28-4 Dynamical Theory of the Electric and Luminiferous Medium. 



control of the polarised molecules arises in appreciable part from the 

 magnetic interaction of their neighbours, thus vitiating the law as 

 well as introducing effects of hysteresis. The well-known model of 

 Ewing would thus represent an ideal perfect ferromagnetic in which 

 the control arises wholly from the latter cause. 



In application of the previous results as to how far physical 

 actions can be considered as transmitted across the aether by elastic 

 stress, the conditions are formulated under which the correlative 

 principle utilised by Poynting is valid, that the actual rate of change 

 with time of the organised or mechanical energy within any region 

 is expressible explicitly as a surface-integral over its boundary. 



The mechanical effects of light-waves are reconsidered in the 

 light of this molecular theory. The conclusion is reached that such 

 effects are wholly associated with absorption of the radiation, that 

 no influence of perfectly transparent media on radiation can pro- 

 voke a mechanical reaction. There is a mechanical force acting on 

 an absorbing mass, in the direction of the incident radiation and 

 equal to E(l— m~ 2 ), where E is the energy absorbed per unit time 

 and m is the real part of the index of refraction. Partial analogies 

 are furnished by the mechanical effects of Hertzian radiation on a 

 medium built up of conducting linear circuits, and of sound waves 

 on a medium formed of a system of resonators. 



As an application of the law of the mechanical forcive on dielec- 

 trics, the changes of dimensions of a condenser under electrification 

 are considered. The problem is found to admit of exact solution if 

 the condenser layer consists of a closed sheet, of any form, but of 

 uniform thickness. In that case the mechanical stress in the 

 material of the sheet proves to be simply of the type of the Faraday- 

 Maxwell stress. The theory is compared with Quincke's experi- 

 mental results : their main features are verified, including those 

 which led Quincke to assign a wholly non-mechanical origin to the 

 effect : but something less than half the change of volume remains 

 over as an intrinsic electric deformation, not due to the transmitted 

 mechanical forces. 



Finally a series of practical illustrations of the mechanical theory 

 are treated, some of which have already been employed for experi- 

 mental measurement, and which are capable of still further applica- 

 tion. The mechanical circumstances attending the refraction of 

 uniform fields of electric force by fluid media are developed. The 

 theory of various arrangements for measuring electric tractions and 

 pressures in fluid dielectrics is worked out. The effect of an electric 

 field on the velocity of ripples on the surface of a conducting or a 

 dielectric fluid is determined : as also are the relations of electric 

 polarisation to vapour tension and fluid equilibrium. The internal 

 mechanical forces in a complete magnetic circuit are examined, and 



