Charges on three equidistant disks 1 47 



to make the observed charges appear to agree nearer with the theory than 



they really did. Which way the second cause should operate I cannot say. 



On the whole it should seem as if the true charge of a cylinder whose 



length is L and diameter D is to that of a globe whose diameter is L nearly 



as | to natural logarithm -=^ , or as -489 to Tabular log. -^ . 



288] EXP. VIII. Let AB, ab and eg (Fig. 19) be three equal thin 

 parallel plates equidistant and very near to each 

 other, and let Cf, the line joining their centers, be 

 perpendicular to their planes, and let all three 

 plates communicate with each other and be posi- 

 tively electrified: it may easily be shewn that ac- 

 cording to the theory the quantity of redundant 

 fluid in the middle plate will be many times less 

 than that in either of the outer plates, or than that 

 which it would receive by the same degree of 



electrification if placed by itself. I therefore took three tin plates, each 

 12 inches square, and placed them as above described, and electrified 

 them by means of a wire fixed to a Leyden jar, the end of the wire being 

 formed in such manner as to touch all three plates at once. As soon as 

 the electrifying wire was taken away I drew away the outer plates, and 

 at the same time approached a pair of cork balls to the middle plate in 

 the same manner as I did to the globe in the first experiment and observed 

 how much they separated, care being taken to take away the electricity 

 of the outer plates as soon as drawn away. I then removed the outer 

 plates and, by the same means that I used in the first experiment, made 

 the quantity of redundant fluid in the jar less than before in a given ratio, 

 and by means of this jar electrified the middle plate by itself and ap- 

 proached the cork balls as before. In this manner I proceeded till I found 

 how much it was necessary to diminish the quantity of redundant fluid 

 in the jar in order that the corks might separate as much as before, and 

 consequently how much less the quantity of redundant fluid in the middle 

 plate when placed between the two other plates was than that which it 

 would have received by the same degree of electrification if placed by itself * . 



The result was that when the distance of the outer plates was \ 



1 1 -05 



to 



inches, the quantity of redundant fluid in the middle plate was about \ 



times less than it would be if electrified in the same degree when placed 

 by itself. 



289] It is plain that according to the theory the quantity of redundant 

 fluid in each of the outer plates should be the same, and that the quantity 



* [Art. 542 and Note 23.] 



