526 
PROFESSOR J. A. EWING ON EXPERIMENTAL 
an angle /3—/3 0 , which he calls the “ permanent set ” of the molecule. Maxwell has 
developed the consequences of this supposition at length, but into these it is not 
at present necessary to follow him, further than to point out that (following up the 
analogy to a strained solid whose elasticity is perfect up to a certain limit, and which, 
when strained beyond that limit, acquires a new limit of elasticity at or near to the 
greatest value of the previously applied stress) we should expect to find, after the 
application of any force X, a purely elastic series of magnetic changes when we 
subsequently remove and reapply X. The movements of each molecule would then 
lie between the angle of previous permanent set /3—/3 0 and the angle /3, and between 
those limits we should find each molecule taking up one and the same direction for 
each value of X, whether that was reached by increment from a lower, or decrement 
from a higher value. In other words, tins modified theory leads us to expect no 
hysteresis in the relation of magnetic intensity to magnetising force when, after its 
first application, a magnetising force is subsequently removed and reapplied. 
§ 4. To test this point, I have examined experimentally the cyclic changes of 
magnetisation which accompany cyclic changes of magnetising force, both when the 
force is confined to one direction, and also when its sign is reversed. The results do 
not support Maxwell’s extension of Weber’s theory, for they show that all changes 
of magnetism caused by changes of magnetising force exhibit that lagging action 
which I have called hysteresis. The word “ retentiveness,” commonly restricted to 
name that property in virtue of which the magnetic metals retain a portion of their 
magnetism when the magnetising force is removed, might with propriety be extended 
so as to designate the resistance to any change of magnetic state (whether increase or 
decrease) which they exhibit whenever the magnetic field in which they are placed 
suffers any change. The existence of residual magnetism when the field is reduced 
to zero is, in fact, only one case of an action which occurs whenever the field is varied 
in any way, namely, a tendency to persistence of previous magnetic state. And my 
experiments show that when the magnetism of iron is altered by varying, not the 
field, but the state of stress of the metal, the same tendency again manifests itself. 
The word retentiveness might be extended so far as to cover this case of lagging also; 
and if hysteresis were found only in these and similar magnetic phenomena, there 
would be no need to invent a name for it. But, as will be shown later, there are 
cases of hysteresis which have, as far as can be seen, nothing to do with magnetic 
condition, and which the quality of magnetic retentiveness, even if understood in the 
widest possible sense, cannot account for. I have therefore found it convenient and 
even necessary to employ a new term, which merely designates this peculiar action 
without implying any theory as to its cause. 
§ 5. As regards the hysteresis which occurs when the magnetism of soft iron is 
changed, my experiments confirm the idea already suggested by other observers, that 
when the molecular magnets of Weber are rotated they suffer, not first an elastic 
and then a partially non-elastic deflection as Maxwell has assumed, but a kind of 
