454 



Dr. J. Hopkiuson. 



[Dec. 16, 



the same charge is j* ^ + ^dx^Jde, hence the mechanical force re- 



C dY 



sisting separation of the plates is —-de. If the dielectric be air, 



I dx 

 J o 



A — =47re, and the attractive force between the plates is < ^ 7re or 



x A 



AY 2 



g — -. If Kp be the dielectric constant as determined bj an experi- 



ment on the force between the plates when the potential difference is 

 V and distance is x, 



.^-fsvs » 



If K be the dielectric constant obtained by direct comparisons of 

 charge and potential, 



K = 5f « 



E * K =J1W£ ^ 



We ordinarily assnme that Yoc xe; if so, Kp/K=l. These results 

 follow quite independently of any suppositions about the nature of 

 electricity, about action at a distance, or tensions and pressures in the 

 dielectric. 



Yet another method of determining the dielectric capacity of fluids 

 has been used by Professor Quincke. Let a bubble of air be intro- 

 duced between the two plates, let the area of the bubble be A l5 and 

 let P be the excess of pressure in the bubble above that in the external 

 air when the potential is Y, allowance being first made for capillary 

 action. 



The condenser now consists of two parts, one a fluid condenser 

 area A — Ai, the other an air condenser area Ai, we have mechanical 

 work done in increasing the area of the bubble from Ai to Ai + dA u 

 with constant charge — 



e = [ S: dA i de > 



but this work is a-PdAp 



whence xF — 



No w 4re = Aj- + (A- A{)f(V), 



