Royal Institution | Saturday 23rd Feb.[1856]
My dear Sir.
Up to the present moment I have been unable to write a word in reply to your last note.2 On Thursday I have a lecture and shall be very busy; but on Friday I am quite at your service. We shall have a long day to prepare, and from the nature of the experiments I infer that we shall be able to make all arrangements comfortably & efficiently. We have a very beautiful means of showing the heat of congelation3 which I will have prepared on Thursday, so that it shall be cool and ready for action on Friday. The subject as far as I can see is an extremely promising one, but remember you have only an hour at your disposal and if you made all the experiments on your list it would be at the rate of an experiment every four minutes. Would this afford you sufficient time for explanation? If out of the fifteen you were to choose ten, these ten would afford so many resting places for the general mass of the audience, and would at the same time allow you a tolerable freedom of exposition. With regard to the experiment on the effect of induced currents I devised the following means of estimating the comparative conductivity of bismuth and copper some time ago. A cube of each metal was taken, and suspended from a twisted string, above the cube and attached to it by a copper wire was a small pyramid with the base horizontal and its four sides were formed by 4 little triangular pieces of looking glass. The mirrors rotated with the cube, and the latter of course was placed between the poles of an electro-magnet. A beam of light was thrown upon the mirror and as it turned round – slowly at first – the images reflected from its sides followed each other in a circle of about 30 feet in diameter. As the motion quickened the patches of light blended themselves into a continuous line. On closing the circuit with copper the effect was astonishing – your own term best describes it the cube was ‘struck dead.’ The idea of the top is a beautiful one if you can realise it – if not we can leave the experiment in the above form.
Very Sincerely Yours | John Tyndall
RI MS JT/1/5/1550–1
LT Transcript Only
The letter was printed, and dated as 1856, in S. P. Thompson, Life of William Thomson Baron Kelvin of Largs, 2 vols. (Macmillan: London, 1910), vol. 1, pp. 313–14.
your last note: probably letter 1186. Thomson had been asked to deliver a Friday Evening Discourse at the RI on 29 February 1856 and Tyndall had agreed to help Thomson set up the experiments (Faraday Correspondence, 5: 3705 and 3079). See W. Thomson, ‘On the Origin and Transformation of Motive Power’, Roy. Inst. Proc., 2 (1854 –8), pp. 199–204.
heat of congelation: the heat abstracted when a fluid changes to the solid state as when water forms ice. Tyndall’s demonstration was probably a version of the one he used later in the 1862 series of lectures on heat. This used a supersaturated solution of sodium sulphate kept quite still overnight. By simply dropping a single crystal of the salt into the solution, the audience could observe the solution slowly solidify. When a thermo-electric pile was held against the flask, the temperature rose rapidly from zero as solidification took place. J. Tyndall, Heat Considered as a Mode of Motion being a Course of Twelve Lectures delivered at the Royal Institution of Great Britain in the Session of 1862 (London: Longman, Green, 1863), pp. 153–4.
Please cite as “Tyndall1199,” in Ɛpsilon: The John Tyndall Collection accessed on