Mr. J. Tatum on Electro-Magnetism. 109 



to pieces of steel, but as respects the direction of their poles; 

 for which purpose, 



Exp. 11, I inclosed a piece of steel in a helix similar to 

 Exiy, 9, which connected the copper and zinc side of the appa- 

 ratus (described in your last number) : in a few moments it be- 

 came magnetic, and its north pole was towards the zinc or 

 positive side of the apparatus. 



Exp. 12. Another piece of steel was inclosed in a helix 

 coiled from right to left. After a few moments it was examined, 

 when its north pole was found to be towards the copper or 

 negative side of the apparatus : so that it appears evident that 

 the poles of the steel, rendered magnetic by either comnion or 

 Voltaic electricity, are determined by the direction of the coils 

 of the helices. 



I cannot help noticing the similarity which appears to exist 

 between the direction in which it is necessary for the electrical 

 current to traverse, in order to render ferruginous bodies mag- 

 netic, and the direction in which I suppose the magnetic influ- 

 ence traverses in magnetic bodies. 



I am aware that pieces of steel mayhscve been rendered mag- 

 netic by passing an electrical charge across them; but I have 

 never produced snch. potaeiful magnets by this means as by the 

 use of the helix ; indeed the experiment has been rather uncer- 

 tain, but after all it is but an imperfect modification of the helix. 



I am fully sensible that an erroneous theory may be ad- 

 vanced to explain the phaenomena of experiments, and I am 

 not so partial to my opinion as to i7isist that I may not labour 

 under some mistaken idea; but I cannot conceive how those 

 movements are produced on the dipping needle, rotatory ap- 

 paratus, &c., if the magnetic fluid passes in straight lines 

 from one part of the needle to the other, or from one part of 

 the connecting wire of the Voltaic apparatus to the other; for, 

 let us suppose AB, Fig. 3, to represent a part of the above 

 wire, and that the magnetic fluid passes in a straight line from 

 A to B ; and let CD represent one end of a dipping needle, 

 in which the fluid passes from C to D — what can occasion the 

 needle to descend in the direction from 1 to 2 when on one side 

 of the wire, but to ascend in the direction from 2 to 1 when on 

 the other side of the wire ; and also to move from right to left 

 when above the wire, but from left to right when below it; 

 and further, that these effects are reversed, if we reverse either 

 the poles of the wire or those of the needle? 



But if it be granted that the fluid rotates, as I have sug- 

 gested, then, to me at least, those movements and rotations are 

 easily explained. I am. Gentlemen, yours, &c. 



Dorset Street. J. TatUM. 



Erratum. — Fn Mr. Tatum's last paper, vol. IxJ. p. iJ45, line 24, after 

 * equator' iiuot ' of the*. 



