William Thomson to Faraday   12 June 1860

Thornliebank | Glasgow | June 12, 1860

My dear Faraday

The insurance trial regarding the Atlantic Telegraph, for which I expected to be summoned to London before this time, has been put off until the end of the month, and I expect to be required to attend between the 25th and the 1st of July¹. I shall call to see you if you are in London, and to ask if you are disposed to come out to Kew along with me, where I shall have to go to look after the recording atmospheric electrometer².

I have made several attempts to discover, if possible, indications of electric force in the air over the surface of two liquids, such as sulphate of zinc and sulphate of copper, separated by a porous partition, but as yet with no result. I think there must be something to be found; and probably strong in such a case as caustic potash and nitric acid, since these two liquids when substituted for acidulated water next the zinc and platinum of a galvanic element increase its electro-motive force very largely.

I yesterday had an opportunity of observing something with my portable electrometer during thunder. No lightning was visible, but I could perceive the instants of the discharges that gave rise to audible sound by sudden motions of the needle. The thunder came about 20 seconds later than an impulse of this kind, several times, from which I judged that it was about 5 miles distant. The motion of the needle was more sudden than that which takes place when the conductor with the match burning is suddenly insulated. When this is done the needle gradually deflects without vibration, and shows nearly the full effect in 5 or 6 seconds. The changes yesterday were so sudden as to leave the needle vibrating, and were therefore inductive beginnings of the electric change in the conductor which the burning match completes. Besides the larger impulses which I was able to connect with the thunder, there was a constant flickering of the needle, which seemed to show that between flash and flash sufficient to make audible thunder, there were countless smaller discharges. On a small scale the same thing is produced, & is indicated by the needle in the same way, when shreds or fibre assist disruption of the air in any “field of electric force” in connection with experimental apparatus.

The ordinary atmospheric changes, although sometimes very rapid, for instance doubling the force in a minute or less, are not instantaneous, and show their effect by a gradual motion of the needle without vibrations. It seems certain that such changes are produced by motions of electrified air, while those I observed yesterday must have been due to discharges.

Believe me, | My dear Faraday | Ever truly yours | William Thomson