M. Gr. Wiedemann on Torsion. 101 



by various forces in the direction of the deformations are 

 directly proportional to the forces, provided that they do not 

 approach too near to the maximum of the first deforming 

 force. The deviations from this in the extreme positions, 

 beyond which the wire behaves more or less like a yet unaltered 

 one (see my memoir of 1858), may arise from the molecules 

 not yet possessing as much mobility in those positions as 

 beyond them. 



If thereupon the deformations are effected in the opposite 

 direction, the molecules are rotated and displaced within new 

 limits, between which homogeneity is not yet restored ; the 

 body behaves more or less like a fresh one ; a displacement 

 of the permanent positions of equilibrium, in the direction of 

 the new temporary deformations, comes in, until, after repe- 

 tition of these, the wire behaves in this direction as it did in 

 the other. 



18. Torsion of different Parts. — When a wire is deformed 

 alternately in opposite directions, it is by no means absolutely 

 inevitable, especially before the body has accommodated itself, 

 that constantly the same molecular groups will be moved and 

 turned. Just as at the demagnetizing of a magnet succes- 

 sion-points may arise (to which I shall recur in another place) , 

 they may also appear, for instance, in a rod which is first 

 twisted and then turned in the opposite direction. With 

 slight torsions this is not immediately perceptible ; with greater 

 ones it is very distinctly so. In order to proves this, one end 

 of a rod of soft copper 15 millims. thick, and 160 millims. 

 long, was fixed in a screw vice, while the other end was 

 twisted 720° and then turned as far back again. In the wave- 

 like coils of the fibres it was distinctly seen that the detorsion 

 was most considerable at the extremities of the rod, and was 

 much slighter towards the middle. Since the molecular 

 groups of such a rod, removed unequal distances from their final 

 positions of equilibrium, in consequence of the elastic after- 

 action simultaneously tend towards them with different and 

 variable velocities, and these motions accumulate, an alter- 

 natingly directed rotation of the free end of the rod may take 

 place, as was observed by Kohlrausch*. Also the more 

 superficial and more central layers of the wire, which in the 

 torsion are unequally stretched and compressed, may be affected 

 in different measure by the detorsion, and may therefore con- 

 dition this interesting phenomenon f . 



* Compare also experiments by Neesen (Pogg. Ann. clvii. p. 584, 

 1876) and 0. E. Meyer (Pogg. Ann. cliv. p. 357, 1875 ; Wied. Ann. iv. 

 p. 252, 1878). 



f Compare also Tresca, Comptes Bendus, lxxiii. pp. 1104, 1153 (1871). 



