Testing by trial plate I 3 1 



246] This way of making the experiment I found much more accurate 

 than the other, for supposing the required surface of the trial plate to be 

 expressed by the number 16, I found that its surface must be increased 

 to about 20 before I could be certain that the pith balls would separate 

 negatively, and that it must be diminished to about 12 before they would 

 separate positively; whereas I found that increasing its surface from 20 

 to 21 would make the balls separate sensibly further, and that diminishing 

 its surface from 12 to n would have the same effect; so that I could 

 determine the required surface of the trial plate at least four times more 

 exactly by the latter method than by the former. 



247] It will be shewn hereafter* that the quantity of deficient fluid 

 in the trial plate is in proportion to the square root of its surface ; conse- 

 quently the redundant fluid in B must exceed, or fall short of, the deficient 

 fluid in the trial plate by about th part, in order that the balls should 

 separate, and moreover the increasing or diminishing the deficience of 

 fluid in the trial plate by about ^ part will make a sensible difference in 

 the separation of the balls. 



248] It is plain that this way of finding the required surface of the 

 trial plate is not just, unless the vials are charged equally in both trials, 

 namely, that in which the balls separate positively and that in which they 

 separate negatively; I therefore fastened an electrometer to the wire Pp, 

 at a sufficient distance from the vials, consisting of two paper cylinders 

 about three-quarters of an inch in diameter and one inch in height, sus- 

 pended by linen threads about eight inches long, and in changing the 

 vials took care always to turn the globe f till these cylinders just began 

 to separate. 



249] In all the later experiments, however, I made use of a more 

 exact kind of electrometer, consisting of two wheaten straws, Aa and Bb 

 (Fig. 30), eleven inches long, with cork balls A and B at the bottom, each 

 one-third of an inch in diameter, and supported at a and b by fine steel 

 pins bearing on notches in the brass plate C, and turning on these pins 

 as centers. This electrometer was suspended by the piece of brass C from 

 the prime conductor, and a piece of pasteboard, with two black lines 

 drawn upon it, was placed six inches behind the electrometer on a level 

 with the balls, in order to judge of the distance to which the balls sepa- 

 rated, the eye being placed before the electrometer at thirty inches distance 

 from them (a guide for the eye being placed for that purpose J), and the 



* [Arts. 284, 479, 682.] f [Of Nairne's electrical machine, see Art. 563.] 



t It is necessary that the eye should always be placed nearly at the same dis- 

 tance from the electrometer, 'as it is evident that the nearer the eye is placed the 

 further the balls will appear to separate. But as the distance of the balls from the 

 eye is so much greater than their distance from the pasteboard, a small alteration 

 in the distance of the balls either from the eye or the pasteboard will make no 

 sensible alteration in the distance to which the balls appear to separate. 



92 



