168 Mr Larmor, On a mechanical representation of a [May 4, 



the equation V 2 V = which of course indicates infinite velocity of 

 propagation or adjustment of disturbances. 



The medium being incompressible, the total volume of all 

 the cavities will remain the same. We may put this necessary 

 property of the motion in evidence by supposing each cavity to 

 be filled with incompressible fluid. The displacement of this 

 fluid at the surface will then be continuous with the displace- 

 ment of the solid and will represent the electric surface density. 

 The inertia of the fluid must not sensibly interfere with the 

 adjustment of the normal displacement ; so that if the rigidity 

 of the solid is taken to be finite, the fluid must be of negligible 

 density. But it is to be borne in mind that, as the fluid re- 

 presents a conductor, the motion of the fluid does not represent 

 electrostatic displacement except on the surface. 



We may thus represent the circumstances of an electric vi- 

 brator by the annexed diagram. 



Two condensers A, B, are represented, with their inner coatings 

 connected by a conducting wire in which the spark gap required 

 for the production of the initial disturbance is usually situated ; 

 and their outer coatings are connected by another wire which 

 may be to earth. Each conducting system forms a cavity in the 

 elastic dielectric, which may be considered to be filled with in- 

 compressible massless liquid ; in this case there are two such 

 cavities with pairs of plane faces opposed to each other. 



The disturbance may be supposed to be originated by getting 

 an excess of liquid into the inner coating of the condenser A ; 

 that involves pushing away the plate of dielectric between the 

 two coatings of this condenser, and therefore removing an equal 

 amount of liquid from the outer coating. For the parts of the 

 conducting surfaces other than the opposed faces are all backed 

 up by thick masses of dielectric, which will not yield sensibly as 

 compared with the thin plates of dielectric which belong to the 

 condensers. The greater mobility of the latter accounts both 

 for the store of energy that the condenser can acquire, and for 

 the equality of the charges of its two coatings. 



