the Directive Power of Magnets, tyc. 231 



Dr. Lamont inferred the proportion of the effects of different 

 steel magnets from the proportion of the effects of different 

 soft-iron bars under the influence of induction. The remark 

 No. 4 (above) goes far, I think, to justify this assumption. 



Appendix* 



Remarking the singularity of the experimental result as to 

 the apparent localization of the attractive pole of a galvanic coil 

 at the very extremity of the coil, I commenced an investigation 

 of the theoretical attraction of a coil, on the laws of galvanic 

 attraction usually received. On my mentioning the subject to 

 my friend James Stuart, Esq., Fellow of Trinity College, Cam- 

 bridge, he kindly undertook, at my request, to prepare a 

 complete theoretical investigation. I am happy in being per- 

 mitted by Mr. Stuart to place before the Royal Society his 

 mathematical discussion of the attraction of the coil, which I 

 am confident will be found to be very complete and of great 

 elegance. I append to it a comparison of the numerical results 

 of the theory with the numerical results of experiment ; and 

 the agreement will be found to be so great as to justify entire 

 confidence in the assumed law of galvanic action and the mathe- 

 matical treatment of it, and a high estimate of the accuracy of 

 the experimental observations. 



Investigation of the Attraction of a Galvanic Coil on a small Mag- 

 netic Mass. By James Stuart, M.A., Fellow of Trinity 

 College, Cambridge*. 



From investigations given by Ampere, we can deduce an ex- 

 pression for the potential U at an external point Q of a closed 

 circular galvanic current carried by a wire of indefinitely small 

 section. Let a be the radius of the circle, let the distance of Q 

 from C, the centre of the circle, be r, and let the line C Q make 

 an angle 6 with the normal to the plane of the circle : then it can 

 be shown that, when r is less than a, 



tt n 7 f . r „ lr» 1.3 r 5 " "1 



L « 2a 3 3 2.4 a b 5 J 



and when r is greater than a, 



tt o yf l'«*ii 1-3 fl4 T3 1.3.5 a 6 -o, 1 



* Abbreviated from the Appendix originally presented and read with 

 the paper. 



