of the Physical Properties of Iron. 



247 



twisted in the position A 4 B 4 . Now if we keep on applying 

 and removing the torsional couple in this way, first in one 



direction and then in the other, the region A 2 A 4 will gra- 

 dually diminish until a minimum is reached. According to 

 Wiedemann, this is exactly what takes place when we allow 

 the wire to vibrate freely ; the permanent position of equili- 

 brium is constantly shifted to and fro. Within the regions 

 Ai A 2 and A 3 A 4 the elasticity is perfect, and there is on 

 the whole no gain or loss of energy. The loss of energy ex- 

 perienced in a torsionally vibrating wire arises from the work 

 expended in the region A 2 A 4 in shifting the permanent 

 position of equilibrium from A 2 to A 4 and back again ; and, 

 provided the amplitudes of the oscillations do not exceed a 

 certain limit, the extent of the region A 2 A 4 is proportional 

 to the amplitude. Wiedemann goes further than this ; for he 

 says what is true with respect to the wire as a whole is true 

 with respect to each molecule of the iron, and that the internal 

 friction is really due to the rotation to and fro of the perma- 

 nent positions of equilibrium of the molecules. We need not 

 stop to discuss here this last point ; but what does seem pro- 

 bable is, that the main part of the loss of energy is experienced 

 as the bar swings from A 2 to A 4 . When, as in my own ex- 

 periments, the deformations produced by the oscillations are 

 very small, it would seem that the positions A 2 , A 4 are really 

 subpermanent rather than permanent ; and if time were given 

 and the molecules agitated, the bar would of itself return to 

 the position A , when the torsional couple was reduced to zero. 

 I have said, that as the wire oscillates, the region A 2 A 4 

 becomes narrower and narrower, and Wiedemann speaks of 

 the period during which the diminution takes place, as u the 

 accommodation period/'' My own experiments have verified 

 the results of those of Wiedemann and Sir William Thomson "*, 



* Proc. Roy. Soc. May 18, 1865. 



