Royal Institution, Nov. 26, 1855
My DEAR SIR,
THE communication from Professor Weber which appears in the present Number of the Philosophical Magazine,1 has reminded me, almost too late, of your own interesting letter on the same subject published in the April Number of this Journal.2 A desire to finish all I have to say upon this question at present3 induces me to make the following remarks, which, had it not been for the circumstance just alluded to, might have been indefinitely deferred.
With reference to the mutual action of a row of bismuth particles, you say that ‘it is perfectly easy to demonstrate that it must be such as to impair the ‘diamagnetization’ when the line of the row is parallel to the lines of the force’4 (the ‘must’, you will remember, is put in italics by yourself). From this you infer, that in a uniform field of force a bar of bismuth would set its length along the lines of force. Further on it is stated that this action is ‘demonstrated with as much certainty as the parallelogram of forces’; and you conclude your letter by observing that ‘the assertions which I5 have made are demonstrable in every case without special experiment, … and are absolutely incontrovertible, as well as incapable of verification, by experiment or observation on diamagnetics’.6
Most of what I have to say upon this subject condenses itself into one 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?
If you can, I, for my part, shall rejoice to learn the process by which such certainty is attained: but if you cannot, it will, I think, be evident to you that the verb ‘must’ is logically ‘defective’.
We know that magnetized iron attracts iron: we know that magnetized iron repels bismuth: this, so far as I can see, is your only experimental ground for assuming that magnetized bismuth repels bismuth, and yet you affirm that an action deduced from this assumption ‘is demonstrated with as much certainty as the parallelogram of forces’.7 Do I not state the question fairly? I can, at all events, answer for my earnest wish to do so.
It is needless to remind one so well acquainted with the mental experience of the scientific inquirer, that the very letters which you attach to your sketch, page 291,8 may tempt us to an act of abstraction – a forgetfulness of a possible physical difference between the n of iron and the n of bismuth9 – which may lead us very wide of the truth. The very term ‘pole’ often pledges us to a theoretic conception without our being conscious of it. You are also well aware of the danger of shutting the door against experimental inquiry on an unpromising subject; and when you apparently do this in your concluding paragraph,10 I simply accept it as a strong way of expressing your personal conviction, that the action referred to is too feeble to be rendered sensible by experiment.11
Believe me, dear Sir, | Most truly yours, | JOHN TYNDALL.
Phil. Mag., 10 (1855), pp. 422–3.
Published under the title ‘On Reciprocal Electrical Induction’.
THE communication … Philosophical Magazine: reproduced here as letter 1039.
your own interesting letter … Journal: reproduced here as letter 1053.
A desire … question at present: This letter to Thomson was written the day before the RS received Tyndall’s paper ‘Further Researches on the Polarity of the Diamagnetic Force’. Thus, far from this letter being Tyndall’s last word on diamagnetism, it was a prolegomena to his RS paper in which he criticised Thomson’s views. See n. 11, below.
it is perfectly … lines of the force: see letter 1053.
I: after which Tyndall inserted ‘[yourself]’, i.e. Thomson.
demonstrated … observation on diamagnetics: see letter 1053.
is demonstrated … parallelogram of forces: Thomson had written: ‘Still, I repeat, this action, although demonstrated with as much certainty as the parallelogram of forces, is so excessively feeble as to be absolutely inappreciable’ (letter 1053).
page 291: the first diagram in letter 1053.
the n of iron and the n of bismuth: the letter n refers to the north pole.
your concluding paragraph: In the final paragraph of letter 1053, Thomson claimed that the principles he had there enunciated ‘are absolutely incontrovertible, as well as incapable of verification, by experiment or observation on diamagnetics’.
the action … by experiment: In his ‘Further Researches on the Polarity of the Diamagnetic Force’ Tyndall demonstrated, contra Thomson, that the action could ‘be rendered sensible by experiment’. Although Tyndall did not mention Thomson in that paper he clearly included him among those ‘investigators, [who] converted this belief [that diamagnetic polarity was not demonstrable] into a certainty’ (Phil. Trans., 146 (1856), pp. 237–60, on p. 238).
Please cite as “Tyndall1158,” in Ɛpsilon: The John Tyndall Collection accessed on