ON THE DIRECTIVE POWER OF MAGNETS. 
493 
a translation, by W. T. Lynn, Esq., of the Royal Observatory, is printed in the Philo- 
sophical Magazine, 1861, November, 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, Cambridge, he kindly undertook, at my request, to prepare 
a complete theoretical investigation. I am happy in being permitted 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 mathematical 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 Magnetic Mass. 
By James Stuart, M.A . , Fellow of Trinity College , Cambridge. 
Received July 26, — Read December 5, 1872*. 
From investigations given by Ampere, we can deduce an expression for the potential 
U at an external point Q of a closed circular galvanic current carried by a wire of inde- 
finitely 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 «, 
U=2rf + 
and when r is greater than a, 
U=2x*(-ipP,4 
1.8 a 4 
2.4 r l 
_ 1_ l 3_ l 5 a p 
x 3 o 4 6 A x 5 ‘ ‘ ’ 
* Abbreviated from the Appendix originally presented and read with the paper. 
3x2 
