i 56 Experiments on Coated Plates 



so that I might have saved the trouble of making two trial plates. How- 

 ever, for the sake of more accuracy, I always chose to make two trial 

 plates and to take the mean of the results obtained by means of each 

 trial plate for the true result. 



299] One reason why this method of trying the experiment is more 

 exact than the former, or that by means of a sliding plate only, is that 

 in the former method I was liable to some error from inaccuracy in judging 

 how much of the tinfoil coating of the trial plate was left uncovered by 

 the sliding brass plate, whereas in this method, as the charge of the sliding 

 plate is but small in respect of that of B, it was not necessary to be accurate 

 in estimating its surface. But I believe the principal reason is that an 

 error which will be taken notice of by and by, and which proceeds from 

 the spreading of the electricity on the surface of the glass, is greater in 

 a sliding plate than in one coated in the usual manner. 



In general I think it required scarcely so great an increase of the charge 

 of the trial plate to make a sensible alteration in the degree of separation 

 of the pith balls in the following experiments as in the preceding, and 

 therefore it should seem as if these experiments were capable of rather 

 more exactness than the former, but this was not the case, as the different 

 trials were found not to agree together with quite so much exactness in 

 these experiments as the preceding. For this reason, and also because 

 they were attended with less trouble, I repeated the experiments oftener, 

 as I not only compared each plate with the trial plate for more times 

 together as I did in the preceding experiments, but in general I repeated 

 the experiment on several different days. 



300] The circumstance which gave me the most trouble in these ex- 

 periments was the spreading of the electricity on the surface of the glass. 

 To understand this, let ABab, Fig. 21, be a flat plate of coated glass, 

 cd and CD being the two coatings, and let CD be positively electrified, and 

 let cd communicate with the ground. 



c d. 



Fig. 21. 



It is plain that the electric fluid will flow gradually from CD and 

 spread itself all round on the surface of the glass, and nearly the same 

 quantity of fluid will flow from the opposite side of the glass into cd, so 

 that those parts of the glass which are not coated gradually become charged, 

 those parts becoming so soonest which are nearest the edge of the glass. 



On discharging the plate the uncoated part of the glass gradually 

 discharges itself, as on the side AB the fluid will flow gradually from the 



