20th Nov. 1855.
My dear Sir
If I can render you any assistance in preparing instruments for the lecture which you have so kindly consented to give us1 it will give me great pleasure to do so.
We are well furnished with electromagnetic apparatus.
You can if you wish make Peltier’s experiment2 in a manner visible to all.
You can also produce fire by friction, and heat by the compression of air. We have contrivances for such experiments.
We have also a very effective manner of showing the heat produced by congelation.
We are well furnished in the line of Frictional Electricity.
You will however mark out your own course, and I can only say that I shall be happy to render you any assistance in my power.
I may take this opportunity of referring to a matter which struck me a few evenings ago, and on which a word from you will doubtless make me clear.
From your proof that the intensity of a magnetic field increases towards the centre of curvature3 I should infer that if the lines of force were parallel straight lines the intensity at right angles to them would be constant. I have a steel horseshoe magnet here in which the lines of force run sensibly parallel from leg to leg almost from top to bottom, yet such a field is not one of constant intensity, for the force increases from the bend towards the poles.
When we examine such a field closely we even find that the lines of force are slightly curved, the centre of curvature being towards the bend, and not towards the poles.
According to this the intensity increases as we recede from the centre of curvature.
If you have not plenty of time at your disposal, don’t trouble yourself thinking on this.
I have just finished a paper on polarity4 which I purpose sending to the Royal Society in a few days. I am now entangled in compression experiments.5
Believe me | Very sincerely yours | J. Tyndall
RI MS JT/1/5/1544–5
Transcript Only
the lecture … give us: Thomson had been invited to deliver a Friday Evening Discourse at the RI on 29 February 1856. His lecture, entitled ‘On the Origin and Transformations of Motive Power’, was later published in Roy. Inst. Proc., 2 (1854–8), pp. 199–204.
Peltier’s experiment: The French physicist Jean Charles Athanase Peltier (1785–1845) performed an experiment in 1834 in which he demonstrated that cold is produced by an electric current flowing from antimony to bismuth. This experiment complemented the discovery thirteen years earlier by Thomas Johann Seebeck (see letter 1142) who found that a temperature difference between two different metals produced a voltage.
your proof … centre of curvature: W. Thomson, ‘Elementary Demonstrations of Propositions in the Theory of Magnetic Force’, Phil. Mag., 9 (1855), pp. 241–8.
a paper on polarity: Tyndall’s paper ‘Further Researches on the Polarity of the Diamagnetic Force’ was received by the RS on 27 November and read at its meeting on 20 December 1855. It was then published in Phil. Trans., 146 (1856), pp. 237–59.
compression experiments: ‘At work at the compression of oxalate of cobalt and oxalate of iron’ (Tyndall’s Journal, 19 November 1855: RI MS JT/2/13c/804). Some of his compression experiments were published in ‘On the Relation of Diamagnetic Polarity to Magne-crystallic Action’, Phil. Mag., 11 (1856), pp. 125–37.
Please cite as “Tyndall1155,” in Ɛpsilon: The John Tyndall Collection accessed on