02 



ELECTRO-MAGNETISM. 



currents on a globe so prepared, the 

 apparatus might be made to represent 

 with great accuracy every circumstance 

 of magnetic dip and direction ; and by 

 employing, instead of a magnetic nee- 

 dle, an electro- dynamic cylinder, all 

 the phenomena of terrestrial magnetism 

 might be exhibited, without the inter- 

 vention of magnetism, by means of elec- 

 tricity alone. 



(299.) The origin of these electrical 

 currents permeating the interior of the 

 earth, and more especially its external 

 layer, may possibly be traced to the 

 action of the solar rays on successive 

 parts of the torrid zone, which taking 

 place from east to west, may excite cur- 

 rents of positive electricity in that direc- 

 tion, and in planes corresponding with 

 the magnetic equator. The probability 

 of such an effect being produced, and 

 the inference from analogy that similar 

 currents may be excited, and even exist 

 permanently in iron and steel, is greatly 

 increased by the recent discoveries of 

 Professor Seebeck, that electric currents 

 may be produced and maintained in cir- 

 cuits formed exclusively of solid con- 

 ductors, by the partial application of 

 heat. This discovery, which leads to a 

 separate department of this science, to 

 which the name of Thermo- Electricity 

 has been given, will be treated of in the 

 next Chapter. 



(300.) A further confirmation of the 

 electro-dynamic theory of magnetism is 

 derived from its applicability to the cu- 

 rious phenomena of magnetic rotations, 

 which have been described in the eighth 

 chapter of the Treatise on Magnetism 

 ( 354 to 360). Soon after the discovery 

 of this new class of facts, M. Arago 

 suggested to M. Ampere the substitution 

 of electro-dynamic cylinders for the 

 magnetic bars in these experiments on 

 the effects of rotation. The first trials 

 made by these two philosophers in con- 

 junction did not lead to any decisive re- 

 sult, in consequence of some defects in 

 the apparatus they employed ; but when 

 these defects were obviated in the sub- 

 sequent experiments which they made 

 with M. Colladon, in which a very short 

 double helix, forming a coil of about 

 two inches in diameter, was used, they 

 succeeded perfectly in obtaining the 

 same results as if magnets had been 

 employed*. Hence we may infer the 

 complete identity between all the effects 



* Ampere, Theorie des Pbenomencs Elcctro-dy- 

 niiciues, p. 196. 



of a common magnet and an electro- 

 dynamic cylinder. 



(301.) We thus find that the theory 

 of Ampere satisfies every condition that 

 is required of a true theory, inasmuch 

 as it affords a complete explanation of 

 all the phenomena, even in their mi- 

 nutest details. It unites the character 

 of simplicity in principle, and compre- 

 hensiveness in its applications ; and by 

 suggesting new combinations, it has led 

 to the discovery of new facts. It also 

 has an important advantage over the 

 theory of tangential forces in presenting 

 greater facility of mathematical investi- 

 gation, and for the comparison of the 

 analytical formulae thence obtained, with 

 the results of experiment ; and thereby 

 affording the most severe test of its 

 accuracy. If the truth of the theory be 

 established, it will effect an important 

 step in the generalization of physical 

 phenomena, by showing that all those 

 formerly referred to the operation of an 

 unknown principle, considered as dis- 

 tinct from electricity and denominated 

 magnetism, are, in fact, essentially elec- 

 tric, and that the two principles are 

 identical, and instead of being the bases 

 of two separate departments of know- 

 ledge, are merely branches of a single 

 and more extended and comprehensive 

 science. 



(302.) It must at the same time be 

 acknowledged that much still remains 

 to be done towards removing the diffi- 

 culties opposed to this as well as other 

 electro-magnetic theories, which are pre- 

 sented by the singular and apparently 

 capricious phenomena of the induction 

 of , magnetism by electrical currents 

 transmitted along conductors, and de- 

 rived either from the voltaic or the com- 

 mon electrical batteries. We allude 

 particularly to the results of the expe- 

 riments of Savary, already noticed in 

 $ 164 to 168, and which have-not yet 

 been sufficiently generalized to 'admit of 

 being explained on any hypothesis, 



CHAPTER XIII. 

 Thermo- Electricity. 



(303.) PROFESSOR SEEBECK, of Berlin, 

 discovered, in the year 1822, that cur- 

 rents of electricity might be produced 

 by the partial application of heat to a 

 circuit composed exclusively of solid 

 conductors. The original experiment 

 which established this fact was first an- 

 nounced in this country, in the Annals 



