The development of theories of electricity and of magnetism naturally proceeded along somewhat similar lines. The conception of effluvia, which emanated from bodies when electrified or mag netized, but eventually returned to them, fell out of favour with the discovery of electric conduction. The one and two fluid theories came into being, and though both found advocates, there seemed to be no conclusive experimental test available, to decide between them. F. V. T. Aepinus applied a one fluid theory with considerable success to magnetic phenomena in his Testamen Theoriae Electricitatis et Magnetisnii (1759). He supposed that at the poles the normal concentration of the magnetic fluid was increased or diminished. Particles of the fluid repelled each other, and attracted particles of iron. It was necessary to suppose that particles of iron (in an abnormal fluid-free state) also exerted mutual repulsion. Coulomb preferred a theory of two magnetic fluids, boreal and austral (corresponding to vitreous and resinous, or positive and negative electricity) to account for the fact that isolated poles do not occur he supposed that the two magnetic fluids, equal in amount, were permanently imprisoned within the molecules of magnetic bodies, and that magnetization consisted in the separation to an extent depending on the applied field of the boreal and austral fluids to opposite ends of each molecule.
The mathematical development of the theory of magnetism, as far as the phenomena then known were concerned, was carried to a mature stage by Simeon Denis Poisson (1781-184o) in a series of memoirs of which the first was published in 1821, following on previous work on electrostatic theory. Poisson adopted Cou lomb's views as the basis of his treatment, but as his results are, to a large extent, independent of the assumption of the existence of two magnetic fluids, resting mainly on the experimentally de termined inverse square law and on the hypothesis that magnetiza tion is a molecular phenomenon, they remain a correct mathe matical formulation of many of the quantitative aspects of mag netic phenomena. He obtained expressions for the magnetic forces due to bodies magnetized in any manner, in terms of sur face and volume integrals involving the intensity of magnetiza tion. He considered the forces inside magnetized bodies, and propounded a quantitative theory of induced magnetism. Poisson's theory, largely freed from arbitrary assumptions by Kelvin, was extended by him and also by Green, Neumann, Kirchoff, Max well and others. Gauss also did valuable work on magnetic theory, and is responsible for an indirect but precise method of confirm ing the inverse square law.
netism was being worked out, those discoveries were made which showed the relation between electricity and magnetism, linking the two together into a single wider science (see ELECTRICITY). In 1819, Hans Christian Oersted (1775-1851) found that a magnetic needle placed parallel to a current-bearing wire tended to set itself at right angles to the current. Oersted had long been looking for some action of electricity on the magnetic needle, but the effect discovered was quite different from what had been anticipated.
The effect taking place Oersted called the "conflict of electricity," which was supposed to perform circles round the conductor and to act only on magnetic particles of matter. The discovery was rapidly followed up. Andre Marie Ampere (1775-1836) investi gated theoretically and experimentally the mutual action of cur rent-bearing circuits (1820). Arago, in the same year, succeeded in magnetizing a piece of iron by the electric current. The whole subject of the mutual action of currents and magnets was shortly afterwards dealt with comprehensively by Ampere in one of the most celebrated memoirs in the history of physics (1825). In this he shows that a current circuit is equivalent in its magnetic effects to a magnetic "shell," magnetized at right angles to the surface, whose boundary coincides with the circuit.
The outstanding work of Michael Faraday (1791-1867) can only be glanced at. In 1831, in no more than ten full days of research between Aug. and Nov., he unravelled all the essential features of electromagnetic induction. "The quantity of electricity thrown into a current," he states, "is directly as the number of (magnetic) curves intersected." While Ampere accepted the idea of action at a distance as a satisfactory basis for mathematical treatment, Faraday constantly attempted to interpret electric and magnetic action in terms of stresses and strains in a medium. He visualized magnetic lines of force in closed curves proceeding through a magnet and pervading space, lines tending to shorten themselves, and repelling each other when side by side. Faraday's views were given mathematical expression by his great follower James Clerk Maxwell (1831-79). It is unnecessary to deal here with the experimental confirmation by Hertz of Maxwell's views as to electromagnetic radiation ; but the synthesis involved in the interpretation of light as an electromagnetic phenomenon is one whose significance cannot be overemphasized.
Dia-, Para- and Ferro-magnetism.—In 1778 S. J. Brugmans observed that bismuth and antimony were repelled from the poles of a magnet, but the importance of this was not clearly realized.