100 M. G. Wiedemann on Torsion. 



of crystals can be directly heard. The molecules then gra- 

 dually take up new positions of stable equilibrium with respect 

 to both their relative situation and the orientation of their sup- 

 posed axes. 



If the deformations take place successively (or even simul- 

 taneously) in different, independent directions — e. g. in two 

 directions perpendicular the one to the other, the molecules 

 may be rendered still more movable by this double motion 

 than if the deformation took place in only one direction, because 

 now the rotations of the molecules are conditioned by both 

 influences. Thus, e. g., we know that by torsion, as well as 

 by stretching, the molecules of magnetic wires are in fact 

 variously rotated. 



The phenomenon that a loaded wire under certain circum- 

 stances, e. g. at higher temperatures, receives through torsion- 

 forces greater temporary and permanent torsion than without 

 loading (§ 12), and that even when temporarily twisted as far 

 as a fixed limit the same holds good for the residual permanent 

 torsion (§ 10), may be in part conditioned hereby. Stretching, 

 however, may also act, in this way : by it the molecules are 

 more forcibly pressed together and are consequently rotated 

 further round their centres of gravity during the torsion, 

 which rotation may then have for its result a further displace- 

 ment of the permanent positions of equilibrium of the con- 

 sequently differently oriented molecules with respect to one 

 another, beyond which the displacements as far as the tempo- 

 rary torsion (1\— P x ) are entirely independent of the load. 

 With repeated torsions the differences of behaviour of the wire 

 with and without a load become less and less as the mobility 

 of the molecules is enhanced ; substantially, therefore, they fall 

 into the accommodation-period (cf. the experiments on oscil- 

 lation, § 24), and by no means universally prevail. 



The proposition advanced by Braun*, that a body which 

 simultaneously undergoes two temporary deformations in two 

 independent directions A and B receives a greater permanent 

 deformation in the direction A than if only the temporary de- 

 formation in the direction A had operated alone, may be quite 

 in harmony with the above experiences, and would probably 

 also hold only for the period of accommodation. 



If a body has lost unhomogeneousness within the limits 

 conditioned by the repeatedly applied, at first unilateral de- 

 formations, and if its molecules have " accommodated " them- 

 selves, the regular phenomena come out more and more. At 

 the same time the displacements of the molecules occasioned 



* Pogg. Ann. clix. p. 312 et seq. (1876). 



