249. If the cylinder DC, be carefully examined whilst within the inductive influence of the positively electrified ball, E, it will be found to have the end c, negatively electric, and at the end D, positively; whilst an intermediate zone, A, will be found neutral and unelectrified, so that the distribution of electricity on its surface may be compared to that in an excited tourmaline (244): whilst things are in this state, and the pith balls standing apart from each other, touch the cylinder DC, with the finger, or any other conducting body connected with the earth, the pith balls will collapse, from the positive electricity running off by the finger to the earth; the negative electricity cannot escape in the same manner, because it is firmly held in the end c, of the cylinder, by the attractive influence of the opposite electricity of the ball, E. Now remove the finger, leaving the conductor insulated, and separate E, to a considerable distance from c, the negative electricity in which, being released from the influence of E, expands itself over DC, and the positive electricity which had been previously combined with it, having been removed by touching it with the hand, it is left in a free state on DC, and the balls F instantly separate with negative electricity. If this experiment be repeated with an excited piece of sealing-wax, amber, or sulphur, instead of the glass tube, E, the same phenomena will occur, with this difference, that the induced electricity will always be of the opposite kind, as would of course, be expected à priori.

250. The application of this inductive influence, furnishes us with the readiest mode of ascertaining the kind of electricity present in any excited substance; for this purpose, excite a glass tube by friction, and hold it about a foot distant from the cap of the gold-leaf electrometer (239); the leaves will diverge with positive electricity, the negative being retained in the cap of the instrument: touch the latter with the finger, the leaves collapse, and the positive electricity escapes to the earth; the negative being retained in

the cap by the attraction of the positively electrified tube. Now, remove first the finger, then the tube, and the gold leaves will diverge with negative electricity; excite, by friction or otherwise, the substances whose electric state is to be examined, and hold it near, but not in contact with, the cap of the electrometer; if the substance be positively electrified, it will attract the negative electricity from the gold leaves into the cap of the instrument, causing the former to collapse; whilst, if it be negative, it will, by repelling the electricity of the same kind already in the electrometer, increase the previously divergent state of the gold leaves. By this process, it becomes exceedingly easy to discover what species of free electricity is present in any excited substance.

251. In these experiments (248-250), the induction takes place through the column of air separating the excited tube from the conductor (248), or electrometer (250). A similar action is capable of taking place when other non-conductors are interposed; these substances, in consequence of their permitting induction to take place through them, have been termed dielectrics. These dielectrics differ considerably in the degree of facility with which they permit induction to take place through them, indicating the existence of a specific inductive capacity. Thus, sulphur, lac, and glass, have much higher inductive capacities than air.*

252. Induction has been demonstrated by Faraday, to be essentially a physical action, occurring between contiguous particles, never taking place at a distance, without polarizing the molecules of the intervening dielectric; thus, in the experiment already detailed (248), a space of six inches occurred between the inducting excited tube and the conductor, whose electricity was affected by its action: we are not to assume from this, that the decomposition of the neutral electric state of the conductor arose from an action at a

On this subject, the admirable papers of Dr. Faraday, in the Philosophical Transactions for 1838 should be consulted, especially § 1252-78.

distance; for most satisfactory evidence has been adduced by Dr. Faraday that the intervening dielectric, air, has its particles of electricity arranged in a manner analogous to those of the conductor DC, by the inducting influence of the glass tube. The theory of induction depending upon an action between contiguous molecules is supported by the fact, which would be otherwise totally inexplicable, that a slender rod of glass or resin, when excited by friction and placed in contact with an insulated sphere of metal, is capable of decomposing the electricity of the latter by induction, most completely, even at the point of the ball equidistant from the rod, and consequently, incapable of being connected with it by a right line; so that we must either consider that induction is exerted in curved lines, or propagated through the intervention of contiguous particles. Now, as no radiant simple force can act in curved lines, excepting under the coercing influence of a second force, we are almost compelled to adopt the view of induction acting through the medium of contiguous particles.

