The magnetism of the earth is likewise able to give a direction to the suspended wire. This direc tion must, in the northern hemisphere, be the same which is produced by a magnet placed below the wire with its austral pole above, and its magnetical axis put in the direction of the dipping needle ; which direction is the same as that which a mag netical needle should tend to give the wire if it were fixed below it, in the same position which the cur rent gives it. Thus the plane CD E F must be directed perpendicularly to the magnetical direc tion; when the current enters at A, the perpendicu lar part F E will be placed towards the west, but towards the east, if the current enters at H.
The same reasoning may be employed in all other cases where a movable uniting wire is exposed to the influence of terrestrial magnetism; for instance, when the wire is suspended in such a way as to permit the particles to move only in vertical planes. Plate DXXII. Fig. 17, represents an arrangement of this kind. A BCD is a wire, whose two ex tremities are wrapt round the ends of a thin axis of some non-conductor, and are terminated by two steel points, a and d, destined to be placed in two steel cups filled with mercury, and communicating with a galvanic apparatus. In order to give it the mobility necessary, it is nearly balanced by a coun terweight at E. When the axis is placed perpen dicularly to the direction of the magnetic needle, and the current enters at a, that is in the west, the plane A BCD will be driven out of its perpendi cular position, and deviate towards north: but if the current enters at d, the deviation will be austral. If the axis A D is placed in the direction of the magnetic needle, the deviation will, in the first case, be towards the west, in the last towards the east. The boreal pole of a magnet, placed below B C pro duces the same phenomena; the deviation goes al ways to the left of the current.
The principle of the multiplier has also been ap plied to the movable uniting wire. Fig. 18 repre sents one of these contrivances invented by Mr. Ampere, and somewhat modified by Professor Van de Ross. On the extremity A of the wire is a steel point, resting in a cup with mercury: B is a part of the wire, which forms spirals, fixed on a circu lar piece of pasteboard, through whose centre it passes at the last, and is prolonged to C, which dips in a cup of mercury. Another apparatus, like wise invented by Mr. Ampere, is represented in plate DXXIII. Fig. 1. The wire passes through a glass tube, from A to B, it is then wrapt around it, and being returned to the extremity A, passes also around C D, and being arrived at D is drawn through the tube, and descends finally to the infe rior cup.
Another apparatus of Mr. Ampere, improved by Mr. Marsh, destined to show the magnctical effect
of the earth upon the uniting wire, is represented Fig. 2; A B is a cup of glass nearly filled with a convenient liquid, containing a galvanical arrange ment, and kept swimming upon a liquid by a piece of cork; the uniting wire is like that of Fig. 1.
In the same manner as a magnet can be made to revolve round the uniting wire, so can a movable uniting wire be made to revolve round a magnet. Fig. 3 shows the principal parts of an apparatus for this experiment; C C C C is a glass cup having a hole through its foot, into which is inserted a cop per tube, soldered to a copper disc, cemented to the foot of the glass. The wire E F is also soldered to another copper disc upon which the glass rests; n s is a magnet inserted in the copper tube. The cup is filled with mercury. At a there is a sort of ball and socket joint, by means of which a wire ab is put in communication with the arm D II of a brass pillar: both the socket and the ball are amal gamated, and a piece of silk fixed to the ball or head of the wire, passes through a hole drilled in the arm D H, and by which the wire a b is sus pended, thereby preserving the contact, and leaving to the latter a perfect freedom of motion. When the current is established, the wire a b will revolve about the magnet. The directions of the rotations are such as the theory indicates.
We have seen that a magnet can be made to turn round its axis. An apparatus has likewise been contrived for producing the same phenomena in a movable uniting wire. For shortness sake we shall here omit the description of it, while we give the description of a very simple turning apparatus in vented by Mr. Ampere, and whereof a perpendicu lar section is exhibited in Fig. 4. AB CD and a b c d are two cylinders of copper, soldered to a bottom of copper, in such a manner that the space between the two cylinders is able to contain a liquid, but the interior cylinder is left open at both its ends. To a and b is soldered a bent copper wire, having a cavity at F. z z is a light cylinder of zinc, to which is also soldered a bent wire, in the middle E of which is a steel point resting in the cavity F, and consequently the cylinder z z will move upon its point of suspension. When the space between the two cylinders is filled with a convenient fluid conductor, an electrical current is established. Now, if a magnet N S is introduced into the cylin drical space of a b c d, the cylinder z z will begin to turn. When the north end (the austral pole) is upwards, the motion is from left to right of the observer, and the contrary with the magnet rever sed; all as it could be predicted from the fundamen tal law of electromagnetism.