Theory of Induced Magnetism. 221 



loaded and partially unloaded down to a given load before 

 being magnetized, its permeability is not the same as when 

 the wire is completely unloaded and reloaded up to the same 

 load. Experimental results of this kind led me in 1884 to 

 write : — " If we apply and remove stress in a w r ire whose 

 magnetic state is entirely neutral, we cause some kind of mole- 

 cular displacement in the relation of which to the applied stress 

 there is hysteresis" * The theory now offered shows how this 

 happens. Hence also the remarkable hysteresis which the 

 thermoelectric quality of iron exhibits with regard to cyclic 

 changes of stress, discovered by Cohn, and more fully described 

 in Phil. Trans. 1886, p. 361. The hysteresis of molecular 

 configuration with respect to stress has been proved to be 

 removable or reducible by vibration. 



From this theoretical explanation of hysteresis in the effects 

 of stress, it at once follows that a cyclic change of stress 

 (provided it be not very small) involves some dissipation of 

 energy in a magnetic metal, whether the piece be magnetized 

 or not. We may expect this dissipation to be most consider- 

 able under conditions which make the magnetic hysteresis 

 large. But it will occur even when there is no external trace 

 of magnetism. 



This of course implies that, in a cyclic process of loading 

 and unloading, work must be spent. There is no perfect 

 elasticity in a magnetic metal, however slowly the process of 

 straining be performed. Under any load there is less strain 

 during application than during removal. This is borne out 

 by experiments on the extension of iron wires (Brit. Assoc. 

 Report, 1889, p. 502). 



The same action occurs to a marked degree in torsional 

 strains. In a twisted specimen there will be a tendency 

 on the part of the molecular magnets to range themselves 

 along lines agreeing more or less with the direction of maxi- 

 mum contraction. Alternate twisting to opposite sides should 

 therefore cause much molecular swinging through unstable 

 positions, with consequent dissipation of energy, even in a piece 

 which is not magnetized. 



Without going at present into details, it may be added that 

 the phenomena of molecular " accommodation " studied by 

 Wiedemann and by H. Tomlinson accord with the theory, 

 and that it seems to lend itself well to explain the very 

 remarkable results which have been obtained by Nagaoka f 



* Phil. Trans. 1885, part ii. p. 614 



t Journal of the College of Science of the University of Tokio, vol. ii. 



1888. 



Phil Mag. S. 5. Vol. 30. No. 184. Sept. 1890. R 



