UPON MAGNETISATION, AND CONVERSELY, IN IRON, STEEL, AND NICKEL. 511 



Magnetic Hysteresis. 



When field varies cyclically, the loop formed by the curves connecting B and H is 

 said to be the result of magnetic hysteresis ; no loop, no hysteresis. 



The effect of vibrations superposed at all stages of the normal loop (A conditions) 

 is, generally speaking, to lessen those differences of magnetisation to which hysteresis 

 without vibrations has already given rise. 



But if the vibrations continue, and the field becomes cyclic, the effects of vibrations 

 cannot be stated concisely in terms of the effects of magnetic hysteresis. To do so, when 

 cyclic field is superposed upon permanently acting vibrations (B conditions), requires as 

 complete a knowledge of the B, H loop as is required without vibrations. 



The loops obtained when vibrations are permanently acting (B conditions) are 

 stable to subsequent "offs" and " ons " of the same vibrations. The loops obtained 

 without vibrations are not stable to superposed vibrations (A conditions). 



Obviously Ewing's statements quoted at the beginning of this paper are not 

 universally applicable. Although his experimental methods, in which field change and 

 tapping alternated, almost certainly precluded the observation of the neutral points, 

 his statements are quite applicable to the A conditions whatever the intensity of the 

 vibrations. They are, however, not applicable when applied to the B conditions, 

 when the intensities of the vibrations are such that their effects have not reached a 

 limiting value. 



Molecular Theory. 



§ 182 of Magnetic Induction in Iron may be consulted. The general nature of the 

 argument is shown by the following quotation : — " Any kind of disturbance that will 

 give the molecular magnets intervals of freedom, or of diminished constraint, will tend 

 to do away with hysteresis." This statement also appears completely applicable if 

 the disturbances be superposed under the A conditions. Under the B conditions I 

 prefer a deduction relative, not to hysteresis, but to the rate of magnetic change with 

 field change, subject to any condition which the molecular theory may demand. 

 However much B v may exceed B in low fields, the ratio B v /B must in high fields 

 approximate to unity. Consequently, if the differential permeability in low fields be 

 greater with than without vibrations, in high fields the reverse necessarily follows. 



My deductions therefore are : (1) that if the cyclic amplitude be not unduly increased 

 the differential permeability will be greater with than without permanently acting 

 vibrations ; (2) that the increase of differential permeability with vibrations may be 

 associated either with increase or decrease of residual magnetisation, coercive force, or 

 hysteresis loss, but that there is greater probability that those magnetic properties 

 which depend upon hysteresis will be increased with vibrations for the same value 



