Heating on the Magnetic Properties of Iron. 259 



already results have been obtained which appear to be of sufficient 

 novelty to warrant publication. 



The hysteresis of the iron was directly measured by means of 

 Professor Ewing's Hysteresis Tester, in which the work spent on a 

 specimen rotating in a magnetic field is observed and is compared 

 with the work spent in rotating standard specimens.* In these 

 experiments the same pair of standards was used throughout for 

 the calibration of the hysteresis tester. The test specimens were 

 all cut from the same sheet f metal and were of soft Swedish 

 transformer plate, having very low initial hysteresis. They were 

 first tested in the annealed state and were then heated in small 

 ovens which were kept hot by means of incandescent lamps. The 

 temperatures of the ovens were observed in most cases by mercury 

 thermometers, but those above 200 C. were measured by a 

 Callendar-Griffiths platinum pyrometer. The specimens were taken 

 out of the ovens from time to time to be tested, and all the tests 

 of hysteresis were made at atmospheric temperature. It was not 

 found possible to keep the temperature of each oven very constant, 

 but when the ovens were once hot, the variation of temperature 

 was rarely more than 10 degrees C. in either direction. To these 

 variations may be ascribed certain irregularities which will be 

 apparent in the observations, but the general character of the 

 changes due to prolonged heating is sufficiently clear. Each specimen 

 consisted of a bundle of seven strips 3 inches long, and about J- inch 

 wide, and each strip was annealed separately by heating it to redness 

 in a Bunsen flame, and allowing it to cool in the air. As the effects 

 of prolonged heating described below were in all cases found to be 

 completely removed by reannealing, the same samples could be used 

 over and over again, and this was in fact done in most cases. In all 

 the experiments the measurement of hysteresis relates to cyclic pro- 

 cesses in which the induction B changes from + 4000 to 4000 

 C.G.S. units. 



The effects produced by baking differ widely at different tempera- 

 tures. Below 40 C. the author has found no evidence of any 

 change. Between 40 C. and about 135 C. the hysteresis simply 

 increases with time, at least during the longest time of heating tried 

 in these experiments. The increase of hysteresis is relatively rapid 

 at first, and becomes slower as times goes on. Curves 1 4, fig. 1 

 show results of this nature by exhibiting the percentage increase 

 in hysteresis after various times of baking. The absolute values of 

 the hysteresis at the different stages are stated in Table I in ergs 

 per cycle per cubic centimetre (for B = 4000) together with the rise 

 expressed as a percentage of the initial hysteresis to the nearest 



* ' Journal Inst. Elect. Eng.,' vol. 24, p. 403 j also ' Min. Proc. Inst. C.E.,' vol. 

 126, p. 206. 



