March 30, 1876] 



NATURE 



437 



EXPERIMENTAL RESEARCHES ON THE 

 EFFECTS OF ELECTRICAL INDUCTION, 

 FOR THE PURPOSE OF RECTIFYING THE 

 THEORY COMMONLY ADOPTED^ 



nPHE theory generally adopted in treatises on Physics and 

 ■*■ Electricity to explain the fundamental fact of electrical in- 

 duction in an insulated cylinder A B, acted on by an electrified 

 body c, is as follows : — It is admitted that on the extremity B of 

 the induced cylinder, that is, the extremity next to the inductor G, 

 is found only the electricity opposite to that of the inductor ; 

 while on the rest of the same cylinder is found only electricity 

 similar to that of the inductor. But these two opposite elec- 

 tricities are both supposed to be endowed with tension, conse- 

 quently they ought to be divided by a neutral line. When it is 

 wished to represent in a graphic manner the electrical distribution 



+ + + -f" + + 



indicated, obtained by induction upon the cylinder A B, this is 

 done by means of Fig. i. In this figure A B represents the 

 induced and insulated cylinder, c the positive inductor, a b the 

 neutral line, ap b the negative induced electricity, which may 

 also be called induced electricity of the first kind, and aq b the 

 positive electricity, or induced electricity of the second kind. 



It is obvious that the explanation commonly adopted for electric 

 induction cannot hold for the following three reasons : — I. Because, 

 as the two opposite electricities possess tension, they ought on 

 that account to neutralise each other as they are present on a 

 conducting cylinder. 2. Because in putting into communication 

 with the earth the extremity B of the induced cylinder, upon 

 which extremity it is admitted that the homonym of the inductor 

 is not found, nevertheless that only is dissipated, while the oppo- 

 site electricity, which is found on the same extremity, remains 

 there entirely, notwithstanding its communication with the earth. 



P\ 



Fig. I. 



3. Because of the two kinds of electricity which coexist upon the 

 induced insulated body, only the homonym of the inductor is 

 dissipated by contact with the air. The expenmental proofs 

 upon which this old theory is based do not prove the facts indi- 

 cated while the action of c is being exerted, contrary to what is 

 stated in treatises on Physics. Bat in what follows we shall give 

 other experimental and inefragable proofs to demonstrate that 

 the explanation indicated is not admissible. 



Melloni ' having discovered that the above-mentioned explana- 

 tion was altogether erroneous, proposed ano'her, which is as 

 follows : — Upon the induced and insulate! cylinder A B is found 

 rueryivhere electricity of the same kind as that of the inductor, i.e. , 

 induced electricity of the second kind ; but to a much greater 

 degree at the extremity A, furthest from the inductor and much 

 less at the extremity B, nearest to the same inductor. As to 

 induced electricity of the first kind, it does not possess any 

 tension, i.e., it is entirely latent (dissimulated) on the extre- 

 mity B, nearest to the inductor, and proceeding from this extre- 

 mity B towards the extremity A, furthest from the same inductor, 

 it always goes on diminishing. It is for this reason that upon 

 the induced insulated body there is a section in which the induced 



+ + + ■!■ -t- -J- + 



electricity of the first kind, entirely concealed, will be equal to 

 that of the second kind, entirely free. Also these two opposite 

 electricities may coexist upon the induced and insulated cylinder, 

 without neutralising each other. 



In order to represent graphically this electrical distribution, 

 obtained by induction upon the cylinder A B, and regarded as 

 true by Melloni, let us make use of Fig. 2. In this figure A B 

 represents the induced and insuhted cylinder, c represents the 

 positive induction, a b the section of electricities equal to each 

 other but of opposite kinds ; m, a, p, b, n, the induced of the 

 first kind, and a, q, b, h the induced electricity of the second 

 kind, i.e. the homonym of the inductor. 



The new theory of Melloni, the truth of which I have proved 

 by means of the experiments afterwards described, does not com- 

 plicate the explanation of the facts which depend upon it. On 

 the contrary, it tends to present them all in a unique and invari- 

 able aspect, the only one which is really natural and conformable 

 to observation. 



In the light of this new theory we see clearly (i) why the two 

 opposite electricities coexisting on the induced and isolated 

 cylinder do not neutralise each other ; (2) if the extremity B, 



+ + 4-4'+4-+-i- 



FlG. 2. 



the nearest to the inductor G, is put into communication 

 with the ground, the homonymous electricity of the inductor 

 alone is lost, and not the opposite electricity ; (3) why, of the 

 two kinds of electricity which are found upon the insulated induced 

 body, there is lost, by contact with the air, only the homonym 

 of the inductor. 



The cylinder A B, insulated and subjected to the influence of c, 

 develops in a state of tension only the electricity homologous to 

 that ot the inducing bod)-. The opposite electricity is com- 

 pletely latent, and becomes sensible only after the suppression 

 of the inductive force. 



' An Exposition of the Two Theories of Electric Induction. By M. Paul 

 Volpicelli. 

 * "Comptes Rendus," t. 39, p, 177 (July 24, 1854). 



When a proof-plane applied to the extremity b of the induced 

 and insulated cylinder is subtracted from its influence, there is 

 always seen the resultant of the t .vo electricities which are found 

 on the same extremity, both of them having become free, on the 

 same proof-plane subtracted from the induction. This resultant 

 may be either positive or negative, or even nil, relatively to the 

 extremity B ; but in each of these three results, we are bound to 

 admit the pre-existence of two component electricities, opposite 

 to each other, one completely latent, the other completely free, 

 upon the same extremity. 



If any clement whatever of the section a b (Fig. 2) can be 

 removed when insulated from the induction it will give a nil 

 resultant. 



Melloni, in the communication above referred to, confesses 



