406 Induced Magnetism [CH. xn 



phenomenon of magnetostriction, analogous to the phenomenon of electro- 

 striction already explained ( 203). Clearly the liquid will expand until the 



pressure is decreased by an amount ^ H 2 at each point, the new pressure 



and the mechanical forces resulting from the magnetic field now producing 

 equilibrium in the fluid. By measuring the expansion of a liquid placed in 

 a magnetic field Quincke has been able to verify the agreement between 

 theory and experiment. 



MOLECULAR THEORIES. 

 Poisson's Molecular Theory of Induced Magnetism. 



475. In Chapter V it was found possible to account for all the electro- 

 static properties of a dielectric by supposing it to consist of a number of 

 perfectly conducting molecules. Poisson attempted to apply a similar 

 explanation to the phenomenon of magnetic induction. 



Poisson's theory can, however, be disproved at once, by a consideration of 

 the numerical values obtained for the permeability //,. This quantity is 

 analogous to the quantity K of Chapter v, so that its value may be estimated 

 in terms of the molecular structure of the magnetic matter. The fact with 

 respect to which Poisson's theory breaks down is the existence of substances 

 (namely, different kinds of soft iron) for which the value of JJL is very large. 

 To understand the significance of the existence of such substances, let us 

 consider the field produced when a uniform infinite slab of such a substance 

 is placed in a uniform field of magnetic force, so that the face of the slab is 

 at right angles to the lines of force. If the value of /JL is very large, the fall 

 of potential in crossing the slab is very small. Throughout the supposed 

 perfectly-conducting magnetic molecules the potential would, on Poisson's 

 theory, be constant, so that the fall of potential could occur only in the 

 interstices between the molecules. In these interstices (cf. fig. 46), the fall of 

 potential per unit length would be comparable with that outside the slab. 

 Hence a very large value of /z could be accounted for only by supposing the 

 molecules to be packed together so closely as to leave hardly any interstices. 

 Samples of iron can be obtained for which p is as large as 4000 ; it is known, 

 from other evidence, that the molecules of iron are not so close together that 

 such a value of p could be accounted for in the manner proposed by Poisson. 



It is worth noticing, too, that Poisson's theory does not seem able, without 

 modification, to give any reasonable account of the phenomena of saturation, 

 hysteresis, etc. 



Weber s Molecular Theory of Induced Magnetism. 



476. A theory put forward by Weber shews much more ability than 

 the theory of Poisson to explain the facts of induced magnetism. 



