Oersted’s discovery
“Let the opposite poles of the galvanic apparatus be joined by a metallic wire…Let the rectilinear part of this wire be placed in a horizontal position over the magnetic needle duly suspended, and parallel to it…These things being thus arranged, the magnetic needle will be moved, and indeed, under that part of the joining wire which receives electricity most immediately from the negative end of the galvanic apparatus, will decline towards the west.” (Oersted’s pamphlet,”Experimenta circa efficaciam conflictus electrici in acum magneticam”, trans. by J.E. Kempe in Journal of Telegraph Engineers, Vol. 5, p459, 1876, qtd. in A Source Book in Physics, by William Magie, p438)
Ampere’s Memoir Proposing Galvanometer
“The ordinary electrometer indicates tension and the intensity of the tension; there was lacking an instrument which would enable us to recognize the presence of the electric current in a pile or a conductor and which would indicate the energy and the direction of it. This instrument now exists; all that is needed is that the pile, or any portion of the conductor, should be placed horizontally, approximately in the direction of the magnetic meridian, and that an apparatus similar to a compass, which, in fact, differs from it only in the use that is made of it, should be placed above the pile or either above or below a portion of the conductor. So long as the circuit is interrupted, the magnetic needle remains in its ordinary position, but it departs from this position as soon as the current is established, so much the more as the energy is greater, and it determines the direction of the current from this general fact, that if one places oneself in thought in the direction of the current in such a way that it is directed from the feet to the head of the observer and that he has his face turned toward the needle; the action of the current will always throw toward the left that one of the ends of the needle which points toward the north and which I shall always call the austral pole of the magnetic needle, because it is the pole similar to the southern pole of the earth. I express this more briefly by saying, that the austral pole of the needle is carried to the left of the current which acts on the needle. I think that to distinguish this instrument from the ordinary electrometer we should give it the name of galvanometer and that it should be used in all experiments on electric currents, as we habitually use an electrometer on electric machines, so as to see at every instant if a current exists and what is its energy.” (memoir presented to the Academy of Sciences on October 2, 1820, in the Annales de Chimie et de Physique Vol. 15, 1820 on page 59, qtd. in A Source Book in Physics, by William Magie, p450-1)
Ampere’s Memoir Proposing Astatic Needles
“Pour se mettre à l’abri de l’influence que le globe terrestre exerce sur l’aiguille aimantée, et pour distinguer dans les mouvemens que le courant galvanique lui imprime, ce qui est dû à l’action directrice et ce qui est dû à l’action attractive ou répulsive, M. Ampère emploie deux appareils, l’un qu’il appelle aiguille astatique, l’autre qu’il nomme appareil pour les attractions et répulsions magnético-galvaniques à aimant mobile.
Aiguille astatique. Llle ressemble à une aiguille d’inclinaison ; comme elle, e’Ie se meut toujours dans le plan perpendiculaire à l’axe qui la traverse et qui contient son centre de gravité; seulement ce plau de mouvement, qui reste toujours vertical dans la boussole d’inclinaison, peut prendre ici toutes les positions possibles. Or, si on met l’axe de l’aiguille astatique, parallèlement aux résultantes des actions du globe, J’aiguille ne pourra se mouvoir que dans le plan perpendiculaire à ces résultantes. Ainsi l’action du globe sera détruite et l’aiguille restera indiflérente dans toutes ses positions,’ c’est-à-dire, qu’elle sera parfaitement astatique. Alors, si on approche un courant galvanique, son action directrice se fait seule sentir sur l’aiguille , et l’expérience montre qu’elle devient toujours exactement perpendiculaire à la direction du courant.” (“Analyse Des Hémoires Lus Par M. Ampère A L’aca Démie DES SCIENCES, DANS LES SÉANCES DES 18 ET 25 SEPTEMBRE, DES 9 ET 3o OCTOBRE l82o”, from Annales Generales des Sciences Physiques, Vol. 6, p238)
Oersted’s Theory of Magnetism
“To the effect which takes place in this conductor and in the surrounding space, we shall give the name of the conflict of electricity…The electric conflict acts only on the magnetic particles of matter. All non-magnetic bodies appear penetrable by the electric conflict, while magnetic bodies, or rather their magnetic particles, resist the passage of this conflict. Hence they can be moved by the impetus of the contending powers.
