THE DIELECTRIC 115 



dielectric consists in the formation of chains of molecules. If each 

 molecule consists of a positively charged part a and a negatively 

 charged part 6, in the entirely unelectrified condition these 

 molecules ab will be turned indifferently in all directions. When 

 the dielectric begins to be strained we may suppose that some, at 

 any rate, arrange themselves in chains between AB and EF, and 

 if AB is positively charged we shall have the negative elements 

 towards AB thus : AB | ab ab ab ab \ EF. As the strain increases 

 we may suppose the links of the chain, as it were, stretched out 

 thus: 



AB \ a b a b a b a 6 | EF 



and conduction implies a breakdown of the chain, the first a going 

 to AB and neutralising part of its positive charge, while its b 

 unites with the a of the second molecule, and so on along the line, 

 the last b going to EF and neutralising part of the negative 

 charge there, thus : 



AB | a ba ba ba b\ EF 



We may suppose that, in the slightly conducting dielectric, a 

 very small fraction of the whole number of chains breaks down per 

 second, the fraction being proportional to the slope of potential. 



To illustrate Maxwell's theory, let us imagine a condenser, 

 ABCD, Fig. 80, with a dielectric of specific inductive capacity K 

 throughout. Let the upper half 



above EF be a slight conductor, while A B 



the lower half is a perfect insulator. 



Let the condenser be charged till 



the strain is, say, 32 throughout, c 



and then let the upper plate be F 10 - 80. 



insulated. Then strain 32 remains 



in the lower half, EFCD. But conduction in the upper half means 

 that the effective strain in it is gradually diminishing. 



Now let us suppose that the condenser remains insulated till 

 strain 16 only remains in the upper half. There will then only be 

 charge + 16 on AB. On EF there will be charge 32 - 16 = 

 -f 16, and on CD there will be 32. If we now discharge the 

 condenser by connecting AB and CD we shall get in the first 

 place + 16 of AB neutralising- 16 of CD. AB is now discharged. 

 But there is still 16 remaining on CD, which will be equally 

 shared between AB and CD. This implies a transfer of 8 from 

 CD to AB, which is equivalent to a further discharge of + 8 from 

 A B, or a discharge of 24 in all. The potential of AB is made 

 equal to that of CD, and the strain is 8 in ABEF and -f 8 in 

 EFCI), while there is a charge + 16 on EF. 



If AB is now insulated, the negative strain 8 between EF and 

 AB gradually breaks down, while the positive strain between EF 



