202-204] Green's Equivalent Stratum 179 



Electrostriction. 



203. It will now be clear that the action of the various tractions on the 

 surface of a dielectric must always be accompanied not only by a tendency 

 for the dielectric to move as a whole, but also by a slight change in shape 

 and dimensions of the dielectric as this yields to the forces acting on it. 

 This latter phenomenon is known as electrostriction. It has been observed 

 experimentally by Quincke and others. A convenient way of shewing its 

 existence is to fill the bulb of a thermometer-tube with liquid, and place 

 the whole in an electric field. The pulls on the surface of the glass result 

 in an increase in the volume of the bulb, and the liquid is observed to fall 

 in the tube. From what has already been said it will be clear that a 

 dielectric may either expand or contract under the influence of electric 

 forces. 



The stresses in the interior of a dielectric, as given in 199, may also 

 be accompanied by mechanical deformation. Thus it has been observed by 

 Kerr and others, that a piece of non-crystalline glass acquires crystalline 

 properties when placed in an electric field. Such a piece of glass reflects 

 light like a uniaxal crystal of which the optic axis is in the direction of the 

 lines of force. 



GREEN'S EQUIVALENT STRATUM 



204. Let 8 be any closed surface enclosing a number of electric charges, 

 and let P be any point outside it. The potential at P due to the charges 

 inside 8 is 



FIG. 56. 



where r is the distance from P to the element dxdydz, and the integration 

 extends throughout 8. By Green's Theorem (equation (101)) 



122 



- F dS, 

 where the normal is now drawn outwards from the surface 8. 



