1191. I had occasion at first to attach the stem to the socket by other means, as a band of paper or a plugging of white silk thread; but these were very inferior to the cement, interfering much with the insulating power of the apparatus.

1192. The retentive power of this apparatus was, when in good condition, better than that of the electrometer (1186.), i.e. the proportion of loss of power was less. Thus when the apparatus was electrified, and also the b.a.l.l.s in the electrometer, to such a degree, that after the inner ball had been in contact with the top _k_ of the ball of the apparatus, it caused a repulsion indicated by 600 of torsion force, then in falling from 600 to 400 the average loss was 8.6 per minute; from 400 to 300 the average loss was 2.6 per minute; from 300 to 200 it was 1.7 per minute; from 200 to 170 it was 1 per minute. This was after the apparatus had been charged for a short time; at the first instant of charging there is an apparent loss of electricity, which can only be comprehended hereafter (1207. 1250.).

1193. When the apparatus loses its insulating power suddenly, it is almost always from a crack near to or within the bra.s.s socket. These cracks are usually transverse to the stem. If they occur at the part attached by common cement to the socket, the air cannot enter, and thus const.i.tuting vacua, they conduct away the electricity and lower the charge, as fast almost as if a piece of metal had been introduced there. Occasionally stems in this state, being taken out and cleared from the common cement, may, by the careful application of the heat of a spirit-lamp, be so far softened and melted as to restore the perfect continuity of the parts; but if that does not succeed in replacing things in a good condition, the remedy is a new sh.e.l.l-lac stem.

1194. The apparatus when in order could easily be exhausted of air and filled with any given gas; but when that gas was acid or alkaline, it could not properly be removed by the air-pump, and yet required to be perfectly cleared away. In such cases the apparatus was opened and emptied of gas; and with respect to the inner ball _h_, it was washed out two or three times with distilled water introduced at the screw-hole, and then being heated above 212, air was blown through to render the interior perfectly dry.

1195. The inductive apparatus described is evidently a Leyden phial, with the advantage, however, of having the _dielectric_ or insulating medium changed at pleasure. The b.a.l.l.s _h_ and B, with the connecting wire _i_, const.i.tute the charged conductor, upon the surface of which all the electric force is resident by virtue of induction (1178.). Now though the largest portion of this induction is between the ball _h_ and the surrounding sphere _aa_, yet the wire _i_ and the ball B determine a part of the induction from their surfaces towards the external surrounding conductors. Still, as all things in that respect remain the same, whilst the medium within at _oo_, may be varied, any changes exhibited by the whole apparatus will in such cases depend upon the variations made in the interior; and these were the changes I was in search of, the negation or establishment of such differences being the great object of my inquiry. I considered that these differences, if they existed, would be most distinctly set forth by having two apparatus of the kind described, precisely similar in every respect; and then, _different insulating media_ being within, to charge one and measure it, and after dividing the charge with the other, to observe what the ultimate conditions of both were. If insulating media really had any specific differences in favouring or opposing inductive action through them, such differences, I conceived, could not fail of being developed by such a process.

1196. I will wind up this description of the apparatus, and explain the precautions necessary to their use, by describing the form and order of the experiments made to prove their equality when both contained common air. In order to facilitate reference I will distinguish the two by the terms App.

i. and App. ii.

1197. The electrometer is first to be adjusted and examined (1184.), and the app. i. and ii. are to be perfectly discharged. A Leyden phial is to be charged to such a degree that it would give a spark of about one-sixteenth or one-twentieth of an inch in length between two b.a.l.l.s of half an inch diameter; and the carrier ball of the electrometer being charged by this phial, is to be introduced into the electrometer, and the lever ball brought by the motion of the torsion index against it; the charge is thus divided between the b.a.l.l.s, and repulsion ensues. It is useful then to bring the repelled ball to the standard distance of 30 by the motion of the torsion index, and observe the force in degrees required for this purpose; this force will in future experiments be called _repulsion of the b.a.l.l.s_.

