236 The Mathematical Electricians of the 



diamagnetism," wrote Weber* in 1852, "the hypothesis of 

 electric molecular currents in the interior of bodies is cor- 

 roborated, and the hypothesis of magnetic fluids in the interior 

 of bodies is refuted." The latter hypothesis is, moreover, unable 

 to account for the phenomena shown by bodies which are 

 strongly magnetic, like iron : for it is found that when the 

 magnetizing force is gradually increased to a very large value, 

 the magnetization induced in such bodies does not increase in 

 proportion, but tends to a saturation value This effect cannot 

 be explained on the assumptions of Poisson,but is easily deducible 

 from those of Weber; for, according to Weber's theory, the 

 magnetizing force merely orients existing magnets ; and when it 

 has attained such a value that all of them are oriented in the 

 same direction, there is nothing further to be done, 



Weber's theory in its original form is, however, open to 

 some objection. If the elementary magnets are supposed to be 

 free to orient themselves without encountering any resistance, 

 it is evident that a very small magnetizing force would suffice 

 to turn them all parallel to each other, and thus would produce 

 immediately the greatest possible intensity of induced magnetism. 

 To overcome this difficulty, Weber assumed that every displace- 

 ment of a molecular circuit is resisted by a couple, which tends 

 to restore the circuit to its original orientation. This assump- 

 tion fails, however, to account for the fact that iron which 

 has been placed in a strong magnetic field does not return 

 to its original condition when it is removed from the field, 

 but retains a certain amount of residual magnetization. 



Another alternative was to assume a frictional resistance 

 to the rotation of the magnetic molecules ; but if such a 

 resistance existed, it could be overcome only by a finite 

 magnetizing force ; and this inference is inconsistent with the 

 observation that some degree of magnetization is induced by 

 every force, however feeble. 



The hypothesis which has ultimately gained acceptance is 

 that the orientation is resisted by couples which arise from the 



* Ann. d. Phys. lxxxvii(1852), p. 145 ; Tyndall and Francis' Sci. Mem., p. 163. 



