POLARITY OF THE DIAMAGNETIC FORCE.

Introduction, 1870.

SOON after the discovery of diamagnetism, Professor Reich, of Freiburg, made the following very important experiment. Placing a ball of bismuth on a torsion balance which had been previously employed in determinations of the density of the earth, he found that magnet bars, on being brought up in a horizontal direction to the case near the ball, produced a very distinct repulsion, both when the north and the south pole were brought near.

But when several similar bars were brought near, half with their north and the other half with their south poles, there was no effect perceptible, or merely a slight one arising from the inequality of the magnets employed."* Prof. W. Webert immediately saw the bearing of this result on the character of diamagnetism. From this single experiment,' he says, 'it might be concluded with the greatest probability that the origin of the diamagnetic force is not to be sought for in the never-changing metallic particles of the bismuth, but in an imponderable constituent moving between them, which on the approach of the pole of a magnet is displaced and distributed differently according to the difference of this pole.' He then inquires into the nature of this imponderable constituent, and into its bearing on the view first enunciated by Faraday, that diamagnetism might be explained by assuming the existence of a polarity the reverse of that of magnetism. He subjects the view to an experimental test, and shows that a bar of bismuth which at a certain distance had no sensible action on a magnetic needle, did exert an action on the same needle when

* Poggendorff's Annalen, vol. lxxiii. p. 60; Phil. Mag. vol. xxxiv. p. 127.

+ Poggendorff's Annalen, January 7, 1848; Taylor's Scientific Memoirs, vol. v

P. 477.

6

placed between the poles of a powerful magnet.* Between the two poles of the horseshoe magnet,' writes Weber, ' a very perceptible and measurable effect is exhibited, viz., a deflection of the needle, owing to one pole being repelled and the other attracted.' He found that when the poles of the influencing magnet were reversed, the deflection produced by the bismuth was reversed also; and that when a piece of iron was substituted for the bismuth, the deflection produced by the magnetic metal was opposite to that produced by the diamagnetic one. Hence he concluded that Faraday's hypothesis was proved. To render the proof more complete, Weber made an exceedingly skilful arrangement to show that induced currents were excited by the diamagnetisation of bismuth as well as by the magnetisation of iron. The proof of diamagnetic polarity appeared, therefore, to be complete.

Faraday, however, again took up the subject. Referring to his hypothesis of diamagnetic polarity, he says the view was 'received so favourably by Plücker, Reich, and others, but above all by W. Weber, that I had great hope it would be confirmed; and though certain experiments of my own did not increase that hope, still my desire and expectation were in that direction.' 'It appeared to me,' he continues, that many of the results which have been supposed to indicate a polar condition, were only consequences of the law that diamagnetic bodies tend to go from stronger to weaker places of magnetic action.' In a paper of great experimental power, he demonstrates that the induced currents ascribed by Weber to the diamagnetisation of bismuth were probably due to a totally different cause; and with regard to Weber's experiment with the bar of bismuth placed between the poles of a magnet, Faraday says, 'I have repeated this experiment most anxiously and carefully, but have never obtained the slightest trace of action with the bismuth. I have obtained action with the iron; but in those cases the action was far less than if the iron were applied outside, between the horseshoe magnet and the needle, or to the needle alone, the magnets being entirely away. On using a garnet, or a weak diamagnetic substance of any kind, I cannot

* The action of the magnetic poles upon the suspended needle was neutralised by a second magnet, the needle being thus rendered sufficiently sensitive to respond to the action of the bismuth,

find that the arrangement is at all comparable, for readiness of indication or delicacy, with the use of a common or an astatic needle, and therefore I do not understand how it could become a test of the polarity of bismuth when these fail to show it.'

'Finally,' he continues, 'I am obliged to say that I can find no experimental evidence to support the hypothetical view of diamagnetic polarity, either in my own experiments, or in the repetition of those of Weber, Reich, or others. I do not say that such a polarity does not exist, and I should think it possible that Weber, by far more delicate apparatus than mine, had obtained a trace of it, were it not that then also he would have certainly met with the far more powerful effects produced by copper, gold, silver, and the better conducting diamagnetics.'

