UPON MAGNETISATION, AND CONVERSELY, IN IRON, STEEL, AND NICKEL. 503 



In nickel, the induction reached is much greater when vibrations are superposed (A 

 conditions) than one-half the induction change on reversals when vibrations are 

 permanently acting (B conditions). Compare figs. XL and VIII. As we have 

 seen, the reverse holds for iron and steel. Compare figs. I. and II. No systematic 

 investigation, however, was made as to how these relative effects, under the A and B 

 conditions, may vary through wide ranges of field, in nickel as compared with iron 

 or steel. 



As the cyclic extremes are departed from, the induction change which occurs when 

 vibrations are superposed follows the field, not the field change. This effect, however, 

 is a decreasing one, and a point is reached when the field is decreasing where super- 

 posed vibrations produce no induction change whatever. 



4>00 



/ ANNEALED 



A. CONDITIONS 



2 V/ FIG. XI 

 //nickel 



FIG IX 



IRON 



B v 



It should be noted that, unless demagnetisation precedes each observation (A con- 

 ditions), thus wiping out the effects of the immediately preceding superposition of 

 vibrations, it is highly improbable that any induction change, opposite in sense to the 

 field change, would be observed. For instance, if, as in Ewing's experimental method, 

 field change and vibrations alternated all round the loop, the possible induction change 

 would be exhausted on the up-curve at or near the cyclic extreme ; and since the effect 

 is a decreasing one, no induction change opposing the field change would be obtained 

 with the same or any less intensity of vibrations when the cyclic extreme is departed from. 



When this neutral point is passed, the superposition of vibrations produces induction 

 change, in the same sense as the field is changing, and continues to do so until the 

 first conditions are reverted to at the other cyclic extreme. 



In low fields the neutral point occurs close to the vertical axis (fig. IX. for iron, 

 fig. XI. for nickel). In high fields it is thrust from the vertical axis, and towards the 



