222 



Mr. C. V. Boys on a 



interesting to note that the curve shown is a natural curve 

 depending on circular functions only. It is independent of 

 the nature of material, such as conductivity, moment of inertia, 

 or of the strength to which the field is made to grow from 

 zero. It is subject to a small error, for I did not destroy the 

 residual magnetism. That the effect of this is appreciable is 

 evident, for in one series of experiments with a disk the throw 

 on making was always from 45° to 33° ; but the first time 

 that the direction of the magnetizing current was changed 

 the throw was from 45° to 35°, after which it was from 45° to 



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33°, as before. This diminution at the first reversal was always 

 the same. I found almost the same fall with the half-crown 

 as with the coil, and with one cell as with ten cells. There 

 were slight differences, but not more than draughts from 

 which I did not shield the needle would have accounted for. 

 The lower angle can be read with precision, but the higher 

 angles become difficult to observe as the damping influence 

 diminishes. The position a = 90° is one of instability ; for how- 

 ever slight a velocity is given to the disk, it will not be brought 

 to rest for a considerable time, owing to the very minute nature 

 of the squares of the cosines of angles nearly equal to 90°. 



The last series of experiments was made with a view to 

 determine whether the strength of field determined by obser- 

 vation of the throw of the disk at breaking agreed with the 



