## Extract

It is well known that if a piece of iron be subjected to a considerable magnetising force and then be tested for permeability correnponding to a lower force, the permeability so obtained may differ widely from the permeability which would have been obtained had the material been previously demagnetised. The effects of previous history have been studied by a good many experimenters, notably by Searle. The object of this part of the present paper is to examine the effect of previous history upon the dissipation of energy by magnetic hysteresis. Suppose in fig. 1 that a piece of iron is carefully demagnetised and that the hysteresis loop No. 1 corresponding to a force H is obtained. This loop is symmetrical about the origin in all respects and its area gives the loss usually referred to in testing work. Suppose that the last value of the force H is positive and that it be increased to a value H_{1}, reversed a few times, and reduced to zero from its positive value. After re-applying the force H and reversing it a sufficient number of times to produce stability, a loop such as No. 2 will be obtained. As is well known, this loop shows a reduced change of magnetic induction and, consequently, a reduced permeability. Suppose, now, that the iron is carefully demagnetised and that a magnetising force supplied by an independent source is applied very gradually, so that when added to the original force H it gives a force H_{3} such that the change of magnetic induction corresponding to a change of the force H is exactly equal to the change observed with loop 2 for the same change of H. The loop obtained will appear as No. 3 in fig. 1. We have now obtained two loops, Nos. 2 and 3, each having the same change of magnetic induction, and each having the same net change of force H. The change from loop 1 to loop 2 has been brought about by inter-molecular force, whereas the change from loop 1 and loop 3 has been brought about by the application of an externally applied constant force H_{3} - H. If the effect of inter-molecular force were capable of being exactly equivalent to the externally applied constant force, one would expect to find that the energy required to perform a complete cycle would be the same in each case; that is, the area of loop 2 would be equal to the area of loop 3.

## Footnotes

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- Received May 4, 1909.

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