20 Prof. W. H. Bragg on the " Elastic Medium " 



ductors) it must be withdrawn from some other part of space, 

 and since the aether cannot be got out of bodies of the second 

 class (dielectrics) it must be drawn from bodies of the first 

 class. In other words, if the dielectric surrounding some 

 conductor be pushed back to any extent, it will encroach to 

 an exactly equal extent on some other conductor or conductors. 



This corresponds to the electrical law that, if any charge be 

 imparted to a conductor, an equal and opposite charge will be 

 induced on some other conductor or conductors. 



2. Let us consider the strain phenomena resulting from the 

 simplest case of charging, viz. pumping in or drawing out 

 aether from a spherical conductor, the conductor being sur- 

 rounded for a great distance by a medium uniform and free 

 from other conductors. 



Let Q be the quantity pumped in, i. e. the charge ; let a 

 be the radius of the sphere. 



Then all round the conductor the medium is pushed back a 



distance -. — r,, and at a distance r from the centre of the 

 4-7ra 2 ' q 



sphere the displacement is j-^* 



Suppose that, when a unit volume of the medium is dis- 

 placed a distance x, the force of restitution is E . x. 



Draw a cone of very small angle from the centre of the 

 sphere. Let it intercept on the surface of radius r an area s, 

 and on that of radius r + Br, Br being small, an area. /. The 

 cone being of very small angle, and Br being small, s = s'. 



The force of restitution due to the displacement of the small 



element of volume so intercepted is E . s . Br . -r-^-v 



477T 2 



Hence if p be the pressure at the surface r,andj9-f-6p that 

 at the surface r 4- Br, 



sj E . Br . 8 . Q 



