Item 126 - Letter from William Thomson

16 pp.

Glasgow - WT feels 'very strongly inclined towards Faraday's view that the substance of a diamagnetic is polar like oxygen or iron, when near a magnet, but that it is less so than the surrounding medium'. John Tyndall 'is in error if he still supposes that either his experiments or Weber's, or any others yet made affords a test as to whether this hypothesis, or true hypothesis that bismuth and the like, have a polarity the reverse of that of iron in the same circumstances. The resultant external force between a diagmagnet and paramagnet is demonstrably the same which ever hypothesis is true; and those experiments are solely indicative of resultant external force' [see WT to James Forbes, 19 March 1857]. Perhaps Faraday doubted Weber's [Wilhelm Weber] experiment gave any other result and he certainly doubted the truth of the interpretation put on Weber's results. 'Tyndall's repetition of Weber's experiment I believe convinced Faraday that the result was genuine'. WT has written a short article in the Cambridge and Dublin Mathematical Journal in May 1846, which 'contains principles and mathematical expressions, founded solely on what Faraday told me in his first paper, which lead in the most obvious way to the determination of all such forces as those which Weber and Tyndall observed'. WT describes what a (mechanical) explanation of electromagnetic induction would entail. Regarding 'Ampere's [André Ampere] theory of real motions in minute circular orbits or vortices, with axes on the whole set in the directions of the line of force, to account for magnetism, I think it is probably true. The magnetic optic discovery seems explicable on no other hypothesis. Since Foucault's [Jean Foucault] exhibition at Liverpool I have been much disposed to look on the gyroscope as an illustration of a magnet. It is of course difficult to see how a current ([?] of matter flowing) through a straight wire can induce among the thermal motions in the surrounding medium, eddies of which the axes are circles in planes perpendicular to the wire; and how eddies in a steel bar magnet with their axes on the whole parallel to its length can induce among the surrounding thermal motions, eddies rooted to the steel at each end...Still it is not beyond expectation that a definite mechanical explanation of such influences may be invented'. If this is done it simply shows that magnetic attraction is a product of pressure along the axes of eddies, caused by centrifugal force. An 'explanation of electromagnetic induction would have to be looked for by considering mechanically the effects produced by moving as whole, pieces of matter among the particles of which these are vortical motions with determined sets'. WT does 'not see how a momentary recoil in the surrounding matter can account for electromagnetic induction'. Nevertheless 'there may be lateral action at or near the boundaries of the conductor in its interior with a reaction causing the external induced current'. WT thinks 'Faraday must be right in supposing both electric action to be conducted through matter and by means of the matter through which it is conducted'. 'Is it credible that matter can act where it is not? Were not those of the schoolmen who demonstrated a universal Plenum right?' WT wonders whether WW can come up with any epithets for recent developments in electrostatics: 'I would like to be able to distinguish between systems in which the electricity to be tested is tried by one, and by two, independently electrified bodies'. He would also like names to distinguish electrometers.