253. This inductive action appears to come into play in every electric phenomenon; thus, in the simple experiment of attracting light bodies by an excited tube (230), the positive electricity in the tube decomposes by induction the electricity of the pieces of paper, repelling their positive fluid; and being thus left in a negative state, they become attracted by the tube, in obedience to the law of mutual attraction between differently electrified bodies. The following experiment illustrates in an interesting manner the development of electricity by induction. Support a pane of dry and warm window-glass about an inch from the table, by means of two books or blocks of wood; and place beneath it several pieces of paper or pith-balls. Excite the upper surface by friction with a silk handkerchief, the electricity of the glass becomes decomposed, its negative fluid adhering to the silk, and its positive to the upper surface of the glass

plate; this by induction acts on the lower surface of the glass, repelling its positive electricity and attracting its negative. The lower surface of the glass, thus becoming virtually electrified by induction through its substance, attracts and repels alternately the light bodies placed beneath it, in a similar manner as the excited tube (230).

254. Into a circular tray of tinned iron, a, about eight or ten inches in diameter and twelve inches deep, pour melted sealing-wax, or a mixture of two parts of shell lac and one of Venice turpentine, until it is filled, and let it cool gradually. A circular plate of stout tinned iron, or brass, about two inches

B

less in diameter than A, is furnished with a glass handle, B, fixed into its centre. Remove the metallic plate from the cake of resin or sealing-wax A, and excite the latter by friction, with a warm and dry piece of flannel; then place on it the plate c: under these circumstances the negatively electrified cake of resin decomposes the natural electric state of c, attracting its positive fluid into the lower surface, and repelling its negative into the upper, by induction. If then c be lifted off by its glass handle, its separated electric fluids will reunite, and it will be found destitute of free electricity; replace c on A, touch the former with the finger, and its positive electricity, repelled by the inductive influence of A, will escape to the earth; then raise c, by its handle B, it will be found to contain positive electricity in a free state, which, on the approach of any conductor, will escape in the form of a vivid spark, the plate resuming its naturally unelectrified state. Again, place c on a, touch it with the finger, negative electricity escapes to the earth; lift off c, approach any conductor towards it, and another spark of positive electricity occurs. This process may be repeated an almost indefinite number of times, the cake a losing none of its electricity by the operation, as it acts solely by its inductive influence on the combined electricities actually

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present in the metallic plate D; indeed, after being once excited, a spark may be obtained from this instrument, during many weeks, without any fresh excitation, and on this account it has been used as an electrifying machine, and was by its inventor, the celebrated Volta, termed electroforo perpetuo. This electrophorus is a most valuable instrument, not only from its affording a beautiful illustration of inductive action but from its yielding a large supply of electricity.

255. A very useful modification of the electrophorus (254), is made by coating a thin pane of glass on one side with tinfoil to within about two inches of the edge. Placing it with the coated side on the table, excite the other surface by friction with a piece of silk covered with amalgam (264), then carefully lifting the glass by one corner, place it on a badly-conducting surface, as a smooth table or the cover of a book, with the uncoated side downwards. Touch the tinfoil with the finger, then carefully elevate the plate by one corner, and a vivid spark will dart from the coating to any conducting body near it; replace the plate, touch it, again elevate it, and a second spark will be produced. An electric jar may be charged, in a few minutes, with an apparatus of this kind only four inches square. This modification of the electrophorus, or electro-lasmus,* as I termed it when I first constructed it several years ago, is a most convenient instrument in the laboratory where electricity is required for endiometric purposes, and where the introduction of an electric machine (260) is inconvenient.

256. If a given quantity of free electricity be communicated to a surface exposing sixteen square inches, and a similar quantity be communicated to another of but four square inches of surface, it is obvious that every square inch of the former will contain but one fourth of that present in every square inch of the latter; hence, although the total quantities

* ἥλεκτρον and έλασμος, lamina.