Abbildungen der Seite
PDF
EPUB

MR. STURGEON's Researches in Electricity, Magnetism, &c. (Continued from page 88.)

To the Editors of the Philosophical Magazine and Journal. Gentlemen,-In no experiment hitherto recorded in the whole history of electro-magnetic rotations, do we find employed, at the same time, both poles of the straight bar magnet. Hence all the rotating phænomena yet elicited, by employing that magnet, are only such as obtain by the reciprocal action of one of its poles and a portion of the conducting wire (under various forms) joining the copper and zinc sides of the battery. And when the poles of the magnet, or the direction of the force in the wire is changed, the results of the experiments in every case appear to be reversed. And in explanation, these, as well as every electro-magnetic phænomenon, are generally considered to proceed from a reciprocal disposition of the wire and magnet to place themselves at right angles to each other, for, when so situated, and at, or near, the centre of the latter, nothing further can be obtained; for both wire and magnet, when free to move, will, in this position, remain at rest.

Convinced as it were that the rotations are in different directions on the opposite poles when employed singly, it would seem as if experimenters had been fully persuaded of the impossibility of obtaining rotations by bringing into action, at the same time, both poles of the above-named magnet; or, whatever else may have prevented them, we have never yet heard of anything of the kind being attempted.

By referring the reader to a postscript in my last paper on thermo-rotations, and which appeared in your journal for April last, it will be there seen that a rotation had been obtained under the above circumstances; and that I had at that time rotated the wire or system of wires in the form of a sphere around, and by the influence of, both poles of an internal magnet.'

*

This experiment, it was observed, first suggested itself on reading Dr. Halley's theory of terrestrial magnetism, and was instituted with a view not only to be somewhat imitative of that philosopher's hypothesis, but likewise to serve in part to exhibit the earth's rotative motion on its axis.

It is by no means intended from this experiment to assert, that the rotatory motion of the earth and planets is really the effect of electro-magnetism, or that the variation of the compass-needle depends upon the theory alluded to; but merely to detail an experimental fact as exhibited by the apparatus in its present imperfect state, the mechanism of which confines the experiment greatly within the limits it might have been extended to. However, it already proves that a galvanized sphere, when free to move and containing within it a magnetic nucleus or kernel, will rotate by the influence of that central magnet; and that also, were the magnet free to move, • See page 88 of this volume.

a probability is manifested that it would rotate at the same time, i. e. they would both rotate at the same time by the reciprocal action of each other.

Having during my leisure hours been busily employed in fitting up an apparatus for other new experiments, I have not, till now, had an opportunity to describe that by which this is made. The experiment, however, has been witnessed during that time by gentlemen eminent in this branch of philosophy, and who have uniformly acknowledged it to be an imposing and pleasing spectacle. It is now offered, with some others which it is presumed are the first of the kind, and which possibly may not be thought uninteresting to some of your readers.

Description of the Apparatus.

Fig. 1 is an elevation of the apparatus; nNn, sSs, a brass cylindrical tube containing seven bar magnets, each of which is eight inches long. These magnets are placed in a frame in the lower part of the cylinder, the upper end of which, being moveable, can be taken off at any time to change their position for the purpose of varying the experiment. Fig. 2 is a transverse section of the cylinder with its magnets.

[merged small][merged small][merged small][merged small][merged small][merged small][graphic][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

As the like poles of the magnets are all placed in the same end of the tube when used, the latter, with its contents, may be considered as a compound cylindrical magnet. At the lower end S of the cylinder is a stout brass stud T, which stands firm in a socket on the top of the foot A. Around this socket is a cell for the purpose of holding quicksilver. Another cell eee fits gently on the outside of the cylinder, and is suspended over its top part by a stout brass wire

ww, the extremities of which communicate with the mercury in the cell eee. Another connecting wire ff communicates with the mercury in both cells. Cc Cc is a copper vessel made in the form of an Ampère's rotating cylinder, seven inches diameter; in this vessel is placed a cylinder of zinc, as represented by the dotted lines zz. On the outer rim, and at 90° from each other, are soldered four brass studs; to the extremities of which are soldered two brass wire circles (one only of which is shewn in the figure) that cross each other at right angles in the pole P; from which junction descends a pivot that runs in a hole made to receive it in the upper part of the connecting wire ww. The two circles are each 9 inches diameter, and form two great circles of the sphere they are intended to represent. The lower parts of the circles are left open, and the points of the wires bent downwards, till they come in contact with the mercury in the lower cell. Two other wires proceed from the upper edge of the zinc cylinder, and terminate in the mercury in the upper cell eee, as shewn in the figure.

