# Historical origin of magnetic monopoles

There are two related historical questions that I'm trying to find answers to:

1. Who was the first to introduce the concept of magnetic monopoles?

2. Griffiths in his textbook Introduction to Electrodynamics refers to a model of magnetic dipole, formed analogously to electric dipole via pair of magnetic monopoles, as a "Gilbert model" (presumably giving a tribute to William Gilbert and his classical work "De Magnete"). Who was the first to consider such model of magnetic dipole?

• The earliest reference that is sometimes mentioned in this context is a letter by Petrus Peregrinus de Maricourt from 1269 (translation available at Project Gutenberg), which contains an early (first?) identification of two distinct poles of magnets, but I'm suspicious if this can be interpreted as the introduction of the concept of isolated magnetic poles.

• Maxwell himself in seminal paper "On Physical Lines of Forces" (1861) (cf. Wikipedia), on the right-hand side of the equation (6) has quantity "$$m$$" which looks like something which in modern notation would correspond to density of magnetic charges, $$\rho_m$$ (please, correct me if I'm misinterpreting this!).

• P. Curie in his paper "Sur la possibilité d'existence de la conductibilité magnétique et du magnétisme libre" (1894) (J. Phys. Theor. Appl. 3, 415-417 (1894)) speculates on possibility of isolated magnetic charges.

• H. Poincaré in his paper "Remarques sur une expérience de M. Birkeland" (1896) (available at bibnum) considers the problem of motion of point electric charge in a field of magnetic monopole (in his words: "Supposons un seul pôle magnétique...").

• J.J. Thomson in his book "Electricity and Matter" (1904) considers isolated (?) "magnetic poles" (see e.g. p. 25 in the copy at the Internet Archive).

• Depends on what you mean by "model". Qualitatively, Gilbert had it already. Or do you mean deriving the magnetic dipole force law from putting two opposite charges together by analogy to the electric dipole? – Conifold Nov 30 '19 at 0:56
• @Conifold OK, Gilbert indeed has concept of two distinct magnetic poles (just as Peregrinus did), but I'm not convinced that he contemplated isolated magnetic monopoles (magnetic point charges). Maybe I'm missing something in Gilbert's work... please point me to a page in "De Magnete"! If we restrict question even further, to a quantitative "Gilbert model" of magnetic dipole with derivation of field or force, then I would be really surprised if anything of this kind appeared before 18th century... – Ivica Smolić Nov 30 '19 at 1:15

The modern concept of magnetic monopole (as a real isolated charge) is due to Dirac in 1931, although Curie speculated about the possibility earlier. Even electric charges, as in particles, only appeared in 19th century, see Wikipedia's Discovery of two kinds of charges. Before that electricity and magnetism were mostly viewed as produced by fluids, one or two. That was Franklin's (one) and Coulomb's (two) view, for example.

The identification of two magnetic poles in magnets is suggested by Peregrinus, and more explicitly by Gilbert. The first quantitative model that interprets magnets as dipoles with the poles attracting/repulsing according to the inverse square law appears in Michell's Treatise of Artificial Magnets (1750). Here is from History of the Theories of Aether and Electricity:

"In this he states the principles of magnetic theory as follows:

"Wherever any Magnetism, is found, whether in the Magnet itself, or any piece of Iron, etc., excited by the Magnet, there are always found two Poles, which are generally called North and South; and the North Pole of one Magnet always attracts the South Pole, and repels the North Pole of another: and vice versa"

This is of course adopted from Gilbert. "Each Pole attracts or repels exactly equally, at equal distances, in every direction." This, it may be observed, overthrows the theory of vortices, with which it is irreconcilable. "The Magnetical Attraction and Repulsion are exactly equal to each other." This, obvious though it may seem to us, was really a most important advance, for, as he remarks,

"Most people, who have mention'd any thing relating to this property of the Magnet, have agreed, not only that the Attraction and Repulsion of Magnets are not equal to each other, but that also, they do not observe the same rule of increase and decrease... The Attraction and Repulsion of Magnets decreases, as the Squares of the distances from the respective poles increase."

