Part 36 (2/2)

1250. The peculiar condition of dielectrics which has now been described, is evidently capable of producing an effect interfering with the results and conclusions drawn from the use of the two inductive apparatus, when sh.e.l.l-lac, gla.s.s, &c. is used in one or both of them (1192. 1207.), for upon dividing the charge in such cases according to the method described (1198. 1207.), it is evident that the apparatus just receiving its half charge must fall faster in its tension than the other. For suppose app. i.

first charged, and app. ii. used to divide with it; though both may actually lose alike, yet app. i., which has been diminished one-half, will be sustained by a certain degree of return action or charge (1234.), whilst app. ii. will sink the more rapidly from the coming on of the particular state. I have endeavoured to avoid this interference by performing the whole process of comparison as quickly as possible, and taking the force of app. ii. immediately after the division, before any sensible diminution of the tension arising from the a.s.sumption of the peculiar state could be produced; and I have a.s.sumed that as about three minutes pa.s.s between the first charge of app. i. and the division, and three minutes between the division and discharge, when the force of the non-transferable electricity is measured, the contrary tendencies for those periods would keep that apparatus in a moderately steady and uniform condition for the latter portion of time.

1251. The particular action described occurs in the sh.e.l.l-lac of the stems, as well as in the _dielectric_ used within the apparatus. It therefore const.i.tutes a cause by which the outside of the stems may in some operations become charged with electricity, independent of the action of dust or carrying particles (1203.).

-- v. _On specific induction, or specific inductive capacity._

1252. I now proceed to examine the great question of specific inductive capacity, i.e. whether different dielectric bodies actually do possess any influence over the degree of induction which takes place through them. If any such difference should exist, it appeared to me not only of high importance in the further comprehension of the laws and results of induction, but an additional and very powerful argument for the theory I have ventured to put forth, that the whole depends upon a molecular action, in contradistinction to one at sensible distances.

The question may be stated thus: suppose A an electrified plate of metal suspended in the air, and B and C two exactly similar plates, placed parallel to and on each side of A at equal distances and uninsulated; A will then induce equally towards B and C. If in this position of the plates some other dielectric than air, as sh.e.l.l-lac, be introduced between A and C, will the induction between them remain the same? Will the relation of C and B to A be unaltered, notwithstanding the difference of the dielectrics interposed between them?[A]

[A] Refer for the practical ill.u.s.tration of this statement to the supplementary note commencing 1307, &c.--_Dec. 1838._

1253. As far as I recollect, it is a.s.sumed that no change will occur under such variation of circ.u.mstances, and that the relations of B find C to A depend entirely upon their distance. I only remember one experimental ill.u.s.tration of the question, and that is by Coulomb[A], in which he shows that a wire surrounded by sh.e.l.l-lac took exactly the same quant.i.ty of electricity from a charged body as the same wire in air. The experiment offered to me no proof of the truth of the supposition: for it is not the mere films of dielectric substances surrounding the charged body which have to be examined and compared, but the _whole ma.s.s_ between that body and the surrounding conductors at which the induction terminates. Charge depends upon induction (1171. 1178.); and if induction is related to the particles of the surrounding dielectric, then it is related to _all_ the particles of that dielectric inclosed by the surrounding conductors, and not merely to the few situated next to the charged body. Whether the difference I sought for existed or not, I soon found reason to doubt the conclusion that might be drawn from Coulomb's result; and therefore had the apparatus made, which, with its use, has been already described (1187, &c.), and which appears to me well-suited for the investigation of the question.

[A] Memoires de l'Academie, 1787, pp. 452, 453.

1254. Gla.s.s, and many bodies which might at first be considered as very fit to test the principle, proved exceedingly unfit for that purpose. Gla.s.s, princ.i.p.ally in consequence of the alkali it contains, however well-warmed and dried it may be, has a certain degree of conducting power upon its surface, dependent upon the moisture of the atmosphere, which renders it unfit for a test experiment. Resin, wax, naphtha, oil of turpentine, and many other substances were in turn rejected, because of a slight degree of conducting power possessed by them; and ultimately sh.e.l.l-lac and sulphur were chosen, after many experiments, as the dielectrics best fitted for the investigation. No difficulty can arise in perceiving how the possession of a feeble degree of conducting power tends to make a body produce effects, which would seem to indicate that it had a greater capability of allowing induction through it than another body perfect in its insulation. This source of error has been that which I have found most difficult to obviate in the proving experiments.

