1854.] 



BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 



Ill 



On some Peculiarities of the Magnetic field: hy Professor Ttndall. 



The Professor saiJ, a piece of soft iron suspended between the flat 

 poles of an electro-magnet set its lai'gest dimension from pole, the 

 residual magnetism of the cores being sufficient to produce the effect. 

 This is the normal deportment of magnetic bodies, but it is by no 

 means universal. By mechanical agency, by pressure for example, 

 the structure of a magnetic body can be so modified that its shortest 

 horizontal dimension sets from pole. Prof. TyndaU exhibited actions 

 of the kind where the body operated on was compressed magnetic dust. 

 In such a body two opposing tendencies were at work, — the tendency 

 due to length, which sought to set the length axial, and the tendency 

 due to structure, which sought to set the line perpcncUcular to the 

 length axial. Between the flat poles the latter tendency was predom- 

 inant, but between pointed poles this was not the case ; here the 

 attraction of the ends of the magnetic mass constituted a mechanical 

 couple of sufficient strength to overcome the directive tendency which 

 was due to structure, and to draw the mass into the axial line. But 

 in raising or lowering the body operated on out of the sphere of this 

 local attraction, by bringing it into a position where the distribution of 

 the magnetic field resembled that existing between the flat poles, the 

 body forsook the axial position and turned into the equatorial. Tlie 

 complementary phenomena were exhibited by bismuth. A normal 

 bar of tliis substance sets its length at right angles to the line from 

 the poles ; but Prof. Tyndall exhibited a bar of this substance, which 

 set between the flat poles exactly as a magnetic body. Such a bar, 

 however, between the points set equatorial. On raising it or lowering 

 it, however, it forsook the equatorial position and set axial. In this 

 case the local repulsion of the ends between the points caused the bar 

 to set equatorial, the influence of length thus predominating over the 

 influence of structure ; but removed from the sphere of this local 

 action, the directive tendency of the mass triumphed and caused the 

 bar to set axial. The bar in this case was cut with its length at right 

 angles to the planes of most eminent cleavage of the bismuth: — it is 

 a proved fact, that these planes while the influence of form is armullcd, 

 always set at right angles to the line piercing the poles, and hence 

 where they are transverse to the length, the bar will set axial. These 

 phenomena were examined in a great number of cases ; bars were 

 taken from substances possessing a directive tendency, and it was so 

 arranged that the directive tendency due to structuj-e was always op- 

 posed to the influence of length ; between the points the former 

 tendency succumbed to the latter, while between the flat poles, or 

 above and below the points, the former was triumphant. It is amusing 

 to observe the strife of these two tendencies in substances possessing a 

 strong directive action. A plate of crystallized carbonate of ii-on, 

 when properly suspended, will wrench itself spasmodically from one 

 position into the other, and find rest nowhere. The simple law which 

 governs all these actions is, that if the body, cut as above, be 

 diamagnetic, its length sets equatorial between the points, but above 

 and below them axial. If the body bo magnetic it sets axial between 

 the points, above and below equatorial. Hence the rotation of a mag- 

 netic body, on being removed from between the points, is always from 

 axial to equatorial ; while the corresponding rotation of a diamagnetic 

 body is always from the equatorial to the axial. The deportment of 

 wood in the magnetic field was next described. Nearly sixty specimens 

 examined by Prof. Tyndall were all diamagnetic ; each of them was 

 repelled by the poles of the magnet ; cubes of each when suspended 

 with the fibre horizontal set between the excited poles, the fibre per- 

 pendicular to the line which unites (he poles. Thinking that wood, on 

 account of its structure, would exhibit those directive phenomena 

 which had been demonstrated in the case of the bodies mentioned at 

 the commencement, bars were taken from nearly forty kinds of wood, 

 the fibre being at right angles to the length of tho bar ; in the centre 

 of the space, between two flat poles, all those bars set their lengths 

 from polo to pole. But Prof. Tyndall afterwards observed the remark- 

 able fact, that homogenous tliaraagnetic bodies did the same. Bars of 

 Bulphur, of salt of hartshorn, of wax, and other diamagnetic substances, 

 when suspended in tho centre of the space between two flat poles, set 

 their lengths from pole to pole. Now, as diamagnetic bodies always 

 take up the position of weakest force, it was proved by these experi- 

 ments, and corroborated by others not cited here, that the true force of 

 the centres of the two flat poles contrary to tbo general opinion hitherto 

 received, was the line of minimum force. 



