The

without the trouble of covering them with thread or varnish. ends of the wires project about two feet, for the convenience of connection.

With one of my small cylindrical batteries, exposing about 150 square inches of total surface, this electro-magnet supports 400 pounds. I have tried it with a larger battery, but its energies do not appear to be so materially exalted as might have been expected by increasing the extent of galvanic surface. Much depends upon a proper acid solution: good nitric or nitrous acid, with about six or eight times its quantity of water, answers very well. With a new battery of the above dimensions, and a strong solution of salt and water, at a temperature of about 190° Fahr., the electro-magnet supported between seventy and eighty pounds, when the first seventeen coils only were in the circuit. With the three exterior coils alone in the circuit, it would just support the lifter, or cross piece. When the temperature of the solution was between 40° and 50°, the magnetic force excited was comparatively very feeble. With the innermost coil alone, and a strong acid solution, this electromagnet supports about 100 pounds: with the four innermost wires, about 250 pounds. It improves in power with every additional coil until about the twelfth, but not perceptibly any further; therefore the remaining eight coils appear to be entirely useless, although the last three of them, independent of the innermost seventeen, and at the distance of half an inch from the iron, produce in it a lifting power of seventy-five pounds.

It is evident from these results, that the exciting power of this miniature battery becomes improved by multiplying the number of conducting wires as far as twelve at least, although the greater part of them be at some considerable distance from the iron; and it is highly probable, although the experiments which I have made on this bar do not satisfactorily prove the fact, that by employing a larger galvanic surface, a much further addition to the number of conducting wires may be advantageously introduced into the circuit, for the excitation of the magnetic energies of soft iron. Perhaps the best arrangement would be to have a separate small battery to each wire.

Mr. Marsh has fitted up a bar of iron much larger than mine, with a similar distribution of the conducting wires to that devised, and so successfully employed by Professor Henry. Mr. Marsh's electro-magnet will support about 560 pounds when excited by a galvanic battery similar to mine. These two, I believe, are the most powerful electro-magnets yet produced in this country.

A small electro-magnet, which I also employed on the lecture - table, and the manner of its suspension, is represented by Fig. 3. The magnet is of cylindric rod iron, and weighs four ounces: its poles are about a quarter of an inch asunder. It is. furnished with six coils of wire, in the same manner as the large electro-magnet before described, and will support upwards of fifty pounds.

Fig. 3.

A

I find a triangular gin very convenient for the suspension of the magnet in these experiments. A stage A A, of thin board, supporting two wooden dishes, is fastened at a proper height to two of the legs of the gin. Mercury is placed in these vessels, and the dependent amalgamated extremities of the conducting wires dip into it; one into each portion. The vessels are sufficiently wide to admit of considerable play for the wires in the mercury without interruption of contact, which is sometimes occasioned by the swinging of the magnet and attached weight; the circuit is completed by other wires, which connect the battery with these two portions of mercury. When the weight is supported as in the figure, if an interruption be made by removing either of the connecting wires, the weight instantaneously drops on the table. The large magnet I suspend in the same way on a larger gin; the weights which it supports are placed one after another on a square board, suspended by means of a cord at each corner from a hook in the cross piece, which joins the poles of the magnet.

With a new battery, and a solution of salt and water at a temperature of 190° Fahr., the small electro-magnet supports sixteen pounds.

I noticed in some of my earliest experiments, that a bar of soft iron which had been intensely magnetized by galvanic action, retained a considerable degree of polarity when the exciting cause had been long removed, a phenomenon now more conspicuously displayed by the employment of larger masses; nor does it appear to be an easy matter to subdue entirely this residual polarity. The poles may be reversed as frequently as we please, but still some, and frequently a considerable degree of polarity remains unneutralized. If the cross piece be permitted to remain attached to the poles when the galvanic connection is broken, the residual polarity will still keep them

together with an astonishing force; as is manifest by the very great weight which the magnet continues to support before the cross piece is disengaged from its poles. A residual polarity, however, still remains; but so enfeebled are its energies by the slightest interruption of polar contact, that the cross piece will seldom be supported a second time without a renewed excitation by the battery.

The vigorous residuum of polarity which retains the cross piece to the magnet, arises from a continued mutual attraction between the two; for whilst the battery operates upon the magnet, and excites it to action, the latter in its turn also excites the magnetism of the iron connecting its poles, which iron becomes as decidedly polar as the magnet itself; a north pole being determined in that end of it which is in connection with the south pole of the magnet, and a south pole in the other end which is connected with the north pole of the magnet. The four magnetic poles thus brought into play, and in vigorous operation on each other, will, if not separated, retain their positions at the points of connection, even though the first exciting cause be entirely withdrawn. But if the connection of the two pieces of iron be in the least interrupted, their magnetism immediately recedes from the extremities, and becomes equally distributed in the metal; the vigour of the poles vanishes, and their magnetism becomes totally incapable of keeping them attached to each other.

