]890.] Molecular Theory of Induced Magnetism. 343 



frictional resistance which opposes the turning of the molecular 

 magnets ; this notion lends itself well to account for the most obvious 

 effects of magnetic hysteresis and the redaction of hysteresis by 

 vibration. On the other hand, it conflicts with the fact that even the 

 feeblest magnetic force induces some magnetism. My object in this 

 paper is to refer to another (and not at all arbitrary) condition of 

 constraint which not only suffices to explain all the phenomena of 

 hysteresis without any notion of friction, but seems to have in it abun- 

 dant capability to account for every complexity of magnetic quality. 



In describing Weber's theory, Maxwell points out that, if each 

 molecular magnet were perfectly free to turn, the slightest magnetic 

 force would suffice to bring the molecules into complete parallelism, 

 and thus to produce magnetic saturation. He continues, " This, 

 however, is not the case. The molecules do not turn with their axes 

 parallel to the force, and this is either because each molecule is 

 acted on by a force tending to preserve it in its original direction, or 

 because an equivalent effect is produced by the mutual action of the 

 entire system of molecules. Weber adopts the former of these sup- 

 positions as the simplest."* 



Weber supposes a directing force to act in the original direction of 

 the molecule's axis which continues to act as a restoring force in that 

 direction after the molecule is disturbed. This assumed constraint is 

 quite arbitrary ; moreover, if it were the only constraint, there would 

 be no residual magnetism when the deflecting force was withdrawn. 

 Accordingly, Maxwell modifies Weber's theory by introducing the 

 further assumption that when the angle of deflection exceeds a 

 certain limit the molecule begins to take permanent set. The de- 

 velopment of this, however, does not agree well with the facts. 



The alternative which is offered in the sentence I have quoted from 

 Maxwell was not followed up by him, and seems to have been very 

 generally overlooked, notwithstanding its obvious freedom from 

 arbitrary assumption. Several writers, notably Wiedemannf and 

 Hughes,^ have recognised the inter-molecular magnetic forces by 

 suggesting that the molecules, when unacted on by any magnetising 

 force from outside, may form closed rings, or chains, " so as to satisfy 

 their natural attraction by the shortest path." But Wiedemann ex- 

 pressly postulates a frictional resistance to rotation, which will prevent 

 this arrangement from being more than approximately attained, and 

 which may be more or IQS overcome by vibration. |j 



* Maxwell, ' Electricity and Magnetism,' vol. 2, 443. 



Wiedemann, ' Galvanism us,' 2nd ed., vol. 2 (1), p. 373. 

 t Hughes, ' Roy. Soc. Proc.,' May 10, 1883. 

 Hughes (loc. cit.). 



I) Wiedemann, ' Phil. Mag.,' July, 1886, p. 52; Elektricitat,' vol. 3. 784 

 785. 



