384 RECENT PROGRESS IN PHYSICS. 



Fig. 24. 



He excited a cake of resin, and having 

 stretched over it a sheet of tin foil at a given 

 distance, removed from the latter, by a touch 

 of the finger, its free negative electricity. Pre- 

 L- . J senting to this apparatus, represented by a dia- 

 gram in Fig. 24, two pith balls suspended on 

 linen threads^ they do not diverge in the 



^ ilil !! !!I I Hil!:i| i iillillllil ! il^i M ^ If «|^ *^^e ^^j^f 0^ which may be the distance 



^^''-^ — 01 the pendulum from the sheet oi tm foil 



charged with latent -{- E. 



KnocJienliauer concluded, from this experiment, that when two 

 opposite electricities render each other latent, they lose all action at 

 a distance^ and stand only in relation to each other ; for it could not 

 be supposed that, in case the opposite latent electricities acted at a 

 distance, thesy effects would perfectly neutralize each other at all 

 points above the tin foil. 



Fechner has completely refuted this objection of Knochenhauer (Pog. 

 An., LI, 321.) He has shown that, by the aid of the suspended pith 

 balls, no electrical action can be obtained above the induced plate, 

 because they are not sufficiently sensitive to weak charges. If a proof 

 plane be substituted for the balls, and touched for an instant with the 

 finger, it shows, when tested by the pile electrometer, that it is really 

 electrified and similarly with 6 6; a proof that the efiect which a a 

 has upon c exceeds the effect which the latent electricity in & 6 has 

 upon c. 



Fechner has repeated this experiment, not only in the above de- 

 scribed manner, but varied in a great many ways, and always with 

 the same result. It will not be necessary to describe all these differ- 

 ent forms of experiment, since^ in speaking of the researches of Fara- 

 day on electrical induction, we shall have to return to some points of 

 Fechner s investigation. 



At the conclusion of the report on these experiments Fechner says : 



" From the preceding experiments we are amply justified in con- 

 sidering the attracting and repelling effects of the inducing, and of 

 the so-called latent, electricity, from the same point of view, namely, 

 as free electricity. Electricity, in becoming latent, is invested with 

 no new properties. If its attraction and repulsion be no longer per- 

 ceptible, this is explained by the fact that they are counterbalanced 

 or overpowered by the opposite effects of the inducing electricity, &c." 



Petrina has attempted again to cast doubt upon the correct views 

 of Fechner, that tension electricity acts through uninsulated con- 

 ductors, (Pogg, Ann., LXI, 116,) without, however, being able to 

 advance anything decisive. According to his view, the electricity 

 which i^ec/mer found in the electrical " shadow " of the upper metal 

 plate, was caused by the curved surface of the cylindrical space above 

 the upper plate becoming electrified by induction, and the inductive 

 action spreading thence inward. 



Petrina has neither established this strange idea nor followed out 



,^l 



