely to an iron wire under similar conditions. This 

 I found to be so. Whether the magnetizing currents were 

 strong or weak, the twist of the nickel wire was always in 

 the direction opposite to that given by Wiedemann for iron. 



I was not aware until I read Wiedemann's recent paper 

 that this experiment had been performed previously *. 

 Wiedemann discusses it fully and admits that the facts 

 accord with Maxwell's explanation (p. 54). But he appears 

 to consider that it is a case of merely accidental coincidence : 

 it is true that when longitudinally magnetized, iron expands 

 while nickel contracts, and it is true that, when spirally 

 magnetized, iron and nickel twist oppositely ; but the 

 two sets of phenomena are quite independent and are 

 not related to one another as cause and effect. The 

 torsion is to be explained, he thinks, by supposing that " the 

 obliquely spiral direction which the molecules take up in con- 

 sequence of the two magnetizations at right angles to each 

 other " is, owing to mtermolecular friction, " accompanied by 

 a displacement of the longitudinal fibres and [of the] sections 

 of the wires." In iron the friction of the longitudinal fibres 

 is the greatest, and thus the torsion in the observed direction 

 is accounted for. In nickel the friction of the sections pre- 

 dominates ; a nickel wire is therefore twisted oppositely to an 

 iron wire. For a more complete statement of Wiedemann's 

 views reference must be made to the paper f. 



* Knott, Proc. Roy. Soc. Edin. 1882-3. Quoted by Wiedemann. 



f I confess that I do not find it easy to follow the suggested explanation. 

 Passing over preliminary difficulties, and assuming that the friction between 

 the polar ends of adjoining molecules is different from that between their 

 sides, it seems to me that the observed torsions could only be accounted 

 for by assuming that in iron the friction of the ends is greater than the 

 lateral friction, while in nickel the lateral friction is greatest. This appears 

 to be directly opposed to Wiedemann's statement ; but though I have 

 considered the question carefully and from several points of view, I can 

 arrive at no other conclusion. It is, however, possible that I may have 

 altogether misunderstood the argument, the more so as the paper referred 

 to is a translation. 



