Prof. G. Wiedemann's Magnetic Researches. 53 



experiments which I made in the year 1858 — showing that 

 an iron or a steel bar, subjected to torsion during or after 

 passing a galvanic current through it 7 becomes magnetic — 

 by assuming that the fibres of the bar assume a spiral 

 arrangement upon torsion, and that so that the current also 

 circulates spirally through it. At the same time he himself 

 mentions the difficulty that the effect of torsion is constant 

 in direction up to the strongest current which can be passed 

 through the wire, whilst aelotropy would result only with 

 feeble magnetization, and moreover is opposed to the action 

 of aelotropy upon the electric conductivity. 



The above view is also opposed by the fact that, upon stop- 

 ping the current through the wire, wires which have thus 

 acquired permanent transversal-circular magnetism have pro- 

 duced in them by torsion a permanent axial magnetism. 

 Moreover the opposite behaviour of nickel and iron (see 

 below) contradicts this assumption*. But that this cause 

 may have some secondary action at the same time as the rota- 

 tion of the molecular magnets is hardly to be doubted. 



§3. 



According to the experiments of Sir W. Thomson!, the 

 behaviour of magnetic bars of iron, on ihe one hand, is opposed 

 to that of magnetic bars of nickel and cobalt, on the other hand, 

 when subjected to longitudinal tension. Whilst the tempo- 

 rary magnetism of the first increases with the load under a 

 certain critical value of the magnetic force, above which it 

 decreases, the temporary magnetism of nickel and cobalt 

 decreases below a certain other (much larger) critical value of 

 the magnetizing force. 



Reciprocally, it follows from the experiments of Barret*, 

 that whilst (as Joule § has shown) iron bars lengthen upon 

 being magnetized, bars of nickel, on the contrary, shorten. 

 Cobalt is reported to form an exception and to behave like 

 iron. In these latter experiments, it is true, the purely 

 mechanico-electromagnetic action of the magnetizing coil 

 upon the bar was not excluded. 



, Quite in agreement with this, Mr. Knott || has found that 

 when a current is passed through a bar of nickel magnetized 



* Compare also E wing's proposal of an experiment on this point, Phil. 

 Mag. (5) xiii. p. 423 (1882); Beibl. vi. p. 809 (1882), 



t Proc. Roy. Soe. xxiii. pp. 445, 473 (1875); xxvii. p. 442 (1878) 

 Phil. Trans, clxvi. (2) p. 693 (1877) ; Beibl. ii. pp. 362, 607 (1878). 



\ Phil. Mag. (4) xlvii. p. 51 (1874) ; Beibl vii. p. 201 (1883). 



§ Phil. Mag. xxx. p. 76, 225 (1847). 



|| Proc. Roy. Sac. Edinb. 1882-83, p. 225 ; Beibl. yiii. p. 399 (1884). 



