*i66 Spreading of charge on Coated Plates 



glass would be as the square of the radius of the circle equal in area to the 

 coated surface divided by twice the thickness of the glass, that is, that 

 the actual charges are in proportion to the computed ones. 



327] Though it seems likely from these experiments that the electricity 

 spreads further on the surface of thin glass than it does on thick, yet I 

 can not be sure that it does, as the difference observed is not greater than 

 what might proceed from the error of the experiment. However, as there 

 seems nothing improbable in the supposition, I shall suppose in the 

 following pages that it does really do so. 



328] When I say that the electricity spreads T ^ ff of an inch on the 

 surface of the glass, I mean that the quantity of electricity thereby spread 

 on the uncoated part of the glass is the same that it would be if it actually 

 spread to that distance, and if all that part of the glass which it spread 

 over was charged in the same degree as the coated part, and consequently 

 that the charge of the plate is the same as if the size of the coating was 

 increased by a ring drawn round it -07 of an inch broad, and that the 

 electricity was prevented from spreading any further. But I would by 

 no means be understood to mean that no part of the electricity spreads 

 to a greater distance than that, as it seems very likely that it does so, 

 but that the part furthest from the coating is less charged with electricity 

 than that nearest to it. 



329] What is said above must be understood of the distance to which 

 the electricity spreads with that degree of strength which I commonly 

 made use of in my experiments, but I also made some trials with the 

 plates A and C to determine to what distance it would spread with two 

 other degrees of electricity. 



If a jar with Lane's electrometer fixed to it* was charged to the higher 

 degree, it would discharge itself when the knobs of the electrometer were 

 at -053 inches distance; when it was charged to the lower degree, it dis- 

 charged itself when they were at about half that distance, or at -027 of 

 an inch; and when it was charged to the usual degree, it discharged itself, 

 as was before said, at -04 of an inch, so that the usual degree of electricity 

 was about a mean between these twof. 



It seemed as if the electricity spread about ^ of an inch further with 

 the stronger degree of electricity than with the weaker, but the experi- 

 ment was not accurate enough to determine it with certainty. 



330] I made an experiment of the same kind to determine whether 

 the electricity spread to the same distance on crown-glass as on this. It 



* [Art. 540, Feb. 16, 1773.] 



f [By Macfarlane's experiments (Trans. R. S. Edin. vol. xxvin, Part n, 1878) 

 the electromotive force required to produce sparks between flat disks at those 

 distances would be 14, n-8, and 9 units respectively.] 



