From William Thomson   Nov 22, 1855

2 College, Glasgow | Nov 22, 1855

My dear Sir

I am much obliged to you for your kind offer¹ to give me apparatus and assistance for experimental illustrations of the subject for my proposed lecture, of which I shall thankfully avail myself. I shall be as much pleased to see as to show fire made by friction, and all the other illustrations you mention will be valuable.

I am anxious to show the stunning effect of an electromagnet on a rotating mass of copper.² If I can get a copper spinning top made and conveniently arranged for spinning between the poles of a Ruhmkorff³ or other powerful electromagnet, it will be curious to see it struck suddenly dead when the magnetism is excited; and I would like too if possible to get a mechanical arrangement to force it round at a rapid rate & to show the elevation of temperature it experiences. I propose to have a square head at the top of the spindle, and a key (like a clock or a large watch key) put on it & kept turning by a short stiff india rubber cord of which the lower end would be attached to the key and the upper end to the lower extremity of a vertical shaft to which a quick motion would be given by machinery. If after starting the motion by the mechanism it is desired to leave the top to itself, all that would be required would be to lower the stand on which it rests, when its own weight would clear it of the key. If I get this plan to succeed, I suppose I may rely on a quick motion apparatus. As soon as I have got it tried, I shall write to you as to the arrangements I would wish to make.

I do not know exactly how the difficulty you state⁴ as to the application of my proposition to the lines of force between the branches of your steel magnet is to be explained. I [would] expect that if the lines of force are really curved with their convexity towards the open space

between the poles, the intensity of the force would in that part of the field, in which they are so bent, increase towards the middle of the bend; and this might be tested by taking a very small needle (say, ¼ of an inch of watch spring) hung by a single fibre, and making it oscillate in the different positions. But I would be much surprised if such curvature of the lines of force is found anywhere except near the bend, and I should expect curvature rather the other way in the space between the terminal halves of the branches. I hope, if not before, we shall be able to settle the difficulty when I see you in London.

I am glad you are going on with compression experiments, and I hope you will get satisfactory results. I have been testing the effect of longitudinal extension on electric conductivity, which in the case of iron wire I find decidedly a diminution although very minute. I mean to try lateral extension and compression.

Once more thanking you for your offered help

I remain, Dear Sir, | Yours very truly | William Thomson

RI MS JT/1/T/14