337 



10. Stratum of air between two parallel or nearly parallel plane 

 or curved metallic surfaces maintained at different potentials. Let a 

 denote the distance between the metallic surfaces on each side of the 

 stratum of air at any part, and V the difference of potentials. It is 

 easily shown that the resultant electrostatic force is sensibly constant 

 through the whole distance, from the one surface to the other ; and 

 being in a direction sensibly perpendicular to each, it must (9) 



be equal to - . Hence ( 4) the electric density on each of the 

 a 



y 

 opposed surfaces is equal to - . This is Green's theory of the 



Ley den phial. 



1 1 . Absolute Electrometer. As a particular case of No. 1 0, let 

 the discs be plane and parallel ; and let the distance between them 

 be small in comparison with their diameters, or with the distance of 

 any part of either from any conductor differing from it in potential. 

 The electric density will be uniform over the whole of each of the 



opposed surfaces and equal to - , being positive on one and nega- 



tive on the other ; and in all other parts of the surface of each the 

 electrification will be comparatively insensible. Hence the force of 



V 2 

 attraction between them per unit of area (5 and 6) will be - -, 



if A denote the area of either of the opposed surfaces ; the whole 



ya 

 force of attraction between them is therefore A- - . Hence, if the 



observed force be equal to the weight of w grains at Glasgow, we 

 have 



and therefore 



Addition, dated April 12, 1860. 



Experiments on precisely the same plan as those of Table I. 

 December 13, have been repeated by the same two experimenters, 

 with different distances from *3 to '6 of an inch between the plates 

 of the absolute electrometer, and results have been obtained con- 

 firming the general character of those shown in the preceding Tables. 



