Another ingenious contrivance, invented by Mr. Barlow, is represented in Fig. 5, where A B Is a rectangular piece of hard wood, C D a wooden pil lar, D E F a piece of stout brass or copper wire, ab a somewhat smaller bent wire, soldered to it at F, through the legs of which passes the axis of a wheel \V, of thin copper, h f is a small reservoir for mer cury, and g i a narrow channel running into it. H is a strong horse shoe magnet. Mercury being now poured into the reservoir h f till the tips of the wheel are slightly immersed in it, and the surface covered with weak dilute nitric acid, let the con nexion with the battery be made at i and D, and the wheel will immediately begin to rotate. If the current or the magnet be inverted, the motion of the wheel will also be reversed. In order to under stand this experiment, it must be remarked, that each radius of the wheel which touches the mer cury, is a part of the uniting conductor, of which one side is repelled by the austral, the other by the boreal pole of the magnet; thus it must either tend to raise or depress each of these radii.
Sir II. Davy has exhibited the rotation of a con ductor by means of mercury. When in a shallow non-conducting vessel containing mercury, the con ductors of a powerful galvanical arrangement are plunged at some distance from the sides, and one of the poles of a strong magnet is brought from be low to the bottom of the vessel, near one of the conductors, the mercury round this conductor will form a vortex about it. The directions of the mo tions are always according to the poles and conduc tors in action, such as the fundamental law indicates.
When a movable part of the uniting wire is pla ced in the direction of the dipping needle, it can not be put in motion by the magnetism of the earth: but when it is placed in another plane, though un der the same inclination, it is put in motion. Pro fessor Pohl at Berlin, has invented an apparatus, represented in Plate DXXIII. Fig. 6, exhibiting this phenomenon. AB is a piece of board, sup ported by screws, by means of which it can be le velled. CD is a wooden pillar, whose superior part is immovable, and has on its top an agate, which serves to support a steel-point, whereupon rests a wire EF, balanced by a counter-weight G. At E is a cavity containing a drop of mercury, by means of which one of the conductors, whereof only a part, II, here is represented, may be made to communi cate with the movable wire. JKL is a circular channel containing mercury, which can be put in communication with the galvanic apparatus through a conductor at M. When a powerful electric cur rent is transmitted through the apparatus, EF can only rest in the position of the dipping needle; in all others, it moves until it arrives at that position, which it nevertheless will leave by the motion al ready obtained. Hence it must still continue to
turn, when it is not stopped, to the position in which it is possible for it to rest. ___ Mutual Action of Electrical Currents.
Mr. Ampere found, soon after the discovery of that two conductors attract each other, when they are transmitting electrical currents of the same direction, but that they repel each other when the currents have opposite directions.
The movable conductor, represented in Plate DXXII. Fig. 16, and already described, may be employed to prove this by experiment. As the current which passes through the movable wire ABCDEFGH, has in CD the opposite direction of that in FE, the same uniting wire, which attracts one of these, will repel the other. This experiment may be exhibited in various shapes; but it does not ap pear that any experiment which could not be made by this simple apparatus is necessary for confirm ing the law above mentioned.
This law may easily be deduced from the funda mental law of electromagnetism, as may be seen by Plate CXXIll. Fig. 7, which represents two parallel currents of equal direction, and expressed by the same signs of which we have made use in the preceding pages of this article. It is here evi dent, that the boreal magnetism at b meets with the austral at a, and that the austral at a meets with the boreal at A, thus the effect must be attraction. In Fig. 8, two currents of opposite directions are represented, where the boreal magnetism at b meets with that at 12, and the austral magnetism at a with the similar at a: which must produce repulsion.
When the currents are not parallel, but form an angle, they attract each other when both are di rected either towards the apex or in the contrary way, but they repel each other when one of the two currents is directed towards the apex at the same time that the other goes off from it. Fig. 9. represents two currents which go from the apex. The boreal magnetism being in one of these directed from a to b; the austral magnetism in the other from to et, the result must be an attraction by which the con ductors, if one of them is movable, are brought to parallelism. The figure represents only one side of the conductors; but the opposite sides, having both their magnetical directions reversed, will like wise be attractive. It is also easily understood, that the opposite magnetical poles are directed against each other, and produce attraction when the current in both conductors goes towards the apex of the angle. Fig. 10. represents two currents hav ing opposite directions with respect to the apex of the angle. Here the similar poles in the magneti cal rotations are directed against each other, and therefore produce repulsion, such as to place both conductors in the opposite ends of one straight line, if one of them is movable.