[ 192 ] 



XXVIII. An Account of some Electrical Experiments and Indue- 

 tions. By J. J. Waterston, Esq. 



[Concluded from p. 98.] 



On the Integral of Electric Force and its mode of distribution in 

 space, as deduced from the elementary laws established by Ca- 

 vendish and Harris. 



51. ''T'O prove that the attraction between parallel electrified 

 A conducting surfaces is constant at all distances if the quan- 

 tity of electricity engaged between them is constant, suppose two 

 Leyden jars or magic panes exactly the same in every respect, except 

 that the glass of one (L) is twice as thick as that of the other (M). 

 If it takes 100 unit-jars to charge L so that the balance electro- 

 meter of Harris connected with its charging side requires 10 

 grains, there will be required 200 unit-jars to charge M to the 

 same intensity as measured by the electrometer. The attractive 

 force between a square inch of the two coatings of M is four 

 times the attractive force between a square inch of the two coat° 

 ings of L. Let us suppose the charge of L to be increased by 

 another 100 unit-jars, making the whole charge 200. The force 

 between a square inch of the two coatings of L is now four times 

 what it was with the charge 100. The force is therefore equal 

 to that of a square inch of M, and the quantity of electricity on 

 a square inch is the same in both, while the distance between 

 the coatings is in L twice that in 1VL The electrometer connected 

 with the charging side of L now requires 40 grains. Suppose 

 the sides of a magic pane to be separated by a dielectric plate 

 capable of compression, and which yields stiffly to the attractive 

 force exerted through it from the opposite sides. The quantity 

 remaining constant, the static force is constant, and the intensity 

 of the free part of the charge diminishes as the square of the 

 distance between the opposite surfaces. 



52. The coated surfaces of the magic pane may be supposed 

 to be perfect concentric spheres, in which case no free electricity 

 is required to enclose and confine the charge, which is wholly 

 engaged and symmetrically placed. This form given to the 

 magic plane simplifies the subject, and enables us without em- 

 barrassment to arrive at an accurate conception of the integral of 

 the force. 



Let the sphere A (fig. 25) with conducting surface be situated 

 concentrically within the spherical shell B, also composed of con- 

 ducting matter. We suppose A to be charged with 100 unit- 

 jars of positive electricity. On the inner surface of B we have 

 negative electricity induced; and the quantity is the same as on 

 A, because they mutually engage ; but the density is less, because 



