130 Experiments on the Charges of Bodies 



vial A, and the trial plate with the outside of the vial a; consequently the 

 body B will be overcharged as it communicates with the overcharged part 

 of the vial A , while the undercharged side communicates with the ground ; 

 and the trial plate will be undercharged, as it communicates with the 

 undercharged side of the vial a, while the overcharged side communicates 

 with the ground. 



Immediately after this operation is performed the wires Rr and Mm 

 are lifted up, so as to cut off the communications of the bodies B and T 

 with the vials, and, instantly after, the wires Dd and D8 are let down, so 

 as to make a communication between the body B and the trial plate. 

 For the sake of expedition this operation was performed nearly in the 

 same manner as the former, by means of the silk strings passing over the 

 pullies L and /, and represented in the figure by dotted lines. I also 

 employed an assistant to turn the electrical machine and to manage the 

 silk strings passing over the pulley H, while I stood ready near D to per- 

 form the last mentioned operation as soon as the wires Ss and Nn were 

 let down, and also to see whether the pith balls separated or not. 



243] From the manner of performing the last mentioned operation it 

 appears that the communication is not made between B and T till after 

 their communication with the vials and all other bodies is cut off; conse- 

 quently, if the quantity of redundant fluid communicated to B is more 

 than sufficient to saturate the redundant matter in T, they will be over- 

 charged after the communication is made between them, and the pith 

 balls at D will separate positively, but if the redundant fluid in B is not 

 sufficient to saturate the redundant matter in T they will be undercharged, 

 and the pith balls will separate negatively. 



244] The balls were made of pith of elder, turned round in a lathe, 

 about one-fifth of an inch in diameter, and were suspended by the finest 

 linen threads that could be procured, about 9 inches long. 



245] In making these experiments I did not open the trial plate to 

 such a surface that the pith balls should not separate at all on making 

 the communication between B and T, and assume that for the size which 

 must be given to the trial plate in order that the deficience of fluid in it 

 should be equal to the redundance in B (or for the required surface of the 

 trial plate, as I shall call it for shortness) ; but I first made the surface of 

 the trial plate such that the deficient fluid therein should exceed the re- 

 dundant in B, and that the pith balls should separate negatively, just 

 enough for me to be sure they separated : I then diminished the surface of 

 the trial plate till I found, on repeating the experiment, that the pith 

 balls separated positively as much as they before separated negatively 

 and the mean between these I concluded to be the required surface of the 

 trial plate. 



