1226. After these results on curved inductive action in air I extended the experiments to other gases, using first carbonic acid and then hydrogen: the phenomena were precisely those already described. In these experiments I found that if the gases were confined in vessels they required to be very large, for whether of gla.s.s or earthenware, the conducting power of such materials is so great that the induction of the excited sh.e.l.l-lac cylinder towards them is as much as if they were metal; and if the vessels be small, so great a portion of the inductive force is determined towards them that the lateral tension or mutual repulsion of the lines of force before spoken of, (1224.) by which their inflexion is caused, is so much relieved in other directions, that no inductive charge will be given to the carrier ball in the positions _k, l, m, n, o, p_ (fig. 110.). A very good mode of making the experiment is to let large currents of the gases ascend or descend through the air, and carry on the experiments in these currents.

1227. These experiments were then varied by the subst.i.tution of a liquid dielectric, namely, _oil of turpentine_, in place of air and gases. A dish of thin gla.s.s well-covered with a film of sh.e.l.l-lac (1272.), which was found by trial to insulate well, had some highly rectified oil of turpentine put into it to the depth of half an inch, and being then placed upon the top of the bra.s.s hemisphere (fig. 110.), observations were made with the carrier ball as before (1224.). The results were the same, and the circ.u.mstance of some of the positions being within the fluid and some without, made no sensible difference.

1228. Lastly, I used a few solid dielectrics for the same purpose, and with the same results. These were sh.e.l.l-lac, sulphur, fused and cast borate of lead, flint gla.s.s well-covered with a film of lac, and spermaceti. The following was the form of experiment with sulphur, and all were of the same kind. A square plate of the substance, two inches in extent and 0.6 of an inch in thickness, was cast with a small hole or depression in the middle of one surface to receive the carrier ball. This was placed upon the surface of the metal hemisphere (fig. 112.) arranged on the excited lac as in former cases, and observations were made at _n, o, p_, and _q_. Great care was required in these experiments to free the sulphur or other solid substance from any charge it might previously have received. This was done by breathing and wiping (1203.), and the substance being found free from all electrical excitement, was then used in the experiment; after which it was removed and again examined, to ascertain that it had received no charge, but had acted really as a dielectric. With all these precautions the results were the same: and it is thus very satisfactory to obtain the curved inductive action through _solid bodies_, as any possible effect from the translation of charged particles in fluids or gases, which some persons might imagine to be the case, is here entirely negatived.

1229. In these experiments with solid dielectrics, the degree of charge a.s.sumed by the carrier ball at the situations _n, o, p_ (fig. 112.), was decidedly greater than that given to the ball at the same places when air only intervened between it and the metal hemisphere. This effect is consistent with what will hereafter be found to be the respective relations of these bodies, as to their power of facilitating induction through them (1269. 1273. 1277.).

1230. I might quote _many_ other forms of experiment, some old and some new, in which induction in curved or contorted lines takes place, but think it unnecessary after the preceding results; I shall therefore mention but two. If a conductor A, (fig. 111.) be electrified, and an uninsulated metallic ball B, or even a plate, provided the edges be not too thin, be held before it, a small electrometer at _c_ or at _d_, uninsulated, will give signs of electricity, opposite in its nature to that of A, and therefore caused by induction, although the influencing and influenced bodies cannot be joined by a right line pa.s.sing through the air. Or if, the electrometers being removed, a point be fixed at the back of the ball in its uninsulated state as at C, this point will become luminous and discharge the conductor A. The latter experiment is described by Nicholson[A], who, however, reasons erroneously upon it. As to its introduction here, though it is a case of discharge, the discharge is preceded by induction, and that induction must be in curved lines.

[A] Encyclopaedia Britannica, vol. vi. p. 504.

