Faraday advanced what has been termed the molecular theory of electricity which as sumes that electricity is the manifestation of a peculiar condition of the molecule of the body rubbed or the ether surrounding the body. Faraday also, by experiment, discovered para magnetism and diamagnetism, namely, that all sol.ds and liquids are either attracted or re pelled by a magnet. For example, iron, nickel, cobalt, manganese, chromium, etc., are para magnetic (attracted by magnetism), whilst other substances, such as bismuth, phosphorus, anti mony, zinc, etc., are repelled by magnetism or are diamagnetic ((Phil. Trans.,' 1845). Brugans of Leyden in 1778 and Le Baillif and Becquerel in 1827 had previously discovered diamagnetism in the case of bismuth and antimony. Faraday also rediscovered specific inductive capacity in 1837, the results of the experiments by Caven dish not having been published at that time. He also predicted (Phil. Mag., March 1854) the retardation of signals on long submarine cables due to the inductive effect of the insula tion of the cable, in other words, the static capacity of the cable.
The 25 years immediately following Fara day's discoveries of electric induction were fruitful in the protnulgation of laws and facts relating to induced currents and to magnetism. In 1834 Lenz and Jacobi independently demon strated the now familiar fact that the currents induced in a coil are proportional to the number of turns in the coil. Lenz also announced at that time the important law that, in all cases of electronsagnetic induction the induced currents have such a direction that their reaction tends to stop the motion that produces.them, a law that was perhaps deducible from Faraday's ex planation of Arago's rotations.
In 1845 Joseph Henry, the American physi cist, published an account of his valuable and interesting experiments with induced currents of a high order, showing that currents could be induced from the secondary of an induction coil to the primary of a second coil, thence to its secondary wire, and so on to the primary of a third coil, etc. (Philosophical Magazine, 1849). Abria published the results of some re searches into the laws of these induced currents, but owing to their complexity the investigation was not productive of very notable results. ((Ann. de Chimie III,' i, 385). About 1850 Kirchoff published his laws relating to branched or divided circuits. He also showed mathe matically that according to the then prevailing electrodynamic theory, electricity would be propagated along a perfectly conducting wire with the velocity of light. Helmholtz investi gated mathematically the effects of induction upon the strength of a current and deduced therefrom equations, which experiment con firmed, showing amongst other important points the retarding effect of self-induction under cer tain conditions of the circuit ((Poggendorf 1851). In 1853 Sir William Thomson
(later Lord Kelvin) (q.v.) predicted as a re sult of mathematical calculations the oscillatory nature cf the electric discharge of a condenser circuit. To Henry, however, belongs the credit of discerning as a result of his experiments in 1842 the oscillatory nature of the Leyden jar discharge. He wrote Am. Phil. Soc.,' Vol. II, pp. 193, 196) : The phenomena require us to admit the existence of a principal dis charge in one direction, and then several reflex actions backward and forward, each more ferb!e than the preceding, until the equilibrium is ob tained. These oscillations were subsequently observed by Feddersen (1857) who using a rotating concave mirror projected an image of the electric spark upon a sensitive plate, thereby obtaining a photograph of the spark which plainly indicated the alternating nature of the discharge. Sir William Thomson was also the discoverer of the electric convection of heat (the uThomsono effect). He designed for elec &cal measurements of precision his quadrant and absolute electrometers. The reflecting gal vanometer and siphon recorder, as applied to submarine cable signaling, are also due to him.
About 1876 Prof. H. A. Rowland of Balti more demonstrated the important fact that a static charge carried around produces the same magnetic effects as an electric current. The importance of this discovery consists in that it may afford a plausible theory of magnetism, namely, that magnetism may be the result of directed motion of rows of molecules carrying static charges.
After Faraday's discovery that electric cur rents could be developed in a wire by causing it to cut across the lines of force of a magnet, it was to be expected that attempts would be made to construct machines to avail of this fact in the development of voltaic currents. (See ELECTRIC MACHINERY; ELECTRIC DIRECT CURRENT; GEN =worts). The first machine of this kind was due to Pixii, 1832. It consisted of two bobbins of iron wire, opposite which the poles of a horse shoe magnet were caused to rotate. As this pro duced in the coils of the wire an alternating current, Pixii arranged a commutating device (commutator) that converted the alternating current of the coils or armature into a direct current in the external circuit. This machine was followed by improved forms of magneto electric machines due to Ritchie, Saxton, Clarke, Stohrer 1843, Nollet 1849, Shepperd 1856, Van Maldern, Siemens, Wilde and others.