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SECOND PERIOD
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SECOND See also:PERIOD .—We now enter upon the second period of See also:electrical See also:research inaugurated by the See also:epoch-making See also:discovery of Alessandro See also:Volta (1745–1827). L. See also:Galvani had made in 1790 his historic observations on the See also:muscular contraction produced in the bodies of recently killed frogs when an electrical See also:machine was being worked in the same See also:room, and described them in 1791 (De viribus electricitatis in motu musculari commentarius, See also:Bologna, 1791). Volta followed up these observations with rare philosophic insight and experimental skill. He showed that all conductors liquid and solid might be divided into two classes which he called respectively conductors of the first and of the second class, the first embracing metals and See also:carbon in its conducting See also:form, and the second class, See also:water, aqueous' solutions of various kinds, and generally those now called electrolytes. In the See also:case of conductors of the first class he proved by the use of the condensing See also:electroscope, aided probably by some form of multiplier or doubler, that a difference of potential (see See also:ELECTROSTATICS) was created by the See also:mere contact of two such conductors, one of them being positively electrified and the other negatively. Volta showed, however, that if a See also:series of bodies of the first class, such as disks of various metals, are placed in contact, the potential difference between the first and the last is just the same as if they are immediately in contact. There is no See also:accumulation of potential. If, however, pairs of metallic disks, made, say, of See also:zinc and See also:copper, are alternated with disks of See also:cloth wetted with a conductor of the second class, such, for instance, as dilute See also:acid or any electrolyte, then the effect of the feeble potential difference between one pair of copper and zinc disks is added to that of the potential difference between the next pair, and thus by a sufficiently See also:long series of pairs any required difference of potential can be accumulated.
The Voltaic See also:Pile: This led him about 1799 to devise his famous voltaic pile consisting of disks of copper and zinc or other metals with wet cloth placed between the pairs. Numerous examples of Volta's See also:original piles at one See also:time existed in See also:Italy, and were collected together for an See also:exhibition held at See also:Como in 1899, but were unfortunately destroyed by a disastrous See also:fire on the 8th of See also:July 1899. Volta's description of his pile was communicated in a See also:letter to See also:Sir See also:Joseph See also:Banks, See also:president of the Royal Society of See also:London, on the loth of See also: Trans., vol. go, pt. 1, p. 405. It was then found that when the end plates of Volta's pile were connected to an electroscope the leaves diverged either with See also:positive or negative See also:electricity. Volta also gave his pile another form, the couronne See also:des tasses (See also:crown of cups), in which connected strips of copper and zinc were used to See also:bridge between cups of water or dilute acid. Volta then proved that all metals could be arranged in an electromotive 1 See also:Modern researches have shown that the loss of See also:charge is in fact dependent upon the ionization of the See also:air, and that, provided the atmospheric moisture is prevented from condensing on the insulating supports, water vapour in the air does not per se bestow on it conductance for electricity.series such that each became positive when placed in contact with the one next below it in the series. The origin of the electromotive force in the pile has been much discussed, and Volta's discoveries gave rise to one of the historic controversies of See also:science. Volta maintained that the mere contact of metals was sufficient to produce the electrical difference of the end plates of the pile. The discovery that chemical See also:action was involved in the See also:process led to the See also:advancement of the chemical theory of the pile and this was strengthened by the growing insight into the principle of the conservation of See also:energy. In 1851 See also:Lord See also:Kelvin (Sir W. See also:Thomson), by the use of his then newly-invented See also:electrometer, was able to confirm Volta's obser- vations on contact electricity by irrefutable See also:evidence, but the contact theory of the voltaic pile was then placed on a basis consistent with the principle of the conservation of energy. A. A. de la Rive and See also:Faraday were ardent supporters of the chemical theory of the pile, and even at the See also:present time opinions of physicists can hardly be said to be in entire accordance as to the source of the electromotive force in a voltaic couple or pile.'
Improvements in the form of the voltaic pile were almost immediately made by W. Cruickshank (1745–1800), Dr W. H.
See also:Wollaston and Sir H. See also:Davy, and these, together with other eminent See also:continental chemists, such as A. F. de See also:Fourcroy, L. J.