1231. As argument against the received theory of induction and in favour of that which I have ventured to put forth, I cannot see how the preceding results can be avoided. The effects are clearly inductive effects produced by electricity, not in currents but in its statical state, and this induction is exerted in lines of force which, though in many experiments they may be straight, are here curved more or less according to circ.u.mstances. I use the term _line of inductive force_ merely as a temporary conventional mode of expressing the direction of the power in cases of induction; and in the experiments with the hemisphere (1224.), it is curious to see how, when certain lines have terminated on the under surface and edge of the metal, those which were before lateral to them _expand and open out from each other_, some bending round and terminating their action on the upper surface of the hemisphere, and others meeting, as it were, above in their progress outwards, uniting their forces to give an increased charge to the carrier ball, at an _increased distance_ from the source of power, and influencing each other so as to cause a second flexure in the contrary direction from the first one. All this appears to me to prove that the whole action is one of contiguous particles, related to each other, not merely in the lines which they may be conceived to form through the dielectric, between the _inductric_ and the _inducteous_ surfaces (1483.), but in other lateral directions also. It is this which gives an effect equivalent to a lateral repulsion or expansion in the lines of force I have spoken of, and enables induction to turn a corner (1304.). The power, instead of being like that of gravity, which causes particles to act on each other through straight lines, whatever other particles may be between them, is more a.n.a.logous to that of a series of magnetic needles, or to the condition of the particles considered as forming the whole of a straight or a curved magnet. So that in whatever way I view it, and with great suspicion of the influence of favourite notions over myself, I cannot perceive how the ordinary theory applied to explain induction can be a correct representation of that great natural principle of electrical action.

1232. I have had occasion in describing the precautions necessary in the use of the inductive apparatus, to refer to one founded on induction in curved lines (1203.); and after the experiments already described, it will easily be seen how great an influence the sh.e.l.l-lac stem may exert upon the charge of the carrier ball when applied to the apparatus (1218.), unless that precaution be attended to.

1233. I think it expedient, next in the course of these experimental researches, to describe some effects due to _conduction_, obtained with such bodies as gla.s.s, lac, sulphur, &c., which had not been antic.i.p.ated.

Being understood, they will make us acquainted with certain precautions necessary in investigating the great question of specific inductive capacity.

1234. One of the inductive apparatus already described (1187, &c.) had a hemispherical cup of sh.e.l.l-lac introduced, which being in the interval between the inner bull and the lower hemisphere, nearly occupied the s.p.a.ce there; consequently when the apparatus was charged, the lac was the dielectric or insulating medium through which the induction took place in that part. When this apparatus was first charged with electricity (1198.) up to a certain intensity, as 400, measured by the COULOMB'S electrometer (1180.), it sank much faster from that degree than if it had been previously charged to a higher point, and had gradually fallen to 400; or than it would do if the charge were, by a second application, raised up again to 400; all other things remaining the same. Again, if after having been charged for some time, as fifteen or twenty minutes, it was suddenly and perfectly discharged, even the stem having all electricity removed from it (1203.), then the apparatus being left to itself, would gradually recover a charge, which in nine or ten minutes would rise up to 50 or 60, and in one instance to 80.

1235. The electricity, which in these cases returned from an apparently latent to a sensible state, was always of the same kind as that which had been given by the charge. The return took place at both the inducing surfaces; for if after the perfect discharge of the apparatus the whole was insulated, as the inner ball resumed a positive state the outer sphere acquired a negative condition.

1236. This effect was at once distinguished from that produced by the excited stem acting in curved lines of induction (1203. 1232.), by the circ.u.mstance that all the returned electricity could be perfectly and instantly discharged. It appeared to depend upon the sh.e.l.l-lac within, and to be, in some way, due to electricity evolved from it in consequence of a previous condition into which it had been brought by the charge of the metallic coatings or b.a.l.l.s.

1237. To examine this state more accurately, the apparatus, with the hemispherical cup of sh.e.l.l-lac in it, was charged for about forty-five minutes to above 600 with positive electricity at the b.a.l.l.s _h_ and B.

