IJ4 The trial plate 



but on the upper side there was only a small coating of tinfoil. I had 

 also fiat plates of brass of different sizes which I could lay on the upper 

 surface, and slip backwards and forwards, and thereby increase or diminish 

 the size of the upper coating at pleasure, for the area of the upper coating 

 is equal to the area of the plate of brass added to that of so much of the 

 tinfoil as is left uncovered by the brass*. 



By this means I could increase or diminish the quantity of deficient 

 fluid on the lower side of the trial plate at pleasure, for I could alter the 

 size of the upper coating at pleasure, and the quantity of deficient fluid 

 on the under side of the plate is not much greater than it would be if the 

 lower coating was no greater than the upper, and consequently depends 

 on the size of that upper coating. 



As it is necessary that the trial plate should be insulated, it was not 

 laid immediately on the bars Nn and Pp, but was supported by sticks 

 of waxed glass fastened to those bars. 



Having by these means found what size it was necessary to give to 

 the upper coating of the trial plate in order that the pith balls should 

 separate positively just sensibly, and what size it was necessary to give 

 to it that they might separate as much negatively, I removed the plate B 

 and placed the plate or plates which I intended to compare with it (or 

 the plate b as I shall call it) in its room and repeated the experiment in 

 just the same manner as before. Then, if I found that the size which it 

 was necessary to give to the upper coating of the trial plate in order to 

 exhibit the same phenomena was the same as before, I concluded that 

 the charge of the plate b was the same as that of B. If, on the other hand, 

 I found that it was necessary to make the area of the upper coating of 

 the trial plate greater or less than before in any ratio, I concluded that 

 the charge of b was greater or less than that of B in the same ratio, for 

 the quantity of deficient fluid on the lower side of the trial plate will be 

 pretty nearly in proportion to the area of the upper coating. 



N.B. In the following experiments it was always contrived so that 

 the charges of the plates to be compared together should be pretty nearly 

 alike, so that if the quantity of deficient fluid on the lower surface of the 

 trial plate was not exactly in this proportion, it would make very little 

 error in the proportion of the charges. 



298] The method above described is that which I made use of in my 

 first experiment, but I afterwards made use of another method a little 

 different from this, and which I found more exact, though rather more 

 complicated, namely, for each set of plates that I wanted to compare 



* N.B. In order to estimate how much of the tinfoil was left uncovered, I drev 

 parallel lines upon it at small equal intervals from each other, and took notic 

 which of these lines the edge of the brass plate stood at. [Arts. 442, 488.] 