See also:Thenard and J. W. See also:Ritter (1776–1810), ardently prosecuted research with the new See also:instrument. One of the first discoveries made with it was its See also:power to electrolyse or chemically decompose certain solutions. See also: These two gases, as See also:Cavendish and See also: De la Rive reviews the subject in his large See also:Treatise on Electricity and Magnestism, vol. ii. ch. iii. The writer made a contribution to the discussion in 1874 in a paper on " The Contact Theory of the Galvanic,See also:Cell," Phil. Mag., 1874, 47, p. 4O1. Sir See also:Oliver See also:Lodge reviewed the whole position in a paper in 1885. " On the Seat of the Electromotive Force in a Voltaic Cell," Journ, Inst. Elec. Eng., 1885, 14, p. 186.
the conductor joining the extremities of the pile; he speaks of it as the electric conflict, and says: " It is sufficiently evident that the electric conflict is not confined to the conductor, but is dispersed See also:pretty widely in the circumjacent space. We may likewise conclude that this conflict performs circles See also:round the See also:wire, for without this See also:condition it seems impossible that one part of the wire when placed below the magnetic needle should drive its See also:pole to the See also:east, and when placed above it, to the See also:west." Oersted's important discovery was the fact that when a wire joining the end plates of a voltaic pile is held near a pivoted magnet or See also:compass needle, the latter is deflected and places itself more or less transversely to the wire, the direction depending upon whether the wire is above or below the needle, and on the manner in which the copper or zinc ends of the pile are connected to it. It is clear, moreover, that Oersted clearly recognized the existence of what is now called the magnetic See also: This discovery of Oersted, like that of Volta, stimulated philosophical investigation in a high degree. Electrodynamics.—On the 2nd of See also:October 182o, A. M. See also:Ampere presented to the See also:French See also:Academy of Sciences an important memoir,' in which he summed up the results of his own and D. F. J. See also:Arago's previous investigations in the new science of See also:electromagnetism, and crowned that labour by the announcement of his great discovery of the dynamical action between conductors conveying the electric currents. Ampere in this paper gave an See also:account of his discovery that conductors conveying electric currents exercise a mutual attraction or repulsion on one another, currents flowing in the same direction in parallel conductors attracting, and those in opposite directions repelling. Respecting this achievement when See also:developed in its experimental and mathematical completeness, Clerk See also:Maxwell says that it was " perfect in form and unassailable in accuracy." By a series of well-chosen experiments Ampere established the See also:laws of this mutual action, and not only explained observed facts by a brilliant See also:train of mathematical See also:analysis, but predicted others subsequently experimentally realized. These investigations led him to the announcement of the fundamental See also:law of action between elements of current, or currents in infinitely See also:short lengths of linear conductors, upon one another at a distance; summed up in compact expression this law states that the action is proportional to the product of the current strengths of the two elements, and the lengths of the two elements, and inversely proportional to the square of the distance between the two elements, and also directly proportional to a See also:function of the angles which the See also:line joining the elements makes with the directions of the two elements respectively. Nothing is more remarkable in the See also:history of discovery than the manner in which Ampere seized upon the right See also:clue which enabled him to disentangle the complicated phenomena of electrodynamics and to deduce them all as a consequence of one See also:simple fundamental law, which occupies in electrodynamics the position of the Newtonian law of See also:gravitation in See also:physical See also:astronomy. In 1821 See also:Michael Faraday (1791–1867), who was destined later on to do so much for the science of electricity, discovered electromagnetic rotation, having succeeded in causing a wire conveying a voltaic current to rotate continuously round the pole of a permanent magnet' This experiment was repeated in a variety of forms by A. A. De la Rive, See also:Peter See also:Barlow (1776-1862), William See also:Ritchie (179o-1837), William See also:Sturgeon (1783–1850), and others; and Davy (Phil. Trans., 1823) showed that when two wires connected with the pole of a battery were dipped into a See also:cup of See also:mercury placed on the pole of a powerful magnet, the fluid rotated in opposite directions about the two electrodes. Electromagnetism.—In 1820 Arago (See also:Ann. Chim. Phys., 282o, 15, p. 94) and Davy (Annals of Philosophy, 1821) discovered independently the power of the electric current to magnetize ' Memoire sur la theorie mathematique des phenomenes electrocic,.~; namiques," Memoires de l'institut, 182o, 6; see also Ann. de (.him., 182o, 15. ' See M. Faraday, " On some new Electro-Magnetical Motions. and on the Theory of See also:Magnetism," Quarterly See also:Journal of Science, 1822, 12, p. 74; or Experimental Researches on Electricity, vol. ii. p. 127.See also:iron and See also:steel. See also:Felix See also:Savary (1797–1841) made some very curious observations in 1827 on the magnetization of steel needles placed at different distances from a wire conveying the See also:discharge of a See also:Leyden See also:jar (Ann. Chim. Phys., 1827, 34). W. Sturgeon in 1824 See also:wound a copper wire round a See also:bar of iron See also:bent in the shape of a horseshoe, and passing a voltaic current through the wire showed that the iron became powerfully magnetized as long as the connexion with the pile was maintained (Trans. See also:Soc. Arts, 1825). These researches gave us the electromagnet, almost as potent an instrument of research and invention as the pile itself (see ELECTROMAGNETISM).
