1895.] in the Magnetic Permeability of Iron. 231 



The slow increase of hysteresis,* wliicli is the subject of this 

 communication, must not be classed with the immediate effect pro- 

 duced by the moderate heating of iron. When the iron of trans- 

 formers is heated, in working, it at first absorbs less energy than 

 when cool, probably because the increase in its resistance lessens the 

 eddies set up in it. This is the case whether the heating is by 

 ordinary means or by magnetic reversals. It is a temporary effect 

 the loss goes up again on cooling. If it is kept heated, a slow change 

 takes place, the loss gradually increasing again, from the decrease of 

 permeability caused by heat.f If the temporary reduction of the loss 

 is entirely accounted for by the increased resistance reducing the 

 eddies, it appears to afford a means of separating the losses caused 

 by hysteresis proper, and those caused by eddies. The ordinary 

 measurement of magnetising loss of course gives the sum of these 

 two. 



The increase of hysteresis shown in the tests, and resulting from 

 the heating, is somewhat greater than the author has observed in 

 transformers where the heat has been caused by the magnetic and 

 electric losses ; possibly this is because in the latter cases the tempe- 

 rature has been somewhat lower. 



All the tests show an increase of loss, the maximum apparently 

 nob yet having been reached. The tests are being continued. 



Conclusions. The conclusions to which these observations lead, so 

 far as they have gone, are : 



1. The effect is not fatigue of the iron caused directly by repeated 

 magnetic reversals it is not " progressive magnetic fatigue." 



2. Neither magnetic nor electric action is necessary to its pro- 

 duction. 



3. It is a physical change resulting from long-continued heating at 

 a very moderate temperature. 



4. It appears to be greater if pressure is applied during heating. 



5. It is not produced when the iron is not allowed to rise more 

 than a few degrees above the ordinary atmosphere. 



6. It is similar to the effect produced by hammering, rolling, or by 

 heating to redness and cooling quickly. 



7. The iron returns to its original condition on re-annealing. 



8. It does not return to its original condition if kept unused and at 

 ordinary atmospheric temperatures, whether the periods of rest arc 

 short or long. 



* This effect may have an important influence on the reliability of measuring 

 instruments having iron portions magnetised by alternate currents. The constant* 

 of such instruments may gradually fall. 



f See Appendix III, p. 212. 



