LESSONS IN ELECTRICITY. 



380 



Jone many things that I have not told you to 

 do, and thus multiplied your experience be- 

 yond what 1 have indicated. You ate almost 

 bure to have caused a bit of iron to hang from 

 tkt end of your magnet, and you hare prob- 

 ably succeeded in causing a second piece to 

 attach itself to the first, a third to the second ; 

 until finally the force lias become too feeble 

 to bear the weight of more. If you have 

 operated with nails, you may have observed 

 that the points and edges hold together with 

 the greatest tenacity ; and that u bit of iron 

 clings more firmly to the corner of your raag- 

 net than to one of its flat surfaces. In short, 

 you will, in all likelihood, have enriched your 

 experience in many ways without any special 

 direction from me. 



Well, the magnet attracts the nail, and that 

 nail attracts a second one. This proves tiiat 

 the nail in contact willi the magnet has had 

 the magnetic quality developed m it by that 

 contact. If it be withdrawn from the mag- 

 net, its power to attract its fellow-nail ceases. 

 Contact, however, is not necessary. A sheet 

 of glass or paper, or a space of air, may exist 

 between the magnet and the nail ; the latter 

 is still magnetized, though uot so forcibly as 

 when in actual contact. The nail then pre- 

 sented to the magnet is itself a temporary 

 magnet. That end which is turned toward 

 the 'magnetic pole has the opposite magnetism 

 of the pole which excites it ; the end most 

 remote from the pole has the same magnet- 

 ism as the pole itself, and between the two 

 poles the nail, like the magnet, possesses a 

 magnetic equator. 



Conversant a.'? you now are with the theory 

 of magnetic fluids, you have already, I doubt 

 not, anticipated me in imagining the exact 

 condition of the iron under the influence of 

 the magnet. You picture the iron as possess- 

 ing the neutral fluid in abundance ; you pic- 

 ture the magnetic pole, when brought near, 

 decomposing the fluid ; repelling the fluid of 

 a like kind with itself, and attracting the un- 

 like fluid ; thus exciting in the parts of the 

 iron nearest to itself the opposite polarity. 

 But the iron is incapable of becoming a per- 

 manent magnet. It only shows its virtue as 

 long as the magnet acts upon it. What, then, 

 does the iron lack which the sluel possesses? 

 It lacks coercive force. Its fluids are sepa- 

 rated with ease, but, once the separating 

 cause is removed, they flow together again, 

 and neutrality is restored. Y our imagination 

 must be quite nimble in picturing these 

 changes. You must be able to see the fluids 

 dividing and reuniting according as the mag- 

 net is brought near or withdrawn. Fixing a 

 definite pole in your imagination, you must 

 picture the precise arrangement of the two 

 fluids with reference to this pole. And you 

 nust not only be well drilled in the use of 

 this mental imagery yourself, but you must 

 bo- able fo arouee the same pictures in the 

 minds of your pupils, and satisfy yourself 

 that they possess this power of placing iiciu- 

 ally before themselves magnets and iron in 

 various positions, and describing the exact 

 magnetic slate of the iron, in each j/ailicultir 



case. The mere facts of magnetism will have 

 their interest immensely augmented by an 

 acquaintance with those hidden principles 

 whereon the facts depend. Still, while you 

 use this theory of magnetic fluids to track out 

 the phenomena and link them together, bo 

 sure to tell your pupils that it is to be regarded 

 as a symbol merely a symbol, moreover, 

 which is incompetent to cover all the facts,* 

 but which does good practical service while 

 we arc waiting for the actual truth. 



This state of excitement into which the soft 

 iron is thrown by the influence of the magnet, 

 is sometimes called "magnetization by in- 

 fluence." More commonly, however, the 

 magnetism is said to bo "induced" in the 

 soft iron, and hence this way of magnetizing 

 is called " magnetic induction." Now, there 

 is nothing theoretically perfect in Nature : 

 there is no iron so soft as not to possess a 

 certain amount of coercive force, and no steel 

 so hard as not to be capable, in some degree, 

 of magnetic induction. The quality of steel 

 is in some measure possessed by iron, and the 

 quality of iron is shared in some degree by 

 steel. It is in virtue of this latter fact that the 

 unmagnetiztd darning needle was attracted 

 in your first experiment ; and from this you 

 may at once deduce the consequence that, 

 after the steel has been magnetized, the ;e- 

 pulsive action of a magnet must be always 

 less than its attractive action. For the re- 

 pulsion is opposed by the inductive action of 

 the magnet on the steel, while the attraction 

 is assisted by the same inductive action. 

 Make this clear to your miuds, and verify it 

 by your experiments. In sume cases you can 

 actually make the attraction due to the tem- 

 porary magnetism overbalance the repulsion 

 due to the permanent magnetism, and thus 

 cause two poles of the Fame kind apparently 

 to attract each other. When, however, good 

 hard magnets act on each other from a suffi- 

 cient distance, the inductive action practi- 

 cally vanishes, and the repulsion of like poles 

 is sensibly equal to the- attraction of unlike 

 ones. 



I dwell thus longon elementary principles, 

 because they are of the first importance, and 

 it is the temptation of this age of unhealthy 

 cramming to neglect them. Now follow me 

 a little further. In examining the distribu- 

 tion of magnetism in your strip of steel, you 

 raised the needle slowly from bottom to top, 

 and found what we called a neutral point at 

 the centre. Now does the magntt really ex- 

 ert no influence on the pole presented to its 

 centre ? Let us pee. 



Let S N, Fig. 1, be your magnet, and let i 

 n represent a particle of north magnetism 

 placed exactly opposite the middJe of the 

 magnet. Of course this is an imaginary case, 

 as you can never in reality thus detach your 

 north magnetism from its neighbor. What is 



* This theory breaks down when applied to diamag- 

 netic bodies, which are repelled by magnets. Like 

 soft iron, fnch bodies nre thrown into a state of tem- 

 porary excitement in virtue of which they are repelled, 

 bur any attempt 10 explain wirh. a repulsion by the de- 

 composition of a fluid -will demonstrate its own 

 futility. 



