136 Experiments on the Charges of Bodies 



259] In dry weather the linen threads by which the pith balls arc 

 suspended are very imperfect conductors, so that the balls are apt not to 

 separate or close immediately on giving or taking away the electricity. 

 To remedy this inconvenience I moistened the threads with a solution of 

 sea-salt, which I found answered the end perfectly well, for the threads 

 after having been once moistened conveyed the electricity ever after very 

 well, though the air was ever so dry. 



260] As the charge of the vials A and a is continually diminishing 

 from the time that the communication between them and the electrical 

 machine is taken away, both by the electricity running along the surface 

 of the vial from the inside to the outside, and by the waste of electricity 

 from the wires rRSs and mMNn and their supports, it is necessary that 

 the operation of electrifying B and T and lifting up the wires rR and mM 

 should be performed as soon as possible, and, above all, it is necessary 

 that the communication should be made between B and T as soon as 

 possible after lifting up the wires rR and mM. This end was obtained 

 very well by the manner, already described, of performing the operation. 



261] Before I begin to relate the experiments, it will be proper to say 

 something more about the accuracy that is to be expected in them. 1 

 before said that increasing or diminishing the surface of the trial plate 

 by ^ of what I called the required surface, i.e., that surface in which the 

 deficience was equal to the redundance in B, made a sensible alteration 

 in the distance to which the pith balls separated. In reality I found that 

 increasing or diminishing it by only ^4 part of the required surface would 

 in general make a sensible alteration, but I could not be certain to nearly 

 so small a quantity, for it would frequently happen that after having 

 determined the surface of the trial plate at which the balls separated to 

 a given degree, that on repeating the experiment a little after, the balls 

 would separate differently from what they did before, and that I was 

 obliged to alter the surface of the trial plate by T '^ and sometimes even | 

 of the required surface in order to make the balls separate in the same 

 degree as before. Therefore, as increasing the surface of the trial plate 

 by ^j part increases the deficience of fluid therein by fa part, it appears 

 that if the bodies B and b really contain the same quantity of redundant 

 fluid, it might seem from the experiments as if B contained fa or even 

 T ^ part more or less redundant fluid than b, so that I am liable to make 

 an error of -fa or -fa part in judging of the proportion of the quantity of 

 redundant fluid in two bodies. I imagine, however, that it will not often 

 happen that the error will amount to as much as 3^. 



262] I do not very well know what this irregularity proceeded from. 

 Part of it might arise from the difference in the strength with which the 

 vials were charged, but 1 believe that part of it must arise from some other 



