54 



MAGNETISM. 



actually reversed. More frequently, how- 

 ever, there arises, from this opposition 

 of powers, considerable irregularity and 

 confusion in the poles of both magnets. 

 Heat, as we have seen, impairs mag- 

 netism : care should therefore be taken 

 to avoid exposing magnets to a high 

 temperature. We should likewise be 

 very cautious to avoid all rough and 

 violent treatment of a magnet ; for we 

 have seen how quickly its virtue is lost 

 by any concussion or vibration among 

 its particles. A fall on the floor, espe- 

 cially if it strike against any hard sub- 

 stance, will materially weaken it : rub- 

 bing with coarse powders, for the purpose 

 of polishing it, and grinding, in order to 

 bring it to any required form, are equally 

 injurious. A natural loadstone will, in 

 like manner, suffer by such an operation ; 

 hence we should attempt to alter its natu- 

 ral form as little as possible ; and when 

 it is necessary to do so, it should be 

 effected very rapidly by cutting it briskly 

 in the thin discs of a lapidary's wheel. 



(214.) Although the loadstone retains 

 its magnetic virtue more tenaciously 

 than any artificial magnet that can 

 be constructed, yet even this body re- 

 quires a certain management for the 

 permanent preservation of its power. 

 For this purpose it should be armed, as 

 it is called; that is, an armature of iron 

 should be applied to both its poles. In 

 order to do this most effectually, we 

 must first ascertain the situation of the 

 poles of the loadstone ; and cutting off 

 all the superfluous parts, give it the 

 shape of a parallelepiped, having the 

 poles in the middle of two opposite sur- 

 faces, and at the same time taking care to 

 preserve the axis, which passes through 

 the poles, of as great a length as can be 

 obtained : for it has been observed, that 

 any curtailment of the magnet in the 

 direction of this line deprives it of force 

 in a greater degree than when shortened 

 in any other direction. 



(215.) Two plates of very soft iron 

 must next be provided, equal in breadth 

 to the surfaces containing the poles, and 

 a little longer than those surfaces ; so 

 that, when applied to them, a portion of 

 each plate shall project beyond the load- 

 stone to a small extent. Infix. 58 ^ r 

 represents the sections of these iron 

 plates affixed to the opposite sides of the 

 loadstone L ; and P p the projecting 

 pieces. These projecting pieces should 

 be much narrower than the other portion 

 of the plates. For loadstones weighing 

 less than an ounce, the lower surfaces 



of the projections need not exceed the 

 tenth of an inch ; and so in proportion 

 for larger loadstones. The thickness of 

 the plates, also, must be regulated by 

 the strength of the loadstone, and can 

 scarcely be determined without previous 

 trial in each particular case. The best 

 way, therefore, is to make them tolera- 

 bly thick at first ; and then file off suc- 

 cessive layers, until we find, by actual 

 experiment on the power of the loadstone 

 after each reduction, that we cease to 

 obtain any advantage ; for the power 

 increases gradually to a certain limit, 

 at which the filing ought to be discon- 

 tinued. The armature of a loadstone 

 should be fixed on it very firmly, by 

 wires, or by an external case, which 

 should be made of any metal which is 

 not susceptible of magnetism. Load- 

 stones are sometimes cut into a spherical 

 shape, in imitation of the earth, and are 

 then called terrella. Their armatures 

 should, in that case, be adapted to the 

 curvature of the surface, and should 

 each cover about a quarter of that 

 surface. 



(216.) The addition of armatures to a 

 loadstone is found to have a very favour- 

 able effect in augmenting its strength, 

 and this increase of strength goes on for 

 a considerable time after they have been 

 applied. But there is another, and a 

 still more important advantage resulting 

 from them, in enabling us to direct the 

 power of the loadstone, and to concen- 

 trate it into a small space. The polarities 

 of loadstones are often diffused over a 

 considerable part of their surface ; and 

 these scattered forces could never be 

 made to bear upon any point on which 

 they are required to act, unless by the 

 intermedium of some substance which 

 might collect and unite them. The iron 

 armatures supply this intermedium. 

 They receive at their expanded part the 

 inductive influence of all the scattered 

 poles residing in the surfaces to which 

 they are applied ; and this influence 

 being transferred to the narrow extre- 

 mity, is there concentrated, and acts 

 with full effect. By this expedient also, 

 the resultant forces, derived from each 