The Rev. Dr. Scoresby stated, that, by subjecting to force ordinary 

 magnets of liardcned steel, as by suddenly bending them, or striking 

 tlieni in particular modes, they may have tlicir poles reversed or bo 

 deprived of their mngaetism, or hardened uou-magnelic steel may be 



instantly rendered magnetic and he considered that these facts, which he 

 had long since made public, should be keptbefore the mind in such inves- 

 tigations as the very original and interesting facts just brought under tho 

 notice of the Section. — Prof. Faraday, after very briefly, yet lucidly, 

 explaining to the Section the leading distinctions between paramagnetic 

 and diamagnetic bodies, and their behaviour in the magnetic field, said 

 that it was conceded on all hands that the explanation was erroneous, 

 which Pliicker had given of the phenomena which he first discovered con- 

 nected with the branch of research to which Prof. TyndaU had just been 

 directing their attention, and which he was so ably hunting down. 

 But when he said the original explanation of Pliicker was erroneous 

 he did not mean that as the slightest disparagement to that philosopher. 

 It was well understood by all who had any pretensions to ecieutifio 

 knowledge since the days of Bacon, that it was through the mist of 

 error that the most important discoveries had to be made, and that in 

 pursuing any research it was much better in the first stages of the 

 inquiry to have erroneous views, than to be without any views that 

 would tend to connect the scattered facts. For his part, he was not 

 ashamed to own that he was a learner, and that in almost every in- 

 stance it was through the clouds of en'or that he arrived at tho 

 conclusions which satisfied him most. And as his mathematical skill 

 and acquirements were by no means such as to entitle him to despise 

 instruction, he should feel particularly gi-ateful to his mathematical 

 friends present. Dr. Whewell and others, If they would explain to him 

 and to the Section the law of distribution of the magnetic force in tho 

 magnetic field, if it was knoivn. — Dr. Whewell explained how tho force 

 would be distributed upon the old theory of magnetic lines ; but he 

 said he was aware, and he believed it was now generally admitted, that 

 this theory must be gi-eatly modified, if not given entii-ely up. But as he 

 saw Prof. W. Thomson in the Section, who had paid particular attention 

 to the developement of the matliematical theory of magnetical and 

 electrical forces, he trusted that that gentleman would favour the 

 Section with his views. — In answer to Prof. Faraday's question, as to 

 the mathematical conditions under which a uniform field of magnetic 

 force may be produced. Prof. W. Thomson remarked that the mathe- 

 matical theory of the distribution of force both afforded a remarkably 

 simple and definite general answer, and pointed out the most convenient 

 practical means of fulfilling these conditions either approximately or 

 rigorously. For, in the first place, it is strictly demonstrable that if 

 the force be rigorously uniform iu some locality, in the neighbourhood 

 of any kind of magnet or electro-magnet, through even one one- 

 thousandth of a cubic inch, in fact, through any finite bulk however ' 

 small, it carmot but be vigorously uniform through every portion of space 

 to which it is possible to go from that locality without passing through 

 the substance of the magnet. Hence, although between flat poles, 

 such as Mr. Faraday first inti'oduced for obtaining uniformity of force, 

 we have in reality a most excellent practical approximation to a uni- 

 form distribution of very intense magnetic force, through a space of 

 several cubic inches, in a locality not only visible but in every way 

 convenient for experimental pm-j)oses : yet it is absolutely impossible 

 that the force can be rigorously unifoiin through the smallest finite 

 bulk of the magnetic field in any such arrangement, or generally, in 

 any locality external to a magnet. If an experimenter wants a rigo- 

 rously uniform field of force, he can only have it in the interior of his 

 magnet ; and he must be contented not to see the action he experiments 

 on at the time it is being produced, unless he will follow the example 

 of Prof. Faraday, who "went into a hollow cubical conductor of 

 electricity and lived in it," and so was enabled to observe some most 

 interesting and important fundamental proper-ties of electrical force. 

 It would be easy to make a hollow electro-magnet, in the interior of 

 which the experimenter could obseiTC with the minutest accuracy the 

 bearings of all kinds and sh.apes of bodies in a vigorously uniform field 

 offeree. All that is necessary to make such a conductor is to take a 

 hollow papier muchd globe, say six feet in diameter, and roll galvanic 

 wire over its surface in a succession of close parallel circles, having 

 their planes at equal distances from one another. A hollow non- 

 magnetic body of any shape, cubical for instance, may have a rigo- 

 rously uniform distribution of magnetic force produced in it-s interior 

 bya suitable distribution of galvanic wire over its surface, determinable 

 according to tlio form of this surface, by the mathematical tlieory from 

 which these results are stated. But it would bo difficult, perhaps 

 practically impossible, to get a sufficient intensity for exhibiting tho 

 forces experienced by diamagnetic or weakly paramagnetic bodies in a 

 uniform field of such extent that the operator could himself enter it ; 

 anil experimenters must bo contented eitlier Avith appro.ximationa to 

 uniformity, such as in the arrangement with Hat poles, so successfully 

 used by Prof. Tyndall iu tho beautiful cxjierimcuts which he had ex- 



