To Thomas Archer Hirst
My dear Tom
Try to find a flaw in this
AB is a lever suspended horizontally by an axis passing through a round which point it is free to turn. A repulsive force acts upon the rectangle rrʹ fixed at the end of the lever – The direction of the force being from NN, it is mechanically impossible that such a force could cause the lever to rotate in the direction of the arrow. Take for example the forces on the two ends of the rectangle – their movements are respectively measured by the intensity of the force multiplied into the distance of its line of direction from the axis of suspension and in the case before us both these movements tend to turn the bar the same way – that is to make it rotate in a direction opposed to the arrow.
If then the lever rotate with the arrow it is an irresistable proof that the force cannot be purely repulsive.
Supposing the force f at r to be equal or very nearly so to that fʹ at rʹ and that the former force is repulsive while the latter is attractive. Then the product f x aʹa is less than the product fʹ x aʺa – the moment of force tending to turn the lever in the direction of the arrow would be therefore in the ascendant and an attraction of the mass towards NN would be the consequence.
I have so arranged matters as to be able to produce this result with a mass of bismuth. The artifice consists of making the field of force such that the repulsion at r is very nearly = that at rʹ both forces thus neutralize each other as regards a motion of translation (as the force of the earth on a magnetic needle) but they are still effective in producing a motion of rotation which evidences itself in the manner described – All this is hastily written but you will see what I mean, and that by it the polarity of Diamagnetic bodies,1 or in other words the simultaneous development of repulsion & attraction is reduced to a physical certainty.
affectionately John
I wish you would call at No 20 Rue Cassette and enquire of Mr Despretz whether the books which I sent him2 have arrived safely. If you wish I will give you a note of introduction to him.
Direct the enclosed3 to Anna.
RI MS JT/1/T/1008
This letter may have been sent with letter 1028 or shortly thereafter. Hirst was in Paris from 30 December 1854 until 12 January 1855.
polarity of Diamagnetic bodies: cf. Tyndall’s Bakerian lecture, ‘On the Nature of the Force by which Bodies Are Repelled from the Poles of a Magnet; to which Is Prefixed an Account of Some Experiments on Molecular Influences’, Phil. Trans., 145 (1855), pp. 1–52, esp. pp. 45–6.
the books which I sent him: see volume 4, letter 0980.
the enclosed: enclosure missing.
Please cite as “Tyndall1029,” in Ɛpsilon: The John Tyndall Collection accessed on