Results slight and uncertain 179 



with DC, and their lower coatings with the ground, and the operation 

 performed as before. I found that I was obliged to change the wire W 

 for one of the same thickness, and only 22 inches long, in order that the 

 pith balls should separate the same as before. 



360] Therefore the charge of the two plates D and E and the wire DC 

 together is to that of the tin cylinder and wire DC together as the charge 

 of a wire J inch thick and 22 inches long to that of a wire of the same 

 thickness and 29 inches long, that is, as i to 1-26, and consequently, as 

 the charge of the wire DC is but small in comparison of that of the two 

 plates, the charge of the two plates will be to that of the tin cylinder pretty 

 nearly in the same proportion of i to 1-26*. 



Having thus found what proportion the charges of the plates and 

 cylinder bear to each other when electrified in a very weak degree, I tried 

 what proportion they bore with the usual degree of electrification. 



361] To this purpose I placed the two plates on the machine repre- 

 sented in Fig. 20 between M and m in the usual manner, and on the other 

 side I placed a sliding coated plate, and found as usual what size must 

 be given to the coating of this plate that the pith balls should just separate 

 positively, and what size must be given to it that they should just separate 

 negatively. 



I then removed the two plates and suspended the tin cylinder so as 

 to touch the wire Mm, but without touching any other part of the machine, 

 and found what size it was necessary to give to the coating of the sliding 

 plate that the pith balls should separate as before. 



By this means the charge of the tin cylinder was found to be to that 

 of the two plates as 1-33 to i. Therefore the charge of the two plates 

 seems to bear pretty nearly the same proportion to that of the cylinder 

 whether the electricity is of the usual strength or very weak. But if we 

 suppose that the electricity spreads -07 inches on [the] surface of glass 

 with the usual degree of electrification, and that it does not spread sensibly 

 with the weak degree of electrification, then the proportion which the 

 charge of the glass plates bears to that of the cylinder should be less with 

 the usual degree of electrification than with the weak one, and that by 

 about A part. 



This difference, however, is not more than what might very well 

 proceed from the error of the experiment. 



362] On the whole, I am uncertain whether the charge of a glass plate 

 would really bear a rather less proportion to that of a globe or other body 

 when the electricity is strong than when it is weak, provided the electricity 



* I believe the true proportion is between that of i to 1-28 and that of i to 1-37, 

 but as the experiment is not capable of much accuracy, I think it needless to 

 trouble the reader with the computation. [See Art. W>6, and Note 25, p. 416.] 



122 



