410 Induced Magnetism [OH. xn 



it gives no account of the phenomenon of retentiveness. Maxwell has 

 shewn how the theory may be modified so as to take account of these 

 two phenomena. He supposes that, so long as the forces acting on the 

 molecules are small, the molecules experience small deflexions as imagined 

 by Weber, but that as soon as these deflexions exceed a certain amount, 

 the molecules are wrenched away entirely from their original positions of 

 equilibrium, and take up positions relative to some new position of equi- 

 librium. It might be, for instance, that 

 originally the molecule had two possible 

 positions of equilibrium, OP and OQ in 

 fig. 117. Suppose the molecule to be in 

 position OP and to be acted upon by a 

 gradually increasing force in some direc- o 



tion OA. At first the molecule will turn FlG - 117 - 



from the position OP towards OA. But it may be that, as soon as the 

 molecule passes some position OR, it suddenly swings round and takes 

 up a position in which it must be regarded as being deflected from the 

 position of equilibrium OQ and not from OP. Let its new position 

 be OS, then the deflexion produced is the angle SOP instead of the 

 angle ROP which would be given by Weber's theory. In this way 

 Maxwell suggested it might be possible to account for the induced mag- 

 netisation increasing more rapidly than the inducing force, i.e. for p, 

 increasing with H. 



If the magnetising force is now removed, the molecule in the position 

 OS will not return to its original position OP, but to the position OQ. It 

 will therefore still have a deflexion QOP, called by Maxwell its " permanent 

 set," and this will account for the " retentiveness " of the substance. 



No molecular theory of this kind can, however, be regarded as at all 

 complete. We shall return to the discussion of molecular theories of mag- 

 netism in the next chapter. 



REFERENCES. 



Physical Principles and Experimental Knowledge of Magnetic Induction : 



WINKELMANN. Handbuch der Physik, ii te auflage, Vol. v (1). 

 On the Mathematical Theory of Induced Magnetism : 



J. J. THOMSON. Elements of Electricity and Magnetism, Chap. vm. 



MAXWELL. Electricity and Magnetism, Vol. n, part in, Chaps, iv and v. 

 On Molecular Theories of Magnetism : 



MAXWELL. Electricity and Magnetism, Vol. n, part in, 430 and Chap. vi. 



