RECENT PROGRESS IN PHYSICS. 401 



The density of the electricity induced on C is always very email ; 

 if it were zero we should have 



m . m' = m" 



If this were rigidly true, we should have m^, m^, m\ &c., for the 

 coefficients of induction for the distances 2, 3, 4 ; m being the coeffi- 

 cient for the distance 1; or, in other words, the distances would be 

 the logarithms of the coefficients of induction. 



But the electricity disguised in C is not zero, though it is very 

 feeble. It becomes imperceptible when is very near B, or even 

 when A is brought very close to B ; for a given distance between A 

 and C, the induced electricity becomes a maximum when B is placed 

 exactly in the middle between them. 



These relations had been already discovered by Fechner, who de- 

 scribes them in his treatise above quoted. 



Fig. 42. liosenschold now sought to determine in what pro- 



^ portion the quantity of electricity disguised on C 



'ffl!SSH!2E22E!2!iro varies, when the intermediate plate B is insulated, 



jj -.- ' and is then put in connection with the ground, 



Ti in i i i Pi i ii»i»#iiiiii iiM!i> The plates A, B, and C, were 6 inches in diameter, 

 _ the distance from A to C was 9 inches, and B was 



-(ii iiiiiniiililmpiiii|iin»iiniiii y mid way between them. When B was uninsulated, 

 '•■■ Eosenschold found that the electricity disguised on C 



was only ^l of what there was when B remained insulated. In the 

 latter case, that is, when B is insulated, according to JRosenschold's 

 experiments, it is quite immaterial whether this plate be present or 

 not. 



When the distance from A to C was half as great, the electricity 

 induced on C, B being uninsulated, was only —^ of that which was 

 disguised there when B remained insulated. 



A and C having been placed at the distance of 2 feet from eacb 

 other, the electricity disguised on C was inconsiderable, but on touch- 

 ing B it amounted to more than half of what was observed when B 

 was insulated. 



Similar experiments were made with three-inch plates. 



When A and C were 9 lines apart, and B was touched, the quan- 

 tity of electricity disguised on C was yV of the quantity on it when B 

 was insulated. 



As a final result of these experiments, it was found that m" difiered 

 very little from mm', as long as the distance of the plates A and C 

 from each other did not exceed from ^ to |- of their diameter. 



With reference to the above ratios -^V? ysr? yVj i* ^^7 ^^ remarked 

 that liosenschold has not stated, as he should have done, how the in- 

 dications of his electrometer used for these measurements, were made 

 properly comparable. 



§ 26. RiESS ON INDUCED ELECTRICITY AND THE THEORY OP CONDENSERS. 



The last labors on this subject which we have to mention here, are those 

 of Biess, published in the YSd vol. of Poggendorff's Annalen, under the 

 title: "Ow Influence Electricity and the Theory of Condensers." By 

 the name influence electricity, Riess designates that which is generally 

 26 s 



