184 



JSIA TUKE 



[June 21, 1883 



cisely a similar north polarity. Thus the iron is homogeneous, 

 and its polarity symmetrical. If we now magnetise this rod 

 to produce a strong south pole at its lower portion, we can 

 gradually reverse this polarity, by the influence of earth's mag- 

 netism, by slightly tapping the upper extremity with a small 

 wooden mallet. If we observe this rid by means of a direction 

 needle at all part'', and successively during its gradual passage 

 from one polarity to the other, there will be no sudden break 

 into a haphazard arrangement, but a gradual and perfectly sym- 

 metrical rotation from one direction to that of the opposite 

 polarity. 



If this rod is placed east and west, having first, say, a north 

 polarity to the right, we can gradually discharge or rotate the 

 molecules to zero, and as gradually reverse the polarity by 

 simply inclining the rod so as to be slightly influenced by earth's 

 magnetism ; and at no portion of this passage from one polarity 

 to neutrality, and to that of the opposite name, will there be 

 found a break of continuity of rotation or haphazard arrange- 

 ment. If we rotate this rod slowly, horizontally or vertically, 

 taking ob ervations at each few degrees of rotation of an entire 

 revolution, we find still the same gradual symmetrical change of 

 polarity, and that its symmetry is as complete at neutrality as in 

 evident polarity. 



In all the^e cases there is no complete neutrality, the longi- 

 tudinal polarity simply beconing transversal when the rod is 

 east and west. F, G, H, I, J, Fig. 1, show this gradual change, 

 H being neutral longitudinally, but polarised transversely. If, 

 in place of the rod, we take a smill square soft iron plate and 

 allow its m decides freedom under the sole influence of the 

 earth's magnetism, then we invariably find the polarity in the 



direction of the magnetic dip, no matter in what position it be 

 held, and a sphere of soft iron could only be polarised in a 

 similar direction. Thus we can never obtain complete external 

 neutrality whilst the molecules have freedom and do not form an 

 internal closed circle of mutual attractions ; and whatever theory 

 we may adopt as to the cause of polarity in the molecule, such 

 as Coulomb's, Poison's, Ampere's, or Weber's, there can exist 

 no haphazard arrangement in perfectly soft iron, as long as it is 

 free from all external causes except the influence of the earth ; 

 consequently these theories are wrong in one of their most 

 essential parts. 



We can, however, produce a closed circle of mutual attraction 

 in iron and steel, producing complete neutrality as long as the 

 structure is not destroyed by some stronger external directing 

 influence. 



Oersted discovered that an external magnetic needle places 

 itself perpendicular to an electric current ; and we should expect 

 that, if tlie molecules of an iron wire pos es.^ed inherent polarity 

 and could rotate, a similar effect would lake place in tbc i iterior 

 of the wire to that observed hy Oersted. Wiedemann first 

 remarked this effect, and it has been known as circular mag- 

 netism. This circle, however, consists really in each molecule 

 having placed itself perpendicular to the current, simply obeying 

 Oersted's law, and thus forming a complete circle in which the 

 mutual attractions of the molecules forming that circle are 

 satisfied, as shown at C, Fig. 1. This wire becomes completely 

 neutral, any previous symmetrical arrangement of polarity 

 rotating to form its complete circle of attractions ; an"! we can 

 thus form in bird iron and steel a neutrality extremely difficult 

 to break up or destroy. We hive evident pro if that this 



neutrality consists of a closed chain, or circle, as by torsion we 

 can partially deflect them on either side ; thus, from a perfect 

 externally neutral wire, producing either polarity, by simple 

 mechanical angular displacement of the molecules, as by right - 

 or left-handed torsion. 



If we magnetic a wire placed east and west, it will retain this 

 polarity until freed by vibrations, as alreidy remarked. If we 

 pass an electric current through this magnetised wire, we can 

 notice the gradual rotation of the molecules, and the formation 

 of the circular neutrality. If we commence with a weak current, 

 gradually increasing its strength, we can rotate them as slowly 

 as may be desired. There is no sudden break or haphazard 

 moment of neutrality : the movements to perfect zero are 

 accomplished with perfect symmetry throughout. 



We can produce a more perfect and shorter circle of attrac- 

 tions by the superposition of magnetism, as at B, Fig. I. If we 

 magnetise a piece of steel or iron in a given direction with a 

 strong magnetic directing power, the magnetism penetrates to a 

 certain depth. If we slightly diminish the magnetising power, 

 and magnetise the rod in a contrary direction, we may reduce it 

 to zero by the superposition of an exterior magnetism upon one 

 of a contrary name existing at a greater depth ; and if we con- 

 tinue this operation, gradually diminishing the force at each 

 reversal, we can easily superpose ten or more distinct symmetri- 

 cal arrangements, and as their mutual attractions are satisfied in 

 a shorter circle than that produced by electricity, it is extremely 

 difficult to destroy this formation when once produced. 



The induction balance affords also some reasons for believing 

 that the molecules not only form a closed circle of attractions, 

 as at u, but that they can mutually react upon each other, so as 

 toclo-e a circle of attractions as a double molecule, as shown at 

 A. The experimental evidence, however, is not sufficient to 



dwell on this point, as the neutrality obtained by superposition 

 is somewhat similar in its external effects. 



We can produce a perfectly symmetrical closed circle of 

 attractions of the nature of the neutrality of C, Fig. I, by form- 

 ing a steel wire into a closed circle, 10 centimetres in diameter, 

 if this wire is well joined at its extremities by twisting and 

 soldering. We can then magnetise this ring by slowly revolving 

 it at the extremity of one pole of a strong permanent magnet ; 

 and, to avoid consequent poles at the part last touching the 

 magnet, we should have a graduating wedge of wood, so that 

 whilst revolving, it may be gradually removed to greater distance. 

 This wire will then contain no consequent points or external 

 magnetism : it will be found perfectly neutral in all parts of its 

 closed circle. Its neutrality is similar to c, Fig. I ; for if we 

 cut this wire at any point we find extremely strong magnetic 

 polari y, being magnetised by this method to saturation, and 

 having retained (which it will indefinitely) its circle of attractions 

 complete. 



I have already shown that soft iron, when its molecules are 

 allowed perfect freedom by vibration, invariably takes the 

 polarity of the external directing influence, such as that of the 

 earth, and it does so even with greater freedom under the influ- 

 ence of heat. Manufacturers of electromagnets for telegraphic 

 instruments are very careful to choose the softest iron and 

 thoroughly anneal it ; but very few recognise the importance as 

 regards the position of the iron whilst annealing it under the 

 earth's directing influence. The fact, h iwever, has long since 

 been observed. 



Dr. Hooke (1684) remarked that steel or'iron was magnetised 

 when heated to redness and placed in the magnetic meridian. I 

 have slightly varied this experiment by heating to redness three 

 similar steel bars, two of which had been previously magnetised 



