IM 



IM.I LA TORI r"i:i -KS.--M \..M. PISM. 



[MAGNETIC ATTRACTION. 



natural loadstone-~but if tho magnetised needle be rubbed 

 against another one, the Utter will also exhibit similar 

 properties to that from which it acquired them. 



The extent of magnetic force so communicated, depends 

 on the size of the loadstone and of the piece of steel which 

 is magnetised. In every instance in which force of any 

 kind is communicated to any body, there is a definite 

 relation between the body possessing and that receiving 

 a force, with respect to their man. This is a natural 

 consequence of the constitution of any mass, because its 

 atoms have, either chemically or dynamically, a fixed 

 amount of attractive power, which they can exert on 

 any proximate atom within their influence. This 

 power can be mathematically calculated. Tho circum- 

 stances may vary considerably, but the laws are in 

 every case constant ; and the attraction of gravitation is 

 perhaps the boat illustration which we can adduce to 

 prove our position. For instance, we may suppose that 

 any body exerts an attractive power on another near 

 to it Such a power must necessarily have some relation 

 to the mass or other qualities of the body which it 

 attracts, and tho distance existing between. This re- 

 lation or comparative effect of force may bo ascertained 

 in a variety of ways. In the case of gravitation, the time 

 occupied by any body in falling from any height, may 

 become a measure of the force. But in the case of 

 magnetism there is no such opportunity of calculating 

 the value or amount of its force. There exists a kind of 

 antagonism in the magnetic effect, which tho earth, and 

 the electric currents which circulate around it, exert. It 

 is true that we can, by tho oscillations of a magnetic 

 needle, approximate towards an idea of tho amount of 

 magnetic attraction which may exist between terrestrial 

 magnetism, and an artificially induced magnetic arrange- 

 ment. But still we are left in doubt as to the exact 

 proportion ; and the varied affections which may exist 

 between tho two phases of the some force, interfere con- 

 siderably. For instance, there is a daily variation in 

 the polar direction of the magnetised needle. This 

 alone is sufficient to show that a considerable uncer- 

 tainty exists between the indications afforded by our 

 best arrangements, and the permanent, or continuous 

 and regular, indications which tho earth affords of its 

 magnetic intensity. 



These remarks may at first sight seem impertinent 

 to the subject- on which we are treating : but such is not 

 the case ; because we shall have to examine into the com- 

 parative power of various magnetic arrangements, and 

 the causes which influence it. 



We may hero remark, that a straight or bar magnet 

 has two ends, which exhibit the greatest amount of 

 magnetic force ; and that the point half-way between 

 the extremities seems entirely destitute of magnetism. 

 Hence, in making an artificial magnet, special attention 

 is paid to these extremities, or poles, as they are fre- 

 quently termed. A horse-shoe magnet is simply a bar 

 magnet bent into the form of a horse-shoe ; and tho 

 advantages which this form presents is, that tho two 

 poles are brought near to each other, and can so exert 

 simultaneously their power on any object presented to 

 Fig. 1. them. 



When a number of horse-shoe mag- 

 nets are bound together, they form 

 what is colled a compound magnet. 

 One of these is illustrated in tho an- 

 nexed engraving, in which K and S 

 represent respectively the north and 

 south polos of tho magnet, and K is 

 what is called the keeper. This is a 

 piece of soft iron placed at tho poles 

 <>f a horse-shoe magnet, for tho pur- 

 pose of preventing any loss of power. 

 We may here remark, that either 

 straight or horse-shoe magnets should 

 always have a piece of soft iron, or a 

 quantity of iron filings, at the poles; 

 which, by their reaction on tho magnet, prevent its losing 

 its attractive force. 

 There are two modes.of making magnets ; the one by 



touch we have already alluded to : that by means of vol- 

 taic electricity wo shall describe under the head of Electro- 

 Magnetism. The simplest mode of making a bar magnet, 

 is that of rubbing a piece of good steel, continually, by 

 means of a compound horse-shoo magnet, such as we have 

 just described. This, however, must bo done in a proper 

 manner ; tho steel boing placed on a level table so as to 

 be rubbed from its centre towards its extremity l>y one 

 pole of the horse-shoo magnet ; and this will induce in it 

 a magnetism of an opposite character. Tho other ex- 

 tremity is then to be rubbed in a similar manner, from 

 tho centre of the steel, by the other pole of tho horse- 

 shoe magnet ; and this is to be repeated until the bar is 

 rendered as magnetic as possible. This, as we have 

 already mentioned, will depend on the quality of steel, 

 and the strength of the magnet employed. Bar mag- 

 nets are best kept in pairs, and tho north polo should 

 be marked on each. In laying them aside, tho north 

 pole of one should be in contact with tho south pole of 

 the other a plan which, like that of the keeper, prevents 

 a loss of power. 



3. MAGNETIC ATTRACTION, REPULSION, AND POLARITY. 

 We have already had to refer, in our previous ob- 

 servations, to the fact, that a magnet has the power 

 of attracting pieces of iron or steel towards it. We 

 have also mentioned, that the extremities of a magnet 

 are termed "poles." Wo shall now enlarge on these mat- 

 ters, and investigate some interesting phenomena, and 

 applications depending on them. 



If a piece of steel, in the shape of a long rod or bar, 

 be magnetised, and then hung by a silk thread so that 

 it may move freely, it will be found to take up a position 

 pointing nearly north and south, if no disturbing cause 

 be present. This result indicates that some force must 

 act on the magnetised needle; and such is tho magnet ism 

 of the earth itself. If, while the bar is thus hanging, 

 another magnet of equal power and size be presented to 

 it, pole to polo, the first needle will be either repelled or 

 attracted. If a north pole of one be presented to the 

 south pole of tho other, or vice vend, then those will 

 attract each other. If, however, the north poles or the 

 south poles be presented to each other, then they will 

 repel instead of attracting, as in the last case. 



From this experiment we learn that similar poles 

 repel, whilst dissimilar poles attract each other ; and 

 this law holds good in every shape or form which a 

 magnet may possess. 



The simplest mode of trying this is by magnetising 

 two sewing-noodles, and floating one of them in water, 

 which is easily done by gently laying it on the surface. 

 The film of air always attached to polished surfaces 

 enables it to swim on the liquid. The polos of the 

 other needle may be alternately presented to either end 

 of that swimming on tho water, and the phenomena of 

 attraction, repulsion, and polarity conveniently studied. 



It will be found, that as tho distance between the 

 two needles is increased, the effect they mutually exer- 

 cise is also decreased ; and the ratio of this decrease will, 

 for any two points in each magnet, bo, like all radial 

 forces, as the square of distances. It must, however, 

 be borne in mind, that we refer here to two mathema- 

 tical points, rather than to tho extension of a magnetised 

 surface : for it is evident that tho magnets, being of con- 

 siderable length and breadth, tho force they exercise 

 will be given off in many directions. Could wo obtain 

 a magnet in tho form of a sphere, then wo should doubt- 

 less find that the force obeyed the laws above-mentioned ; 

 but as, in all ordinary forms of magnets, the length 

 exceeds tho breadth and hence the laws of force must 

 bo of unequal length so the law of radial forces will 

 not have an exact application practically. These obser- 

 vations apply equally to the attracting and attracted 

 body. 



It must also be kept in mind, that the poles, or chief at- 

 tracting portions of a magnet, are not at its extremities. 

 They exist at a short distance nearer to the centre, and 

 cqui-distant from that point ; and thus, if the two ends 

 of a horse-shoe or bar magnet were slightly hollowed 

 out between tho two sides, the chief centre of attraction 



