Discovery of Specific Inductive Capacity 21 



with a circular coating with that of the same plate with a new coating of 

 nearly the same area, but cut into strips, so that its perimeter was very 

 much greater than that of the circular coating. 



In this way he found that if we suppose a strip of uniform breadth added 

 to the coating all round its boundary, the capacity of this coating, sup- 

 posing the electricity not to spread, will be equal to that of the actual 

 coating as increased by the spreading of the electricity. The most probable 

 breadth of this strip he found to be 0-07 inch for thick glass and 0-09 

 for thin. 



When this correction was applied to the areas of the coatings of the 

 different coated plates, the computed charges of plates made of the same 

 kind of glass were found to be very nearly in the same ratio as their 

 observed charges. 



But the observed charges of coated plates were found to be always 

 several times greater than the charges computed from their thickness and 

 the area of their coatings, the ratio of the observed charge to the com- 

 puted charge being for plate glass about 8-2, for crown glass about 8-5, 

 for shellac about 4-47, and for bees' wax about 3-5. Thus Cavendish not 

 only anticipated Faraday's discovery of the Specific Inductive Capacity 

 of different substances, but measured its numerical value in several sub- 

 stances. 



The values of the specific inductive capacity of various substances as 

 determined by different modern observers are compared with those found 

 by Cavendish in the table in Note 27. 



To make it certain, however, that the difference between the observed 

 and calculated capacities of coated plates really arose from the nature of 

 the plate and not from some error in the theory, Cavendish determined 

 the capacity of a "plate of air," that is to say a condenser consisting of 

 two circles of tinfoil on glass with air between them. The capacity of a 

 plate of air was found to be much less than that of a plate of glass or of 

 wax of the same dimensions, but it seemed to be about T ' T in excess of the 

 calculated value. This discrepancy will be discussed in Note 17. 



These may be considered the principal results of the investigations 

 with coated plates, but the following list of collateral experimental re- 

 searches will show how thoroughly Cavendish went to work. 



A question of fundamental importance in the theory of dielectrics is 

 whether the electric induction is strictly proportional to the electromotive 

 force which produces it, or in other words, is the capacity of a condenser 

 made of glass or any other dielectric the same for high and for low 

 potentials? 



The form in which Cavendish stated this question was as follows * : 

 "Whether the charge of a coated plate bears the same proportion to that 

 of a simple conductor, whether the electrification is strong or weak." 



* Art. 526. 



