187, 188] DIELECTRICS AND MAGNETIZABLE BODIES. 365 



Now by 145 we have 



W F >- F 3 F F 3 -F 4 y.^-y. 

 (3) F ^ -5' f = ~^~' ~dT 



so that we have the equations 

 Solving these for F 3 and F 4 



y = VldsJy + F 2 ( + 



from which we get for the force outside the slab 



In the electrical case if F l5 F 2 , are the surfaces of conductors, 

 the density on the upper plate V l is 



w " 



and the capacity of the condenser of area S 



v u / V V 



" \ ~ * 2 



By measuring the capacity with the slab and with it removed 

 we may determine the dielectric constant of the slab in terms of 

 that of the surrounding medium. If d and d 2 are zero, the 

 capacity is 



which is, as was stated in 184, proportional to the dielectric 

 constant. The apparent surface density on the upper face of the 

 slab is 



so that the intensity of polarization is 



47r [^ (d, 

 which is in the direction of the force if fa > fj^. 



