1893.] the Electric and Lwniniferous Medium. 453 



somehow produced, of the dielectric system on the conductors is 

 dWjdcj). 



The stress in the cetJier between two electrified bodies consists of a 

 tangential traction on each element of area, equal in magnitude to 

 the tangential component of the electric force at that place, and at 

 right angles to its direction. The stress in the material of the 

 dielectric is such as is produced in the ordinary manner by the 

 surface tractions exerted on the^material by the conductors that are 

 imbedded in it. The stress in the dielectric of Faraday and Maxwell 

 has no real existence ; it is in fact such a stress as would be felt by 

 the surface of a conductor used to explore the field, when the con- 

 ductor is so formed and placed as not to disturb the electric force in 

 the dielectric. The magnetic stress of Maxwell is simply a mathe- 

 matical mode of expression of the kinetic reaction of the medium. 



The transfer of a charged body across the field with velocity not 

 large compared with the velocity of electric propagation carries with 

 it the whole system of electric displacement belonging to the body, 

 and therefore produces while it lasts a system of displacement currents 

 in the medium, of which the circuits are completed by the actual 

 flow of charge along the lines of motion of the different charged 

 elements of the body. 



The phenomena of the electrostatic polarisation of dielectrics were 

 at one time provisionally represented by Faraday as due to the 

 orientation of electric polar elements of the medium by the electric 

 force, just as magnetisation is actually due to the orientation of the 

 magnetic polar elements by the magnetic force of the field ; and this 

 theory was developed at length by Mossotti. At a later period 

 Maxwell himself (' Dynamical Theory,' 11) compared the electric 

 displacement in a dielectric medium to an actual displacement of the 

 electric charge on conducting molecules imbedded in it a conception 

 mathematically equivalent to the above. In a previous paper* I have 

 explained by simple reasoning that this view is inconsistent with the 

 -circuital character of the electric current, a conclusion in agreement 

 with that of von Helmholtz, who adopted this idea in his generalised 

 theory of electrodynamics. It is therefore necessary to obtain a 

 complete view of this matter from our present standpoint. The 

 polarised molecule, with its positive and negative ions, is as we have 

 seen a reality ; but if the current is to remain circuital, the action of 

 the electric force of the field must not affect the actions between the 

 constituent vortices which are the cause of their orientation, nor 

 the distribution of the electric charges on the atoms, so much as to 

 produce any sensible electric displacement of this kind. These re- 

 strictions might be secured by taking the two poles of the molecule 



* " On the Theory of Electrodynamics," ' Eoy. Soc. Proc.,' vol. 49, 1891, p. 522. 