It is sufficiently evident from the preceding facts that the electric conflict is not confined to the conductor, but dispersed pretty widely in the circumjacent space.
From the preceding facts we may likewise infer that this conflict performs circles; for without this condition it seems impossible that the one part of the uniting wire, when placed below the magnetic pole, should drive it towards the east, and when placed above it towards the west; for it is the nature of a circle that the motions in opposite parts should have an opposite direction. Besides, a motion in circles, joined with a progressive motion, according to the length of the conductor, ought to form a conchoidal or spiral line; but this, unless I am mistaken, contributes nothing to explain the phenomena hitherto observed.
All the effects on the north pole above-mentioned are easily understood by supposing that negative electricity moves in a spiral line bent towards the right, and propels the north pole, but does not act on the south pole. The effects on the south pole are explained in a similar manner, if we ascribe to positive electricity a contrary motion and power of acting to the south pole, but not upon the north.”
–“Experiments on the Effect of a Current of Electricity on the Magnetic Needle,” Annals of Philosophy, October 1820
Ampere’s Theory of Magnetism
“We now turn to the examination of this last action and of the action of two magnets on each other and we shall see that they both come under the law of the mutual action of two electric currents, if we conceive one of these currents as set up at every point of a line drawn on the surface of a magnet from one pole to the other, in planes perpendicular to the axis of the magnet, so that from the simple comparison of facts it seems to me impossible to doubt that there are really such currents about the axis of a magnet, or rather that magnetization consists in a process by which we give to the particles of steel the property of producing, in the sense of the currents of which we have spoken, the same electromotive action as is shown by the voltaic pile, by the oxidized zinc of the mineralogists, by heated tourmaline, and even in a pile made up of damp cardboard and discs of the same metal at two different temperatures. However, since this electromotive action is set up in the case of a magnet between the different particles of the same body, which is a good conductor, it can never, as we have previously remarked, produce any electric tension, but only a continuous current similar to that which exists in a voltaic pile re-entering itself in a close curve. It is sufficiently evident from the preceding observations that such a pile cannot produce at any of its points either electric tensions or attractions or repulsions or chemical phenomena, since it is then impossible to insert a liquid in the circuit; but that the current which is immediately established in this pile will act to direct it or to attract or repel it either by another electric current or by a magnet, which, as we shall see, is only an assemblage of electric currents.
It is thus that we come to this unexpected result, that the phenomena of the magnet are produced by electricity and that there is no other difference between the two poles of a magnet than their positions with respect to the currents of which the magnet is composed, so that the austral pole is that which is to the right of the currents and the boreal pole that which is to the left.” -Ampere’s presentation to the Academy of Science on October 2, 1820
Notes on Joseph Henry’s Lectures on Ampere’s Theory
“Thus far we have a mere collection of facts, and we might go on after the manner of the books, accumulating these to almost any extent. But facts without an explaining theory will be but confusion. We have a theory most fertile in its consequences. Ampere, the discoverer of the 3rd fact is the author of a theory which explains all the facts of common magnetism and of electro-magnetism. The fact that currents passing the same way attract each other, and those passing opposite ways repel each other is an ultimate one. His hypothesis is that magnetism consists in currents of electricity. A magnet consists of currents of electricity revolving around the bar. When the S end of a magnet (fig 6) is brought near the S end of a second magnet, the currents turn in opposite directions whence the repulsion between them. When dissimilar ends are brought near to each other, these currents turn in the same direction whence their attraction. Ampere made hundreds of deductions from this theory, all of which he found to agree exactly with the facts. Professor Henry was the first in this country who studied Ampere’s theory, and made some new deductions from it, and experiments proving them; owing to which it was that he was called to the Professorship in Princeton. If he has done anything for science, it has ever been by the proper use of a theory. Ampere supposes the currents flowing in the same direction, by their lateral action to carry along + rarify the electricity between, when by the external pressure so to speak, the magnetic bars are brought together. Again currents flowing in opposite directions crowd in the fluid + increase its pressure between them, when the bars separate. This theory was too fanciful to meet with much favor at first, but all the English philosophers have at length come to adopt it. If it is without analogy to any other theory, the facts + motions on which it is founded are also unlike any previously known phenomena. We need not suppose these currents to be constant, but to be set in motion by magnetizing the bar. In treating this subject, we shall take Ampere’s supposition, endeavoring to get a clear idea of the consequences flowing from it.
