ELECTRICAL PRESSURE. 9 1 



The two surfaces being infinitely near, we may write 



~~dn H ' 

 which gives 



- 



dn 



Calling R the action relative to unit volume of the dielectric 

 at a point, we have 



R-I# 



2 an 



The result is accordingly the same as if the forces were exerted 

 on the dielectric itself, and as if the force for unit volume were 



determined by a potential equal at every point to --; hence the 



volume element tends to be drawn in the direction towards which 

 the function p increases. 



104. But, under these conditions, the volume-element cannot 

 be in equilibrium ; it is therefore necessary to bring other forces into 

 play. It is sufficient if we assume that the element experiences at 

 each point of its surface a perpendicular pressure p analogous to 

 hydrostatic pressure, and that the level surfaces corresponding to the 

 pressure p l coincide with the electrical equipotential surfaces. The 

 volume element will experience an upward pressure. 



-- -, 

 dn dn 



which will balance the resultant pressure R if 



, 

 dn 



or 



A =IA 



The true action on the volume-element ddn consists then of a 

 pressure p l = -p acting on the whole surface, and of a tension p on the 



