156 Dr. E. Taylor Jones on 



From the induction-throws obtained with the two coils, the 

 mean induction in the isthmus and the field-intensity just 

 outside the isthmus (which by the principle of continuity of 

 tangential magnetic force is equal to the magnetic force just 

 inside the isthmus) could be calculated. 



It was assumed that the field in and near the isthmus was 

 so nearly uniform that the induction and magnetic force were 

 constant over the section of the isthmus, and the latter equal 

 to the field in the annular space between the two coils. 



Theory. 



The results obtained with this arrangement can be inter- 

 preted in the following way according to Maxwell's theory. 



The mechanical force X acting in the direction ox on a 

 body magnetized with an intensity whose components are 

 A, B, C, and placed in a field of magnetic force of components 

 «, ft 7, is * 



X=X 1 + X 2+ X 3 =jJ|(Aj + B| + Gg)^^ & . (1) 



Applying this to the case of the bar C 2 , taking the origin 

 at the outer end (cf. fig. 1) and ox along the axis of the bar — 

 also considering for the present only the first term Xj of the 

 integral, and assuming that A and a are independent of y 

 and z in each section of the bar — we have after integrating by 

 parts, 



where Z = length, and S = section of the bar. 



At the end (x = l) between the pole-pieces we have A=I, 

 while «=H + 2wI, corresponding to the action of the pole- 

 pieces and of the free end of the bar C,. 



The distribution of a along the axis was found by throwing 

 out a small exploring coil from different points on the axis. 

 The results showed that a certain region existed in which a 

 was always very small and nearly constant. The ordinates of 

 the curves abc and def, fig. 1, represent the values of a at 

 points on the axis, with magnetizing currents of 7 and 25 

 amp. respectively. 



The length of the bar C 2 was so chosen that it reached to 



* Maxwell, ' Electricity and Magnetism,' vol. ii. § 639. 