1198. One of the inductive apparatus, as, for instance, app. i., is now to be charged from the Leyden phial, the latter being in the state it was in when used to charge the b.a.l.l.s; the carrier ball is to be brought into contact with the top of its upper ball (_k_, fig. 104.), then introduced into the electrometer, and the repulsive force (at the distance of 30) measured. Again, the carrier should be applied to the app. i. and the measurement repeated; the apparatus i. and ii. are then to be joined, so as to _divide_ the charge, and afterwards the force of each measured by the carrier ball, applied as before, and the results carefully noted. After this both i. and ii. are to be discharged; then app. ii. charged, measured, divided with app. i., and the force of each again measured and noted. If in each case the half charges of app. i. and ii. are equal, and are together equal to the whole charge before division, then it may be considered as proved that the two apparatus are precisely equal in power, and fit to be used in cases of comparison between different insulating media or _dielectrics_.

1199. But the _precautions_ necessary to obtain accurate results are numerous. The apparatus i. and ii. must always be placed on a thoroughly uninsulating medium. A mahogany table, for instance, is far from satisfactory in this respect, and therefore a sheet of tinfoil, connected with an extensive discharging train (292.), is what I have used. They must be so placed also as not to be too near each other, and yet equally exposed to the inductive influence of surrounding objects; and these objects, again, should not be disturbed in their position during an experiment, or else variations of induction upon the external ball B of the apparatus may occur, and so errors be introduced into the results. The carrier ball, when receiving its portion of electricity from the apparatus, should always be applied at the same part of the ball, as, for instance, the summit _k_, and always in the same way; variable induction from the vicinity of the head, hands, &c. being avoided, and the ball after contact being withdrawn upwards in a regular and constant manner.

1200. As the stem had occasionally to be changed (1190.), and the change might occasion slight variations in the position of the ball within, I made such a variation purposely, to the amount of an eighth of an inch (which is far more than ever could occur in practice), but did not find that it sensibly altered the relation of the apparatus, or its inductive condition _as a whole_. Another trial of the apparatus was made as to the effect of dampness in the air, one being filled with very dry air, and the other with air from over water. Though this produced no change in the result, except an occasional tendency to more rapid dissipation, yet the precaution was always taken when working with gases (1290.) to dry them perfectly.

1201. It is essential that the interior of the apparatus should be perfectly free from _dust or small loose particles_, for these very rapidly lower the charge and interfere on occasions when their presence and action would hardly be expected. To breathe on the interior of the apparatus and wipe it out quietly with a clean silk handkerchief, is an effectual way of removing them; but then the intrusion of other particles should be carefully guarded against, and a dusty atmosphere should for this and several other reasons be avoided.

1202. The sh.e.l.l-lac stem requires occasionally to be well-wiped, to remove, in the first instance, the film of wax and adhering matter which is upon it; and afterwards to displace dirt and dust which will gradually attach to it in the course of experiments. I have found much to depend upon this precaution, and a silk handkerchief is the best wiper.

1203. But wiping and some other circ.u.mstances tend to give a charge to the surface of the sh.e.l.l-lac stem. This should be removed, for, if allowed to remain, it very seriously affects the degree of charge given to the carrier ball by the apparatus (1232.). This condition of the stem is best observed by discharging the apparatus, applying the carrier ball to the stem, touching it with the finger, insulating and removing it, and examining whether it has received any charge (by induction) from the stem; if it has, the stem itself is in a charged state. The best method of removing the charge I have found to be, to cover the finger with a single fold of a silk handkerchief, and breathing on the stem, to wipe it immediately after with the finger; the ball B and its connected wire, &c. being at the same time _uninsulated_: the wiping place of the silk must not be changed; it then becomes sufficiently damp not to excite the stem, and is yet dry enough to leave it in a clean and excellent insulating condition. If the air be dusty, it will be found that a single charge of the apparatus will bring on an electric state of the outside of the stem, in consequence of the carrying power of the particles of dust; whereas in the morning, and in a room which has been left quiet, several experiments can be made in succession without the stem a.s.suming the least degree of charge.

