board, on which are erected the two brass pillars p p, one of which supports the horse-shoe magnet M. The other carries a pair of parallel projecting arms, on the extremities of which the axis of a copper disc c is supported. To the lower end of the disc is attached a weight or bob to give it a vibrating tendency; so that by this arrangement the disc can be made to vibrate freely in its own plane between the arms of the apparatus. A quadrantal screen s e, of thin brass, is attached to the front arm, and consequently a quarter of the disc is hid behind it. When at rest, and in the position represented by Fig. 9, the quarter behind the screen is divided equally by a radial line drawn on its surface. When an experiment is to be made with this instrument, the bob is first to be brought to the point o of the arms, where there is a contrivance for retaining it in that position, or releasing it at pleasure. When in that position, the index line le appears from behind the screen. The first part of the experiment is made without the presence of the magnet M. When the trigger is pulled, the bob falls from the point o, and performs a certain number of vibrations before the total disappearance of the index line le. This number being noted, the bob is again brought to the drop at o. The magnet is now placed on the stage, with its poles close to the surface of the disc. (The figure represents the magnet in such a position that the disc may vibrate between its poles). The trigger is again pulled, and the vibrations are counted till the index line is again lost sight of behind the screen. With a thin copper disc, eight inches in diameter, vibrating between the poles of a horse-shoe magnet, the mean of six trials without the magnet, and of six with the magnet, were as below: Exp. 8.-Without the magnet... 160 vibrations. Exp. 9.-With a similar disc of zinc : Without the magnet...150 vibrations. It would appear by these two experiments, that copper is more affected by the presence of the magnet than zinc. This, however, is not always the case, for I have frequently found discs of zinc more affected than copper. There is also a difference even in the same kind of metal and since a great deal depends upon the power of the magnet, as well as upon the character of the metal, it is plain that in all experiments of this kind, especially when intended for comparing the results with different metals, the same magnet ought to be employed, and the experiments made whilst it has the same standard power. With a very powerful magnet I have frequently reduced the number of vibrations from 150 to 30, and in some cases still lower. Hearing that Messrs. Herschel and Babbage had obtained some curious results by cutting the revolving disc in several of its radii, I also made some experiments with a disc of copper similarly cut. Fig. 10, (Plate III), will represent the manner in which the disc was cut. An experiment was first made whilst the disc was whole, and the means of six trials were as below: Exp. 10. Without the magnet..150 vibrations. With the magnet ... 45 vibrations. The disc was now cut with a pair of scissors, as at No. 1 Fig 10, and an experiment made; next at No. 2, and another experiment made; and so on till nine slits were cut in the disc and in every case the vibrations were performed between the poles of a horseshoe magnet, as shown in Fig. 9. I also tried an annular disc of copper, the inner diameter of whicn was six inches, and the external diameter eight inches. This rim was cemented to a disc of pasteboard, and vibrated between the poles of a horse-shoe magnet. Exp. 12.-Without the magnet, 178 vibrations. With the magnets, 140 vibrations. The results of this experiment show that the central parts of copper discs are very much concerned, either in receiving or in transmitting the magnetic impressions; for those impressions, in whatever way they may operate, were much less efficient in this annular rim than in any of the former modifications of the disc. Exp. 13.-A disc of zinc was vibrated in this experiment; and the horse-shoe magnet, when employed, was laid on its edge on the stage, so that both poles were presented to the same side of the disc. Without the magnet, 120 vibrations. With the magnet, 90 vibrations. Two bar magnets were next employed, the disc first vibrating between their north poles, and afterwards between a north and a south pole. Exp. 14.-Between two north poles, 112 vibrations. Between a north and a south pole 80 vibrations. The bar magnets were next placed with both north poles on one side of the disc, and afterwards with both south poles on one side of it. Exp. 15.-With north poles 109 vibrations. With south poles 102 vibrations. I need not here remark, that no other experiments in this interesting inquiry have presented such an extraordinary discrepancy of results, as those I have just described. They prove in the most decisive manner that the energies of the magnet become curiously modified by every change of its position, with reference to the vibrating plate on which they are exercised; notwithstanding which, there does not appear to be any position in which it can be placed (provided it be sufficiently powerful), that would entirely neutralize its influence on the metal. The most favourable position in which the magnet can be placed for displaying its influence, appears to be that in which its north and south poles are presented to the opposite sides of the disc; and the position of the poles, which appears to be the least favourable for such a display, is when the poles that are presented to the opposite sides of it are of the same nature. This curious circumstance is very different from anything which I had observed in my experiments on discs of iron; for with that metal it had always appeared that when poles of the same name were presented to any point in the edge of the disc, the one above and the other below, the dipping needle invariably indicated the greatest polarity in the iron; and least of all, when the edge of the disc was placed between the poles of a horse-shoe magnet. Besides, the iron exhibits vigorous polarity whilst at rest; but not a trace of polar action could be detected in copper or zinc, unless those metals were in motion. The only opportunity then of discovering the distribution of polarity in them, was whilst they were in that condition, either vibrating or revolving on an axis. I began this tedious inquiry by suspending a magnetic needle near to a vibrating disc of copper, sometimes when the magnet was in one position, and sometimes when in another. The needle was evidently affected in whatever position the magnet was placed; but I soon found that it would oscillate with the plate whether the magnet was present or not. I might perhaps have expected that this would be the case, by taking into consideration that the needle itself would magnetize the disc. I observed, however, that the