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VII. On the Reciprocal Action of Diamagnetic Particles. 

 Letter from Prof. Thomson to Prof, Tyndall. 



My dear Sir, Glasgow College, Dec. 24, 1856. 



I HAVE been prevented until to-day, by a pressure of business, 

 from replying to the letter you addressed to me in the 

 Number of the Philosophical Magazine published at the begin- 

 ning of this month. 



You ask me the question, " Supposing a cylinder of bismuth 

 to be placed within a helix, and surrounded by an electric cur- 

 rent 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 t" 



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 expe- 

 rience the opposite force to that experienced by a little ball of 

 iron, that is, the same force as a little steel maguet held with its 

 axis reverse to the lines of force; and in no case has a discre- 

 pance, or have any indications of a discrepance, from this law 

 been observed. I feel therefore in my own mind a certain con- 

 viction, 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 bis- 

 muth is really acted on by the same force as that which a little 

 reverse maguet, 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 bythe neighbouring end of the bismuthbar; 

 and the attraction or the repulsion will preponderate according 

 as the attracted or the repelled part is nearer. There is then cer- 

 tainly 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 



