STRAIN ON THE PIIYSICAL PROPERTIES OF MATTKI! 



25 



to quote one of WIEDEMANN'S experiments, so that some idea may be formed of the 

 extent of the permanent deformation produced by a given torsional stress. 



Experiment XXI. 



An annealed brass wire 48 centims. in length and 2 centims. in diameter, ln:nli>d 

 with 10'46 kilos., and subjected to torsional stress, increased by small amounts at a 

 time. Scale and mirror were used, and a displacement of the image through 

 34'9 scale-divisions corresponded to 1 of rotation of the mirror. 



An examination of the fourth column shows that the consecutive values of 

 permanent deformation produced by consecutive equal loads form roughly a 

 geometrical progression, of which the common ratio is 1'42. It may then be roughly 

 calculated that the permanent twist produced by the first 10 grammes cannot be 

 greater than '14, whilst the total torsion is 104. Thus in this experiment the 

 permanent torsion produced by the smallest load is less than yooth of the total ; but 

 in some of the author's own experiments, in consequence of a much greater length of 

 wire being used, and that, too, of much smaller diameter, the temporary deformations 

 vary, on the average, between -fatii and g^th of the smallest in WIEDEMANN'S experi- 

 ments, so that the permanent torsion of the wire itself would escape observation should 

 the attempt be made to detect it by the method of statical torsion. 



[October, 1887. Nevertheless, the experiments described in the author's paper 

 seem to place beyond doubt the soundness of WIEDEMANN'S hypothesis that even 

 with the most minute torsional deformations the molecules of a torsionally oscillating 

 wire are permanently twisted first to this side and then to that, and that this is the 

 main cause of the internal friction.t For how else can the enormous increase of the 

 internal friction which can be effected by interrupted magnetisation be accounted for ? 

 Had a magnetic elastic limit been proved to exist, there would have been some little 

 difficulty in accepting WIKIUM \\s's views, for in the author's experiments on internal 

 friction, previously alluded to, the logarithmic decrement was found to be independent 



WIEDKMANN designates as temporary torsion what is here put down a* total torsiou. 

 t Provided the amplitudes of the oscillations do not exceed a certain limit. 

 MDCCCLX \\V11 1 . A. E 



