Electromagnetic Stress. 263 



than when they are separated at one side, thus enclosing a 

 wedge-shaped gap. 



This was the case in the present experiments up to induc- 

 tions of about 14,000 C.Gr.S. 



For higher inductions the tractive force is greatest when 

 the surfaces are everywhere in contact, the upper core and 

 coil therefore assuming naturally the proper position, pro- 

 vided, of course, the screw-adjustment is first sufficiently good. 

 For inductions up to about 14,000 C.Gr.S., therefore, the 

 lower bar was in unstable equilibrium when in good contact 

 with the upper, for higher inductions in stable equilibrium ; 

 at about 14,000 the equilibrium was indifferent, and the 

 nature of the contact was found within wide limits to have 

 no influence on the Tractive Force. 



As regards the results showing better agreement with the 

 theory than before at higher inductions, I attribute this to a 

 better method of testing the screw-adjustment of the upper 

 coil, and partly, perhaps, to the improved state of the contact- 

 surfaces after polishing. 



At this stage the weights were all smaller than those 

 calculated from Maxwell's expression, but their square roots 

 were approximately proportional to the induction for inductions 

 up to about 14,000 units, being still smaller at higher in- 

 ductions, the deviation from the theoretical values increasing 

 to about 3 per cent, at B = 20,000. 



The uniformity of these results led me to believe that the 

 errors, if any existed, were not accidental but due to some 

 cause which acted always in the same way. Accordingly, I 

 tried the effect of increasing the distance between the coils, 

 thus leaving a greater part of the core near the plane of 

 contact unsurrounded with coil-windings. I found that the 

 effect of increasing this distance by about 2 millim. was very 

 slight for inductions up to about 14,000 C.G.S., and for 

 higher inductions was a diminution of Tractive Force of a 

 few hundred grammes, varying with the induction — differences 

 of the same order as the differences between the above observed 

 and calculated values. I could also increase the Tractive 

 Force by an amount of the same order by putting between the 

 coils a few extra turns of wire. As the actual distance between 

 the windings of the two coils (including the end-plates and 

 the space occupied by the small guiding ring) in the above 

 experiments was about 5 millim., it was clear that the 

 observed weights were on this account smaller than the 

 theoretical values ; in other words, that the induction, as 

 calculated from the magnetization-curve, was greater than the 

 actual induction across the surface of contact. 



This result can be easilv explained ; for when the induction 



T2 