Things being thus arranged, when dilute acid is poured into the copper vessel, the galvanic action takes place throughout every part of the great circles which represent the sphere; ascending from the copper in the upper hemisphere HH from the equator EE to the pole P; and descending in the lower hemisphere h h from the equator to the mercury in the lower cell, which is the place of the other pole of the sphere. From the poles it is transmitted to the mercury in the upper cell eee, by the conducting wires ww and ff; and both circuits are completed by the wires from the zinc cylinder terminating in this cell. Thus two galvanic circuits are formed by one apparatus; and hence likewise is solved the apparent anomalous phænomena of galvanized wires or cylinders rotating in different directions on the opposite poles of a magnet. This delusive appearance is completely relative, and similar to our notions of up and down; but with respect to the magnet, and in reality, the rotation is the same on both poles: for example, let any person turn first towards the north, and then towards the south; by observing the heavenly bodies, they would appear to revolve to the left in the former, and to the right in the latter case; yet the rotative motion of the earth (which is the cause of their apparent motion) is constantly the same way. In this experiment both poles of the magnet are acted on at the same time by similar galvanized wires, and the machine rotates by the joint influence of the former on the excited wires of the latter. Owing, however, to the weight and size of the machine, the rotation is but slow at the beginning of the experiment, but soon acquires considerable velocity, and the generating force, added to the acquired momentum, keeps it in motion for a length of time.

Well aware of the difficulties that would arise at the present day in any endeavour to support an hypothesis of the earth's being hollow, and containing within it a spherical magnet, as was the opinion of Dr. Halley, I have confined myself to a detail of the experi

ment only, without obtruding either remark or opinion. The success of this experiment, however, so satisfactorily confirmed my expectations, that the principle upon which it was made could hardly fail to intimate its applicability to others of a like nature; one of which being intimately connected with that I have just described, this may, perhaps, be considered no improper place for its detail.

The experiment for rotating the magnet on its axis, it is well known, was first made by M. Ampère; but the manner in which as yet it has been exhibited, like all the other similar experiments, shews the action only on one-half of the magnet at one time;-and by reversing the poles (everything else remaining as before) it appears to rotate in a contrary direction.

I hope, however, that I have satisfactorily proved that this apparent contrariety of rotation is merely relative, and that the real motion is the same in both cases. If possible, however, the experiment I am about to describe will confirm this property in a still more satisfactory manner.

The rotation of the magnet on its axis by subjecting both poles at the same time to the influence of similar electric currents.

Fig. 3 is an elevation of the apparatus; and, with the exception of the magnet NS, is entirely of brass-work. A is a round foot, from which rise the two cheeks BB, for the purpose of supporting the annular cell qq holding quicksilver; and likewise the cross piece R, which is screwed tight down by the knobs k k, and thus the apparatus is kept firm and steady. The magnet passes through the opening in the centre of the cell qq, and communicates with the mercury there contained by means of a wire soldered to its equator, and at right angles to its axis. C The poles are furnished with pivots; the lower of which at S runs in a small cup on the top of the foot A; and the upper

B

3.

R

9

A

one at N runs in the lower extremity of a screw nail passing through the cross piece R, and whose head is made into the form of a cup for holding mercury. ZC are two wires proceeding respectively from the zinc and copper side of a battery: the former communicating with the mercury in the cell qq, and consequently with the equator of the magnet; the latter communicates with the upper pole N in the manner shewn in the figure. Thus, one-half of the magnet forms part of the galvanic circuit. The other half, from its equator to the lower pole S, forms likewise part of another similar galvanic circuit, by means of the wires Z'C' which proceed respec

tively from the zinc and copper sides of another battery, and communicate with the equator and pole in the same manner as the former. By subjecting the magnet in this manner to the influence of like electric currents, it is rotated with an astonishing velocity, but which may in a moment be retarded by interrupting the circuit of either battery; and again accelerated by renewing the contact. This variation of the experiment proves to demonstration the utility of employing both poles at the same time, and is another proof of the rotations on opposite poles, in the old experiments, being merely relative; for the poles of the magnet are here both connected with the copper side, and the equator with the zinc side of the respective batteries. Another advantage in the manner of making this experiment is, that the glass vessel and mercury for floating the magnet are here not used; therefore the whole of the magnet is in view; whereas in the old mode, only a small portion of the magnet, about the thickness of a quill, was visible above the surface of the mercury. It can be no small gratification to those who are in the habit of giving public lectures, to be enabled to exhibit this experiment to the satisfaction of a large audience; for as the lecturer can now have his rotating magnet of almost any size he pleases, and likewise of any figure, this interesting experiment may be viewed from the remotest part of the lecture-room. Another inconvenience I have almost entirely removed both in making this and every other electro-magnetic experiment. My batteries are of such a peculiar construction as not to annoy the experimenter by the escape of hydrogen; neither is the expense of making the experiments more than one-fortieth of any other method yet made public; yet the apparatus in general is of large dimensions; for instance, the sphere in the former experiment is 9 inches diameter; and the magnet in the latter is 8 inches long.

I should now proceed to the description of other new experiments, were I not confident that I have already intruded upon your valuable pages. That task must therefore be deferred for the present. Some of the minor of those experiments are,--The rotation of the cylinder by the influence of an external magnet;-Ampère's cylinders rotated independent of each other's weight;-Thermo-rotation on both poles of the magnet;-Electro-magnetic bells, &c.

In summing up the results of the two detailed experiments, it appears from the first,

That similiar electrized wires rotate in the same direction round both poles of the magnet;

That both the copper and zinc cylinders are here carried round the magnet in the same direction;

That a sphere, conducting similar currents of electricity from its equator to its poles, will rotate by the influence of an internal magnet.

And from the second it is evident that, had the magnet in the first experiment been free to move, it would likewise have rotated

« ZurückWeiter »