644 Mr. R. Beattie on the Hysteresis of JXickel and 



the magnet had been brought to a standstill, combined with 

 a knowledge of K and V, ought to have been sufficient to 

 determine W. But, practically, perfect symmetry was, of 

 course, unattainable, and 6 varied considerably during a 

 rotation so that a mean value had to be taken. The plan 

 actually followed was to rotate the magnet clockwise 15° at 

 a time, the position of the pointer P being read off on a 

 circular scale at each halting-place ; then to rotate the magnet 

 counter-clockwise, taking readings every 15° as before. Half 

 the difference between the means of the two sets of readings 

 gave the proper value of 9. The constant K was obtained by 

 loading the suspended system with a mass of known moment 

 of inertia, and observing its periodic time when making 

 torsional oscillations about OW (1) in the loaded, (2) in the 

 unloaded condition. 



In determining the intensity of magnetization it was 

 assumed that the disk, closely similar in form to a flattened 

 ellipsoid of revolution, and placed in an approximately uni- 

 form field, would be pretty uniformly magnetized, and that 

 its demagnetizing factor might, in these circumstances, be 

 taken as 



N = 7T 2 



(2) 



where c is the thickness and a the diameter of the disk. To 

 obtain the intensity two distinct experiments were needed. 

 One of these was made by removing the disk and rotating 

 between the poles of the magnet an exploring coil connected 

 with a ballistic galvanometer. This gave the strength, H, of 

 the applied field. For the other experiment a few turns of 

 wire were wound diametrically round the disk and connected 

 with the ballistic galvanometer. The disk thus over-wound 

 w T as then placed in the field with its plane horizontal and 

 quickly turned through 180°, the diameter of winding placed 

 perpendicular to the lines of force. The resulting galvano- 

 meter-throw r measured the total magnetic flux, Y, through 

 the coil, and the intensity of magnetization, I, was readily 

 deduced from the equation 



F " A > H (3) 



1 = 



4ttA< 



■A,N' 



in which A x is the area included within the coil wound dia- 

 metrically round the disk, and A 2 is the area of a diametrical 

 cross- section of the disk taken perpendicular to its plane. 



The experiments on nickel were carried out with a disk 

 •0525 cm. thick and 3*97 cms. in diameter. The results are 



