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Similar results were obtained by the use of an iron wire, two feet long and one-twelfth of an inch in diameter. Five pairs of the nitric acid battery produced an increment of the thirty thousandth part of an inch; and when only one pair of the battery was employed, I had an increment very slightly less, viz., the thirty-three thousandth part of an inch.
This increment does not appear to depend upon the thickness of the bar; for an electro-magnet made of iron, three feet long and one inch square, was found to expand under the magnetic influence to nearly the same extent, compared with its length, as the wires did in the previous experiments.
I made some experiments in order to ascertain the law of the increment. Their results proved it to be very nearly proportional to the intensity of the magnetism and the length of the bar.
Trial was made whether any effect could be produced by using a copper wire, two feet long and about one-tenth of an inch in diameter ; but I need scarcely observe that the attempt was unattended with the slightest success.
A very good way of observing the above phenomena is to examine one end of an electro-magnet with a powerful microscope, while the other end is fixed. The increment is then observed to take place with extreme suddenness, as if it had been occasioned by a blow at the other extremity. The expansion, though very minute, is indeed 80 very rapid, that it may be felt by the touch ; and if the electromagnet be placed perpendicularly on a hard elastic body, such as glass, the ear can readily detect the fact that it makes a slight jump every time that contact is made with the battery.
When one end of the electro-magnet is applied to the ear, a distinct musical sound is heard every time that contact is made with, or broken from, the battery; another proof of the suddenness with which the particles of iron are disturbed.
With regard to the application of this new force to the movement of machinery, I have nothing favourable to advance. calculation on the basis of the modulus of elasticity of iron will shew that an electro-magnet, consisting of a bar of iron one inch square and three feet long, will exert a force of about ten pounds over the space of the twenty thousanth part of an inch, every time that contact with the voltaic battery is made or broken, providing the transmitted current is capable of saturating the iron. If, therefore, contact be made, and broken a hundred times per second, for an hour together, we shall have only fifteen pounds raised to the height of one foot. This force is, therefore, far too minute for the movement of machinery; and the duty per ound of zinc is vastly less than that of the common electro-magnetic engine.
We shall now examine the bearing of this new property of the electro-magnet on magnetic theory. We shall consider it in connection with the hypothesis of Ampère and Æpinus.
The theory of Ampère refers the phenomena of magnetism to the
attraction and repulsion of currents of electricity moving in the same or contrary directions. Fig. 1, plate 4, represents the section of six particles of a magnetized bar of iron, according to (perhaps) the best modification of that philosopher's theory. The black circles represent atoms of iron, and the shades around them represent atmospheres of electricity moving in planes at right angles to the axis of the magnet.
This theory affords a good explanation of most cases of magnetic attraction. But the physical conditions which are demanded by it are impossible, and contrary to the analogy of nature, for it must necessarily suppose motion, or at least an active force to be continued against antagonist forces, for an indefinite length of time, without loss, in order to explain the phenomena exhibited by a hard steel magnet.
The only way in which it can account for the fact that iron, after receiving a certain quantity of magnetism, is incapacitated from receiving a further supply, or becomes saturated, is by supposing that the electricity which revolves around each atom of iron has a centrifugal tendency. The velocity of the electric currents around the atoms of iron will tend to be proportional to the inductive influence which urges them, and if the electricity be not endowed with centrifugal force, it is difficult to say why it should refuse to travel beyond a certain velocity; and in that case the phenomenon of saturation is left unexplained. If, however, the momentum of electricity, and its consequent centrifugal tendency when rotated, be supposed to exist, the currents will be prevented from going beyond a certain velocity by their interference with one another.
These principles are, however, less successful in accounting for the increase of length which we have noticed in a bar of iron when under the magnetic influence ; for as the electricity is supposed to revolve in a plane at right angles to the axis of the bar, the divergence of the fluid from each atom of iron by centrifugal force would have the effect of shortening the mass of iron, which is directly opposed to experience.
If we now turn to a corrected theory* of Æpinus, we shall see that it explains very well, facts which are unaccounted for by the former theory.
Let the black circles in Fig 2 represent six atoms of iron ; the shades around them atmospheres of magnetism; and the white rings over these still rarer atmospheres of electricity. Further, let the space between the atoms be supposed to be filled with calorific ether in a state of vibration, or otherwise, to be occupied with the oscillations of the atoms themselves. Such a state of things will probably give a good idea of part of a bar of unmagnetized steel or iron. Now, if an inductive influence be applied to the atoms represented in Fig. 2, the magnetism is supposed to accumu
See Dr. Roget's Treatise on Magnetism (s. 133).
late on one side of the atoms of iron, as represented by Fig. 3, and the bar is rendered magnetic.
Such a theory seems to me to afford a natural and complete expression of facts. It supposes nothing which we cannot readily comprehend, except the existence and elementary properties of matter, which are necessarily assumed by every theory, and which the Great Creator has placed utterly beyond the grasp of the human understanding.
When all the magnetism of each atom of iron is accumulated to one side, such atom may be said to be saturated with magnetism.
It is obvious, that when the magnetism is accumulating at one side of each particle, the bar will increase in length in the direction of its polarity, and decrease in a direction at right angles to this. The former fact we have proved by experiment, and there can be little doubt that a very delicate apparatus would exhibit the diminution of the thickness of a bar of iron in consequence of the communication of magnetic virtue.
Our theory will also account for the fact, that at a certain degree of heat all the magnetic power of iron is destroyed. I have before observed that the space left between the magnetic particles in Fig. 3 represents the room taken up by their vibration. This vibration is called heat, and will, of course, increase in violence and extent with the increase of the temperature of the bar. Now, it is natural to suppose that the atoms of iron have far greater weight and inertia than the atmospheres of magnetism and electricity which surround them; therefore these atmospheres will be in a state of vibration, while the atoms of iron remain in a state of comparative quiescence; and when their vibration has reached a certain extent, the inductive influence will not be able to arrange the magnetism in any
definite direction with regard to the atoms of iron.
The retentive power may be explained by supposing the magnetism to adhere to the atoms of iron to a certain extent. And if we make another supposition, viz., that an atom of iron, on combining with an atom of carbon, loses its attraction for magnetism on that side which is next the carbon, the superior retentive power of steel in comparison with that of iron is explained.
Ladies and gentlemen, I have had the honour of laying before you a theory of magnetism, first originated by Æpinus, and somewhat modified by myself. In my opinion it affords a simple and comprehensive expression of facts, which is the chief value of any theory. Whether or not such a state of things as I have assumed really exists in nature, it will require further experiments to demonstrate.
MR. Sturgeon's Researches in Electricity, Magnetism, fc.
(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 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 syge tem 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 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.
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 ends 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