Ampere had already previously shown that a See also:spiral conductor or solenoid when traversed by an electric current possesses magnetic polarity, and that two such solenoids See also:act upon one another when traversed by electric currents as if they were magnets. Joseph See also: F. E. See also:Lenz (1804–1865) and M. H. von See also:Jacobi (1801–1874). J. P. See also:Joule found that magnetization did not increase proportionately with the current, but reached a maximum (Sturgeon's Annals of Electricity, 1839, 4). Further investigations on this subject were carried on subsequently by W. E. See also:Weber (1804–1891), J. H. J. See also: Oersted's discovery in 1819 was indeed epoch-making in the degree to which it stimulated other research. It led at once to the construction of the See also:galvanometer as a }neaps of detecting and measuring the electric current in a conductor. In 182o J. S. C. Schweigger (1779–1857) with his " multiplier " made an advance upon Oersted's discovery, by winding the wire conveying the electric current many times round the pivoted magnetic needle and thus increasing the deflection; and L. See also:Nobili (1784–1835) in 1825 conceived the ingenious See also:idea of neutralizing the directive effect of the See also:earth's magnetism by employing a pair of magnetized steel needles fixed to one See also:axis, but with their magnetic poles pointing in opposite directions. Hence followed the astatic multiplying galvanometer.
Electrodynamic Rotation.—The study of the relation between the magnet and the See also:circuit conveying an electric current then led Arago to the discovery of the " magnetism of rotation." He found that a vibrating magnetic compass needle came to See also:rest sooner when placed over a See also:plate of copper than otherwise, and also that a plate of copper rotating under a suspended magnet tended to See also:drag the magnet in the same direction. The See also:matter was investigated by See also: See also:Herschel, Peter Barlow and others, but did not receive a final explanation until after the discovery of electromagnetic See also:induction by Faraday in 1831. Ampere's investigations had led electricians to see that the force acting upon a magnetic pole due to a current in a neighbouring conductor was such as to tend to cause the pole to travel round the conductor. Much ingenuity had, however, to be expended before a method was found of exhibiting such a rotation. Faraday first succeeded by the simple but ingenious See also:device of using a light magnetic needle tethered flexibly to the bottom of a cup containing mercury so that one pole of the magnet was just above the See also:surface of the mercury. On bringing down on to the mercury surface a wire conveying an electric current, and allowing the current to pass through the mercury and out at the bottom, the magnetic pole at once began to rotate round the wire (Exper. Res., 1822, 2, p. 148). Faraday and others then discovered, as already mentioned, means to make the conductor conveying the current rotate round a magnetic pole, and Ampere showed that a magnet could be made to rotate on its own axis when a current was passed through it. The difficulty in this case consisted in discovering means by which the current could be passed through one See also:half of the magnet without passing it through the other half. This, however, was overcome by sending the current out at the centre of the magnet by means of a short length of wire dipping into an See also:annular groove containing mercury. Barlow, Sturgeon and others then showed that a copper disk could be made to rotate between the poles of a horseshoe magnet when a current was passed through the disk from the centre to the circumference, the disk being rendered at the same time freely movable by making a contact with the circumference by means of a mercury trough. These experiments furnished the first elementary forms of electric motor, since it was then seen that rotatory See also:motion could be produced in masses of See also:metal by the mutual action of conductors conveying electric current and magnetic See also:fields. By his discovery of See also:thermoelectricity in 1822 (Pogg. Ann. Phys., 6), T. J. Seebeck (1770–'831) opened up a new region of research (see THERMO-ELECTRICITY). James See also:Cumming (1777–1861) in '823 (Annals of Philosophy, 1823) found that the thermo-electric series varied with the temperature, and J. C. A. See also:Peltier (1785–1845) in 1834 discovered that a current passed across the junction of two metals either generated or absorbed See also:heat. See also:Ohm's Law.