Precisely the same kind of reasoning will explain the cause of that well known deterioration of magnetic force which invariably takes place by removing the cross piece from a highly excited steel magnet. The excitation in this case, whether it be performed by the operation of a magnet, or by the more gradual process of adding small weights to those already suspended, is carried on whilst the poles and the cross piece are in contact; and both become excited at the same time. The magnet and cross piece now operate on each other with a gradually increasing vigour; the poles of the one becoming more and more energetic as the power of the opposing poles of the other become further exalted. And this reciprocal

increase of action will be exercised on each other till a maximum of mutual attraction is obtained, at which time the magnet will support its greatest load. If more weight be added, the mutual attractive forces of the magnet and cross piece will be overpowered, and the two will separate. The magnetism of the iron no longer displays polarity, and that of the steel partially recedes from its extremities; but in consequence of the character of the metal, a considerable polarity is still displayed by the magnet: its energies, however, are very much diminished, and when the cross piece is replaced at the poles, the effort to stimulate it to polarity is proportionably diminished. The reciprocal attractions now operate with impaired forces, and consequently the load which can be supported is much less than before.

A compound steel magnet in my possession weighs nine pounds, which, when well magnetized, will support 120 pounds; but if

weights be added till the cross piece falls off, the power is reduced to about seventy-five pounds.

Fig. 4 is a representation of an electro-magnetic sphere, mounted on a mahogany frame, consisting of a stout base board and two upright pillars, to the upper extremities of which is fixed a cross piece, or stage. On the centre of the stage, and directly over the sphere, is placed a dipping needle; and near to the extremities are inserted the lower ends of two wires, bent twice at right angles, as seen in the figure; the upper extremities of these wires are finely pointed, and support two horizontal needles.

The sphere N S is a cast-iron shell, about eight inches in diameter, and weighs sixty-eight pounds. Its surface is divided into three parts, which, for convenience, may be called tropical and polar, having one of the former and two of the latter. The tropical region Fig: 4.

N

B

K

S

is covered with four coils of copper wire in separate strata, insulated from each other, and also from the metallic sphere, by means of slips of silk; the exterior coil, being uncovered, is seen in the figure. The coils are prevented from slipping towards the poles by means of stout iron rings soldered to the sphere, one at each tropical circle.

The two sets of extremities of the conducting wires are soldered, one to each of the two copper discs or wheels B B, through the centres

of which the extremities of the horizontal axis of the sphere pass. The lower edges of the wheels dip into portions of mercury placed in two semicircular vessels c c, which slide into the side of the pillars a little below the holes which receive the axis of the sphere.

By this arrangement the extremities of the wires which encircle the globe will constantly be in connection with the mercury in the two cups, in whatever position the poles may be placed. The connections with the battery are accomplished by two wires joining its plates, and the two portions of mercury in the vessels c c.

If this apparatus be so placed, before the battery be attached, that the needles stand at right angles to the vertical plane of the frame, and the dip of the central needle is counterbalanced, they will be arranged nearly parallel to each other, as their distance prevents them being affected by the ball.

If the axis joining the poles of the sphere be now brought into a horizontal position, by turning the key K, which enters the horizontal axle, the equator will be directly under the dipping needle. With this position of the apparatus, make the galvanic connections so that the pole of the sphere which faces the north may assume north polarity, or that species of polarity which is natural to the northern regions of the earth. The two horizontal needles will deviate from their former positions, having their north ends drawn towards the sphere the central needle, being placed on the equator will remain undisturbed. If the north pole of the sphere be now gradually turned upwards, the horizontal needles will be directed still more towards this pole, and the north end of the central needle will incline. The angles of deviation and dip will increase more and more as the north pole of the sphere advances towards the zenith; and when it has arrived at that point, those angles will have arrived at their maximum. The central needle will stand vertically, exhibiting a dip of 90° over the pole; and the two horizontal needles will point almost directly towards it. By permitting the sphere to resume its first position as gradually as it left it, the needles which are influenced by it will be observed to recede as gradually to their former positions.

If now the galvanic connections be reversed, the whole of the needles become reversed also, because of a change in the polarity of the sphere; and the dipping needle, after a few oscillations, will settle in a horizontal position.

By experimenting in this way with various positions of the sphere, it will be found to operate on the needle in a manner highly imitative of the earth's magnetism on different parts of its surface.

The two polar regions of the sphere exhibit a diffused polarity, the centres of which are nearly, perhaps exactly, in the poles of its equator. There are, however, in the polar regions several points which exhibit distinct polarity; and although of the same character as the general pole in which they are situated, will draw a delicate needle held near to them from the direction which it takes when