William Thomson to Faraday   31 October 1859

2 College, Glasgow | Oct 31,1859

My dear Sir

The diagrams which came to you by post¹ were from Professor Clerk Maxwell. He wished to speak of them to you at Aberdeen² but I suppose did not find an opportunity in the closely packed time.

The curves represent lines of force, or lines of motion of a fluid, for various cases in which their directions are all parallel to one plane. In every such case the set of curves which cut a set of lines of force perpendicularly are themselves a set of lines of force for another case; and therefore each of the diagrams bears a double interpretation. The best electrical application is to lines of atmospheric-electric force in the neighbourhood of a long straight mound, or mountain ridge, or trench, with a section of any form - In such a case you will see that, except near the ends of the line of elevation or depression, the lines of force will be all parallel to one plane, namely to any plane perpendicular to that line. Among Maxwell’s diagrams you will find one, for instance, which shows the lines of electric force in the neighbourhood of a long semicylindrical mound. The surfaces which we mathematicians call surfaces of equal potential (after George Green’s³ definition) will be shown in the diagram by the lines perpendicular to the lines of force. In the case of the semicylindrical mound, the surfaces of equal potential commence, at the earth, with the plane & curved surface of the earth, & approximate to parallel planes, at greater & greater heights. The curves showing these surfaces, are [like] lines of motion of water round a long cylindrical bar carried in a direction perpendicular to its length.

I am now having a reflection electrometer made for Kew, to act as a self recording instrument for atmospheric observation. The same kind of instrument will I believe be the best for general experimental purposes in which an electrometer of the highest sensibility is required. It will also answer for lecture illustrations, but for elementary instruction perhaps one such as you saw at Aberdeen⁴ showing its whole action and construction, and distracting the mind less from the electrical topic, may be preferred. I could have an instrument of either kind constructed for you if you please, and it would be a great pleasure to me to take charge of it should you wish to have one made. The last which I had made (much improved on the one you saw) cost £11. I do not think the appliances for reflection will add much to the cost, but I shall soon be able to have an exact estimate sent should you desire it.

Believe me | Yours very truly | William Thomson

Prof Faraday

Endorsed by Faraday: Thomson on Dellmans⁵ mode of observing electricity - British Association Aug 1856 p17 of notices | Athenaeum - No. 1505⁶.