238 



MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



periments, the subsequent effect on the electrometer being quite independent of the 

 presence of the air. 



67. The law according to which the force of electrical attraction varies, when ex- 

 erted between bodies at different distances, has been justly considered by many pro- 

 found philosophers an important object of physical research : it may be satisfactorily 

 arrived at by the methods of experiment so frequently referred to in the course of this 

 paper (20.). The results are for the most part of an extremely simple kind, without 

 any complication ; and being strikingly illustrative of an influential law, applicable to 

 many forces in nature, they may not be altogether undeserving of attention. 



{v.) A weight of eighteen grains being placed in the pan t, fig. 9, the parallel and 

 even surfaces of the opposed bodies m m were placed at 05 of an inch distant. A 

 given quantity of electricity was now accumulated in the jar E, the attractive force of 

 which just balanced the given weight, the quantity being determined by means of the 

 unit jar u. The distance between the bodies m m being now increased to an inch, 

 that is, to twice the former distance, and the same quantity again accumulated in 

 the jar, the attractive force was found equivalent to 4*5 grains precisely. In like 

 manner a weight of two grains exactly balanced the former force, when the distance 

 between the bodies was increased to 1*5 inch, or three times the first. 



In these experiments, the attractive forces varied as the squares of the respective 

 distances inversely with great precision; a law of much importance in its conse- 

 quences, but which has received but comparatively little elucidation from methods of 

 research not involving complicated conditions. 



68. By substituting the electrometer, fig. 2, for the balance, the same law is imme- 

 diately arrived at, either with simple electrified conductors, in the way already de- 

 scribed (14.), or otherwise by means of a coated jar, as in the preceding case. When 

 the electrometer is employed, we may compare readily the force in degrees with the 

 distance between the attracting surfaces, the quantity being constant, and hence, as 

 before, arrive at results which present nothing but the mere effects of the law under 

 investigation, as given in the following Table. 



Table III. 



The approximations in the above Table are so close, that the numerical results may 

 be taken as exact. 



69. The law observable in the preceding investigations is immediately apparent 



