240 Profs. W. E. Ayrton and John Perry. [Mar. 21, 



It will be observed that the conventional specific inductive capacity 

 of shell-lac does not follow our law that small specific inductive capacity 

 accompanies high specific resistance. We are inclined, however, to 

 think that the real capacity may be far greater than this conventional 

 one, since M. Boltzmann proved in 1874 that the true specific induc- 

 tive capacities of sulphur and resin were respectively twice and one 

 and a half times as great as the capacities for these substances given 

 in books. Up to the present time we have not ourselves had an 

 opportunity of finding the true specific inductive capacity of shell-lac. 



In the preceding table the resistance of ebonite is given in accord- 

 ance with the rule stated in our last paper, that is, the resistance given 

 in that paper has been reduced in the ratio of 1*7 to 3'15. The specific 

 inductive capacity of Hooper's india-rubber is usually given as 3*1 ; 

 using, however, the numbers given on page 67, Clark and Sabine 

 Electrical Tables for the capacities of a sheet of air, and of the india- 

 rubber one square foot in area, and T oV o mc h in thickness, we find the 

 ratio to be 3 - 3. 



Following out the relationship shown in the above table, we may 

 consider a highly conducting metal as a dielectric of extremely great 

 specific inductive capacity, just as air, which has an extremely great 

 resistance, is a dielectric having (as far as experiment has yet shown) 

 the least specific inductive capacity. But as the resistance of the air 

 although very great is not infinite, so it is reasonable to conceive the 

 existence of a space more highly insulating than when filled with air, 

 and at the same time having a less specific inductive capacity. 



When we examine the phenomena of induction in this way we might 

 expect to find a simple relation between the values of specific resist- 

 ance and specific inductive capacity, but further consideration shows 

 that the received notions of both these qualities of dielectrics are so 

 badly defined that the first step is to establish clearness in our concep- 

 tions before endeavouring to determine in what exact way specific 

 inductive capacity is a function of specific resistance. 



As regards the capacity of an air condenser there cannot be much 

 vagueness, for there is neither perceptible absorption nor true conduc- 

 tion, although further on we shall give our reasons for believing there 

 is some absorption. But in all cases in which the dielectric is not 

 a gas there are absorption and conduction phenomena of a complicated 

 kind, and experiment hitherto has not accurately defined what is meant 

 by capacity in such cases. 



We are now carrying on a series of observations on induction phe- 

 nomena in flint glass jars and other condensers with dielectrics having 

 a greater and less specific conductivity than glass. It is an investiga- 

 tion which progresses very slowly, as some of the single experiments 

 last nearly a month. Certain curves obtained for time risings and 

 fallings of potential in condensers bore striking resemblances to the 



