102 M. G. Wiedemann on Torsion. 



19. Permanent Deformations. — There is, finally, the va- 

 riously answered question to be discussed, whether the defor- 

 mation (torsion) which remains long after the removal of the 

 deforming force is to be attributed to a new position of stable 

 equilibrium of the molecules, corresponding to the state of the 

 body at the time, or whether it is merely a transitional stage 

 through which the body tends, though extremely slowly, to 

 the position of absolute zero which it possessed before that 

 deformation. The former of these two views, however, is 

 most probably the true one ; for, first, otherwise a body could 

 not receive any lasting alteration of shape at all without being 

 submitted to change of temperature, &c, but all bodies would 

 inevitably return gradually to their original form. A second 

 ground for the view that the permanent torsion reached by a 

 wire when repeatedly twisted in one direction corresponds to 

 a permanent position of equilibrium is, that transient torsions 

 in the same direction, produced by weaker forces, reckoned 

 from that position onwards are proportional to the forces. 

 Lastly, one must remember that at the opening of a current 

 temporarily magnetizing a steel rod the molecular magnets, 

 which have been deflected from their position of absolute rest, 

 suddenly spring back with great velocity into new permanent 

 positions of equilibrium, corresponding to the permanent mag- 

 netization, without the latter becoming, either at once or in 

 the course of time, perceptibly less. 



20. Elastic Afteraction. — It w^as to be foreseen, and is con- 

 firmed by the experiments, that with repeated deformations 

 the molecules always reach their temporary as well as perma- 

 nent position of equilibrium the more quickly the oftener the 

 body is deformed in the respective direction. The elastic 

 afteraction declines therefore with different rapidity in the 

 different periods of the accommodation of a body. Further, 

 if a body, e. g. a wire, has been frequently deformed (twisted) 

 in alternately opposite directions, it is not unimportant for the 

 course of the elastic afteraction whether the wire be once more 

 twisted in the direction of the last torsion and then again re- 

 turns to its previous permanent torsion, or whether it be 

 twisted in the opposite direction and its permanent position 

 of equilibrium shifted. Consequently a wire cannot be at 

 once brought up to a determined elastic state, for the obser- 

 vation of the elastic afteraction, by frequently twisting it 

 hither and thither, but there is also the direction of the new 

 torsion to be considered. In this sense, probably, to the pre- 

 vious careful and particular investigations on the elastic after- 

 action further observations will have to be added, which will 



