4-3 



MAGNETISM. 



MAGNETISM. 



the greatest development of magnetism by influence, we see the advan- 

 tage of using iron or steel bars at a red heat. [LIGHTNING.] 



Magnetism may be developed in iron, steel, cobalt, and nickel, by 

 other means than the influence of bodies already magnetised, as twisting, 

 hammering, electrical discharges, and galvanic currents. [ELECTRO- 

 MAGNETISM.] If we place a bar of iron in a vertical position, and give 

 it a series of slight blows with a hammer or poker, it will acquire a 

 feeble degree of magnetism ; hence it happens that the anvils and other 

 tools employed in smithies are endowed with magnetism. In all such 

 cases the mechanical operations tend to bestow a coercive power, while 

 the terrestrial magnetism separates the fluids in the body. 



Cavallo, Benett, and Coulomb remarked the indications of magnetism 

 given by various substances, as copper, silver, &c. It is particularly 

 observable in hammered copper, and scarcely perceptible when the 

 copper has been cast. Coulomb formed very fine needles of various 

 substances, and suspending them by silk strings between the opposite 

 poles of two powerful loadstones, found that they were acted on by the 

 latter. This phenomenon was attributed to the existence of minute 

 quantities of iron, or iron compounds in those different bodies, and the 

 intensity of the magnetic action Coulomb found from direct experi- 

 ments to be proportional to the quantities of iron contained in the 

 bodies, and he afterwards applied this principle to discover the 

 quantity of iron contained in impure metals. Faraday has however 

 shown in the discovery of diamat/netism, that a much deeper truth 

 was involved in the subject, as will be noticed more fully presently. 



From the preceding observations on the properties of the magnetic 

 fluids it will be easy to understand the principles upon which the 

 various modes of constructing artificial magnets are founded, which we 

 shall now briefly notice. The earliest method of magnetising a bar of 

 hard iron or steel was by drawing it throughout its whole extent at 

 right angles over one of the poles of a strong magnet. In this case if 

 we suppose that pole which contains the austral fluid to be used, the 

 first contact with the bar decomposes its neutral magnetisms, attracting 

 to the point of contact the boreal and repelling the austral; the 

 successive parts of the bar are subject to a similar decomposition of 

 their fluids, but it is evident that the effect of each previous decom- 

 position neutralises the succeeding, except at the extremities ; the 

 magnetism thus developed is therefore feeble, and apparent only at the 

 extremities of the bars, or in some consecutive points formed by 

 peculiarities in the material of the bar, or in the mode of operation. 

 Ur. G. Knight greatly improved the mode of magnetising bars in the 

 following manner : he joined two strongly magnetised bars by their 

 ends bearing contrary names, and placing on them in the direction of 

 their length a small steel bar heated to a cherry-red heat, with its 

 middle on the point of junction of the magnetic bars, he made each of 

 them to rub on the corresponding extremity of this steel bar, and the 

 latter when removed was found to be strongly magnetised. In this 

 method not only does the presence of the second magnet favour the 

 decomposition of the magnetic fluids, but the intensity of the action of 

 the magnetic forces is greatly increased by the elevated temperature 

 of the steel bar. 



Du Uamel placed two steel bars of equal length parallel to each 

 other, connecting their corresponding extremities by pieces of soft iron 

 interposed ; then taking two bundles of magnetic bars, he united their 

 poles of contrary name near the middle of one of the steel bars, and 

 by inclining the bundles made one of them pass towards one extremity 

 of the other bar, the second passing in tbe contrary direction, and 

 then successively repeated the operation, when both the steel bars 

 became strongly magnetised, but with contrary magnetisms at the 

 corresponding extremities of each. In this method the decomposition 

 of the neutral magnetisms of the interposed pieces of soft iron adds to 

 the effect produced by the contact of the magnetised bundles with the 

 steel bars. 



Epinus, adopting a similar method, preferred interposing strong 

 magnets instead of soft iron, the relative position of the poles of the 

 two magnets being reversed ; Coulomb combined the advantages of 

 these different methods by composing his magnetised bundles of bars 

 at a cherry-red heat. A fine steel needle may be very strongly 

 magnetised by being placed in the axis of a wire twisted into the form 

 of a helix, the extremities of which are brought in contact with the 

 wires of a powerful galvanic battery. The poles of a bar magnetised 

 to saturation are near its extremities, within generally a few linos, 

 while the intensity becomes insensible at the distance of a few inches ; 

 in a thin bar the intensity may be represented by the difference of the 

 ordmatea of two logarithmic curves, the origin of one being at the 

 austral, and of the others at the boreal extremity of the needle. 



When bodies containing neutral magnetisms are made to rotate 

 rapidly round an axis, the magnetism becomes developed and acts on 

 the needle ; thus a plate of copper made to revolve rapidly in a 

 horizontal plane will influence a compass-needle placed over it, and 

 produce in it a rotation in the same direction, on which subject several 

 valuable observations have been made by M. Arago, Sir John Herschel, 

 and others. It has also produced a second mathematical memoir from 

 M. Poisson. in which the mechanical force generated by rotation is 

 introduced into the general equations deduced from his theory of the 

 distribution of magnetism in bodies. The phenomena ar however 

 now absorbed into MAGNETO-ELECTRICITY. 