(fig. 104.) above and within. It was then discharged, opened, the sh.e.l.l-lac taken out, and its state examined; this was done by bringing the carrier ball near the sh.e.l.l-lac, uninsulating it, insulating it, and then observing what charge it had acquired. As it would be a charge by induction, the state of the ball would indicate the opposite state of electricity in that surface of the sh.e.l.l-lac which had produced it. At first the lac appeared quite free from any charge; but gradually its two surfaces a.s.sumed opposite states of electricity, the concave surface, which had been next the inner and positive ball; a.s.suming a positive state, and the convex surface, which had been in contact with the negative coating, acquiring a negative state; these states gradually increased in intensity for some time.

1238. As the return action was evidently greatest instantly after the discharge, I again put the apparatus together, and charged it for fifteen minutes as before, the inner ball positively. I then discharged it, instantly removing the upper hemisphere with the interior ball, and, leaving the sh.e.l.l-lac cup in the lower uninsulated hemisphere, examined its inner surface by the carrier ball as before (1237.). In this way I found the surface of the sh.e.l.l-lac actually _negative_, or in the reverse state to the ball which had been in it; this state quickly disappeared, and was succeeded by a positive condition, gradually increasing in intensity for some time, in the same manner as before. The first negative condition of the surface opposite the positive charging ball is a natural consequence of the state of things, the charging ball being in contact with the sh.e.l.l-lac only in a few points. It does not interfere with the general result and peculiar state now under consideration, except that it a.s.sists in ill.u.s.trating in a very marked manner the ultimate a.s.sumption by the surfaces of the sh.e.l.l-lac of an electrified condition, similar to that of the metallic surfaces opposed to or against them.

1239. _Gla.s.s_ was then examined with respect to its power of a.s.suming this peculiar state. I had a thick flint-gla.s.s hemispherical cup formed, which would fit easily into the s.p.a.ce _o_ of the lower hemisphere (1188. 1189.); it had been heated and varnished with a solution of sh.e.l.l-lac in alcohol, for the purpose of destroying the conducting power of the vitreous surface (1254.). Being then well-warmed and experimented with, I found it could also a.s.sume the _same state_, but not apparently to the same degree, the return action amounting in different cases to quant.i.ties from 6 to 18.

1240. _Spermaceti_ experimented with in the same manner gave striking results. When the original charge had been sustained for fifteen or twenty minutes at about 500, the return charge was equal to 95 or 100, and was about fourteen minutes arriving at the maximum effect. A charge continued for not more than two or three seconds was here succeeded by a return charge of 50 or 60. The observations formerly made (1234.) held good with this substance. Spermaceti, though it will insulate a low charge for some time, is a better conductor than sh.e.l.l-lac, gla.s.s, and sulphur; and this conducting power is connected with the readiness with which it exhibits the particular effect under consideration.

1241. _Sulphur._--I was anxious to obtain the amount of effect with this substance, first, because it is an excellent insulator, and in that respect would ill.u.s.trate the relation of the effect to the degree of conducting power possessed by the dielectric (1247.); and in the next place, that I might obtain that body giving the smallest degree of the effect now under consideration for the investigation of the question of specific inductive capacity (1277.).

1242. With a good hemispherical cup of sulphur cast solid and sound, I obtained the return charge, but only to an amount of 17 or 18. Thus gla.s.s and sulphur, which are bodily very bad conductors of electricity, and indeed almost perfect insulators, gave very little of this return charge.

1243. I tried the same experiment having _air_ only in the inductive apparatus. After a continued high charge for some time I could obtain a little effect of return action, but it was ultimately traced to the sh.e.l.l-lac of the stem.

1244. I sought to produce something like this state with one electric power and without induction; for upon the theory of an electric fluid or fluids, that did not seem impossible, and then I should have obtained an absolute charge (1169. 1177.), or something equivalent to it. In this I could not succeed. I excited the outside of a cylinder of sh.e.l.l-lac very highly for some time, and then quickly discharging it (1203.), waited and watched whether any return charge would appear, but such was not the case. This is another fact in favour of the inseparability of the two electric forces (1177.), and another argument for the view that induction and its concomitant phenomena depend upon a polarity of the particles of matter.

