« PreviousContinue »
were assumed to coincide, or nearly so, the difference between theory and observation was the least possible. It is evident, therefore, that the earth does not act as if it were a permanently magnetic body, the distinguishing characteristic of which is, to have two poles at a distance from one another. Mr. Barlow has investigated this subject with much skill and success. He first proved that the
magnetic power of an iron sphere resides in its surface; he then inquired what the superficial action of an iron sphere in a state of transient magnetic induction, on a magnetised needle, would be, if insulated from the influence of terrestrial magnetism. The results obtained, corroborated by the profound analysis of M. Poisson, on the hypothesis of the two poles being indefinitely near the centre of the sphere, are identical with those obtained by M. Biot for the earth from M. de Humboldt's observations. Whence it follows, that the laws of terrestrial magnetism deduced from the formulæ of M. Biot, are inconsistent with those which belong to a permanent magnet, but that they are perfectly accordant with those belonging to a body in a state of transient magnetic induction. It appears, therefore, that the earth is to be considered as only transiently magnetic by induction, and not a real magnet. Mr. Barlow has rendered this extremely probable by forming a wooden globe, with grooves admitting of a copper wire being coiled round it parallel to the equator from pole to pole. When a current of electricity was sent through the wire, a magnetic needle suspended above the globe, and neutralized from the influence of the earth's magnetism, exhibited all the phenomena of the dipping and variation needles, according to its positions with regard to the wooden globe. As there can be no doubt that the same
phenomena would be exhibited by currènts of thermo, instead of voltaic, electricity, if the grooves of the wooden globe were filled by rings constituted of two metals, it seems highly probable that the heat of the sun may be the great agent in developing electric currents in or near the surface of the earth, by its action upon the substances of which the globe is composed, and, by the changes in its intensity, may occasion the diurnal variation of the compass and the other vicissitudes in terrestrial magnetism evinced by the disturbance in the directions of the magnetic lines, in the same manner as it influences the parallelism of the isothermal lines. That such currents do exist in metalliferous veins appears from the experiments of Mr. Robert Fox in the Cornish coppper-mines. However, it is probable that the secular and periodic disturbances in the magnetic force are occasioned by a variety of combining circumstances. Among others, M. Biot mentions the vicinity of mountain chains to the place of observation, and still more the action of extensive volcanic fires, which change the chemical state of the terrestrial surface, they themselves varying from age to age, some becoming extinct, while others burst into activity.
It is moreover probable that terrestrial magnetism may be owing, to a certain extent, to the earth's rotation. Mr. Faraday has proved that all the phenomena of revolving plates may be produced by the inductive action of the earth's magnetism alone. If a copper plate be connected with a galvanometer by two copper wires, one from the centre and another from the circumference, in order to collect and convey the electricity, it is found that, when the plate revolves in a plane passing through the line of the dip, the galvanometer is not affected; but as soon as the
plate is inclined to that plane, electricity begins to be developed by its rotation; it becomes more powerful as the inclination increases, and arrives at a maximum when the plate revolves at right angles to the line of the dip. When the revolution is in the same direction with that of the hands of a watch, the current of electricity flows from its centre to the circumference; and when the rotation is in the opposite direction, the current sets the contrary way. The greatest deviation of the galvanometer, amounted to 50% or 60°, when the direction of the rotation was accommodated to the oscillations of the needle. Thus a copper plate, revolving in a plane at right angles to the line of the dip, forms a new electrical machine, differing from the common plate-glass machine, by the material of which it is composed being the most perfect non-conductor; besides, insulation, which is essential in the glass machine, is fatal in the copper one. The quantity of electricity evolved by the metal does not appear to be inferior to that developed by the glass, though very different in intensity.
From the experiments of Mr. Faraday, and also from theory, it is possible that the rotation of the earth may produce electric currents in its own mass. In that case, they would flow superficially in the meridians, and if collectors could be applied at the equator and poles, as in the revolving plate, negative electricity would be collected at the equator, and positive at the poles; but without some. thing equivalent to conductors to complete the circuit, these currents could not exist.
Since the motion, not only of metals but even of fluids, when under the influence of powerful magnets, evolves electricity, it is probable that the gulf stream may exert a sensible influence upon the forms of the lines of magnetic
variation, in consequence of electric currents moving across it, by the electro-magnetic induction of the earth. Even a ship passing over the surface of the water, in northern or southern latitudes, ought to have electric currents running directly across the line of her motion. Mr. Faraday observes, that such is the facility with which electricity is evolved by the earth's magnetism, that scarcely any piece of metal can be moved in contact with others without a development of it, and that consequently, among the arrangements of steam engines and metallic machinery, curious electro-magnetic combinations probably exist, which have never yet been noticed.
What magnetic properties the sun and planets may have, it is impossible to conjecture, although their rotation might lead us to infer that they are similar to the earth in this respect. According to the observations of MM. Biot and Gay-Lussac, during their aërostatic expedition, the magnetic action is not confined to the surface of the earth, but extends into space. A decrease in its intensity was perceptible, and as it most likely follows the ratio of the inverse square of the distance, it must extend indefinitely. It is probable that the moon has become highly magnetic by induction, in consequence of her proximity to the earth, and because her greatest diameter always points towards it. Should the magnetic, like the gravitating force, extend through space, the induction of the sun, moon, and planets must occasion perpetual vibrations in the intensity of terrestrial magnetism, by the continual changes in their relative positions.
In the brief sketch that has been given of the five kinds of electricity, those points of resemblance have been pointed out which are characteristic of one individual power; but
as many anomalies have been lately removed, and the identity of the different kinds placed beyond a doubt, by Mr. Faraday, it may be satisfactory to take a summary view of the various coincidences in their modes of action on which their identity has been so ably and completely established by that great electrician.
The points of comparison are attraction and repulsion at sensible distances, discharge from points through air, the heating power, magnetic influence, chemical decomposition, action on the human frame, and lastly the spark.
Attraction and repulsion at sensible distances, which are so eminently characteristic of ordinary electricity, and, in a lesser degree, also, of the voltaic and magnetic currents, have not been perceived in either the thermo or animal electricities, not on account of difference of kind, but entirely owing to inferiority in tension; for even the ordinary electricity, when much reduced in quantity and intensity, is incapable of exhibiting these phenomena.
Ordinary electricity is readily discharged from points through air, but Mr. Faraday found that no sensible effect took place from a battery consisting of 140 double plates, either through air or in the exhausted receiver of an airpump, the tests of the discharge being the electrometer and chemical action,-a circumstance entirely owing to the small degree of tension, for an enormous quantity of electricity is required to make these effects sensible, and for that reason they cannot be expected from the other kinds, which are much inferior in degree. Common electricity passes easily through rarefied and hot air, and also through flame. Mr. Faraday effected chemical decomposition and a deflection of the galvanometer by the transmission of voltaic electricity through heated air, and ob