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cate, however, with sufficient exactness for my present purpose, that the strength of a magnet, as compared with its weight, is very much less for large magnets than for small ones. It is not difficult to explain this general fact. When one bar of iron is magnetized by another, according to the laws of ordinary magnetic induction, the action is superficial. The most we can do is to bring the surfaces of the two bars into contact on one side. If the mass of iron is thick, the interior portions are so far removed from the inducing magnet, that they receive only a part of the magnetic development of which they are susceptible. This difficulty is removed, if we begin by dividing the thick bar into a number of thin pieces. These may be magnetized separately, and to saturation. When we come to reunite them, we encounter another difficulty. Each piece tends to induce a magnetic state, opposite to its own, in its neighbours ; a state, accordingly, which is opposite to that which its neighbours have already acquired. As soon as the pieces are brought together, a part of the magnetism, originally developed, becomes latent again ; and the united strength of all is not so great as the sum of the powers possessed by the parts when tried separately.
“ In 1820, it was discovered that the conducting-wire of a galvanic battery possessed magnetic properties, and was capable of inducing magnetism in a bar of soft iron placed at right angles to its own length.
This elementary force, first announced by Arago, was soon multiplied in a wonderful degree by the application of Schweigger's principle to it. At length large masses of iron, which defied the ordinary methods of touch, were magnetized to saturation. Magnets which acquire their magnetism from the induction of electricity (electro-magnets) have been made of such a size and power as to lift ten thousand pounds. Professor Henry gave an account, in Silliman's Journal, of an electro-magnet constructed by him, as early as 1831, which, weighing twenty-one pounds, was able to lift seven hundred and fifty pounds ; that is, more than thirty-five times its own weight. I frequently experiment with electromagnets of a half-pound weight, which lift one hundred and seventyfive times their own weight. With a good current, magnets no heavier than this may be made to lift five hundred times their own weight.
“At the present day, the old statical theory of magnetism has been supplanted by the electro-dynamical theory of Ampere. According to the views of this eminent physicist, all magnets are in one sense electro-magnets. In the electro-magnet properly so called, the inducing currents are obtained from a galvanic battery, and are made to flow through a wire which is wound many times around the iron to be magnetized. Each particle of the iron is supposed to contain, even in its unmagnetic state, currents of electricity, circulating around it. As the direction of these currents is not definite, the particle exhibits no magnetic polarity. As soon, however, as it is exposed to the action of the battery currents, these native currents of the iron assume a direction parallel to one another and to that of the currents in the external wire. When its currents are thus directed, the iron has the properties of a magnet. The magnetism of iron, however developed, consists simply in the magnetic properties of these electrical currents. These currents, moreover, are not created by the inducing agent, but only directed. They are always flowing, but not always in parallel directions. If we take a magnet, in which the currents are already direct. ed, and draw it over a piece of common iron, the currents of the latter are turned round so as to be parallel to those of the former. The only difference between a magnet made in this way, by ordinary touch, as it is called, and an electro-magnet, consists wholly in the source of the inducing and directing currents. In one case, we take them from a " battery ; in the other, we use those of a permanent magnet.
“ If this be so, the difference which is observed in the strength of an electro-magnet and an ordinary magnet must proceed from a cor. responding difference in the inducing currents. The battery currents have a greater magnetizing power than the currents of a well-magnetized bar of steel, either because they are stronger in themselves, or because they act from a more favorable position. Now, I have shown by direct experiment that the currents from the galvanic battery are not in themselves so abundant as those which are flowing around a piece of magnetized steel. We suspend a delicate magnetic needle and oscillate it, first in front of one extremity of a helix, through which a battery current is flowing, and then at the same distance from one pole of a steel magnet. From the rapidity of the oscillations we can easily calculate the relative magnetic forces of the helix and the steel bar. It will be necessary to eliminate that part of the motion which belongs to the earth's influence. This is done by oscillating the same needle, when removed from the action both of the helix and steel bar. From this experiment we learn that the battery currents are not nearly so magnetic, and therefore not so abundant, as those of the steel bar. A helix possesses directive power like a compass-needle, but much feebler than a very weak needle of steel, although the current from a
powerful battery flows in the helix. Two helices attract and repel as two magnets ; a single helix and a magnet attract and repel as two magnets. In both cases, particularly in the first, the action is much weaker than when we experiment on two magnets. In all the electrodynamical motions, it is well known that those are the weakest which are produced by the reciprocal action of currents alone, and that a great gain is effected when we substitute for one or both of the currents some kind of iron or steel magnet. Moreover, in electro-dynamical induction, the same superiority appears on the side of the currents of the steel magnets. The currents induced by such magnets are much stronger than those induced by a battery current. When the battery current flows around a piece of soft iron, making it an electro-magnet, then we have the best possible source for induced currents. From all these facts, many of which are familiar, I infer that the battery currents, although possessing a greater magnetizing power than those attached to a steel magnet, are, nevertheless, of less intrinsic energy.
