Ampère's electrodynamic molecular model

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Abstract
In the eighteenth century, the study of electricity and magnetism became integral parts of the general structure of science. The Newtonian focus upon the strange attractive power of gravitational masses tended to direct attention towards other manifestations of attraction with the hope that some light might be thrown upon the mechanism of attraction in general. As the investigation of magnets and electrically charged bodies progressed, two separate, but very similar, theories were gradually constructed which could explain the observed magnetic and electrical effects. Both magnetism and electricity could be viewed as the result of the action of imponderable fluids upon one another. Thus, a magnet contained an austral and a boreal fluid so dispersed throughout the body of the magnet that the austral fluid was at its densest at the south end, and the boreal at the north end. The particles of each fluid were mutually repellent, whereas particles of opposite sign attracted one another. Similarly, electric phenomena could be reduced to the action of a positive and negative electric fluid in which particles of the same charge repelled one another. There were two rather disquieting aspects of these theories. The action of the particles of the imponderables was at a distance, and the mechanism of this action remained as mysterious in 1800 as it had in Newton's day. The theories also seemed to multiply entities unnecessarily. Was it really necessary to have separate electric and magnetic fluids? Were there not enough analogies between magnetic and electric action to permit the explanation of both in terms of a common cause? This question became so obtrusive in the 1780's that the Bavarian Academy of Science offered a prize for the best paper on the analogy between electricity and magnetism. The various memoirs were collected together by the Dutch physicist, J. H. van Swinden and published in three volumes(l) in 1784. Van Swinden's conclusion, after an exhaustive study of all the supposed analogies, was that electricity and magnetism were essentially different and separate phenomena. In the course of his analysis, he also suggested that one of the desiderata for both electric and magnetic science was a precise knowledge of the laws of attraction and repulsion by which the respective imponderable fluids worked. This appears to have been the stimulus for Charles Coulomb's classical researches. The wonderful precision and ingenuity with which he attacked the problem by means of his torsion balance is well known. The fact that both electrical and magnetic action followed an inverse square law, differing only in the constants, might seem, at first sight, to support the idea of the essential similarity of electricity and magnetism. But Coulomb, armed with his precise methods of measurement, pursued his quarry into the very depths of the intimate structure of matter itself. Electricity and magnetism, he found, were two fundamentally different things, requiring two different kinds of fluids to explain them. The electric fluids penetrated the intermolecular spaces of bodies and flowed from one body to another under the proper urging. The magnetic fluids, on the other hand, were always contained within molecules of magnetic substances and could not flow from one molecule to another.(2) Each fluid acted only upon its opposite; the two different kinds of fluids had no effect upon one another. Hence, he concluded, there was only a similarity and not an identity of action of the electric and magnetic fluids.

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