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, which we may represent by the letter. N ami S. When. 



formed ot this material U suspended boruontally on pivot 

 farnuhcd with u ste cap for the purpose of diminuhing friction, it 

 usiimsi a particular direction, Marly north and south, f MAUXCT.] 

 The pole X, at the north extremity of the needle thui adjusted, U 

 commonly called the nortA p4e of the needle ; the other, 8, the <oU 

 ftlf, though the contrary name*, a* uaed by Dr. Gilbert, would be 

 more correct in connection with the theory of magnetism. 



If we now bring a piece of eoft iron near the pole X, it will be 

 attracted to that pole and become attached to it, so that the exertion 

 of a mechanical force U neoeatary to separate them. In thui way a 

 magnet held vertically will curtain a piece of iron, provided the weight 

 of the iron doee not exceed the magnetic force. The pole S has a 

 rimilar attractive power on iron ; the cause of this attractive power i* 

 called MAGNETISM. 



We have obaerved that in a magnetic needle placed horizon tally on a 

 pivot, the pole N u turned northwards, and S southwards, nearly : if 

 raco a needle be attached to a piece of cork floating on water, it will 

 adjust rUelf to thia direction, the deviation of which from the true 

 north and sooth line U the dtflitation of the needle. If now we invert. 

 the iitiun of the needle, so that S is brought into the place pre- 

 viously occupied by N, and rice vend, the needle and cork will make a 

 complete revolution, and acquire its original position. Hence we see 

 a distinction between the magnetisms predominant at N and at S ; the 

 former U called Atutral, and the latter Boreal magnetism. It will be 

 easy to observe the analogy between the mutual relations of the two 

 magnetisms, and those of positive with negative electricities. 



We must insulate a conducting electrised substance in order to pre- 

 serve ita electricity, but this is not necessary in the case of a magnet; 

 each fragment of the latter is itself a magnet, possessing its north and 

 south poles, and the same view may be extended to its constituent 

 particles. A non-conducting energy, called the eoercire power, exists 

 therefore in magnetic substances, by which the loss of magnetism when 

 developed is prevented, and by which also the poles N and S are situate 

 in a determinate position relative to the body of the magnet. This is 

 not the case with soft iron, which has not the coercive force. 



The effect of the magnetic forces is considerably modified when 

 substances which are capable of readily becoming magnetic by influence 

 are situated near the magnet ; and a similar effect takes place by the 

 decomposition of the neutral electricities when under the influence of 

 an electrised body. [ELECTRICITY.] The transmission of the magnetic 

 force through interposed bodies may be observed familiarly by placing 

 a common sewing-needle on a smooth horizontal board, and moving a 

 strong magnet underneath the board : the needle will roll or revolve 

 along the board according to the peculiar motions given to the magnet. 



Let us next consider the action of magnets on each other. For this 

 purpose make two magnets or magnetic needles to float on water, 

 distinguishing the poles of one as before by N and S, and of the other 

 by y and S'. Bring either the pole N near to N', or S to S' : the 

 needles or magnets will separate to a greater distance, and with the 

 greater energy the nearer these poles are placed to each other. On 

 the contrary, if we bring N and S' near each other, the needles will 

 approach and unite those points, and the same happens when the 

 pouu N' and 8 are made contiguous : hence this law magnctitmt of 

 At tame name art mutually repubirt; tkote of contrary nama art 

 mutually atlraciirt. In the article ELECTRICITY, we have shown that 

 the same law is true with respect to the two electricities. 



The mass of the globe contains various sources of magnetism 

 [MAGXET] ; and since a magnetic needle freely suspended acquires a 

 determinate position, it follows from this law that the magnetiam at 

 the south extremity 8 is Boreal, that is, of the same name as the 

 terrestrial magnetism which is predominant in the northern hemisphere, 

 being repelled therefrom ; and the uuguetism at the north extremity 

 N u for a like reason Austral. The law of magnetic force at different 

 diitsnos is expressed by the inverse square of the distance : one mode 

 of verifying this law U by observing the times of the oscillations of a 

 small fine wire, sunpended in a pliine perpendicular to the magnetic 

 meridian (in order to neutralise the magnetising influence of the earth), 

 and subjected to the action of a powerful magnet. 



We can, by combining these laws, explain the manner in which soft 

 iron, cobalt, sad nickel are attracted or suspended by a magnet 

 TbeM metals, when chemically pure, contain both the austral and 

 boreal magnetism in a combined state, in consequence of their want of 

 coercive power. When a piece of soft iron U brought near the pole N, 

 which contains the austral fluid, the austral magnetism of the iron is 

 repelled to the farther extremity, and the boreal attracted to the nearer 

 extremity of the iron relative to the point N : this disposition of the 

 fluid takes place immediately, and the law of force above announced 

 relative to the distances causes the attraction of the fluid at N, on the 

 boreal fluid of the toft iron, to nosed its repulsion on the austral, 

 which u more remote from N : the total effect, in virtue of this excess, 

 m therefor* necessarily attractive. When the iron however is removed 

 trout this influence, iu natural magnetums again recornbine. This will 

 not be the case if, instead of soft iron, we use hardened iron or steel : 

 the decomposition of the natural magnetism* takes place with greater 

 difficulty, in consequence of the coercive power which protects their 

 actual disposition j but if we use a powerful magnet at one extremity 

 of a steel needle, or, which U more effectual, a pair of strong magnets 



at both extremities, the north pole of one and the south pole of the 

 other being brought in contact with the needle, the decomposition will 

 be partially effected, and will likewise be retained by the same coercive 

 power which opposed its development ; and agreeably with the magnetic 

 laws of repulsion and attraction, that point of the needle in contact 

 with the south pole will become a north pole of the needle, and the 

 other a south 'pole. This method of producing magnetism is liable to 

 the objections both of producing feeble magnetism and also producing 

 consecutive points. 



