Note 5.- on zero of potential 369 



Hence, since F 2 = F t 



A + B = (n - i) (3 - n) a a &"- 3 , 



and we find 



A = i ( - i) (3 ~ n) 



When M = 2, . _ i 2 < 



46 277 ' 



46 277 ' 



In this case, however, we can carry the approximation further, for it is 

 shown in Note 20 that 



A - B=-(a + --. b log ~ ) 



TT\ 277 5 bJ 



It is shown in "Electricity and Magnetism," Art. 202, that when two disks 

 are charged to equal and opposite potentials, the density near the edge of each 

 disk is greater than at a distance from it, and the whole charge is the same as 



if a strip of breadth had been added all round the disk. 



277 



Hence A + B = 1. ,(a + }* 



2b \ 277/ 



a 2 a b 



and ^=<L + A a + _I_ 



40 477 477" 



D 2 i i , /. a i\ 



B = -j- a = b [ log r I . 



40 477 477 2 \ b 47 



NOTE 5, ART. 90. 

 [On zero of Potential. See Note 7.] 



This proposition seems intended to justify those experimental methods in 

 which the potential of the earth is assumed as the zero of potential. 



Cavendish, by introducing the idea of degrees of electrification, as dis- 

 tinguished from the magnitudes of overcharge and undercharge, very nearly 

 attained to the position of those who are in possession of the idea of potential*. 

 But the very form of the phrases " positively or negatively electrified," which 

 Cavendish uses, confers an importance on the limiting condition of "no electrifi- 

 cation," which we hardly think of attributing to "zero potential." For we 

 know that all electrical phenomena depend on differences of potential, and that 

 the particular potential which we assume for our zero may be chosen arbitrarily, 

 because it does not involve any physical consequences. 



[* See vol. n of this Edition, Preface.] 



c. p. i. 24 



