72. I consider all bodies as insulators which cause discharges through them to be more difficult than through a vacuum, and which, by their interposition within a circuit, can prevent that propagation of the oppositely polarizing undulations which would otherwise ensue. This furnishes a good mean of discrimination between insulators and conductors, the criterion being that a discharge ensues more readily as there is more of the one and less of the other in the way: that the one leads the waves where they would not go, the other impedes their going where they would proceed. Both in the case of disruptive discharge through air, producing a spark, or of a deflagrating discharge through wire, causing its explosion, there is a dispersion of intervening ponderable particles; and yet there is this manifest discordancy, that in one case the undulatory process of transfer is assisted, in the other resisted. The waves follow the metallic filament with intense attraction, while they strive to get out of the way of those formed by the aeriform matter, as if repelled. Hence the term disruptive, from dirumpo, to break through, was happily employed by Farraday to designate spark discharges.
73. The zigzag form of the disruptive spark shows that there is a tendency in the aeriform particles to turn the waves out of that straight course, which, if unresisted or facilitated, they would naturally pursue. On the one hand, the aerial filaments being unsuitable for the conveyance of the electric waves, these are forced by them out of the normal path—first in one direction, then in another; while, on the other hand, the finest metallic filament furnishes a channel for the electric waves, so favourable that this channel is pursued, although the consequent polarization of the conducting particles be so intense as to make them fly asunder with explosive violence. Even when a bell-wire has been dissipated by lightning, it has been found to facilitate and determine the path of the discharge.
74. The various forms of the electric spark, resulting from varying the gas through which it may be made to pass, agreeably to the researches of Farraday, is explained by the supposition that the peculiarities of the spark is partially the consequence of the polarizability of the gaseous atoms through which the discharge is made, and varies, accordingly, in its appearance.
Difference between Frictional Electricity and Galvanic does not depend on the one being superior as to Quantity, the other as to Intensity; but on the different Degrees in which the Ethereo-ponderable Atoms of the Bodies affected are deranged from their natural state of Neutralized Polarity.
75. I infer that all magneto-polar charges are attended by an affection of ponderable particles; and that the reason why the most intense statical charge does not affect a galvanometer is, that it is only when appositely excited bodies are neutralized by the interposition of a conductor as during a discharge, that ethereo-ponderable particles are sufficiently polarized to enable them to act upon others in their vicinity, so as to produce a polar affection the opposite of their own. (54.58.) In this way dynamic induction is consistently explained, by supposing that the waves of polarization, in passing along one conductor, produce, pari passu, the opposite polarization in the proximate part of any neighbouring conductor suitably constituted, situated and arranged to allow it to form a part of a circuit.
76. It is only during the state of the incessant generation and destruction of what has been called the two electricities, that the circuit, which is the channel for the passage of the polarizing waves, is endowed with electro-magnetic powers. It was, no doubt, in obedience to a perception of this fact, that Oersted ascribed the magnetism of a galvanized wire to a conflict of the electricities. Undoubtedly, that state of a conductor in which, by being a part of an electrical circuit, it becomes enabled to display electro-magnetic powers, is so far a conflict of the two electricities, as the affections of matter, which are denominated electrical, consist of two opposite polar forces, proceeding, agreeably to the language of Farraday, in opposite directions from each side of the source, and conflicting with each other, so as to be productive of reciprocal annihilation.
77. That a corpuscular change in conductors is concomitant with their subjection to, or emancipation from, a galvanic current, is proved by an experiment of Henry’s, which he afforded me an opportunity on one occasion of witnessing. I allude to the fact that sound is produced whenever the circuit is suddenly made or suddenly ruptured. By I. P. Marrian it has been observed that a similar result takes place during the magnetization or demagnetization of iron rods, by the alternate establishment or arrestation of galvanic discharges through wires coiled about them so as to convert each into an electro-magnet. Mr. Marrian represents the sound as resembling that produced by striking a rod upon one of its ends.[74][75] Sounds from this source were observed by Dr. Page in 1838. See Silliman’s Journal for that year, vol. xxxiii.
78. Thus it appears that there is an analogy between the state of matter which involves permanent magnetism, and that which constitutes a galvanic current, so far as this, that either by one or the other, during either its access or cessation, an affection of the ponderable particles concerned ensues, sufficient to produce sound.
79. Simultaneously with the production of sounds, as above stated, by the opening or closing of the galvanic circuit through a metallic rod or the coils of an electro-magnet, secondary waves are induced, called secondary currents. It seems reasonable to ascribe these waves to the same shifting of the poles, which produces the sonorific undulations.[76]