In a very exhaustive and beautiful memoir translated by myself from Poggendorff's Annalen, vol. lxxxvii., p. 145,* Prof. Weber returns to the subject of diamagnetism, and considers four possible assumptions to account for the origin of the diamagnetic effects:

1. The internal cause of such effects may be referred to the existence of two magnetic fluids which are more or less independent of the ponderable matter which carries them.

2. They may be due to the existence of two magnetic fluids, which are only capable of moving in connexion with their ponderable carriers (rotatory molecular magnets).

3. They may be due to the existence of permanent molecular currents formed by the electric fluids, and which rotate with the molecules.

4. They may be due to the existence of electric fluids, which can be thrown into molecular currents.

He sup

Weber decides in favour of the fourth hypothesis. poses that by the act of magnetisation molecular currents are generated in diamagnetic bodies; which currents, like those of Faraday, have a direction opposed to that of their generators. But Faraday's currents are of vanishing duration, being immediately extinguished by the resistance of the conductors through which they move. Diamagnetism, however, would require permanent molecular currents to account for it. Weber

Scientific Memoirs, published by Taylor & Francis, New Series, vol. i. p. 163.

secures this permanence by supposing that the induced molecular currents move in channels of no resistance* round the molecules. This assumption enables him to link all the phenomena of diamagnetism together in a satisfactory manner. While recognising the extreme beauty of the hypothesis, I should hesitate to express a belief in its truth.

Weber also again applied his wonderful experimental skill to the subject of currents induced by the act of diamagnetisation; and in my opinion, fairly met all the requirements of the case; but neither his labours nor those of Poggendorff and Plücker produced conviction in the mind of Faraday. The notion of a distinct diamagnetic polarity was also opposed by others. Prof. von Feilitzsch, for example, contended, on theoretic grounds, and backed his contention by definite experiments, that the magnetic excitement of bismuth and of iron were one and the same. This was also the view of M. Becquerel. Matteucci subsequently entered the field as an ardent opponent of diamagnetic polarity.

The following investigations bear upon this subject.

* This, indeed, is involved in Ampère's theory of molecular currents. See Letter of Prof. Weber further on.

THIRD MEMOIR.

ON THE POLARITY OF BISMUTH, INCLUDING AN EXAMINATION OF THE MAGNETIC FIELD.*

THE polarity of bismuth is a subject on which philosophers have differed and on which they continue to differ. On the one side we have Weber, Poggendorff, and Plücker, each affirming that he has established this polarity; on the other side we have Faraday, not affirming the opposite, but appealing to an investigation which is certainly calculated to modify whatever conviction the results of the above-named experimenters might have created. It will probably have occurred to those occupied experimentally with diamagnetic action that whenever the simple mode of permitting the body experimented with to rotate round an axis passing through its own centre of gravity, can be applied, it is preferable in point of delicacy to all others. A crystal of calcareous spar, for example, when suspended from a fine fibre between the poles, readily exhibits its directive action, even in a field of weak power; while to establish that peculiar repulsion of the mass which is the cause of the directive action, even with high power and with the finest torsion balance, is a matter of considerable difficulty. In the knowledge of this and in the fact of my having a piece of bismuth, whose peculiar structure suggested the possibility of submitting the question of diamagnetic polarity to a new test, the present brief enquiry originated.

In December 1847 a paper on 'Diamagnetic Polarity' was read before the Academy of Sciences in Berlin by Professor Poggendorff, the result arrived at by the writer being, that a bismuth bar, suspended horizontally and occupying the equatorial position between two excited magnetic poles, was transversely magnetic-that side of the bar which faced the north pole possessing north polarity, and that side which faced the south pole possessing south polarity; the excitation being thus

*Phil. Mag., Nov. 1851.