due to the permanency of electric currents. Electric currents may very possibly, either directly or indirectly, magnetize the terrestrial globe; but we have no reason whatever to believe that such currents are essential to give retention of polarity to steel.

The introduction of polar magnetic lines into the theory of electro-magnetism would simplify the explanation of the phenomena, and reduce them to the principles of magnetics; and experiments may be shown in both sciences which are favourable to such a conclusion, independently of any consideration that would reconcile to identity the electric and magnetic matter.

If it can be admitted as an universal maxim in nature, that when one species of matter is impregnated with, contains, or is charged with another, the charged body must necessarily be of a grosser texture than the substance with which it is charged, or that the latter should be more subtle than the former; then it is possible that the magnetic matter, which is the most subtle we are acquainted with in nature, may insinuate itself into the pores of the electric; and the latter become charged with the former, as decidedly, under some circumstances, as a piece of iron is naturally charged with them both.

I shall not, however, on the present occasion, advance further into speculative suppositions of this kind, which, however curious they may appear in themselves, are perhaps not of much interest in the present stage of our knowledge of physical operations.

In the positions which I have advanced, I have carefully avoided every consideration that could possibly embarrass the mind, or prevent them from being understood. They would virtually, however, have been but very little affected, by taking into account the magnetism of the metal as an intermediate agent in the process of excitation; but they are much simplified by omitting those remote laws, which would be better exhibited separately, and as a distinct class, which may be admitted, or rejected, at pleasure, without affecting the calculations of the experimenter.

The paper, of which the above theory formed a part, was originally published in the London and Edinburgh Phil. Mag., vol. ii. of the present series. The remote laws of electro-magnetism, here spoken of, will he found illustrated in the following article.

L.

Application of the preceding Theory, and of the laws of Electro-magnetism, to the explication of Phenomena. By WILLIAM STURGEON, Lecturer on Experimental Philosophy, at the Honourable East India Company's Military Seminary, Addiscombe, &c. &c.

1st. The most simple application of this theory is in the production of electric currents by the motions of conducting bodies in the vicinity of a bar magnet. Let O, fig. 68, plate IX. represent a transverse section of an endless metallic wire, situated in the magnetic atmosphere of the bar, N, S, whose polar lines are consequently some on one side, and some on the other, of that portion of the wire represented by the section O. If now, the wire O be made either to approach the bar, or to recede from it, it will have to pass through some of those exciting magnetic lines. Hence, by position 2, the electric fluid in the wire will be put into motion.

If the wire be made to approach the steel bar, it will then advance on those magnetic lines situated between them, and according to positions 6 and 7, the direction of its electric current will be from the spectator, looking at the figure, to behind the paper on which it is printed, and where the wire is supposed to be continued. But if the wire be made to recede from the magnetic bar, its electric fluid will be excited by the impressions of those polar magnetic lines which are exterior to it, and the current will flow towards the spectator, or in the opposite direction to the former current. It is obvious, however, that, although the currents thus produced will flow in directions the reverse of each other in the wire, and also with regard to the position of the magnetic bar, they still observe one and the same direction with reference to those magnetic lines which impel the electric fluid into motion. Precisely the same phenomena would be displayed if the wire were stationary, and the magnet put into motion.

Remark. By inspection of fig. 68, it will appear obvious that the currents will observe these directions in all cases where the advance and recession of the wire are between the extremities of the bar, and in a plane perpendicular to its axis.

2d. If the wire be kept perpendicular to the axis of the magnet and passed down the side of the latter from the upper extremity N, to its centre; the efficient magnetic lines will have the same relation to the wire, as those to the advancing wire in the first application; and the current will be from the spectator. But if the wire be continued in its downward

motion farther than the centre of the magnet, it will then advance on the efficient magnetic lines in the opposite direction to that whilst moving down the first half; and the current thus produced will be towards the spectator. By moving the wire in the opposite direction, or from the lower to the upper extremity of the magnet, then, because of its advancing upon the magnetic lines in precisely the same order as before whilst moving downwards from N to S, the currents thus produced, will observe the same directions whilst the wire passes the first and second halves of the magnet respectively. So that the current produced opposite the lower half will be from the spectator; that produced opposite the upper half will be towards the spectator. Hence this practical

Rule. If a wire be placed at the centre of a magnet, and at right angles to its axis: then if it be moved parallel to itself either towards the north or the south pole, the electric current produced in that wire will always observe one and the same direction. But if the wire be moved from either pole towards the centre of the magnet, the current produced will be in the opposite direction to the former: the directions of both currents being conformable to the law laid down in positions 6 and 7 of the theory. And as the same arrangement of magnetic lines is observed on every side of a magnet, the rule holds good in the motions of rings, or endless flat helices placed on the axial bar. Or, when those forms of conductors are stationary, by the motions of a magnet in their axes. In these cases the exciting impressions take place on every side of the ring, or helix.

