Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/237

Rh 219 MAGNETISM fTlHE word magnetism is derived from the Greek word J_ pdyvris, which was applied to an ore of iron possessing a remarkable attractive power for iron, and supposed to have been originally found near the town of Magnesia, in Lydia. 1 Thus Lucretius writes: Quern Magueta vocant patrio tie nomine Graii, Mugnetum quia fit patriis in finibus ortus. This name is said by Plato 2 to have been given to it by Euripides, and he adds that most call it the Heraclean stone. It is needless here to criticize the above or other derivations that have been given for the word ; we merely remark that it is now applied to all the phenomena kindred to that which first drew attention to the magnetic iron ore, viz., a selective attraction for iron. In the following article we shall give, in the first place, a sketch of the leading phenomena of strongly magnetic b 0( ji eSi y e SQa ]] then describe a provisional theory suf ficient to render a general account of these phenomena, and shall afterwards proceed to render this theory more pre cise, to develop it to its necessary conclusions, and to com pare these with experiment, indicating where the theory is either incorrect or incomplete. Then we shall discuss the paramagnetic and diamagnetic properties of all bodies, as expounded by Faraday; an account will be given of the con nexion between the magnetic and the other physical proper ties of bodies ; and, lastly, we shall endeavour to give some idea of the different physical theories that have been pro posed in order to give something more than a mere short hand record of the facts of observation. LEADING PHENOMENA. iagnetic It appears, from what Lucretius says in the passage now- above quoted, 3 that the Greeks and Romans were aware, ients &quot; i ron &amp;gt; but also that it endued iron in contact with it with its own peculiar property. Thus an iron ring will hang suspended by the attraction of a loadstone, and from that ring another, and so on, up to a certain number, depending on the power of the stone and the weight, &c., of the rings. They were also aware that the attraction was confined to iron, or at all events was not indiscriminate, and that it was not destroyed by the intervention of other bodies, such as brass, between the magnet and the iron. It appears, too, from the passage Fit qnoque ut a lapide hoc ferri natura recedat Interdum, fugerc atque sequi consueta vicissim, &c. that they had an idea that, under certain circumstances, the attraction might be replaced by a repulsion. If, how ever, we understand aright the latter part of Lucretius s somewhat obscure description of what seems to have been an actual experiment of his own, this notion was in reality a hasty generalization, not justified by the observed facts. 4 In any case there seems no warrant for assuming, as some have done, that the ancients had any definite conception of magnetic polarity. What they wanted in definite experimental knowledge they supplied by an abundant use of the imagination. 1 Gilbert, De Magnete, lib. i. chap. ii. , says, &quot;Magnesia ad Mse- audrura&quot; ; but it is uncertain whether this or Magnesia ad Sipylum is meant. 2 Ev ry Xidu fyv Ei/pnri Srjs fj.tv Mayvririv uivofj-aatv, oi 8e irool Hpa/cAeiai (Ion, 533 D). See Munro s Lucretius, vol. i. p. 662. The other name is from Heraclea in Lydia. 3 Bk. vi. line 906 #., and 1042 sq.; comp. Plato, Ion, ut supra, whom there is reason to think he is quoting. 4 See below, p. 225. We are told, for instance, that the magnet attracts wood and flesh, which was certainly beyond their powers of observation ; that it is effective in the cure of disease ; that it affects the brain, causing melancholy ; that it acts as a love philtre ; that it may be used in testing the chastity of a woman ; that it loses its power when rubbed with garlic, but recovers it when treated with goat s blood ; that it will not attract iron in the presence of a diamond, and much else that was eagerly copied by the wonder-loving writers of the Middle Ages. The science of magnetism made no real progress till the invention of the mariner s compass. The early history of this instrument is very obscure. According to some autho rities it was invented in China, and found its way into Europe probably through Arabian sources. The light thrown by recent researches on the literature of the Chinese has apparently thrown doubt upon their claim to this invention, 5 although the knowledge of the loadstone and its attractive property may have been older among them than even among the Greeks. The first accounts of the compass in Europe go back to the 12th century, and, although the instrument described is very rough, it is not spoken of as a new invention. In its earliest form it seems to have consisted simply of an iron needle which was touched with the loadstone and placed upon a pivot, or floated on water, so that it could turn more or less freely. It was found that such a needle came to rest in a position pointing approximately north and south (some accounts say east and west, in which case there must have been a cross piece on the needle to indicate what was pro bably the important direction for the mariner). As these compasses were made of iron (steel was not used till much later), and were probably ill-pivoted, they must have been very inaccurate ; and the difficulty of using them must have been much increased by the want of a card, which was a later addition made apparently by the Dutch. It is unnecessary to enter into more detail here respect ing the early history of the compass, as the matter has been very fully treated in the article COMPASS. 6 We pro ceed therefore to show the bearing of the invention upon the science of magnetism. It will at once be seen that it involves two scientific discoveries of capital importance : first, that the loadstone can transmit to iron with which it comes in contact a permanent property like its own ; and, secondly, that a loadstone or magnet if suspended freely will turn so that a certain direction in it assumes a fixed position relative to the geographical meridian, a certain part of the magnet turning always towards the north, and the part opposite towards the south. These opposite parts of the magnet are called its &quot; poles.&quot; To fix our ideas we shall describe a process by which we might definitely determine this direction in the magnet. Following the example of Gilbert, let us consider a spherical magnet. Our reason for dealing with this form in the first instance is to make it perfectly clear that the phenomena depend essentially on something apart from the form of the body. We shall suppose that the magnet is homogeneous as to its mass, so that its centre of gravity 5 See Mffilendorff, Z. D. M. G., xxxv. 70. 6 It may be mentioned that the statement that Peter Adsiger, in a letter written in 1296, mentions the magnetic declination, appears to be a mistake, arising from the mistranscription of a title. See Wenckebach, quoted by Lament, Handbuch des Magneiistnus, p. 449. The passage from Are Frode, quoted by Hansteen, and alluded to in last edition of this encyclopaedia, appears also to be of doubtful antiquity. See Poggendorff, Oeschichte der Physik, p. 99. Begin ning of magnetic science. Mariner s compass. Axis of a magnet experi mentally denned.
 * dge ot no j. on jy {.j^ t^g l oac i s tone, or magnetic iron ore, attracted