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Ampere’s theory which explains all these motions, rests as we have seen on the one fact. Currents in the same direction attract, in opposite directions repel, and the one hypothesis. Around every atom of the magnet currents of electricity are moving at right angles to the magnets length. The currents around the interior atoms (fig 5) neutralize each other’s effects, and those around the exterior atoms produce as their resultant the currents around the magnet. We say that these currents move around each atom of the magnet, because no matter into how many pieces we break a magnet, each portion still magnetic, ie. has these currents.
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We can now explain Oersted’s experiment (p171). The needle is turned at right angles to the connecting wire by the force which tends to bring the currents in the needle parallel, and in the same direction with that through the wire.
The simple galvanometer is but such a magnetic needle, around which in the direction of its length is placed a coil of wire. The least galvanic current in the wire turns the needle towards the position perpendicular to the wire. The surrounding coil of wire should not be made too long, or there will be too great a retardation of the electricity, which is feeble.” –William J. Gibson’s student notebook
Modern Understanding of Magnetism
“For some reason connected with the quantum mechanics of the structure of the iron atom, it is energetically favorable for the spins of the adjacent iron atoms to be parallel. This is not due to their magnetic interaction. It is a stronger effect than that, and moreover, it favors parallel spins whether like this ^^ or like this >> (dipole interactions don’t work that way). Now if atom A (Fig. 11.27) wants to have its spin in the same direction as that of its neighbors, atoms B, C, D, and E, and each of them prefers to have its spin in the same direction as the spin of its neighbors, including atom A, you can readily imagine that if a local majority ever develops there will be a strong tendency to ‘make it unanimous,’ and then the fad will spread.
Accident somehow determines which of the various equivalent directions in the crystal is chosen, if we commence from a disordered state–as, for example, if the iron is cooled through its Curie point without any external field applied. Pure iron consists of body-centered cubic crystals. Each atom has eight nearest neighbors. The symmetry of the environment imposes itself on every physical aspect of the atom, including the coupling between spins. In iron the cubic axes happen to be the axes of easiest magnetization. That is, the spins like to point in the same directions +- x, +- y, +- z. This is important because it means that the spins cannot easily swivel around en masse from one of the easy directions to an equivalent one at right angles. To do so, they would have to swing through less favorable orientations on the way. It is just this hindrance that makes permanent magnets possible.
An apparently unmagnetized piece of iron is actually composed of many domains, in each of which the spins are all lined up one way, but in a direction different from that of the spins in neighboring domains. On the average over the whole piece of “unmagnetized” iron, all directions are equally represented, so no large-scale magnetic field results. Even in a single crystal the magnetic domains establish themselves. The domains are usually microscopic in the everyday sense of the word. In fact they can be made visible under a low-power microscope. That is still enormous, of course, on an atomic scale, so a magnetic domain typically includes billions of elementary magnetic moments…The division (into domains) comes about because it is cheaper in energy than an arrangement with all the spins pointing in one direction. The latter arrangement would be a permanent magnet with a strong field extending out into the space around it. The energy stored in this exterior field is larger than the energy needed to turn some small fraction of the spins in the crystal, namely, those at a domain boundary, out of line with their immediate neighbors. The domain structure is thus the outcome of an energy-minimization contest.” -Berkeley Physics, p440