1204. Experiments should not be made by candle or lamp light except with much care, for flames have great and yet unsteady powers of affecting and dissipating electrical charges.

1205. As a final observation on the state of the apparatus, they should retain their charges well and uniformly, and alike for both, and at the same time allow of a perfect and instantaneous discharge, giving afterwards no charge to the carrier ball, whatever part of the ball B it may be applied to (1218.).

1206. With respect to the balance electrometer, all the precautions that need be mentioned, are, that the carrier ball is to be preserved during the first part of an experiment in its electrified state, the loss of electricity which would follow upon its discharge being avoided; and that in introducing it into the electrometer through the hole in the gla.s.s plate above, care should be taken that it do not touch, or even come near to, the edge of the gla.s.s.

1207. When the whole charge in one apparatus is divided between the two, the gradual fall, apparently from dissipation, in the apparatus which has _received_ the half charge is greater than in the one _originally_ charged.

This is due to a peculiar effect to be described hereafter (1250. 1251.), the interfering influence of which may be avoided to a great extent by going through the steps of the process regularly and quickly; therefore, after the original charge has been measured, in app. i. for instance, i.

and ii. are to be symmetrically joined by their b.a.l.l.s B, the carrier touching one of these b.a.l.l.s at the same time; it is first to be removed, and then the apparatus separated from each other; app. ii. is next quickly to be measured by the carrier, then app. i.; lastly, ii. is to be discharged, and the discharged carrier applied to it to ascertain whether any residual effect is present (1205.), and app. i. being discharged is also to be examined in the same manner and for the same purpose.

1208. The following is an example of the division of a charge by the two apparatus, air being the dielectric in both of them. The observations are set down one under the other in the order in which they were taken, the left-hand numbers representing the observations made on app. i., and the right-hand numbers those on app. ii. App. i. is that which was originally charged, and after two measurements, the charge was divided with app. ii.

App. i. App. ii.

b.a.l.l.s 160

. . . . 0 254 . . . .

250 . . . .

divided and instantly taken . . . . 122 124 . . . .

1 . . . . after being discharged.

. . . . 2 after being discharged.

1209. Without endeavouring to allow for the loss which must have been gradually going on during the time of the experiment, let us observe the results of the numbers as they stand. As 1 remained in app. i. in an undischargeable state, 249 may be taken as the utmost amount of the transferable or divisible charge, the half of which is 124.5. As app. ii.

was free of charge in the first instance, and immediately after the division was found with 122, this amount _at least_ may be taken as what it had received. On the other hand 124 minus 1, or 123, may be taken as the half of the transferable charge retained by app. i. Now these do not differ much from each other, or from 124.5, the half of the full amount of transferable charge; and when the gradual loss of charge evident in the difference between 254 and 250 of app. i. is also taken into account, there is every reason to admit the result as showing an equal division of charge, _unattended by any disappearance of power_ except that due to dissipation.

1210. I will give another result, in which app. ii. was first charged, and where the residual action of that apparatus was greater than in the former case.

App. i. App. ii.

b.a.l.l.s 150

. . . . 152 . . . . 148 divided and instantly taken 70 . . . .

. . . . 78 . . . . 5 immediately after discharge.

0 . . . . immediately after discharge.

1211. The transferable charge being 148 - 5, its half is 71.5, which is not far removed from 70, the half charge of i.; or from 73, the half charge of ii.: these half charges again making up the sum of 143, or just the amount of the whole transferable charge. Considering the errors of experiment, therefore, these results may again be received as showing that the apparatus were equal in inductive capacity, or in their powers of receiving charges.

1212. The experiments were repeated with charges of negative electricity with the same general results.

1213. That I might be sure of the sensibility and action of the apparatus, I made such a change in one as ought upon principle to increase its inductive force, i.e. I put a metallic lining into the lower hemisphere of app. i., so as to diminish the thickness of the intervening air in that part, from 0.62 to 0.435 of an inch: this lining was carefully shaped and rounded so that it should not present a sudden projection within at its edge, but a gradual transition from the reduced interval in the lower part of the sphere to the larger one in the upper.