1255. _Induction through sh.e.l.l-lac._--As a preparatory experiment, I first ascertained generally that when a part of the surface of a thick plate of sh.e.l.l-lac was excited or charged, there was no sensible difference in the character of the induction sustained by that charged part, whether exerted through the air in the one direction, or through the sh.e.l.l-lac of the plate in the other; provided the second surface of the plate had not, by contact with conductors, the action of dust, or any other means, become charged (1203.). Its solid condition enabled it to retain the excited particles in a permanent position, but that appeared to be all; for these particles acted just as freely through the sh.e.l.l-lac on one side as through the air on the other. The same general experiment was made by attaching a disc of tinfoil to one side of the sh.e.l.l-lac plate, and electrifying it, and the results were the same. Scarcely any other solid substance than sh.e.l.l-lac and sulphur, and no liquid substance that I have tried, will bear this examination. Gla.s.s in its ordinary state utterly fails; yet it was essentially necessary to obtain this prior degree of perfection in the dielectric used, before any further progress could be made in the princ.i.p.al investigation.

1256. _Sh.e.l.l-lac and air_ were compared in the first place. For this purpose a thick hemispherical cup of sh.e.l.l-lac was introduced into the lower hemisphere of one of the inductive apparatus (1187, &c.), so as nearly to fill the lower half of the s.p.a.ce _o, o_ (fig. 104.) between it and the inner ball; and then charges were divided in the manner already described (1198. 1207.), each apparatus being used in turn to receive the first charge before its division by the other. As the apparatus were known to have equal inductive power when air was in both (1209. 1211.), any differences resulting from the introduction of the sh.e.l.l-lac would show a peculiar action in it, and if unequivocally referable to a specific inductive influence, would establish the point sought to be sustained. I have already referred to the precautions necessary in making the experiments (1199, &c.); and with respect to the error which might be introduced by the a.s.sumption of the peculiar state, it was guarded against, as far as possible, in the first place, by operating quickly (1248); and, afterwards, by using that dielectric as gla.s.s or sulphur, which a.s.sumed the peculiar state most slowly, and in the least degree (1239. 1241.).

1257. The sh.e.l.l-lac hemisphere was put into app. i., and app. ii. left filled with air. The results of an experiment in which the charge through air was divided and reduced by the sh.e.l.l-lac app. were as follows:

App. i. Lac. App. ii. Air.

b.a.l.l.s 255.

0 . . . .

. . . . 304 . . . . 297 Charge divided.

113 . . . .

. . . . 121 0 . . . . after being discharged.

. . . . 7 after being discharged.

1258. Here 297, minus 7, or 290, may be taken as the divisible charge of app. ii. (the 7 being fixed stem action (1203. 1232.)), of which 145 is the half. The lac app. i. gave 113 as the power or tension it had acquired after division; and the air app. ii. gave 121, minus 7, or 114, as the force it possessed from what it retained of the divisible charge of 290.

These two numbers should evidently be alike, and they are very nearly so, indeed far within the errors of experiment and observation, but these numbers differ very much from 145, or the force which the half charge would have had if app. i. had contained air instead of sh.e.l.l-lac; and it appears that whilst in the division the induction through the air has lost 176 of force, that through the lac has only gained 113.

1259. If this difference be a.s.sumed as depending entirely on the greater facility possessed by sh.e.l.l-lac of allowing or causing inductive action through its substance than that possessed by air, then this capacity for electric induction would be inversely as the respective loss and gain indicated above; and a.s.suming the capacity of the air apparatus as 1, that of the sh.e.l.l-lac apparatus would be 176/113 or 1.55.

1260. This extraordinary difference was so unexpected in its amount, as to excite the greatest suspicion of the general accuracy of the experiment, though the perfect discharge of app. i. after the division, showed that the 113 had been taken and given up readily. It was evident that, if it really existed, it ought to produce corresponding effects in the reverse order; and that when induction through sh.e.l.l-lac was converted into induction through air, the force or tension of the whole ought to be _increased_. The app. i. was therefore charged in the first place, and its force divided with app. ii. The following were the results:

App. i. Lac. App. ii. Air.

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