Henry’s Explanation

Acceptance of the Theory

Joseph Henry’s Explanation of the Galvanometer

It is Ampere’s theory that Professor Henry took to the lecture hall in order to explain the electromagnetic phenomenon displayed by magnets, currents, and of course, the galvanometer. An excerpt from William J. Gibson’s notes on his lectures on the subject is as follows:

Figure 6

“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.

Figure 5

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.

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, 1842-1843)

photograph taken of Gibson’s notebook, click to enlarge

Another student notebook by Henry Cameron dated four years later includes these notes on the same subject.  It also includes brief notes on the ether components of Ampere’s theory:

“Ampere instead of accumulating facts as had been done in other branches of science, at once proposed a hypothesis and proceeded to verify it by deductions, and thus discovered new facts. VC. The theory it is true is fanciful, but the facts are strange and new. This theory depends or rests upon one fact, and one hypothesis—The fact—that currents in the same direction attract—in opposite repel.—The hypothesis—that magnetism consists in currents of electricity around each atom of the iron, and at right angles to the length of the bar. This theory serves to explain a great many facts. How the currents are generated VC his theory does not attempt to explain. We shall endeavor to do it hereafter. Thus unlike poles of magnets attract because when they are put together, the currents around them are then going in the same direction whereas if like are put together, the currents are opposite and repel. VC. If we break it are two poles. VC. It also explains why the needle turns at right angles, because in that direction the currents around the needle are going the same way as that in the wire. The magnetism of the earth is no longer an ultimate fact; but is referred to currents around the earth in the direction in which the sun moves. We hope to be able to refer the developments of these currents to the influence of heat. By this theory we are enabled to explain the dip of the needle VC. Why do currents in the same direction attract? We suppose that there is a plenum of the ethereal medium, and from the fact that we suppose it to pervade all space, the atoms must press with great force, upon one another, and consequently the elasticity is very great, it being a function of this pressure. We infer great elasticity in this from the immense velocity in light, as sound in air. When currents are induced in wires, something like the lateral action of air takes place, and a small degree of exhaustion between the wires allows the immense pressure without to be perceptible and hence the attraction is explained upon the chemical principles. it is mere pressure probably. In the case of opposite currents, there is as it were a conflict VC hence repulsion. But a slight degree of exhaustion of the ethereal medium is necessary to develop electrical phenomenon.” (Henry Cameron’s student notebook, 1846-7)

The VC’s that dot these notes may mean “Vi coactus” which is Latin for “under constraint.” This was historically used to indicate that one was forced to sign, which would make them a humorous addition by Cameron.