1214. This change immediately caused app. i. to produce effects indicating that it had a greater aptness or capacity for induction than app. ii. Thus, when a transferable charge in app. ii. of 469 was divided with app. i., the former retained a charge of 225, whilst the latter showed one of 227, i.e. the former had lost 244 in communicating 227 to the latter: on the other hand, when app. i. had a transferable charge in it of 381 divided by contact with app. ii., it lost 181 only, whilst it gave to app. ii. as many as 194:--the sum of the divided forces being in the first instance _less_, and in the second instance _greater_ than the original undivided charge. These results are the more striking, as only one-half of the interior of app. i. was modified, and they show that the instruments are capable of bringing out differences in inductive force from amongst the errors of experiment, when these differences are much less than that produced by the alteration made in the present instance.

-- iv. _Induction in curved lines._

1215. Amongst those results deduced from the molecular view of induction (1166.), which, being of a peculiar nature, are the best tests of the truth or error of the theory, the expected action in curved lines is, I think, the most important at present; for, if shown to take place in an unexceptionable manner, I do not see how the old theory of action at a distance and in straight lines can stand, or how the conclusion that ordinary induction is an action of contiguous particles can be resisted.

1216. There are many forms of old experiments which might be quoted as favourable to, and consistent with the view I have adopted. Such are most cases of electro-chemical decomposition, electrical brushes, auras, sparks, &c.; but as these might be considered equivocal evidence, inasmuch as they include a current and discharge, (though they have long been to me indications of prior molecular action (1230.)) I endeavoured to devise such experiments for first proofs as should not include transfer, but relate altogether to the pure simple inductive action of statical electricity.

1217. It was also of importance to make these experiments in the simplest possible manner, using not more than one insulating medium or dielectric at a time, lest differences of slow conduction should produce effects which might erroneously be supposed to result from induction in curved lines. It will be unnecessary to describe the steps of the investigation minutely; I will at once proceed to the simplest mode of proving the facts, first in air and then in other insulating media.

1218. A cylinder of solid sh.e.l.l-lac, 0.9 of an inch in diameter and seven inches in length, was fixed upright in a wooden foot (fig. 106.): it was made concave or cupped at its upper extremity so that a bra.s.s ball or other small arrangement could stand upon it. The upper half of the stem having been excited _negatively_ by friction with warm flannel, a bra.s.s ball, B, 1 inch in diameter, was placed on the top, and then the whole arrangement examined by the carrier ball and Coulomb"s electrometer (1180. &c.). For this purpose the b.a.l.l.s of the electrometer were charged _positively_ to about 360, and then the carrier being applied to various parts of the ball B, the two were uninsulated whilst in contact or in position, then insulated[A], separated, and the charge of the carrier examined as to its nature and force. Its electricity was always positive, and its force at the different positions _a, b, c, d,_ &c. (figs. 106. and 107.) observed in succession, was as follows:

at _a_ above 1000 _b_ it was 149 _c_ 270 _d_ 512 _b_ 130

[A] It can hardly be necessary for me to say here, that whatever general state the carrier ball acquired in any place where it was uninsulated and then insulated, it retained on removal from that place, notwithstanding that it might pa.s.s through other places that would have given to it, if uninsulated, a different condition.

1219. To comprehend the full force of these results, it must first be understood, that all the charges of the ball B and the carrier are charges by induction, from the action of the excited surface of the sh.e.l.l-lac cylinder; for whatever electricity the ball B received by _communication_ from the sh.e.l.l-lac, either in the first instance or afterwards, was removed by the uninsulating contacts, only that due to induction remaining; and this is shown by the charges taken from the ball in this its uninsulated state being always positive, or of the contrary character to the electricity of the sh.e.l.l-lac. In the next place, the charges at _a_, _c_, and _d_ were of such a nature as might be expected from an inductive action in straight lines, but that obtained at _b_ is _not so_: it is clearly a charge by induction, but _induction_ in _a curved line_; for the carrier ball whilst applied to _b_, and after its removal to a distance of six inches or more from B, could not, in consequence of the size of B, be connected by a straight line with any part of the excited and inducing sh.e.l.l-lac.

