10 ELECTRICITY [HISTORY. discoveries of Galvani and Yolta were destined, in their turn, to pass into the shade, and the intellectual enterprise of the natural philosophers of Europe was directed to new branches of electrical and magnetical science. Guided by Magnetic theoretical anticipations, Professor H. C. Oersted of Copen- ictioii of hagen (Experimenta circa e/ectum conflictus electrici in slectric acum inagneiicam) in 1820 discovered that the elec- ji s . trical current of a galvanic battery, when made to pass severed through a platinum wire, acted upon a compass needle placed >y below the wire. He found that a magnetic needle placed Dersted. ^ Q fa Q neighbourhood of an electric current always places itself perpendicular to the plane through the current and the centre of the needle ; or, more definitely, that a magnetic north pole, carried at a constant distance round the current in the direction of rotation of an ordinary cork-screw advancing in the positive direction of the current, would always tend to move in the direction in which it is being carried. Slectro- Scarcely had the news of Oersted s discovery reached lyna- France when a French philosopher, Ampere, set to work to mcs. develop the important consequences which it involved. j^jfg Physicists had long been looking for the connection be-
- heory. tween magnetism and electricity, and had, perhaps,
inclined to the view that electricity was somehow to be explained as a magnetic phenomenon. It was, in fact, under the influence of such ideas that Oersted was led to his discovery. Ampere showed that the explanation was to be found in an opposite direction. He discovered the ponderornotive action of one electric current on another, and by a series of well-chosen experiments he established the elementary laws ef electrodynamical action, starting from which, by a brilliant train of mathematical analysis, he not only evolved the complete explanation of all the electro magnetic phenomena observed before him, but predicted many hitherto unknown. The results of his researches may be summarized in the statement that an electric current in a linear circuit of any form is equivalent in its action, whether on magnets or other circuits, to a magnetic shell bounded by the circuit, whose strength at every point is constant and proportional to the strength of the current. By his beautiful theory of molecular currents, he gave a theoretical explanation of that connection between electri city and magnetism which had been the dream of previous investigators. If we except the discovery of the laws of the induction of electric currents made about ten years later by Faraday, no advance in the science of electricity can compare for completeness and brilliancy with the work of Ampere. Our admiration is equally great whether we contemplate the clearness and power of his mathematical investigations, the aptness and skill of his experiments, or the wonderful rapidity with which he elaborated his dis covery when he had once found the clue. lecent In 1821 Faraday, who was destined a little later to do n-ogress so much for the science of electricity, discovered electro- , , magnetic rotation (Quarterly Journal, xii.), having suc- Ivna- ceeded in causing a horizontal wire carrying a current to iiics. rotate continuously across the vertical lines of a field of magnetic force. The experiment was very soon repeated in a variety of forms by De la Rive, Barlow, Ritchie, Sturgeon, and others ; and Davy (Phil. Trans.), in 1823, observed that, when two wires connected with the pole of a battery were dipped into a cup of mercury placed on the pole of a powerful magnet, the fluid metal rotated in op posite directions about the two electrodes. The rotation of a magnet about a fixed current and about its own axis was at once looked for, and observed by Faraday and others. The deflection of the voltaic arc by the magnet had been observed by Davy in 1821 (Phil Trans.); and in 1840 Walker observed the rotation of the luminous discharge in a vacuum tube. For many beautiful expsriments on the influence of the magnet on the strata, &c., in vacuum tubes, we are indebted to Pliicker, De la Rive, Grove, Gassiot, and others who followed them. One of the first machines in which a continuous motion Electr was produced by means of the repulsions and attractions magm between electromagnets and fixed magnets or electro- en s in magnets was invented by Ritchie (Phil. Trans., 1833). The artifice in such machines consists in reversing the polarity of one of the electromagnets when the machine is near the position of equilibrium. For a general theory of these machines, showing the reasons why they are not useful as economic motive powers, see Jacobi (Memoire sur r Application de I* Electro-mag netisme au Mouvement des Machines, Potsdam, 1835), and Joule (Mech. Mag., xxxvi.). Electro-magnetic engines have, however, found a restricted use in scientific workshops, such as Froment s, in driving telegraphic apparatus, <kc. In 1820 Arago (Ann. de Chim. ct de Phys., t. xv.) and Magn< Davy (Annals of Philosophy, 1821) discovered indepen- izatioi dently the power of the electric current to magnetize iron ^ y , . and steel. Savary (Ann. de Chim. et de Phys., t. xxxiv., curreE 1827) made some very curious experiments on the alter nate directions of magnetization of needles placed at differ ent distances from a wire conveying the discharge of a Leyden jar. The dependence of the intensity of magnet ization en the strength of the current was investigated by Lenz and Jacobi (Pogg. Ann., xlvii., 1839), and Joule found that magnetization did not increase proportionately with the current, but reached a maximum (Sturgeon s Ann. of EL iv. 1839). The farther development of this subject, which really belongs to magnetism, has been carried on by Weber, Miiller, Von Waltenhofen, Dub, Wiedemann, Quin- tus Icilius, Riecke, Stoletow, Rowland, and others. The use of a core of soft iron, magnetized by a helix surround ing it, has become universal in all kinds of electrical ap paratus. Electromagnets of great power have in this way been constructed and used in electrical researches by Brewster, Sturgeon, Henry, Faraday, and others. The most illustrious among the successors of Ampere was Recen Wilhelm Weber. He greatly improved the construction of F r g re the galvanometer, and invented the electro-dynamometer. , To these instruments he applied the mirror scale and tele- dyna- scope method of reading, which had been suggested by mics. Poggendorff, and used by himself and Gauss in magnetic measurements about 1833. In 1846 he proceeded with his improved apparatus to test the fundamental laws of Am pere. The result of his researches was to establish the truth of Ampere s principles, as far as experiments with closed circuits could do so, with a degree of accuracy far beyond anything attainable with the simple apparatus of the original discoverer. The experiments of Weber must be looked upon as the true experimental evidence for the theory of Ampere, and as such they form one of the corner-stones of electrical science. While experiment was thus busy, theory was not idle. In Theor 1845 Grassmann published (Pogg. Ann., Ixiv.) his Neue of Theorie der Electrodynamik, in which he gives an elemen- ^ tary law different from that of Ampere, but leading to the same results for closed circuits. In the same year F. E. Neumann published yet another law. In 1846 Weber announced his famous hypothesis connecting electro- statical and electrodynamical phenomena. Much has been written on the subject by Carl Neumann, Riemann, Stefan, Clausius, and others. Very important are three memoirs by Helmholtz, in Crelle s Journal (1870-2-4), in which a general view is taken of the whole question, and the works of his predecessors are critically handled. We shall have occasion, in the body of the article, to refer to the dynamical theory of Clerk Maxwell, which promises
to effect a revolution in this part of electrical science.