3d. The same laws of excitation apply to the phenomena exhibited by the employment of a horse-shoe magnet as to those exhibited by that of a straight bar: and may easily be understood by an attention to what has been said respecting the latter kind. If, for instance, one portion of an endless wire were to be placed between the branches of the horse-shoe magnet represented by fig. 62, and perpendicular to its plane: then a motion of that part of the wire, parallel to itself, from the bend of the magnet towards its poles, would advance it nearly perpendicularly on those magnetic lines situated immediately between the branches: and also on those curved magnetic lines which are above and below the space between them, and whose poles are in the same direction; as may be understood by looking at fig. 63. Hence, by positions 6 and 7, the current produced in that part of the wire would be from the spectator looking at fig. 62. But if the wire were to be moved in the opposite direction, or from the poles towards the bend of the magnet, the current in the same part No 4, May, 1837.

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of the wire would be towards the spectator: both currents with this magnet observing the same laws as with the straight one: and invariably referrible to the polar positions of those magnetic lines on which the wire advances.

Imagine, now, the endless wire to be a ring which will move freely on either branch of the magnet, and its motions similar to those before described. Then, although the directions of the currents and motions of the ring will still observe the same relation to each other as before; each current, excepting in that part of the ring immediately between the branches of the magnet, will appear to change its direction, by the motions being made on this or that particular branch; as may be understood by looking at fig. 69, where the arrows indicate the directions of one and the same current in the ring when placed on different branches of the magnet. The imagination may possibly be assisted by considering the original ring to be split, or composed of two flat rims laid side by side, and susceptible of separation, excepting on one side, where they would still be held together. These two portions of the original ring being now placed on the poles of the magnet, one on each as in fig. 69, and moved from the poles towards the bend, would each carry a branch current from that excited in the whole part of the original ring situated more immediately between the magnetic poles. Behind the magnet, one of these branch currents would flow towards the spectator's right, and the other towards his left: whilst the main current between the branches of the magnet would flow directly from him. Reversing the motion of the wire would be attended with the usual vicissitudes in the direction of the current.

The apparent anomaly exhibited by these phenomena, which is a mere deception arising from the figure of the magnet requiring those parts of the ring not immediately between its branches to be placed towards the right of the spectator in one case, and towards his left in the other, has been productive of much mystery, wherever a perfect knowledge of the fundamental laws has been wanting. The same illusion is effected by operating with the poles of a bar magnet, provided they be placed in one and the same direction (say upwards) during the time they are employed.

All that has been said about rings apply to helices of every description.

By discovering a similar illusion in M. Ampère's beautiful experiment in which a magnet rotates on its axis, I was enabled to rotate a large magnetic bar, by causing two electric currents to traverse, each half its length, from its poles to its centre, or conversely; which currents, according to the views

previously taken of the nature of the action, ought to have counteracted each other's effect.

4th. Let the bar N, S, fig. 68, be a cylinder of soft iron, and O the section of a wire placed at right angles to it. If, now, the iron bar be converted into a magnet, similar to that represented by the figure, by the application of the poles of permanent magnets at its extremities, polar magnetic lines will start from its surface on every side; and those on the right side will advance upon the wire O, and thus give the exciting impressions, and cause an electric current in the wire, whose direction will be from the spectator (See positions 6 and 7.). But when the permanent magnetic poles are withdrawn from the iron cylinder N, S, the electric current in the section O will be reversed; being then caused by those magnetic lines exterior to the wire, which, by rushing towards their native bar, give the exciting impressions.

To prevent circumlocution, it may be convenient to call the first-mentioned motion of the magnetic lines, the magnetic distention, and the latter motion of those lines, the magnetic collapsion. The electric currents excited by either the expansion or collapsion will continue to flow during the whole time the magnetic lines are in motion; but will cease to exist when those lines have become stationary.

By considering O the section of one side of a ring, or of one convolution of an endless helical wire, placed on the iron bar; it will be easily understood that by the magnetic distention the exciting magnetic lines will advance upon the inner surfaces of those conductors, and give impressions on every side alike: and by the collapsion those lines will advance upon their outer surfaces; again giving the exciting impressions on every side. Hence, during either a distention or a collapsion, a ring receives more exciting impressions than a straight wire, and a helix more than a ring: whilst the same law holds good in every case.

Hitherto the application of the proximate laws of magneticelectricity appears exceedingly simple, whether the exciting magnetic lines be permanent or transient. But before we proceed to the application of those laws which are not so easily perceivable, it will be necessary that some explication of them be given in the simplest experimental process in which they are found to operate. By the proximate laws of magnetic electricity alone, we can account for the production and direction of an electric current in a helix enclosing an iron bar, by the conversion of that bar into a temporary magnet; but those laws do not furnish us with means sufficient to comprehend the converse fact, viz: That a similar current in