needle was not only more powerfully affected during the presence of the magnet, but that it would, in some positions, move in an opposite direction to that of the disc. Moreover, the deflections evidently varied by placing the needle on opposite sides, indicating something like a north polarity on one side of the disc, and a south polarity on the other side. Neither were these appearances uniform in all parts of the same side; for in some places there appeared a north polar action, and in others as decided a south polar action. It would be unecessary, however, to describe the various observations which I made by these first experiments; it will be sufficient to remark, that it was soon discovered that this mode of experimenting was by no means the best adapted for such an inquiry; for although this arrangement of the apparatus showed most decidedly that the magnetic force in the copper was distributed in a very peculiar manner, yet, for want of command over the vibratory motions of the disc and the needle, it was impossible to trace it with precision in the area of the metallic surface. I next placed a disc of copper horizontally, so that it could be oscillated or rotated in its own plane on a vertical axis; and by erecting a thin stage over it, a common compass needle could be placed over any part of its surface; and as the axis was connected with a multiplying wheel and band, a motion of any required velocity could be given to the disc. By this apparatus, most of my experiments were made; and I very soon fouud that no very great delicacy in the suspension of the needle was necessary, and that one mounted on a pivot, was much better adapted for the investigation than one suspended by a silken film. The investigation, however, was exceedingly tedious, and required the most rigid observation to reconcile the phenomena to any determined law, or to trace the various curves on the surface of the discs, over which the needle would deviate in any required direction with reference to these lines, by a constant standard direction in the motion of the disc. And what still further increased the difficulty, it was found that the distribution of the magnetic polarity varies with almost every difference in the dimensions of the plate. It also varies by the velocity, and again by the distance of the magnetic poles from the centre of motion; so that upon the whole, although some rule may be observed by any one arrangement, yet the same rule is not applicable in all cases. A few experiments will show in what manner the distribution of polarity in the surface of the disc may be ascertained. Exp. 16.-Let a copper disc of about eighteen inches in diameter be placed so as to be rotated in its own plane on a vertical axis; and let a horse-shoe magnet be placed with its south pole above, and its north pole below the edge of the disc, reaching about two inches beyond the edge towards the centre. Let a compass needle be placed on a stage directly over the centre of the disc; and by a proper arrangement of magnets, let its south pole be directed to the south pole of the magnet. Turn the wheel, and the needle will move in the direction of the plate, but will not perform a revolution. If the vibrations of the needle be attended with corresponding motions of the plate, it may be made to sweep half a circle. Exp. 17.-Let now the north pole of the needle be turned towards the south pole of the magnet, and again turn the wheel. In this case the needle will move in the opposite direction to that of the disc. Exp. 18.-Let the needle be placed over, and just within the left edge of the disc, and not more than 90° from the magnet, Fig. 11. Let it also be permitted to repose with its axis at right angles to the diameter over which its pivot is placed. Turn the disc in the direction indicated by the large exterior arrow, and the south pole will be deflected towards the edge of the disc. Reverse the revolving mo tion of the plate, and the south pole will be deflected towards the centre of it. Exp. 19.-Let the needle be placed below the plate, and in the same vertical plane as before. The deflections answering to the motions of the disc in this case will be opposite to those when the needle was above. In this way the needle may be placed opposite to various parts of the disc, and it will be found that the deflections vary in different places; and over some places no deflection will be observed by the motion of the disc in one direction, although a considerable deflection will be given by the motion being reversed. It would be very difficult to account for these extraordinary phenomena by any known laws of magnetics, and almost as difficult to reconcile them, with our present knowledge, to the laws of electromagnetism. When these experiments were first intended to be published, I had arranged them under the head of Polar Magnetic Streams; but I have since thought that the Distribution of Magnetic Polarity will be a much more appropriate term. It would, however, be no great stretch of the imagination to suppose a disturbance of the electric fluid by magnetic action, as it would be only a kind of reaction to that which takes place in electro-magnetism. If this be really the case (although I do not at present assert that it is so), the electric force would rush from the magnetic poles in the direction of the small arrows in Fig. 11, when the plate rotates in the direction indicated by the large exterior arrow. This force is the most energetic, on that side which is nearest the poles ; it becomes diffused in the other parts of the disc, especially if it be large, and attenuated so as to have scarcely any action on the needle on the extreme parts opposite to the magnet. It returns to the magnet again by various windings, and becomes more compact- in proportion to its approach. This is what the needle indicates to be taking place in the general surface of the disc; so that the deflections near to the left edge are different to those which are observed nearer to the centre on that side of the magnet. The force, therefore, appears as if it were first projected, or driven from the magnetic poles in an opposite direction to that in which the plate revolves, but soon divides itself into two distinct tides, which sweep the area of the plate, recurving to the poles again in opposite directions, as indicated by the two systems of arrows in Fig. 11. The line of greatest energy is the resultant of the two systems of forces emanating from the left side of the magnetic pole; and is a curve determined amongst the feathers of the ascending arrows. It branches off with the aggregate of each of the two recurving systems of force, returning near to the edge on the left hand, but more in the area of the plate on the right hand, side of the magnet. There are also neutral lines, or lines in which the needle would constantly be arranged by the operation of these forces, if unsolicited |