224 Mr. W. M. Mordey. On Slow Changes [Jan. 17, 



III. " On Slow Changes in the Magnetic Permeability of Iron." 

 By WILLIAM M. MORDEY. Communicated by Professor 

 SILVANUS P. THOMPSON, F.R.S. Received December 19, 

 1894. 



When iron is magnetised for a long time by rapidly alternating 

 currents, its magnetic permeability is usually reduced, a gradual 

 increase taking place in the amount of energy absorbed in producing 

 a given magnetisation. This effect has been observed in connection 

 with the working of transformers on alternate-current systems. 

 Although it has been known for some time to a few electrical en- 

 gineers, the author is not aware that any investigation or proof has 

 been published as to the cause of this change.* 



When the author first became aware of this increase he investigated 

 the subject in connection with the work of the Brush Electrical 

 Engineering Company, and to that company acknowledgments are 

 due for permission to publish the results so far obtained. It may be 

 mentioned that the investigation is still in progress. 



In the first place explanations were sought in direct connection 

 with the magnetic or electric actions that take place. 



The explanation that first suggested itself was that eddy currents 

 were being set up in the coils by leakage or partial failure of the in- 

 sulation between adjacent portions of the conductor, or that eddy 

 currents were being set up to an increased degree in the iron by 

 disturbance or change of the insulating material interposed between 

 the thin plates of iron of which the transformers were composed. 



* The following are the only published references to this subject, so far as known 

 to the author : 



In the ' Electrician ' of December 7th, 1894, the subject is introduced by Mr. G-. 

 W. Partridge, with some examples of the increase and a statement that the effect is 

 due to a " molecular change or fatigue in the iron." 



There is also a note by Professor Ewing referring to Mr. Partridge's article, also 

 ascribing the effect, if confirmed, to " a progressive magnetic fatigue." 



In the same journal for December 14th, 1894, there is an article on the subject 

 by Dr. Fleming, stating that the effect is not general, that he has failed to find it in 

 some cases, and pointing out that differences of temperature at the times of testing 

 may account for the variations observed. 



Mr. Ki. T. Smith states that in one sample of iron he has found considerable 

 increase after about 240,000,000 reversals. 



Mr. O. T. Blathy, of Buda-Pesth, states that he is familiar with the effect, that the 

 loss in magnetising transformers increases from 20 to 25 per cent., and that artificial 

 heating of the transformer for several hours to about 150 C. will have the same 

 effect, which he states is permanent. 



Sir David Salomons also states that he has referred to the effect in a work 

 written some months ago. 



