On capacity of a condenser with curved plates 389 



body would be 5-668 inches. Cavendish placed it horizontally 50 inches from 

 the floor. The inductive action of the floor would increase its capacity to 

 5-994 inches; Cavendish, by comparison with his globe, makes it 5-844. 



To compare with this he had two wires each 36 inches long and o-i inch 

 diameter. 



The capacity of one of these at a distance from any other body would be 

 2-8697 inches, or the two together would be 5-7394 inches. 



The two wires were placed parallel and horizontal at 50 inches from the 

 floor. Each wire was therefore influenced by the other wire, and also by the 

 negative images of itself and the other wire. 



The denominator of the fraction expressing the capacity is therefore 



Wire Other Own Other 



itself wire image image 



18 6-2724 + 0-8256 0-1759 - 0-1754 = 6-7467 



24 6-2724 + 0-6596 - 0-1759 - 0-1733 = 6-5828 



36 6-2724 + 0-4672 - 0-1759 0-1678 = 6-3959 



The numerator of the fraction which expresses the capacity of both wires 

 together is 36, so that the capacity of the two is 



From Cavendish's 

 results 



At 18 inches 5-334 4-967 



24 5-469 5-026 



36 5-629 5-277 



Wire of 72 inches 5-994 5'844 



NOTE 14, ART. 155. 

 Lemma XVI. 



If we suppose the plate AB to be overcharged and the plate DF to be 

 equally undercharged, the redundant fluid in any element of AB being numeri- 

 cally equal to the deficient fluid in the corresponding element of DF, then 

 what Cavendish calls the repulsion on the column CE in opposite directions 

 becomes in modern language the excess of the potential at C over that at E. 

 Hence the object of the Lemma is to determine approximately the difference of 

 the potentials of two curved plates when their equal and opposite charges are 

 given, and to deduce their charges when the difference of their potentials is 

 given. [Compare Green's formula, Essay, 8.] 



NOTE 15, ART. 169. 

 On the Theory of Dielectrics. 



Cavendish explains the fact discovered by him, that the charge of a coated 

 glass plate is much greater than that of a plate of air of the same dimensions, 

 by supposing that in certain portions of the glass the electric fluid is free to 

 move, while in the rest of the glass it is fixed. 



