1889.] On Time-lag in the Magnetisation of Iron. 



283 



that the proportion of creeping'to immediate effect is much the same 

 with high as with low magnetisation. 



One may refer, in this connexion, to the energy which is dissipated 

 through hysteresis, in performing a small cycle "by alternately 

 applying and removing a very small force The action is the 



same in kind whether there is or is not additional magnetisation. 



The energy dissipated in each cycle is — and vanishes when 



the increment and decrement of |j go on pari passu with the incre- 

 ment and decrement of |$. 



Consider now fig. 1. When the repeated cyclic changes of 1) are 

 indefinitely rapid and go on without pause, so that creeping has 

 not time to occur, a single straight (or sensibly straight) line such 

 as OA represents the relation of the change of magnetism to the 

 (very small) change of magnetising force, during both increment 

 and decrement. The rapidity of the action prevents any loop from 

 being formed, and there is consequently no sensible dissipation of 

 energy through hysteresis. This state of things is perhaps nearly 

 realised in the case of a vibrating telephone diaphragm, or, in regard 

 to circumferential magnetisation, by an iron conducting wire in a 

 telephone circuit. Again, let the cycle be performed indefinitely 

 slowly. In that case the magnetism, at every stage of the cycle, creeps 

 up or down to a steady value. A sensibly straight line, such as OB, 

 represents the relation of § to p during both increment and decre- 

 ment; and there is again no dissipation of energy. But with any 

 frequency of alternation lying between these extremes of infinitely 

 fast and infinitely slow, a loop will be formed, since the creeping will 

 take effect most considerably at and near the ends of the range (the 

 time-rate of change of Ij being least there), and there will be dis- 

 sipation of energy. When the limits and mode of variation of |p are 

 specified, there must be some particular frequency which will make 

 the energy dissipated per cycle a maximum. 



The phenomena described in the paper have been reproduced in 

 several specimens of annealed iron wire,, of course with quantitative 

 differences. As to the amount of magnetic creeping much depends 

 on the annealing of the specimen. Another piece of iron wire cut 

 from the same bundle as the piece with which these experiments were 

 made, and annealed at another time, showed almost exactly the same 

 susceptibility to magnetism as the first piece, so far as immediate 

 effect went ; but in it the subsequent creeping up was decidedly less 

 (in the proportion of about 4 to 5). 



When the iron is hardened by mechanical strain the phenomena of 

 creeping vanish almost completely. A specimen from the same 

 bundle was annealed, and showed much creeping. It was then put 

 in the testing machine and pulled until it took a set of 1 or 2 mm. 