The consideration of the distribution of magnetism throughout the 



globe has led to various explanatory hypotheses since the time of 

 Halley ; the position, the number, and the motions of the points which 

 may be regarded as poles of terrestrial magnetism, have been all 

 subjects of discussion and of opinions formed on inconclusive grounds. 

 But the consideration of this subject as well as an account of the dip, 

 I variation, and intensity, at different parts of the earth, as well as the 

 consideration of the magnetic equator and poles, and the magnetic 

 observatories that have been erected in different places will be best 

 considered under TERRESTRIAL MAGNETISM. 



Some important discoveries in magnetism have been made of late 

 years, and some knowledge has been gained as to the molecular 

 changes produced by the magnetic condition. Mr. Joule has shown 

 that a bar of iron in being magnetised increases in length, while 

 its breadth diminishes. The greatest elongation observed by him 

 amounted to the ISO-thousandth of the length of the bar. He 

 also states, that the elongation is proportional to the square of the 

 developed magnetic intensity. He has also found that iron wires of a 

 certain tension diminish in length instead of increasing, and that at a 

 certain tension, there is no alteration in length. The sounds produced in 

 making an electro-magnet are noticed under ELECTRO-MAGNETISM. Cir- 

 cular magnetic polarisation is a new and interesting branch of physical 

 science, which we owe to the genius of Faraday. A few words on the 

 subject will suffice here : if in the course of a ray of light polarised by 

 reflection, a bar of heavy glass (silico-borate of lead) be placed, it will be 

 seen by a Nicol's prism that the ray is still polarised, and in the same 

 plane as before. If, however, the poles of a powerful electro-magnet be 

 brought near the bar of glass, in such a position that the direction of the 

 ray through the glass is parallel to the straight line which joins the poles, 

 a remarkable effect will be produced on making the electro-magnet. 

 Supposing the Nicol's prism to be in the position of complete ex- 

 tinction, so that no light is transmitted through it, and the voltaic 

 current be then sent through the coils of an electro-magnet, the light 

 will immediately reappear through the prism, and will continue as 

 long as the electro-magnetic force be maintained, and no longer. [PO- 

 LARISATION]. In the course of these researches, Faraday noticed that 

 when the bar of heavy glass was suspended between the poles of the 

 electro-magnet, no sooner was the magnet made, by connecting the 

 coils with the battery, than the bar of glass ceased to swing in- 

 differently, but moved round and took up a position at right angles to 

 the direction that would be taken by a bar of soft iron placed in the 

 same part of the magnetic field. The bar made a few vibrations round 

 this position, and finally settled in it ; and when displaced, it returned 

 to it, and settled as before in this position, which Faraday calls the 

 equatorial, to distinguish it from the position assumed by an iron bar 

 under the same circumstances, and which he terms the axial. 



On examining other bodies, it was found that every known sub- 

 stance when brought under the influence of a powerful electro-magnetic 

 force, assumed either an axial or an equatorial position with respect to 

 the poles. Faraday was led to regard all substances in nature as 

 magnetic : those bodies which took up an axial position, or were 

 attracted by the poles, he termed para-maynetic, while those bodies 

 which were repelled, and consequently took up an equatorial position, 

 he termed dia-magnetic. A piece of luematitic iron ore which is not 

 acted on by an ordinary magnet will poiut axially when suspended 

 between the poles of an electro-magnet ; and a sheet of writing-paper 

 rolled up into a short cylinder will also point axially in consequence of 

 the minute portion of iron or of cobalt which it contains. The salts 

 of the magnetic metals, when the latter form the base, were also found 

 to be magnetic, such for example as crystals of proto-sulphate of iron, 

 and solutions of such salts in water contained in a thin glass tube (the 

 glass not being magnetic) assumed an axial position. Solutions of 

 sulphate of nickel and sulphate of cobalt behaved in a similar manner 

 and the salts of chromium and manganese were also found to be mag- 

 netic, whence it was inferred that the metals themselves are so. A 

 stick of phosphorus assumes the equatorial position, as do also bismuth 

 and antimony ; and organic substances, such as slices of wood, apple, 

 potato, flesh, &c. These are are dia-magnetic, but not strongly so. 

 Sulphur and india-rubber are decidedly repelled. Among liquids, 

 alcohol and ether are dia-magnetic. and water still more so. Even the 

 different gases and vapours were found to be acted on by the magnet, 

 and among other modes of showing the action, soap-bubbles were 

 blown with each gas in succession upon the end of a capillary tube, 

 and being suspended near the pole of the magnet the circuit was com- 

 pleted, when the bubble was attracted or repelled according as the gas 

 was magnetic or dia-magnetic. Oxygen was found to be powerfully 

 magnetic : all other gases were found to be dia-magnetic, with the ex- 

 ception perhaps of nitrous oxide. Elevation of temperature was found 

 to increase the dia-magnetic condition, a stream of hot oxygen being 

 dia-magnetic in an atmosphere of cold oxygen. Faraday has suggested 

 that this diminution of oxygen in magnetic intensity, as the tempera- 

 ture rises, may account in some way for the diurnal variations of the 

 needle. A diminished pressure was found to lower the dia-magnetic 

 force ; the magnetic power of oxygen being diminished by a diminu- 

 tion of density. Nitrogen, which is a dia magnetic, does not seem 

 to undergo any change in its magnetic relations, by variations in 

 density and temperature. In experiments of this nature the gases 

 were enclosed in thin glass tubes in connection with a delicate torsion 

 balance. The flame of burning bodies, as of a taper for example, when 



