260 Mr. E. Taylor Jones on 



an opening through which passed the lower projecting end of 

 the tube in the other coil, and to this was attached a hook and 

 scale-pan. 



The current, obtained from a battery of accumulators, was 

 measured by a Siemens and Halske's torsion-galvanometer, 

 standardized by electrolysis. 



The magnetization was measured in independent experiments 

 by the magnetometer method. 



The ellipsoid and coils were placed in the magnetic east and 

 west line east of a delicate magnetometer read by telescope 

 and scale. 



The intensity of the earth's horizontal field was measured 

 by the method of Gauss and by the tangent-galvanometer 

 method, a Kohlrausch's local-variometer being found useful 

 for measuring its variations. 



The magnetization-curve was first obtained,, showing the 

 relation between I and H, allowance being made, as usual, for 

 the demagnetizing-force of the ellipsoid calculated from the 

 expression given in Maxwell's ' Electricity and Magnetism ' 

 (Sect. 438). The intensity of magnetization I was calculated 

 in absolute units from the formula* 



where h == earth's horizontal field, 0= deflexion of needle, 

 v = volume and e = eccentricity of ellipsoid, d= distance of 

 needle from centre of ellipsoid (the former being in the pro- 

 longation of the axis of the latter), and n=~, c being the 

 semiaxis of the ellipsoid. 



This curve being determined once for all, the induction B 

 corresponding to any current could be calculated, since 

 B = H + 47rIt. All precautions were taken to determine the 

 curve as accurately as possible, the torsion of the magneto- 

 meter-fibre, the exact position of the needle with respect to 

 the axis of the ellipsoid, the length of a division on the scale, 

 the effect of the compensating-coil at all parts of the scale (by 

 deflecting with permanent magnets), and the temperature of 

 the rooni during the local-variometer experiments all being 

 carefully examined. The residual magnetism was very small, 

 being less than a sixtieth of the total magnetization when 

 the ellipsoid was strongly magnetized. Each current was 



* This was obtained from Thomson and Tait's expression for the 

 potential at an external point due to an ellipsoid of attracting matter "by 

 differentiating twice with respect to the distance of the point. (Cf. 

 Hoessler, Dissertation, Zurich, 1892). 



f Where H= Field due to coils — demagnetizing-force of ellipsoid. 



