

Note 2 1 : capacity of two circles on the same axis 4 1 1 



We may apply this result to estimate the correction for the thickness of 

 the square plates used by Cavendish. The factor by which we must multiply 

 the thickness in order to obtain the correction for the diameter of an infinitely 



thin plate of equal capacity is log T . 



27T O 



a i a 



Tin plate 600 1-017 



Hollow plate n 0-381 



Portland stone, &c 30 0-540 



Slate 75 0-686 



The correction is in every case much smaller than Cavendish supposed. 



NOTE 21, ARTS. 277, 452, 473, 681. 



Calculation of the Capacity of the Two Circles in Experiment VI. 

 The diameter of one of the circles was 9-3 inches, so that its capacity when 

 no other conductor is in the field is = 2-060. The distance between their 



7T 



centres was 36, 24, and 18 inches, which we may call c lf c a , and c 3 . 



The height of the centres of the circles above the floor was about 45 inches, 

 so that the distance of the image of the circle would be about 90 inches and 

 that of the image of the other circle would be about 



r = (90* + c)* . 

 Hence, if P is the potential of the circles when the charge of each is i, 



p _ TT i 2 a 2 o _! _ _i 



2a c 3 c s 



where the first term is due to the circle itself, the second and third to the other 

 circle, as in Note 11, and the two last to the images of the two circles. We thus 

 find for the three distances 



PI = 0-3438, P 2 = 0-3567, P 3 = 0-3689. 



The capacity is 2P" 1 , and the number of "inches of electricity," according 

 to the definition of Cavendish, is 4P -1 , 



or 11-636, H-2I2, 10-844, 



for the three cases. 



The large circle was 18-5 inches in diameter and its centre was 41 inches 

 from the floor, so that its charge would be 12-69 inches of electricity. 



Hence the relative charges are as follows: 



Calculated 



The large circle i-ooo i-ooo 



The two small ones at 36 inches -917 -899 



24 -884 -859 



18 -855 -811 