This great discovery, which is the basis of the mathematical theory of Magnetism, was deduced partly from his own observations, and partly from those of previous investigators (e.g. Dr. Brook Taylor and P. Muschenbroek), who, as he observes, had made accurate experiments, but had failed to take into account all the considerations necessary for a sound theoretical discussion of them.

Interestingly, the inverse cube law for dipoles that follows from it was noted experimentally already by Newton in Principia, see Is Coulomb's law the earliest mathematical formula describing electricity? Aepinus suggested identifying the poles with shortfalls and excesses of the magnetic fluid in 1759, with a mechanistic explanation for the inseparability of the poles, and Brugmans and Wilcke instead introduced two opposing fluids, called "boreal" and "austral". Michell's dipole model was supported by Mayer and Lambert, and became standard after Coulomb's experiments (1777), whose sources were not particle charges, but boreal and austral fluids jointly locked within molecules. Here is from Whittaker again:

"Coulomb rendered great services to magnetic theory. It was he who in 1777, by simple mechanical reasoning, completed the overthrow of the hypothesis of vortices. He also, in the second of the Memoirs already quoted, confirmed Michell's law, according to which the particles of the magnetic fluids attract or repel each other with forces proportional to the inverse square of the distance.

Coulomb, however, went beyond this, and endeavoured to account for the fact that the two magnetic fluids, unlike the two electric fluids, cannot be obtained separately; for when a magnet is broken into two pieces, one containing its north and the other its south pole, it is found that each piece is an independent magnet possessing two poles of its own, so that it is impossible to obtain a north or south pole in a state of isolation.

Coulomb explained this by supposing that the magnetic fluids are permanently imprisoned within the molecules of magnetic bodies, so as to be incapable of crossing from one molecule to the next; each molecule therefore under all circumstances contains as much of the boreal as of the austral fluid, and magnetization consists simply in a separation of the two fluids to opposite ends of each molecule. Such a hypothesis evidently accounts for the impossibility of separating the two fluids to opposite ends of a body of finite size. The same idea, here introduced for the first time, has since been applied with success in other departments of electrical philosophy."

Ampere's "molecular currents" replaced Coulomb's magnetic fluids with circulating electric ones, see What is the history of electric current and resistance? There was little motivation to ponder magnetic charges before the existence of electric ones was accepted in the second half of 19th century.

• Thank you for this detailed answer! One side remark: some authors (see e.g. Merrill, McElhinny, McFadden (eds.): "The Magnetic Field of the Earth: Paleomagnetism, the Core, and the Deep Mantle", page 4, and notes in Encyclopedia.com) ascribe the origin of the term "magnetic pole" ("polus lapidis") to Peregrinus. – Ivica Smolić Nov 30 '19 at 11:46
• Comments are not for extended discussions, so I've moved several comments on this post to chat. – Danu Dec 4 '19 at 22:32
• @ConsigliereZARF As mentioned in the comment I left, comments are not for extended discussions. – Danu Dec 4 '19 at 23:40
• Hey, folks, I'm stepping in a neutral third (fourth?) party. I agree with the decision to move the comments to chat - it's a much better venue for extended discussion about substantial revisions to an answer. Back-and-forths in comments take up way too much space and often make it impossible for any other users to get a word in. Chat is more conducive to this type of thing - and that's why the comments were moved, so the discussion can be continued there in a more convenient locale. cc @ConsigliereZARF and Conifold. – HDE 226868 Dec 5 '19 at 0:09
• Folks, please stop. If you want to make a short, constructive suggestion on Conifold's answer, that's great. If not, please just don't. Off-topic comments are likely to be deleted, as on all Stack Exchange sites. Thanks. – HDE 226868 Dec 5 '19 at 14:22