From William Thomson   Dec. 24, 1855.

Glasgow College, Dec. 24, 1855.

MY DEAR SIR,

I HAVE been prevented until to-day, by a pressure of business, from replying to the letter¹ you addressed to me in the Number of the Philosophical Magazine published at the beginning of this month.

You asked me the question, ‘Supposing a cylinder of bismuth to be placed within a helix, and surrounded by an electric current of sufficient intensity; can you say, with certainty, what the action of either end of that cylinder would be on an external fragment of bismuth presented to it?’

In answer, I say that the fragment of bismuth will be repelled from either end of the bar provided the helix be infinitely long, or long enough to exercise no sensible direct magnetic action in the locality of the bismuth fragment. I can only say this with the same kind of confidence that I can say the different parts of the earth's atmosphere attract one another. The confidence amounts in my own mind to a feeling of certainty. In every case in which the forces experienced by a little magnetized steel needle held with its axis reverse along the lines of force, and a fragment of bismuth substituted for it in the same locality of a magnetic field, have been compared, they have been found to agree. In a vast variety of cases, a fragment of bismuth has been found to experience the opposite force to that experienced by a little ball of iron, that is, the same force as a little steel magnet held with its axis reverse to the lines of force; and in no case has a discrepance, from this law been observed. I feel therefore in my own mind a certain conviction, that even when the action is so feeble that no force can be discovered at all on the bismuth by experimental tests, such, in regard to sensibility, as have been hitherto applied, the bismuth is really acted on by the same force as that which a little reverse magnet, if only feeble enough, would experience when substituted in its place. Now there is no doubt of the nature of the force experienced by the steel magnet, or by a little ball of soft iron, in the locality in which you put the fragment of bismuth. One end of a magnetized needle will be attracted, and the other end repelled by the neighbouring end of the bismuth bar; and the attraction or the repulsion will preponderate according as the attracted or the repelled part is nearer. There is then certainly repulsion when the steel magnet is held in the reverse direction to that in which it would settle if balanced on its centre of gravity. In every case in which any magnetic force at all can be observed on a fragment of bismuth, it is such as the steel magnet thus held experiences. Therefore I say it is in this case repulsion. But it will be as much smaller in proportion to the force experienced by the steel magnet, as it would be if an iron wire were substituted for the bismuth core. Yet in this case the repulsion on the bismuth is very slight, barely sensible, or perhaps not at all sensible when the needle exhibits most energetic signs of the forces it experiences. You know yourself, by your own experiments, how very small is even the directive agency experienced by a steel magnet placed across the lines of force due to the bismuth core. You may judge how much less sensible would be the attraction or repulsion it would experience as a whole, if held along the lines of force; and then think if the corresponding force experienced by a fragment of bismuth substituted for it is likely to be verified by direct experiment or observation. I think you will admit that it is ‘incapable of verification,’ as well as ‘incontrovertible’ by any collation of the results of experiments hitherto made on diamagnetics. As to the concluding paragraph of my letter which you quote, you do me justice when you say you accept it as an expression of my ‘personal conviction that the action referred to is too feeble to be rendered sensible by experiment.’ I will not maintain its unqualified application to all that can possibly be done in future in the way of experimental research to test the mutual action of diagmagnetics under magnetic influence. On the contrary, I admit that no real physical agency can be rightly said to be ‘incapable of verification by experiment or observation;’ and I will ask you to limit that expression to experiments and observations hitherto made, and to substitute for the concluding paragraph of my letter the following statement, written for publication three days later, and published in the same Number of the Magazine as that to which you communicated my letter (Phil. Mag. April 1855, p. 247).² ‘The mutual influence’ between rows of balls or cubes of bismuth in a magnetic field, ‘and its effects’ in giving a tendency to a bar of the substance to assume a position along the lines of force, ‘are so excessively minute, that they cannot possibly have been sensibly concerned in any phaenomena that have yet been observed; and it is probable that they may always remain insensible, even to experiments especially directed to test them.’³

I remain, my dear Sir, | Yours very truly, | WILLIAM THOMSON.

Dr. Tyndall.

Phil.Mag. 11 (1856), pp. 66–7

Printed heading: VII On the Reciprocal Action of Diamagnetic Particles. | Letter from Prof. THOMSON to Prof. TYNDALL | Glasgow College, Dec. 24, 1855.