20 
Davies.—Some Electric and Elastic Analogies . 
medium can be conceived to which minute rotations give a quasi elas¬ 
ticity, such that the magnetic rotation of the plane of polarization of 
light can be explained thereby. On this view the quantity denoting 
permeability in the magnetic problem, and which seems to be the ana¬ 
logue of that which denotes rigidity in the elastic problem, would have 
very different values within and without an iron core. Within it would 
be smaller than without, so that the resistance to rotation would be less in 
soft iron than in copper or air. “ In the place of the soft iron we must 
suppose ether of vastly less rigidity than that of the ether through the 
rest of space, whether copper or air. *' * * To represent the case of a 
soft iron core of permeability 300, suppose the value of n [the rigidity] 
for the ether in the space corresponding to the soft iron core to be 3 ^ of 
its value elsewhere, and let the circuital forcive * be the same as that in 
the former case.” * * * ‘In this case the rotation, and therefore the 
energy of the ether within the core is 300 times what it is in the same 
region without the core, except near the ends.’ | 
* By “ forcive ” Sir William Thomson seems here to mean the system of 
reactions aroused by absolute rotations in the ether just as elastic re¬ 
actions are aroused by displacements in an elastic solid. Electricity in 
motion he seems to regard as a go-between for ordinary matter and 
ether. The latter is supposed to have no rigidity to ordinary slides — at 
least such as are involved in the ordinary movements of the ether; i. e., 
those movements that are caused by the passage of bodies through it at 
ordinary velocities. 
•[Thomson’s Reprint of Mathematical and Physical Papers, Vol. III. 
Note.— The whole subject of the properties and modes of motion of 
various ethers has recently been very clearly and powerfully treated by 
vector methods by Oliver Heaviside, in a series of articles in the London 
“ Electrician.” The old elastic solid theory of the luminiferous ether is 
shown to be quite inadequate to explain electro-magnetic relations. 
