Exp. The current of ethereal matter, which has been shewn to revolve about the earth, descends in its progress as before described, from the highest parts of the atmosphere, and hence ought, at much greater elevations than the one given as an instance, to produce the effects of magnetism.

PH. 199. Electrified magnets attract iron as when not electrified.

Exp. The diffusion of the electric fluid over the surface of the magnet does not destroy the arrangement of its superficial atoms, and hence does not destroy the magnetical effects.

PH. 200. When the magnet is exposed to a continuous current of electric fluid, it is considerably influenced by its action, which changes the direction of the needle, which, when not restrained by other causes, is found to take a position nearly at right angles to its course.

Exp. This is in exact accordance with all the explanations we have given in reference to magnetism, and it may be observed, that all these explanations agree so remarkably, as greatly to corroborate each other, but the particular effects of the mutual actions of electricity and magnetism belong to the next section, in which the subject will be investigated.

SECTION X.

ELECTRO-MAGNETISM.

THE branch of science, denominated Electro-magnetism, because it includes the consideration of many phenomena, in which electrical and magnetical effects are combined, will, with other noble achievements, remain a lasting monument of the industry, skill, and success of the philosophers, who grace the annals of modern times.

The honour of first exhibiting the leading facts, which belong to the class of phenomena comprehended under this designation, is due to M. Oersted, Professor of Natural Philosophy, and Secretary to the Royal Society of Copenhagen, who made the discovery of a marked and mutual action between the magnet and the wire which joins the extremities of the voltaic pile, when in action. The subject has been pursued, with laudable zeal, by the philosophers of Britain, and those of the Continent. Several

theories have already been proposed to account for the facts which experiment has developed, but we need none of them, and shall proceed to notice the principal phenomena, which have been well established.

The sixteen phenomena which are first explained, with the exception of the sixth, are taken from Demonfer rand's Manual of Electro-dynamics, translated by Mr. Cumming. The term Electro-dynamics has been preferred by some, as an appellation to denote this branch of science.

Mutual Action of Condutors.

PH. 1. If a slender copper wire, armed with steel points, and covered with silk, except at the steel points, and bent into the form abcde, fig. 95, have its two horizontal parallel branches, ab, ed, placed so as to float in two parallel channels of mercury, separated by a nonconductor, and the ends of the channels towards a and e be connected, the one with the positive, and the other with the negative end of a very powerful voltaic arrangement, the wire will move in the direction ab, ed, till it is stopped by the ends of the channels.-Demonferrand's Elect.-Dyn. by Cumming, sect. v. p 11.

Exp. This motion is produced by a cause similar to that by which the electrical fly moves; since there is a repulsion between the fluid leaving the mercury and entering the wire at one end, (prop. 14, cor. 9, sect. ii.), and the reverse at the other, both equally conspire to promote the same effect; hence, the wire moves as above stated. Steel points are most suitable for this purpose,

because the iron does not amalgamate with the mercury; hence there is not perfect contact, and the small spark passes as above noticed. The voltaic action must be powerful, and the wire small, otherwise the repulsion will not be sufficient to produce the motion of the wire in the mercury.

PH. 2. If two wires, sufficiently near to each other, be connected with the extremities of a voltaic apparatus, so that the electric current may pass through them both, and towards the same parts, the wires will attract each other; and if one, or both, have great freedom of motion, they will approach and adhere together.

Exp. Let AB, CD, fig. 87, be the two wires, in which the electric currents are moving from A to B, and from C to D; that is, let A and C communicate with the positive, and B and D with the negative end of the battery. Now, since an electric current is flowing along AB, A being positive, and B negative, the air on every side of AB will be affected to some distance, (prop. 27, sect. ii.), and the atmospherules of the air around it will be extended towards the negative end, (see ph. 62, sect. vii.), and the current of ethereal matter, which is continually passing in the atmosphere, (ph. 10, sect. ix.), will be directed the same way to some distance, in the neighbourhood of the wire, on all its sides. The same applies to the wire CD. Hence, the atmospherules of the atoms of the air between the wires will be most of all extended in the direction of the motion, and the current will flow most freely on the sides of the wires which face each other; hence, the entire action will be such, that the two currents will tend to become one; and, since by this means the resistance is in part removed from the interior sides of the wires, while it remains on the opposite sides, they ought to approach, and hence arises the

attraction; and this will evidently hold, whether the wires be parallel or not, provided they are not far distant,

PH. 3. If the two wires meet at any angle, as ABC, or ABC', fig. 88, and both the currents be moving toward the angular point B, or both from it, they will still attract each other; and if one, or both the wires have great freedom of motion, they will approach, and adhere, as in the case of parallel wires.

Exp. If both the currents flow towards B, they will come near enough to produce the effects stated in ph. 2, and hence the wires will approach, and the angle ABC, or ABC', will be diminished, consequently, the effect will be increased, because of the greater proximity of the wires when meeting in a smaller angle, therefore the wires will continue to approach till they coincide. If the currents flow from B towards A and C, the same reason applies, for still the atmospherules of the atoms of the air are elongated the same way by both wires, and the general current of ethereal matter always in the air, is here directed in the same way by both, so that the attraction equally takes place: or, if the air be removed, the general body of ethereal matter present, even in the best vacuum we can produce, will similarly contribute to cause this attraction.

PH. 4. If the two wires, as in ph. 2, are so connected with the battery, that the current in the one moves in the opposite direction to that in the other; such as would arise by connecting A and D, fig. 87, with the positive, and B and C with the negative end of the battery; the wires will repel each other; that is, if one or both be free to move, they will recede to a certain distance.

Exp. Here the currents of ethereal matter, along and in the space between the wires, are in opposite directions, and therefore tend to put such other ethereal matter, as is in their vicinity, into these opposite directions; hence,