290 Historical Sketch of Electro-magnetism. [Oct. 



axis of the helix, but is perpendicular to it. It is probable that 

 it becomes magnetic by some indirect action' of the apparatus. 



Finally, a paper was read on July 5 to the Royal Society by 

 Sir H. Davy on the magnetic phenomena produced by electri- 

 city, and their relation to heat occasioned by the same agent ; 

 but this has not yet been published, nor any account of it given 

 to the world, so that I am unable to state what facts it may 

 contain. 



Such, Sir, is an imperfect account of the experiments made on 

 this subject since Oersted's discovery, that 1 have been able to 

 get access to. With regard to what had been done before that 

 time, though many philosophers had dwelt on the relation of 

 electricity to magnetism, and theorised upon it, yet very little 

 else than opinion can be found in their writings. I cannot, I 

 think, do better than copy the note at the commencement of 

 Sir H. Davy's first paper, to show how little had been done at 

 that time, and with that I shall finish this historical sketch of 

 facts ; and endeavour, in the remainder of this letter, to give a 

 somewhat familiar account of the different theories of electro- 

 magnetism that now exist. 



" M. Hitter asserted, that a needle composed of silver and 

 zinc arranged itself in the magnetic meridian, and was slightly 

 attracted and repelled by the poles of a magnet, and that a 

 metallic wire, after being exposed in the voltaic circuit, took a 

 direction NE and SE. His ideas are so obscure that it is often 

 difficult to understand them ; but he seems to have had some 

 vague notion that electrical combinations, when not exhibiting 

 their electrical tension, were in a magnetic state, and that there 

 was a kind of electro-magnetic meridian depending upon the 

 electricity of the earth. — (Annales de Chimie, torn. lxiv. p. 80.) 

 Since this letter has been written, Dr. Marcet has been so good 

 as to send me from Genoa some pages of Aldini on Galvanism, 

 and of Izarn's Manual of Galvanism, published at Paris more 

 than 16 years ago. M. Mojou, sen. of Genoa, is quoted in these 

 pages as having rendered a steel needle magnetic by placing it 

 in a voltaic circuit for a great length of time. This, however, 

 seems to have been dependent merely upon its place in the mag- 

 netic meridian, or upon an accidental curvature of it; but M. 

 Romagnesia, of Trent, is stated to have discovered that the pile 

 of Volta caused a declination of the needle : the details are 

 not given; but if the general statement be correct, the author 

 Could not have observed the same fact as M. Oersted, but merely 

 supposed, that the needle had its magnetic poles altered after being 

 placed in the voltaic circuit as a part of the electrical combi- 

 nation." 



{To be continued.) 



