152 Mr. S. E. Eoget, Effects of prolonged 



or would show some degree of immunity to temperature effects. It 

 was found that the susceptibility to change at moderate temperatures 

 was not by any means entirely removed by prolonged heating at high 

 temperatures. The subsequent action of low temperatures was, how- 

 ever, slower than in freshly annealed iron, especially after the high 

 temperature had been applied for a considerable time, also it appeared 

 that temperatures at 500 or 600 C. produced less effect on the 

 susceptibility to subsequent change at lower temperatures than was 

 produced by more moderate degrees of preliminary heating. A com- 

 plete series of experiments on this point, however, has not yet been 

 made, only a few samples having been re-heated in this way, but the 

 results obtained with these were consistent, and pointed to the above 

 conclusions. 



The latter part of the action at the higher temperatures resembles an 

 incomplete annealing, as there appears to be little difference between 

 the state after the iron has been heated for a few hours at a tempera- 

 ture just short of the " critical " temperature at which ferromagnetic 

 quality disappears, and that of the same material which has been 

 heated above this temperature so as to become completely annealed. 



The above experiments, and those in my previous paper, refer to one 

 particular brand of iron, all the specimens having come from the same 

 sheet. A few examples, showing how widely different is the behaviour 

 of different brands of steel and iron, are given in figs. 2 and 3. The 

 data for these diagrams are to be found in Tables II and III, where 

 the absolute values of the hysteresis in ergs per cubic centimetres per 

 cycle are given together with the rise expressed as a percentage of the 

 initial hysteresis. 



Figs. 2 and 3 relate to various samples of commercial iron and steel, 

 some of which were supplied by makers in this country and some from 

 America. Fig. 2 shows the effects of heating at 120 C., and fig. 3 

 shows the effects of heating specimens of the same iron at 230 C, 

 The curves, numbered alike in both figures, refer to the same material. 

 All these samples were initially in the annealed state. No. 1 is a piece 

 of the iron used in the previous experiments, and is given here for the 

 sake of comparison. No. 2 is a sample of special transformer steel of 

 equally low initial hysteresis. The action of heat on it is similar in 

 general characteristics to the action of No. 1, but much less in degree. 

 No. 3 is practically " non-ageing," even in the annealed state, and 

 although not of such low initial hysteresis as some of the other speci- 

 mens, would be the most suitable for transformers on account of its 

 immunity from change by prolonged heating. No. 4 is a sample of 

 sheet-iron not specially made for transformers ; it is of poor magnetic 

 quality, but is interesting as showing, at 120 C., effects which require a 

 higher temperature in the other brands of iron ; the initial rise is very 

 rapid, and the subsequent fall of hysteresis is clearly shown, even at 