The quantities of the austral and boreal magnetic fluids in all mag- 

 netic bodies are equal ; for when we bestow magnetic qualities on iron 

 or steel by the influence of loadstones, hammering, sudden cooling in 

 the magnetic meridian, 4c., no new magnetism is communicated ; but 

 the natural magnetisms, which previously neutralised each other, are 

 now decomposed. Again, if a magnetic needle be freely suspended by 

 its centre of gravity, the action of terrestrial magnetism produces no 

 linear motion, but only imposes a direction on the magnetic axis : now 

 all the boreal fluid in the globe attracts all the austral fluid of the 

 needle, and rice rertd, while the like fluids in both repel ; hence a 

 motion of progression would be generated, unless the resultant of the 

 repulsive forces on the needle was exactly equal and of an opposite 

 direction to the resultant of all the attractive forces ; and the rotatory 

 motion of the needle shows that the points of application of these 

 forces are different ; but the intensity of terrestrial magnetism may be 

 regarded as uniform throughout the extent of the needle, and ite 

 direction paralli.'l. In order therefore that the resultants should be 

 equal and contrary, the sums of the boreal and austral forces of the 

 needle must be equal In this respect magnetism resembles the natural 

 electricities of all substances. 



The development of magnetism in bodies, whether by terrestrial 

 action or the influence of loadstones, is analogous to the decomposition 

 of the natural electricities in a system of conducting bodies separated 

 by non-conductors under the influence of an external body and their 

 own natural action : hence when magnetism is communicated by a 

 loadstone, even when in contact, the latter loses none of its own mag- 

 netism, as it acts solely by influence ; whereas in conducting electrised 

 bodies, contact will communicate electricity : the coercive force of 

 magnets therefore extends even to their surfaces. In fact the reaction 

 of the substance magnetised by influence tends to a further decompo- 

 sition of the fluids of the magnetising body, and this gives it greater 

 energy, unless when it is magnetised to saturation, that is, when the 

 internal magnetic forces are equal to the coercive power ; for then any 

 further development of the fluids would be only temporary, and a 

 reunion would take place immediately. 



The dipping-needle is a magnetic needle, the opposite poles of which 

 possess equal magnetic intensities. It is attached to the centre of a 

 vertical circle, and its motion is confined to the plane of this circle. 

 Tim circle has a motion in azimuth about a vertical axis, and within a 

 fixed horizontal circle, both circles being graduated. When the vertical 

 circle is turned round its axis until the needle acquires a vertical 

 position, the plane of the circle is then perpendicular to that of the 

 magnetic meridian, and hence by means of the horizontal circle the 

 position of this meridian plane becomes known. The vertical circle 

 with the needle is now brought to coincide with the meridian plane, 

 and the angular depression of the north pole of the horizon, or more 

 strictly of the magnetic axis, may be read off the graduated limb of the 

 veitical circle, and measures the dip. The right Hue joining the north 

 and south poles of the needle is nearly coincident with the magnetic 

 axis, but the latter may be ascertained more accurately by inverting 

 the needle and taking the mean direction between its two positions of 

 equilibrium. There are other methods of adjusting the dipping- 

 needle, but in every method it requires great delicacy and minuto 

 attention to all parts of the adjustment. 



If we place a bar of soft iron, suspended by a collection of bilk 

 strings at ita middle, in a direction parallel to the magnetic axis of the 

 dipping-needle, the action of terrestrial magnetism will have full effect 

 on the bar, ita natural magnetisms will be decomposed, and it will 

 acquire a polarity similar to that of the needle, its poles repelling the 

 similar polos of the needle, and attracting the contrary poles. Its wont 

 of coercive power prevents it from retaining the polarity of its different 

 parts when the bar is moved into other positions ; for if we invert the 

 position of the bar, that point which was primitively the north pole 

 will now become the south, and vice vend, under the effects of a new 

 decomposition of its magnetisms by terrestrial influence. If however 

 the bar be left for a long time in the direction of the magnetic axis, or 

 if it be heated to a red heat and suddenly cooled by immersion in 

 water, it will acquire a coercive force, and become permanently mag- 

 netic. Iron crosses, weathercocks, Ac., which have been long kept in a 

 fixed position, or have been struck by lightning, acquire magnetic 

 properties in the manner above described. 



It is a remarkable circumstance connected with the change of mole- 

 cular disposition caused by the action of heat, that if we gradually heat 

 a bar of iron, the intensity of its action on a magnet increases, and 

 arrives at a maximum when the bar U brought to a cherry-red heat 

 VM'|I higher degrees of heat the intensity is diminished, and is totally 

 inappreciable when the bar has reached a bright white heat ; on cooling 

 it recovers its powers of action by similar steps, and the same law holds 

 true if the magnet be heated instead of the bar. Hence iu producing 



