LAWS OF STATICAL ELECTRICITY. 221 



with the unit of measure u, the number of charges was noted corresponding to an 

 accumulation, the attractive force of which operating between the two plane sur- 

 faces, m m', was equivalent to a force of 4-5 grains. 



When the quantity of electricity accumulated was doubled, the force amounted to 

 exactly 18 grains ; three times the accumulation balanced a force of 40-5 grains, and 

 so on. 



(e.) When a second and precisely similar jar was connected with the former, so as 

 to double the extent of coating, similar quantities, measured as before, only exhibited 

 one fourth of the previous forces respectively. 



With three similar jars, that is, with three times the surface, the force was only one 

 ninth part of the respective forces first observed. 



By substituting the electrometer, fig. 2, in place of the balance, the march of the 

 attractive force may be gradually observed, so as to exhibit the above results by 

 minute degrees, thereby furnishing very interesting experiments. 



21. The physical causes of these effects are not very apparent ; they seem, how- 

 ever, to have some connexion with the following fact. The force exerted between 

 two given substances, is more or less diminished by the presence of a neutral or other 

 body sharing in the attraction. 



Thus, the excited balls of an electrometer tend to close when an uninsulated 

 neutral body is brought near them. 



The attractive force evinced by any description of electrometer in connexion 

 with a charged conductor, will apparently diminish when a neutral body is presented 

 toward the conductor. 



A similar phenomenon is observable when neutral bodies are interposed between 

 two conductors, mn, fig. 10; one, n, being permanently electrified, and influencing the 

 other, m, by induction. An interv^ening plate, p, appears to operate as a screen, and to 

 arrest, as it were, to a greater or less extent, the inductive influence ; and such will also 

 be the case when the plate p is applied near any other part of the electrified conductor 

 n, without coming between m and n. This eff'ect is strikingly analogous to the opera- 

 tion of screens in diminishing the force of a revolving magnet on metallic discs*. 



It may be likewise remarked, that when a neutral conductor, m, fig. 10, is exposed 

 to the inductive action of an excited body, n, and is at the same time touched with an 

 uninsulated conductor, it will have its original quantity of electricity either increased 

 or diminished according as the electricity of the excited body n is positive or negative. 



Now an electroscope, A, will not indicate the change which has been effected in 

 the conductor m so long as it remains exposed to the influence of the excited body ; 

 but if we remove the excited body, or otherwise make it neutral, then the electroscope 

 A will immediately diverge. 



It is not essential here to enter upon the theoretical explanations of these pheno- 

 mena, the mere facts being alone requisite ; for whatever theory may be considered 



* Philosophical Transactions for 1831, p. 497. 

 MDCCCXXXIV. 2 G 



