RESEARCHES IN MAGNETISM. 
533 
when we reverse the change of magnetising force from increment to decrement, or 
vice versa, the magnetisation begins to change very gradually relatively to the change 
of <§, no matter how fast it may have been changing, in the opposite direction, before. 
So much is this the case that the curves, when drawn to a scale such as that of the 
figure, appear in all cases to start off tangent to a line parallel to the axis on which 
is measured, wdienever the change of >§ is reversed in sign. 
§ 12. In a large number of other experiments I have examined the effects of 
varying the field in all possible ways. The accompanying changes of 33 always 
exhibit static hysteresis with respect to the variations of <§. The curves connecting 
these quantities always form loops as in fig. 2, and the characteristic mentioned in the 
last sentence of § 11 appears to be quite general. It is scarcely necessary to point 
out that this is not what we should expect from the theory of retentiveness suggested 
by Maxwell (§ 3). On the other hand, it is just what we should expect if we 
suppose that there is a static frictional resistance to the rotation of Weber’s 
magnetic molecules. 
An interesting point may be noticed with regard to the small loops, two of which 
are shown in the diagram (fig. 2). Suppose that either on the ascending or the 
descending branch of the main curve we begin to form such a loop by removing the 
magnetising force, taking as the starting point a place in the main curve such that 
when Ip becomes zero 33 is also zero. Evidently this could be done by selecting a 
suitable starting point either in the positive part of the ascending branch of the main 
curve or in the negative part of the descending branch. Then when Ip is reduced to 
zero the piece combines entire freedom from magnetisation with absence of magnetising 
force, but its condition is widely different from that of a previously unmagnetised piece. 
In particular, it is unsymmetrical as regards susceptibility to magnetisation in the two 
longitudinal directions, being much more ready to take magnetism of an opposite sign 
from that which it last possessed than to take magnetism of the same sign. The 
curves of 2> and <!p in the two quadrants, if we imagine them both to be drawn, would 
meet in a sharp angle at the origin, instead of being continuous, as they would be in a 
previously unmagnetised specimen. This is indeed only one example, though a very 
striking one, of the fact that in consequence of hysteresis a condition of no 
magnetism, in a field of no force, is capable of being reached by many processes, some 
of which will leave the metal ready to show a startling want of neutrality when it is 
subsequently magnetised in one or the other direction. 
§ 13. Residual Magnetism in Soft Iron .—A feature in these and other early experi¬ 
ments which caused me much surprise was the largeness of the residual magnetism. 
In fig. 1, 87 per cent., and in fig. 2, 81 per cent, of the total induced magnetism 
remains when the magnetising force has been completely removed ; and in other 
examples I have found the residual magnetism of soft iron to be 90 and even 93 per 
cent, of the induced magnetism. It is generally stated in the best text-books that 
the magnetic condition of soft iron disappears almost wholly when the inducing field 
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