CHAP. VII., 4.] 



ELECTRICITY. OERSTED AMPERE. 



177 



(796.) The theory of Ampere rejects all but push-and- 

 utual p u n forces, such as are commonly recognized in me- 

 ectric chanical physics. These forces are mutual, and be- 

 mductors. long to electric currents. A permanent magnet is 

 a congeries of minute parallel and circular currents, 

 all acting in the same direction, which is at right 

 angles to the magnetic axis or line of force. Grant- 

 ing this for a moment, Oersted's experiment shows 

 that the current in the conductor acts on the currents 

 in the magnet ; and as a magnet places itself trans- 

 versely to a conductor, the currents in the magnet 

 tend to place themselves parallel to that in the con- 

 ductor. Do we then find such properties in move- 

 able electric conductors alone 1 Have they any 

 mutual action ? Does that mutual action tend to 

 produce parallelism ? And if so, may it be farther 

 analysed into direct attractions or repulsions of the 

 several parts of the electric currents upon one 

 another ? All these questions were answered by 

 Ampere affirmatively after due appeal to experiment. 

 Two copper wires connected with voltaic circuits, 

 and suspended with the requisite degree of freedom, 

 approach when the currents have a similar direction, 

 but are repelled when the direction is opposite in 

 the two. When two moveable conductors are placed 

 at right angles, or indeed at any angle, they tend to 

 parallelism. All the usual phenomena of a magnet 

 may be imitated by a long helix of copper wire 

 through which electricity is made by some artifice 

 continually to circulate. The position of the poles 

 is the same as in a real magnet, and the name of 

 pole is determined by the direction (right or left 

 handed) in which the helix is wound. Such an in- 

 strument, not containing one particle of iron, is at- 

 tracted and repelled by a steel magnet, obeys the 

 directive influence of the earth, gives transverse 

 motion to an electric conductor near it, in short, 

 does whatever magnetized iron does. 



Thus, in the mutual action cf electric currents 

 of ^ >or *^ e P nenomena f static electricity are wholly 

 ie inverse unlike) we recognize the great discovery of Ampere. 

 juare of A new science was formed, which he called electro- 

 dynamics, which he proceeded to develope with great 

 skill and success. MM. Biot and Savary found that 

 the electro-magnetic force exerted by an indefinite 

 straight conductor and needle, varies inversely as the 

 simple distance from the conductor; but looking 

 to the elementary actions of each portion of the cur- 

 rent, it will be found that this corresponds to the 

 usual physical law of the inverse square of the dis- 

 tance between the magnetic and the electric ele- 

 ment. 



Whilst Ampere was pursuing his inquiries into 

 the properties of electric currents, others were vary- 

 ing, in a great variety of ways, Oersted's fundamental 

 experiment. A great number of beautiful mechani- 

 cal arrangements were invented, particularly by the 

 elder De la Rive and by Mr Faraday. The latter, 

 however, had the sole merit of effecting a most singu- 





(798.) 

 [r Fara- 

 ay and E 

 i Rive 

 llectro- 

 lagnetic 

 stations. 



lar kind of motion, that in which a magnet float- 

 ing in mercury is made to revolve continuously 

 around a central conducting wire, and in like man- 

 ner a conductor may be made to revolve round 

 a fixed magnet ; nay, stranger still, a magnet 

 acting at once as conductor and magnet, revolves 

 with great velocity on its own axis when an elec- 

 tric stream is made to traverse one half of its 

 length. These astonishing experiments, which, in 

 an earlier age, might have founded a new sect of 

 astronomers and replaced the theory of Vortices, 

 offered also considerable difficulties in the applica- 

 tion of Ampere's theory. They were, however, ulti- Accounted 

 mately removed by Ampere himself, who analysed *? rb ^ re 

 with great skill the mechanical conditions of each 

 case, and interpreting them into the language of his 

 theory, showed how continuous rotations might be 

 produced, according to the laws which he had esta- 

 blished, by electric currents alone suitably arranged ; 

 and he effected by most ingenious experimental combi- 

 nations purely electro-dynamic rotations. Some other 

 experiments, in which magnets seemed to produce a 

 different effect from electro-dynamic cylinders, pre- 

 sented a more serious obstacle, which, however, was 

 removed by a rigorous demonstration of the effects 

 which must ensue, if we regard the elementary mole- 

 cules of a magnet as very small, and consequently the 

 entire magnet as a collection of indefinitely small and 

 correspondingly numerous electro-dynamic cylinders. 

 By means of four critical experiments, Ampere de- 

 termined completely the elementary laws of the 

 mutual action of currents, including that previously 

 established by Biot and Savary in the case of a mag- 

 net and a conductor. This investigation was one of 

 great intricacy, and was carried out with remarkable 

 skill. Ampere had the field almost to himself, Savary 

 making some contributions ; and, what is remarkable, 

 little or nothing has been added either to the theory, 

 or to the deductions from it, since his death. The 

 progress of the science of electro-magnetism has 

 been so astonishingly rapid since the year 1820, that 

 one set of phenomena after another has for the 

 time attracted almost exclusive notice. The disco- 

 very of diamagnetism will probably lead to a recon- 

 sideration of Ampere's theory as applicable to all 

 matter in a more general form. 



This rapid succession of interesting topics has pre- (799.) 

 vented attention from being perhaps sufficiently di- Great 



rected to the importance of Ampere's labours. He erit , of 



, ., i Ampere, 

 is at least as well entitled as any other philosopher 



who has yet appeared, to be called " the Newton of 

 Electricity." 



Ampere was of an amiable, though rather eccentric (800.) 



. ,_ _ TTica ilaot-l 



character. His absence of mind was proverbial, and 

 his style is somewhat cumbrous and obscure. But 

 he was devoted to science, the promotion of which 

 was ever his first consideration, and he evidently 

 himself possessed great clearness in his conceptions. 

 He died on the l?th May 1836. 



i His death. 