1245. Although inclined at first to refer these effects to a peculiar masked condition of a certain portion of the forces, I think I have since correctly traced them to known principles of electrical action. The effects appear to be due to an actual penetration of the charge to some distance within the electric, at each of its two surfaces, by what we call _conduction_; so that, to use the ordinary phrase, the electric forces sustaining the induction are not upon the metallic surfaces only, but upon and within the dielectric also, extending to a greater or smaller depth from the metal linings. Let _c_ (fig. 113.) be the section of a plate of any dielectric, _a_ and _b_ being the metallic coatings; let _b_ be uninsulated, and _a_ be charged positively; after ten or fifteen minutes, if _a_ and _b_ be discharged, insulated, and immediately examined, no electricity will appear in them; but in a short time, upon a second examination, they will appear charged in the same way, though not to the same degree, as they were at first. Now suppose that a portion of the positive force has, under the coercing influence of all the forces concerned, penetrated the dielectric and taken up its place at the line _p_, a corresponding portion of the negative force having also a.s.sumed its position at the line _n_; that in fact the electric at these two parts has become charged positive and negative; then it is clear that the induction of these two forces will be much greater one towards the other, and less in an external direction, now that they are at the small distance _np_ from each other, than when they were at the larger interval _ab_. Then let _a_ and _b_ be discharged; the discharge destroys or neutralizes all external induction, and the coatings are therefore found by the carrier ball unelectrified; but it also removes almost the whole of the forces by which the electric charge was driven into the dielectric, and though probably a part of that charge goes forward in its pa.s.sage and terminates in what we call discharge, the greater portion returns on its course to the surfaces of _c_, and consequently to the conductors _a_ and _b_, and const.i.tutes the recharge observed.

1246. The following is the experiment on which I rest for the truth of this view. Two plates of spermaceti, _d_ and, _f_ (fig. 114.), were put together to form the dielectric, _a_ and _b_ being the metallic coatings of this compound plate, as before. The system was charged, then discharged, insulated, examined, and found to give no indications of electricity to the carrier ball. The plates _d_ and _f_were then separated from each other, and instantly _a_ with _d_ was found in a positive state, and _b_ with _f_ in a negative state, nearly all the electricity being in the linings _a_ and _b_. Hence it is clear that, of the forces sought for, the positive was in one-half of the compound plate and the negative in the other half; for when removed bodily with the plates from each other's inductive influence, they appeared in separate places, and resumed of necessity their power of acting by induction on the electricity of surrounding bodies. Had the effect depended upon a peculiar relation of the contiguous particles of matter only, then each half-plate, _d_ and _f_, should have shown positive force on one surface and negative on the other.

1247. Thus it would appear that the best solid insulators, such as sh.e.l.l-lac, gla.s.s, and sulphur, have conductive properties to such an extent, that electricity can penetrate them bodily, though always subject to the overruling condition of induction (1178.). As to the depth to which the forces penetrate in this form of charge of the particles, theoretically, it should be throughout the ma.s.s, for what the charge of the metal does for the portion of dielectric next to it, should be close by the charged dielectric for the portion next beyond it again; but probably in the best insulators the sensible charge is to a very small depth only in the dielectric, for otherwise more would disappear in the first instance whilst the original charge is sustained, less time would be required for the a.s.sumption of the particular state, and more electricity would re-appear as return charge.

1248. The condition of _time_ required for this penetration of the charge is important, both as respects the general relation of the cases to conduction, and also the removal of an objection that might otherwise properly be raised to certain results respecting specific inductive capacities, hereafter to be given (1269. 1277.)

1249. It is the a.s.sumption for a time of this charged state of the gla.s.s between the coatings in the Leyden jar, which gives origin to a well-known phenomenon, usually referred to the diffusion of electricity over the uncoated portion of the gla.s.s, namely, the _residual charge_. The extent of charge which can spontaneously be recovered by a large battery, after perfect uninsulation of both surfaces, is very considerable, and by far the largest portion of this is due to the return of electricity in the manner described. A plate of sh.e.l.l-lac six inches square, and half an inch thick, or a similar plate of spermaceti an inch thick, being coated on the sides with tinfoil as a Leyden arrangement, will show this effect exceedingly well.

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.