—In 1827 Dr G. S. Ohm (1787–1854) rendered a great service to electrical science by his mathematical investigation of the voltaic circuit, and publication of his paper, See also:Die galvanische Kette mathematisch bearbeitet. Before his time, ideas on the measurable quantities with which we are concerned in an electric circuit were extremely vague. Ohm introduced the clear idea. of current strength as an effect produced by electromotive force acting as a cause in a circuit having resistance as its quality, and showed that the current was directly proportional to the electromotive force and inversely as the resistance. Ohm's law, as it is called, was based upon an analogy with the flow of heat in a circuit, discussed by See also:Fourier. Ohm introduced the definite conception of the See also:distribution along the circuit of " electroscopic force " or tension (Spannung), corresponding to the modern See also:term potential. Ohm verified his law by the aid of thermo-electric piles as See also:sources of electromotive force, and Davy, C. S. M. Pouillet (1791–1868), A. C. See also:Becquerel (1788–1878), G. T. See also:Fechner (1801–1887), R. H. A. Kohlrausch (1809–1858) and others laboured at its See also:confirmation. In more See also:recent times, 1876, it was rigorously tested by G. Chrystal (b. '851) at Clerk Maxwell's instigation (see Brit. Assoc. See also:Report, '876, p. 36), and although at its original enunciation its meaning was not at first fully apprehended, it soon took its place as the expression of the fundamental law of See also:electrokinetics. Induction of Electric Currents.—In 1831 Faraday began the investigations on electromagnetic induction which proved more fertile in far-reaching See also:practical consequences than any of those which even his See also:genius gave to the See also:world. These advances all centre round his supreme discovery of the induction of electric currents. Fully See also:familiar with the fact that an electric charge upon one conductor could produce a charge of opposite sign upon a neighbouring conductor, Faraday asked himself whether an electric current passing through a conductor could not in any like manner induce an electric current in some neighbouring conductor. His first experiments on this subject were made in the See also:month of See also:November 1825, but it was not until the 29th of See also:August 183' that he attained success. On that date he had provided himself with an, iron See also:ring, over which he had wound two coils of insulated copper wire. One of these coils was connected with the voltaic battery and the other with the galvanometer. He found that at the moment the current in the battery circuit was started or stopped, transitory currents appeared in the galvanometer circuit in opposite directions. In ten days of brilliant investigation, guided by clear insight from the very first into the meaning of the phenomena concerned, he established experimentally the fact that a current may be induced in a conducting circuit simply by the variation in a magnetic field, the lines of force of which are linked with that circuit. The185 whole of Faraday's investigations on this subject can be summed up in the single statement that if a conducting circuit is placed in a magnetic field, and if either by variation of the field or by See also:movement or variation of the form of the circuit the See also:total magnetic See also:flux linked with the circuit is varied, an electromotive force is set up in that circuit which at any instant is measured by the See also:rate at which the total flux linked with the circuit is changing. Amongst the memorable achievements of the ten days which Faraday devoted to this investigation was the discovery that a current could be induced in a conducting wire simply by moving it in the neighbourhood of a magnet. One form which this experiment took was that of rotating a copper disk between the poles of a powerful electric magnet. He then found that a conductor, the ends of which were connected respectively with the centre and edge of the disk, was traversed by an electric current. This important fact laid the See also:foundation for all subsequent inventions which finally led to the production of electromagnetic or See also:dynamo-electric See also:machines. Additional information and CommentsThere are no comments yet for this article.
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