Electromagnetic Stress. 261 



reversed, and half the double deflexion of the needle taken as 

 the deflexion corresponding to the mean of the two currents, 

 which were generally the same*. 



The " lifting " experiments were then proceeded with : — 



The upper coil was first suspended vertically and symme- 

 trically above the opening H, in the platform P (fig. 1), the 

 lower bar and coil being thus supported (when the current 

 was made) so that its tube T passed freely through the 

 opening H. 



Enough weights were placed in the scale-pan to nearly 

 overcome the stress, the remainder being slowly poured in in 

 the form of fine shot until the lower coil fell. The current 

 was then read off and the shot weighed. 



This was repeated for a number of currents ranging from 

 1 to 10 amperes (the corresponding inductions ranging from 

 6000 to 20,000 C.G.S.); the adjustment of the upper coil 

 being made before every reading, and the surfaces carefully 

 cleaned with a soft dry brush. 



The " reversed " readings were also taken, as in the mag- 

 netometer experiments. 



The inductions corresponding to the currents used were 

 calculated from the magnetization-curve. The inductions and 

 the square roots of the observed weights were plotted in a 

 diagram along with the straight line representing Maxwell's 

 law. 



First Experiments. 



The first experiments were made with the contact-surfaces 

 of the ellipsoid turned truly plane but not polished. 



The curve representing the observed results was at low 

 inductions considerably above the straight line (the observed 

 weights being greater than those given by Maxwell's law), 

 soon crossing it and remaining below it for high inductions, 

 the difference increasing with the induction. 



The correction due to the excess of area of the coils over 

 that of the core was found by removing the cores and sup- 

 porting the lower coil in the same position on a balance. 



* (1) It may be objected that the magnetization in the u lifting " 

 experiments was not the same as in the magnetometric experiments, on 

 account of the longitudinal pull existing in the former ; but a small cal- 

 culation shows that, with the weights used, this effect was in general 

 very small, and could, except perhaps at the highest inductions, be 

 neglected. 



(2) The magnetization-curve was determined both before and after 

 the ellipsoid was cut, and the demagnetizing-force (which was affected 

 by the shortening of the ellipsoid after the cutting) calculated in both 

 cases. The curve for the cut ellipsoid was lower than the other; but 

 the difference was very slight, especially at high fields. 



Phil. Mag. S. 5. Vol. 39. No. 238. March 1895. T 