“ Whence, then, the questioni recurs, does this superior efficiency of the weaker currents in imparting magnetism to iron proceed ? One cause, without doubt, is the favorable position in which the inducing currents act upon the dispersed currents of the unmagnetic iron. The superficial action of one piece of iron or steel upon another, to which we have already referred, when interpreted by the light of Ampere's theory, amounts to this. The inducing currents of the original magnet and the induced or directed currents of the other bar touch, like any two circles external to one another, only at a single point. If the circulation was around the whole mass of each bar, these circles must still rapidly separate from one another. When we add to this that the flow is about each single particle in each bar, it is obvious that the currents which direct and those which are directed are, for the most part, so remote from one another, and so oblique, as to act at a very great mechanical disadvantage. Moreover, the portions of the circuit which are opposite to the adjacent portions exert a contrary action to that of the latter, and diminish the small result which otherwise might be produced. In electro-magnetizing, the battery current flows wholly round the piece of iron to be magnetized. Throughout the whole circulation, every portion of it is near to at least some part of the iron, so as to act favorably upon it. When we magnetize iron by touch, we can, it is true, turn the different sides up and touch it on all of them. We might even make the original magnet hollow, and insert the bar to be magnetized inside of it, so that all parts should be touched at the same time. Even then the position, as will appear upon reflection, will not be so favorable as where the battery current flows around the bar of soft iron ; inasmuch as two concentric systems of small circles cannot be brought into such close proximity as one system of small circles concentric with a single large circle. We may select an experiment in which these comparatively weak currents from the battery no longer enjoy their favorable position, and then their weakness is plainly manifested. Let a hollow cylinder of iron be taken and placed outside of the helix, instead of inside. In this case, the battery current which flows in the helix, though intrinsically possessing the same mag. netic power as before, produces little or no effect on the external iron cylinder.
“ Another cause of the superiority of electro-magnets is connected with a peculiarity in the position of the poles of permanent steel mag. nets. These poles, if the bar has any considerable thickness, are at a little distance inside of the extremities of the magnetic axis. This displacement has been well explained by the interference of contiguous currents in different portions of the thickness of the bar ; their mutual action prevents the planes of motion around the individual particles of iron near the extremities from being strictly parallel to one another, or perpendicular to the magnetic axis. In the electro-magnet, the currents of the iron are maintained in a strictly parallel direction by the controlling and ever-present activity of the battery current, the direction of which is preserved uniform by the rigidity of the wire. Every one knows how rapidly the power of a magnet diminishes with the distance from its pole, and may understand, therefore, how much of force is lost if the pole is inside of the extremity of the bar, and inaccessible. The pole of the electro-magnet is at the extremity of the bar; we may bring the keeper into actual contact with it ; and for this reason, also, it must appear superior to ordinary magnets. In consequence of this difference in the position of the pole, or the seat of maximum force, the power of the electro-magnet diminishes as the square of the distance from the extremity, while that of the common magnet only diminishes at the same rate as the distance from the extremity increases; the law being, in both cases, that the force diminishes as the square of the distance from the pole increases."
Three hundred and thirteenth meeting,
December 6, 1848. — MONTHLY MEETING. The President in the chair.
The Corresponding Secretary read a letter from Professor Salisbury, of Yale College, accepting the fellowship of the Academy, which was conferred at the last quarterly meeting.
He also announced the donation of the first volume of the Smithsonian Contributions to Knowledge, accompanied by several documents stating the purposes of the Smithsonian Institution, and making certain inquiries respecting the library, &c., of the American Academy.
Hon. Nathan Hale was appointed a member of the Committee on Meteorology, to fill the vacancy left by the decease of Dr. Hale.
Mr. Everett announced that, on the 25th of November last, Mr. George P. Bond discovered a new comet, the seventh which he had discovered independently of other observers. In this instance, such was the velocity of the comet, and its position in respect to a star in the field of view, that Mr. Bond was able to see the comet actually move ; — this, it was said, had never before been a matter of direct observation in the case of any of the heavenly bodies.
Mr. Everett also announced the transmission of the King of Denmark's comet-medal to Miss Mitchell.
The committee on the distribution, &c., of the Academy's publications made a report upon the subject, and proposed the following resolves, which were adopted.
1. “ That the forthcoming volume of the Memoirs of the Academy (Vol. III., New Series) be furnished, on application, to Fellows of the Academy resident in the United States at such distance from Boston that they are not liable to the payment of annual dues, on the payment of three dollars per copy, and the preceding volumes at two dollars per copy.
2. “ That the fourth article of the third chapter of the Statutes be amended, so that it shall read as follows, viz. :— It shall be the duty of the Corresponding Secretary, with the advice and consent of the