1220. To suppose that the upper part of the _uninsulated_ ball B, should in some way be retained in an electrified state by that portion of the surface of the ball which is in sight of the sh.e.l.l-lac, would be in opposition to what we know already of the subject. Electricity is retained upon the surface of conductors only by induction (1178.); and though some persons may not be prepared as yet to admit this with respect to insulated conductors, all will as regards uninsulated conductors like the ball B; and to decide the matter we have only to place the carrier ball at _e_ (fig.

107.), so that it shall not come in contact with B, uninsulate it by a metallic rod descending perpendicularly, insulate it, remove it, and examine its state; it will be found charged with the same kind of electricity as, and even to a _higher degree_ (1224.) than, if it had been in contact with the summit of B.

1221. To suppose, again, that induction acts in some way _through or across_ the metal of the ball, is negatived by the simplest considerations; but a fact in proof will be better. If instead of the ball B a small disc of metal be used, the carrier may be charged at, or above the middle of its upper surface: but if the plate be enlarged to about 1-1/2 or 2 inches in diameter, C (fig. 108.), then no charge will be given to the carrier at _f_, though when applied nearer to the edge at _g_, or even _above the middle_ at _h_, a charge will be obtained; and this is true though the plate may be a mere thin film of gold-leaf. Hence it is clear that the induction is not _through_ the metal, but through the surrounding air or _dielectric_, and that in curved lines.

1222. I had another arrangement, in which a wire pa.s.sing downwards through the middle of the sh.e.l.l-lac cylinder to the earth, was connected with the ball B (fig. 109.) so as to keep it in a constantly uninsulated state. This was a very convenient form of apparatus, and the results with it were the same as those just described.

1223. In another case the ball B was supported by a sh.e.l.l-lac stem, independently of the excited cylinder of sh.e.l.l-lac, and at half an inch distance from it; but the effects were the same. Then the bra.s.s ball of a charged Leyden jar was used in place of the excited sh.e.l.l-lac to produce induction; but this caused no alteration of the phenomena. Both positive and negative inducing charges were tried with the same general results.

Finally, the arrangement was inverted in the air for the purpose of removing every possible objection to the conclusions, but they came out exactly the same.

1224. Some results obtained with a bra.s.s hemisphere instead of the ball B were exceedingly interesting, It was 1.36 of an inch in diameter, (fig.

110.), and being placed on the top of the excited sh.e.l.l-lac cylinder, the carrier ball was applied, as in the former experiments (1218.), at the respective positions delineated in the figure. At _i_ the force was 112, at _k_ 108, at _l_ 65, at _m_ 35; the inductive force gradually diminishing, as might have been expected, to this point. But on raising the carrier to the position _n_, the charge increased to 87; and on raising it still higher to _o_, the charge still further increased to 105: at a higher point still, _p_, the charge taken was smaller in amount, being 98, and continued to diminish for more elevated positions. Here the induction fairly turned a corner. Nothing, in fact, can better show both the curved lines or courses of the inductive action, disturbed as they are from their rectilineal form by the shape, position, and condition of the metallic hemisphere; and also a _lateral tension,_ so to speak, of these lines on one another:--all depending, as I conceive, on induction being an action of the contiguous particles of the dielectric, which being thrown into a state of polarity and tension, are in mutual relation by their forces in all directions.

1225. As another proof that the whole of these actions were inductive I may state a result which was exactly what might be expected, namely, that if uninsulated conducting matter was brought round and near to the excited sh.e.l.l-lac stem, then the inductive force was directed towards it, and could not be found on the top of the hemisphere. Removing this matter the lines of force resumed their former direction. The experiment affords proofs of the lateral tension of these lines, and supplies a warning to remove such matter in repeating the above investigation.