396 RECENT PROGRESS IN PHYSICS. 



and the induced + E of tlie ball B act simultaneously upon the 

 carrier wherever it may be, the effect of the cylinder preponderates, 

 and hence the carrier must be charged with induced -f- E at 6 as well 

 as at e. 



This case is perfectly analogous to that already mentioned in § 17. 

 Faraday explains the matter thus : The proof ball is electrified at 

 h as well as at e by induction ; but since it is impossible to connect by 

 a straight line the shellac cylinder with either h or e, the induction 

 must take place around B through the air, consequently there must 

 be induction in curved lines. To arrive at this conclusion, Faraday 

 must naturally suppose that no inductive action can take place 

 through a conductor. 



That no induction can take place through metal, Faraday believes 

 he can prove. 



Fig. 40. A metallic plate, C, fig. 40, was held above the 



shellac cylinder, and touched for a moment, so 

 i^ that it should be charged by induction with + E. 



A proof plane, or a small proof ball, was now 

 ^ gQ held at a, close to the middle of the plate, and 



^^ " ■ J^ touched for a moment, when it gave no indica- 



tion of a charge ; hence Faraday concluded, that 

 the electricity of the shellac cylinder cannot act 

 inductively through the metal plate ; but when 

 the proof ball was raised to about the distance of 

 h, it received a positive charge, which, according 

 to his view, showed that induction could take 

 place in curved lines around the plate upon point h. 



Fechner has described the same experiment in a somewhat different 

 form, in the memoir already cited, (Pog. Ann. LI, 321.) He has 

 shown that the phenomena, as Faraday describes them, are necessary 

 consequences of the known laws of induction. 



Fechner says : ' ' That the maximum of the effect is seen at some 

 distance from the upper plate,* is not at all surprising. For all 

 points of the upper plate, the influence of the negative electricity 

 which it contained, must be in exact equilibrium with that of the 

 positive electricity of the lower plate ; otherwise, more or less elec- 

 tricity would be decomposed in the upper plate, and accumulate more 

 than the case shows. By elevating the proof plane above the upper 

 plate, its distance from the points of the upper plate increases in 

 another proportion, than from those of the lower plate ; hence the 

 influence of the latter begins to predominate. Yet the increase of the 

 action with the elevation of the proof plane cannot go beyond a cer- 

 tain maximum, because at a greater distance the action of each plate 

 separately would disappear." 



These phenomena, consequently, are not a proof of induction in 

 curved lines ; and, in general, it may be asserted that Faraday has 

 not presented a tenable proof of his hypothesis, namely, that induc- 

 tion takes place through the contiguous particles of the intervening 

 insulator. 



§ 24. Faraday's theory of induction. — F araday endeavors, in the 



* Fechner used (instead of a shellac C3dinder) an insulated and positively electrified me- 

 ta ic plate, -nbich he termed the lower plate. 



