Prof. L. T. More on Electro striction. 7 



The greatest confusion in the correct interpretation of the 

 forniulje arises, I think, from what may be called a too material 

 view of the ?ether. It has proved so convenient a medium 

 on which to hang our equations that we are apt to forget that 

 it has, and must necessarily have, so far as we are concerned, 

 a hypothetical existence ; and a mechanical interpretation of 

 one of its functions invariably breaks down either per se, or 

 when it is compared with other functions. Witness Maxwell's 

 attempt to explain mechanically displacement-currents of 

 electricity, or the abaiidonment of the elaborately developed 

 elastic-solid theory of light. But if the functions ascribed to 

 the ?ether itself are contradictory, they become hopelessly 

 involved when the aether is associated with matter. Even if 

 Faraday's hypothesis, that an electric charge produces a 

 strain in the ?ether, be granted, it does not at all follow that it 

 will be accompanied by unbalanced mechanical forces in matter 

 immersed in the gether. In fact, every experiment has failed 

 which attempted to show a connexion between the two when 

 the effects were static or kinetic, if the velocity was compa- 

 ratively small. When the velocity of the kinetic energy 

 involved is as great as or greater than that of light, the presence 

 of matter in the asther does seem to affect the action. Certainly 

 Kerr's effect, and others of the same kind, can be as readily 

 explained as purely mechanical or thermal actions. 



The most recent and complete of these theories of electro- 

 striction is by Sacerdote, who gives for the relation between 

 the electrical field and the elongation of a cylinder having 

 adherent armatures 



In this equation a is the inverse of Young's modulus and ki 

 the coefficient of variation of the dielectric constant K, pro- 

 duced by a tractile force acting perpendicularly to the lines 

 of force. The remaining letters have their usual significance. 

 The value of ki has never been determined with any accuracy. 

 Indeed, even its sign is a matter of dispute, some obtaining a 

 positive value, and others a negative one. It is introduced 

 into the equation on the ground that the relation is a reciprocal 

 one. If the dielectric constant of a substance increases with 

 a tractile force applied perpendicularly to the lines of force, it 

 is assumed, when a dielectric is charged, that the field of force 

 should produce a tractile force, accompanied by a corre- 

 sponding elongation. But it should be remembered that 

 this coefficient is not yet determined, and even if it were, the 

 reciprocal relation may not exist. In a general way, the 



