154 Mr. S. E. lioget. Effects of prolonged 



more hysteresis than if left in the state in which it was submitted by 

 the manufacturer. 



The author has no information as to the treatment by which this 

 remarkable degree of "non-ageing " quality had been produced. The 

 general characteristics of the action at 230 C. on the different samples 

 (see fig. 3) are much the same, differing only in degree. No. 3 is, again, 

 little changed by prolonged heating. 



It seems from these and other tests, that brands of transformer steel, 

 which are practically " non-ageing," are obtainable commercially, but 

 they are not (at least in these examples) of such low initial hysteresis 

 as the " Swedish iron," which was formerly considered the best mate- 

 rial for transformers. The effects of annealing vary much in differ- 

 ent samples. All the samples tested after annealing have been 

 found to be more liable to change in that state than in the state in 

 which they were supplied by the makers. The method of annealing 

 and rate of cooling may have much to do with the " non-ageing " 

 quality of the material. Incidentally the experiments have given 

 some evidence that samples of iron may undergo a slight change in 

 hysteresis, even if kept at atmospheric temperature for three or four 

 years. 



It may be convenient to briefly summarise the chief effects of pro- 

 longed heating on the magnetic properties of iron which have been 

 observed in these and the previous experiments. 



1. Material in the annealed state is more liable to change than in a 

 harder state. 



2. All the changes produced by prolonged heating are completely 

 removed by re-annealing. 



3. The heating need not be continuous ; the same cumulative effect 

 is produced by a number of short periods at a given temperature as by 

 a continuous heating at the same temperature. 



4. The effect may be regarded as being due to two actions super- 

 posed, one tending to increase the hysteresis, this action being the 

 more prominent at lower temperatures ; the other analogous to an 

 incomplete annealing, tending to decrease the hysteresis, this action 

 predominating at higher temperatures. 



5. The liability of the material to increase in hysteresis at moderate 

 temperatures is not removed by prolonged heating at high tempera- 

 tures. 



6. The change is confined to the lower part of the B.-H. curve, the 

 saturation value of the magnetisation being substantially unaltered. 



7. The effect is produced equally, whether the iron is or is not 

 exposed to the air during heating. 



In conclusion the author wishes to express his thanks to Professor 

 Ewing, for placing at his disposal the facilities which have enabled 

 these experiments to be carried out, and for much other kind help. 



