CHAPTER Y. 



DIELECTKICS AND INDUCTIVE CAPACITY. 



125. MENTION has already been made ( 84) of the fact, discovered 

 originally by Cavendish, and afterwards rediscovered by Faraday, that the 

 capacity of a conductor depends on the nature of the dielectric substance 

 between its plates. 



Let us imagine that we have two parallel plate condensers, similar in all 

 respects except that one has nothing but air between its plates while in the 

 other this space is filled with a dielectric of inductive capacity K. Let us 

 suppose that the two high -potential plates are connected by a wire, and also 

 the two low-potential plates. Let the condensers be charged, the potential 

 of the high-potential plates being V lt and that of the low-potential plates 

 being F . 



Then it is found that the charges possessed by the two condensers are not 

 equal. The capacity per unit area of the air-condenser is l/47rd; that of the 

 other condenser is found to be K/4t7rd. Hence 

 the charges per unit area of the two condensers 

 are respectively 



The work done in taking unit charge from the 

 low-potential plate to the high-potential plate is 

 the same in either condenser, namely Fj F , so 

 that the intensity between the plates in either 

 condenser is the same, namely 



_ , 

 V 



FK,42. 



d 



In the air-condenser this intensity may be regarded as the resultant of the 

 attraction of the negatively charged plate and the repulsion of the positively 



charged plate, the law of attraction or repulsion being Coulomb's law - . 



82 



